The Release of Soluble End Products of Metabolism 111

Copepods

Colattukudy), pp. 50-91.

Kilvington, C. C. (1977). In. XI. Lipids in Calanus 3 The release of soluble end D Marine Biological Associa- products of metabolism A. E. M., and Blaxter, ROBERT LE BORGNE f marine zooplankton by no gairdneri), Marine Bio- l i , J. R. (1982). Fatty acid i Transactions, 10, 463-4. Introduction i the triacylglycerol, wax The organic matter assimilated by a copepod undergoes a series of caeca of rainbow trout i metabolic reactions that leads to the production of living matter (as s and wax esters. Com- 1 growth, moults, storage and gametes) and energy. The latter is prod- I uced by the oxidation of organic compounds by molecular oxygen ! during the respiration process. End products of these reactions enter the external medium through two physiological mechanisms that are usually studied. separately: (1) respiration, stricto sensu, which is concerned with oxygen consumption and the release of carbon dioxide (CO,); (2) excretion, which deals with the liberation of the remaining Fatty acid synthesis and end products. When catabolism is complete, the end products are 37-79. inorganic (ammonium, phosphate) , whereas when it is incomplete or sho, K., and Fujita, S. when the end products are further transformed before excretion (e.g. !&catilis as a living feed urea synthesis), the compounds released are organic. Because both of the Japanese Society for ! excretion and respiration lead to the elimination of catabolic end products, because they have much in common in terms of the methods , and Yone, Y. (1979). used for their measurement and the factors influencing them are j Brachionus plicatilis and identical, it seems logical to consider both releases in the same 'letin of the Japanese Society chapter. le intact lipids of petrel Respiratory exchanges (i.e. oxygen uptake and CO, release) of copepods take place on the tegument and at the rectum. Excretion is achieved by the maxillary glands and two to four nephrocytes that are located superficially in the cephalic region. Soluble excretion, secretion and moulting losses are distinct processes that are difficult to I differentiate when the products released by a marine animal are analysed. Such processes, however, give rise to compounds that differ from those of defaecation, a process concerned with the release of particulate matter than has not been assimilated (see Chapter 6). During the last 20 years, many studies have been made of the respiration and excretion of planktonic animals, mainly because of the

l availability of satisfactory analytical methods and because excretion ! has been recognized as playing an important role in the functioning of ORSTOM Fonds Documentaire

I i

I 110 The Biological Chemistry of Marine Copepods The relei marine ecosystems. This latter point was stressed by Ketchum (1962) main problem with following the studies of Harris and Riley ( 1956) and Harris ( 1959), so in the amounts of s( giving rise to many studies of the contribution made by zooplankton as small as possible, excretion to the nutritional needs of phytoplankton. In addition to this to be so large that tl kind of study, it has been found useful to combine respiration and the incubation. In c excretion rates in order to characterize the type of substrate being 1 the length of the ex

oxidized by zooplankton to produce its energy (Conover and Corner, ~ Thus, for a given tl 1968) and to assess the efficiency of utilization of ingested organic I tion, the longer t matter (Butler, Corner, and Marshall 1969, 1970). influencing the rate Both aspects of the release of soluble end products will therefore be considered. The first aspect is concerned with the proportions of the Short incubations a different kinds of compounds released; the second deals with rate The advantages of measurements and the part played by respiration and excretion in the and short incubatil cycling of organic matter. Most of the studies described here have hours, are that they been concerned with marine copepods, but occasional reference is I closed systems, and made to related work with freshwater species and with other groups in larly as far as diel the marine plankton. ever, to stress effects

c introduction into t Methods of measuring copepod respiration and excretion crowding. Moreove animal concentrati Measurements of respiration and excretion are expressed as rates, i.e. errors. Such considc amounts of oxygen or energy consumed, or CO,, nitrogen or phos- rates, and their vari phorus released per unit of time and body weight. Two kinds of tion experiments (: methods have been generally used: the assay of enzymatic activity and I 1974, for Calanus he1 the incubátion technique. Enzyme assays allow respiration rates to be pages ppicus; Ikeda inferred from measurements of the activity of the electron transport Acyocalanus gibber, C system (Packard 1971) and those of ammonium excretion from glut- plumata). amate dehydrogenase activity (Bidigare, King, and Biggs 1982). In the incubation technique, the measurements are those of concentrati- Crowding and confine ons of compounds consumed or released by animals which are left in a of animal concenkr closed, vesse! for a certain period of time. Values from enzymatic that in the natura assays have to be calibrated by incubation experiments to produce the assembling numbe actual rate in the environment (Packard and Williams 1981). Enzy- effect is caused by 1 matic assays are reviewed by Mayzaud in Chapter 5. Accordingly, here, by the ratio betwec detailed attention is given only to the incubation technique, which is flask. According tc still widely used and is the only one providing information about the decrease in rates di types of compounds released by copepods. the animals are sir During an incubation experiment, the respiration and excretion of This may explain tl one or several animals causes variations in the concentrations of oxy- crowding effect. Fi gen or soluble end products, which can be analysed either at regular et al. (1982a) betwc time intervals or at the beginning and the end of the experiment. The A.gibber at concent i .-

Copepods The release of soluble end products of metabolism 111

#edby Ketchum (1962) main pr,oblem with this technique is to measure significant differences ) and Harris ( 1959), so in the amounts of soluble compounds so that analytical errors will be made by zooplankton as small as possible, while at the same time not allowing these changes :ton. In addition to this to be so large that they cause drastic alteration to the seawater during mbine respiration and the incubation. In other words, a compromise must be found between ype of substrate being the length of the experiment and the concentration of the copepods. (Conover and Corner. Thus, for a given temperature and species, the lower the concentra- ln of ingested organic tion, the longer the incubation, for both are important factors 370). influencing the rates. )ducts will therefore be the proportions of the Short incubations and high animal concentrations xond deals with rate ln and excretion in the The advantages of experiments that use high animal concentrations and short incubation times, lastirig from several minutes to a few s described here have masional reference is hours, are that they avoid the starvation effect which often happens in Id with other groups in closed systems, and they reflect the actual metabolic activity, particularly as far as diel rhythms are concerned. They are sensitive, however, to stress effects resulting from the capture of the animals and their introduction into the vessel, and to the effects of confinement and id excretion crowding. Moreover, physiological state is harder to control at higher animal concentrations and short incubations can lead to analytical expressed as rates, i.e. errors. Such considerations also explain why respiration and excretion O,, nitrogen or phos- rates, and their variability, are generally higher at the start of incuba- veight. Two kinds of tion experiments (see Nival, Malara, Charra, Palazzoli, and Nival mzymatic activity and 1974, for Calanus helgolandicus, Temora sblifera, Acartia clausi and Centro- respiration rates to be pages typicus; Ikeda, Hing Fay, Hutchinson, and Boto 1982a, for the electron transport Auocalanus gibber, Calanopia elliptìca , Eucalanus subcrassus and Pontellìna i excretion from glut- plumata). and Biggs 1982). In :those of concentrati- Crowding and conjnement effects. These are consequences of the increase ials which are left in a of animal concentrations in laboratory experiments compared with thes from enzymatic that in the natural environment. The crowding effect results from iments to produce the assembling numbers of animals far above normal; the confinement Villiams 1981). Enzy- effect is caused by the small size of the vessel and may be represented 7 5. Accordingly, here, by the ratio between the volume of the animal and the volume of the n technique, which is flask. According to Marshall (1973), referring to Zeiss (1963), the nformatioii about the decrease in rates due to crowding would depend upon whether or not the animals are similarly concentrated in their natural environment. ttion and excretion of This may explain the conflicting results’in the literature relating to the oncentrations of oxy- crowding effect. For instance, no difference was observed by Ikeda ysed either at regular et a¿. (1982a) between the ammonium or phosphate excretion rates of -the experiment. The A.gibber at concentrations of one individual in 4 ml and 34 individuals The Biological Chemistry of Marine Copepods The in 100 ml, or for C. ell$tica at similar concentrations. By contrast, , ported that thi Razouls (1972) reported that T. stylifeera displays decreases in respira- ;ADP)/ (ATP tion rate of 22 per cent and 30 per cent when the concentration is and recovers ' increased from 50 to 100 and from 50 to 200/100 ml, respectively. intense energy Corresponding decreases for C. typicus respiration rates were 7 and 22 larly, the EC ( per cent. Similarly, Smith and Whitledge (1977) showed that urea I that of animal excretion rate was about three times as high for Pleuromamma spp. at less disturbed 1-2 animals/100 ml than at 3-4 animals/lOO ml. They did not, how- I other hand, st ever, quote any differences for rates of respiration and ammonium for the higher excretion. As far as the confinement effect is concerned, Menzel and difference was Ryther (1961) found that at equal concentrations of animals, the or remained u respiration rate was lower in vessels of a volume less than 500 ml. larly, Gardner rate of Eucalan Stress effects. In most studies dealing with the influence of incubation first 10 min ol time on metabolic rates, an important decrease is observed at the animals. Anot beginning, followed by a steady rate or a slower decrease. This is I transferred fro shown in Fig. 3.1 for the ammonium excretion rate of T.stylifeera. The decrease may be ascribed to abnormally high rates at the beginning, because of the stress effect. This is followed by normal values, after 80 which starvation lowers the level of the metabolism. Several studies have provided evidence that the high rate values at O the beginning could be caused by the stress effect or the acclimatiza- 401 tion of the animals. Skjoldal and Båmstedt (1977), for example, re- I A

1.5-

O O Time (h) Fig. 3.1. Decrease in ammonium excretion rate (e) of Temoru stylifeera with Fig. 3.2. Kinel length of incubation expressed as pg at. NH,-N per individual per hour N/mg ash-free (from Nival et al. 1974, fig. 4). Paffenhöfer 191 I ! Copepods .. ' The release of soluble end products of metabolism 113 ntrations. By contrast, ported that the energy charge oSzooplankton (EC), i.e. the (ATP f 7s decreases in respira- $ADP)/(ATP + ADP + AMP) ratio, is low just after the net haul, :h the concentration is and recovers to a normal value 24 h later. Low EC is ascribed to 10/100ml, respectively. intense energy utilization by the stressed animals after capture. Simi- ion rates were 7 and 22 larly, the EC of zooplankton caught during 30-min hauls is less than 377) showed that urea that of animals obtained from 3-min hauls and therefore likely to be 3r Pleuromamrna spp. at less disturbed by the sampling conditions (Skjoldal 1981). On the nl. They did not, how- other hand, starvation effects have not been found to be responsible 'ation and ammonium for the higher excretion rates at the beginning of incubation, for no oncerned, lvf enzel and difference was observed by Ikeda et al. (19824 when A. gibber was fed itions of animals, the or remained unfed before the excretion experiment (O to 4 h). Simi- ne less than 500 ml. larly, Gardner and Paffenhöfer (1982) found that the higher excretion rate of Eucalanus pileatus (Fig. 3.2) was a constant feature during the nfluence of incubation first 10 min of their experiments, whatever the feeding status of the ise is observed at the animals. Another kind of stress is thermal, caused when animals are wer decrease. This is rate of T.spylifeera. The transferred from the environmental temperature to a different experi- ates at the beginning, normal values, after Ammonium Aism. 40 he high rate values at ct or the acclimatiza- o;;; E.-m for exampie, re- al U), r Y

al a -c.., -U -m m 5 c c

7 7 Time after feeding (min) f Temora stylifeera with Fig. 3.2. Kinetics of ammonium and amino acid excretion rates (pmole :r individual per hour N/mg ash-free dry wt/h) of Eucalanuspileatus after feeding (from Gardner and Paffenhöfer 1982, fig. 1). 114 The Biological Chemistry of Marine Copepods The relei mental temperature. Thus, an ‘overshoot’ in the respiratory rate of that the animals wi Calanusfinmarchicus was found by Halcrow (1963) at the beginning of crowding and conf his experiment, normal values being recovered 6-8 h after the tempera- starvation effects, ture increase. Leaving animals in the experimental nedium before levels and an incrt starting the actual measurements is a possible solution to the stress or bacterial activity, ’ acclimatization effects. But other problems then arise, namely starva- I types of released tion, when copepods are not fed, and chemical changes in the incuba- dependent so that tion water (oxygen, ammonium, organic compounds) when the accli- mental conditions. matization period is too long. 1 I Starvation effects. Or The physiological state of the copepods. This is also a crucial factor. The mals encounter in values for metabolic rates should be those of normal, healthy animals, long-term starvatic and this physiological state should be sustained throughout the period altered to varying of incubation. However, the higher the animal concentration in Most of the stud short-term experiments, the harder this is to control. In addition to I have not made clea the influence on rates, ratios between the excretions of nitrogen and experiments are du phosphorus are lowered, for dead or injured animals release phos- , ment, with starva phate more rapidly than ammonium (Mullin, Perry, Renger, and Comparing the kin Evans 1975; Ikeda et al. 1982~).The same effect can also be observed Conover (19566) c( for crustaceans when moulting, phosphate release by the cladoceran similar conclusion Daphnia spp. then being seven times higher than normal (Scavia and respiration rates o McFarland 1982), and that ofEuphausia spp. even higher (Ikeda and observed for the fi Mitchell 1982). starvation. Howe\ The source of the excretion products. When excretion is measured under 1 recently achieved 1 feeding conditions, soluble compounds can sometimes arise from ammonia excretior sources other than excretion. Leakage from faecal pellets is one pos- I flow cell interface1 sibility; another is food broken up by the animals during its cap- Fed and starved ture. Errors introduced by these processes are best avoided for both 10 min of the expel are concerned with unassimilated food (Butler et al. 1969, for N and stress or acclimati: P excretions by Calanus spp.; Lampert 1978, for the release ofdissolved remaining higher organic carbon by Daphnia spp.). The possibility of mixing true excre- 1 O-min period wa: tion with soluble material released from unassimilated food is more the stress: for exa critical at the beginning of the experiment, and in short incubations. effect than the sim The animals should therefore be left without food so that their guts Starvation effect are empty before the measurements start. if the animals arc natural levels of Long incubations and low animal concentrations unnaturally conce Most of the problems encountered in short incubations may be elimi- copepods feed, foc nated or reduced with longer experiments: thus, there is less variabil- soon occur. Anotf ity of the results, lower stress and less release from unassimilated food. tured algae is that Moreover, the lower concentrations used in long incubations mean phate, even in the opepods The release of soluble end products of metabolism 115 le respiratory rate of that the animals will be healthier and this will diminish the effects of 1) at the beginning of crowding and confinement. However, other problems arise, namely B h after the tempera- starvation effects, changes in the seawater such as lowered oxygen ?rital medium before levels and an increase in those of toxic compounds, and enhanced lution to the stress or bacterial activity, which changes both the metabolic rates and the arise, namely starva- types of released products. All these effects are temperature- langes in the incuba- dependent so that the extent of their influence can vary with experi- inds) when the accli- mental conditions.

a crucial factor. The Starvation effects. Only starvation periods of several hours, which ani- nal, heal thy animals, mals encounter in their natural environment, are dealt with here: hroughout the period long-term starvation, during which the animal's physiology can be ia1 concentration in altered to varying degrees, will be discussed in Chapter 5. ntrol. In addition to Most of the studies of the kinetics of respiration and excretion rates .ions of nitrogen and have not made clear whether the higher values at the beginning of the nimals release phos- experiments are due to stress or represent normal rates in the environ- Perry, Renger, and ment, with starvation effects subsequently inducing lower rates. can also be observed Comparing the kinetics of respiratory rates of fed and unfed A. clausi, se by the cladoceran Conover (19566) concluded that the decrease was due to starvation. A normal (Scavia and similar conclusion was drawn by Ikeda (19776) using the kinetics of hn higher (Ikeda and respiration rates of Acartia tonsa over 9 h: the systematic decrease he

I 'observed for the first hour'was considered to be the consequence of

1 starvation. However, discrimination between the two effects was . i is measured under rècently achieved 'by Gardner and Paffenhöfer (1982) using E. pileatus, metimes arise from ammonia excretion being measured every 3 min with an incubation al pellets& one pos- 1 flow cell interfaced with high-performance liquid chromatography. nals during its cap- Fed and starved animals displayed higher rates during the first )est avoided for both 10 min of the experiment and this may therefore be clearly ascribed to 3 t al. 1969, for N and stress or acclimatization. Later, rates decreased over time (Fig. 3.2), le release of dissolved remaining higher for fed animals than those not fed. In fact, the of mixing true excre- 10-min period was probably variable, depending on the intensity of nilated food is more the stress: for example, long plankton hauls would have a stronger in short incubations. effect than the simple addition of water into the experimental vessel. od so that their guts Starvation effects can be reduced either with shorter incubations or if the animals are fed during the experiment. In the latter, using natural levels of food will lead to problems because animals are s XI unnaturally concentrated in the experimental vessel: thus, when the ations may be elimi- copepod's feed, food levels rapidly diminish and, effects of starvation there is less variabil- soon occur. Another problem when the food used is natural or cul- i unassimilated food. tured algae is that plants accumulate excreted ammonium and phos- g incubations mean phate, even in the dark, which leads to underestimated rate values. 116 The Biological Chemistry of Marine Copepods

These, however, may be corrected for algal uptake, which is a func- and excretion. tion of the nutrient concentration, by using a Michaelis-Menten relations to remai tionship (see Takahashi and Ikeda 1975, for Metridia paczfica). This method has been improved more recently by Lehman (1980) and used by Miller and Landry (1984) for Calanuspanficus. Here, equal rates of uptake by phytoplankton at any cell density were ensured by inoculat- ing the seawater in the experimental and control vessels with ammonium and phosphate in excess at the start of the experiment. Finally, when copepods feed carnivorously, since animal prey are respiratory surfaces used, their excretion needs to be subtracted from that of the copepods. rate is higher. Iked This was done by Corner, Head, Kilvington, and Pennycuick (1976) explanation for high for Calanus spp. feeding on nauplii of the barnacle Elminius modestus, that copepods captu although in this case no significant nitrogen excretion by the prey clear why ingestion could be observed, even at the highest prey concentrations. rate, unless the utili

Chemical alteration to the seawater medium. During incubation, the com- Bacteria. Bacterial ac position of the seawater in a closed vessel is modified by the animals’ respiration and excretion. The oxygen partial pressure decreases duced with the ani whereas nitrogen and phosphorus concentrations increase. The influ- tion of excreted or ence of the latter on the physiology of experimental animals does not seem to have been studied so that conclusions can only be made about the effects of reduced oxygen levels. introduced at higher I Ikeda (1977a) found no significant decrease in respiration rate for ity. oxygen saturations above 70-80 per cent. Possibly, this rather high Depressed activity level was due to the short incubation time (20 min to 4 h) and the the effect of which w; small volume of the vessel (6.8 ml) used in his experiments. Thus, in a C. finmarchicus (Mars1 previous review of the literature, Ikeda (1970) quoted a limiting level results obtained with of 50 per cent saturation for epipelagic copepods. Le Borgne (1982a) 1978). Marshall (19; observed no significant difference between respiration or excretion TVinkler method for ( rates of mixed zooplankton incubated in seawater of 80-86 per cent or biotics affect the ultra 54-68 per cent oxygen saturation (animals being obtained from the obser.vations, togethe same sample). The value of the oxygen saturation under which respi- their physiological prl ration decreases appears to be influenced by temperature, the activity respiration and excre of the animal (Ikeda 1970),and its habitat. Thus, the limiting level for The measurements o; the bathypelagic copepod Gaussiaprinceps, which is adapted to water of substantial desaturation, is lower than that of epipelagic copepods The incubation teci (Childress 1977). measurable increase One possible way of avoiding the problem of chemical changes in oxygen level in the s the seawater medium is the ‘continuous flow technique’, which consists with the measuremen of circulating water through experimental and control containers and will now be briefly dj ! analysing its oxygen or nutrient concentrations following respiration Respiration. The resp Copepods The release of soluble end products of metabolism 117

>take, which is a func- and excretion. This technique, which allows the experimental condi- Lichaelis-Men ten relations to remain constant, has been used, among others, by Corner hietridia paczfca) . This (1961) for the feeding of C.finmarchicus, Ikeda (19716) for the feeding :hman (1980) and used and respiration of Calanus cristatus, Gerber and Gerber (1979) for the is. Here, equal rates of respiration and excretion of small copepods and Undinula vulgaris, and re ensured by inoculat- Gardner and Paffenhöfer (1982) for the excretion of E. pileatus. It control vessels with seems, however, that flow intensity influences the measured rates. art of the experiment. Feldmeth (1971) considers that under conditions of rapid flow, the ;ince animal prey are respiratory surfaces become more oxygenated and so the respiration n that of the copepods. rate is higher. Ikeda (1971b), however, observed no difference. His .nd Pennycuick ( 1976) explanation for higher ingestion rates (but not for respiration rates) is iacle Ejminius modestus, that copepods capture more food from faster-flowing systems. It is not excretion by the prey clear why ingestion but not respiration would change with the flow Incentrations. rate, unless the utilization of food is spasmodic. Bacteria. Bacterial activity is always observed in incubation experi- ; incubation, the com- ments, even with filtered water, because microheterotrophs are intro- idified by the animals' duced with' the animals. The result of such activity is the mineraliza- al pressure decreases tion of excreted or secreted organic products, with concomitant over- ns increase. The influ- estimation of the inorganic fraction on the one hand and of respiration ental animals does not rate on the other (Mayzaud 19736). These errors are more likely to be an only be made about introduced at higher temperature becáuse of increased bacterial activity. in respiration rate for Depressed activity may 'be obtained by the addition of antibiotics, sibly, this rather high the effect of which was tested on Temora longicornis (Berner 1962) and O min to h) and the 4 C.finmarchicus (iMarshall and Orr 196 1). Other studies compared the xperiments. Thus,'in a results obtained with and without 'antibiotics (e.g. Ikeda 1970; Roger quoted a limiting level 1978). Marshall (1973) mentions that penicillin interferes with the 3s. Le Borgne (1982a) Winkler method for oxygen and Butler et al. (1969) found that anti- spiration or excretion biotics affect the ultraviolet (UV) method for nitrogen'analysis. These er of 80-86 per cent or observations, together with the fact that copepods need bacteria for ing obtained from the their physiological processes, are reasons why many workers studying on under which respi- respiration and excretion do not use antibiotics. nperature, the activity s, the limiting level for The measurements of respiration and excretion rates i is adapted to water of The incubation technique discussed is a means of producing a f epipelagic copepods measurable increase of catabolic end products, or a diminution of oxygenJ level in the seawater medium. The methodology concerned if chemical changes in with the measurement of these variations is a further problem, which inique', which consists will now be briefly discussed. control containers and s following respiration Respiration, The respiratory end product is carbon dioxide (CO,) 118 The Biological Chemistry of Marine Copepods The release o which is rarely determined because the amounts released by copepods reaction of Moore and are low compared with its concentration in seawater. This leads to the urease technique of analytical errors. A possible solution is the use of labelling by '*C, as Mortgan and Cundy (1 performed by Copping and Lorenzen (1980) in experiments with Radioactive methods C.pacificus. This method is precise but needs a known and constant excretion by C.finmarch specific activity of I4CO2in order to convert the measured, labelled and Lean (1973) for the amounts into total quantities of carbon dioxide (see Conover and and Lorenzen (1980) us Francis 1973). used for mixed microzc The usual way of assessing respiration rate is to measure the Laws (1979), Glibert ( diminution of oxygen in the seawater medium. The consumed oxygen The radioactive method is then converted into CO, using the respiration quotient (RQ) which feed on labelled food, wf is the ratio between the amount of released CO, and that of consumed animals feeding on lab oxygen. The RQ is equal to 1 when carbohydrates are degraded, but measured with this tec is less than 1 when the substrates are lipids or proteins. As yet, no ments so that the anim measurements of RQ seem to have been made for marine copepods specific activity (Conov although the need for this was pointed out much earlier (Corner and This brief survey of Cowey 1968). Nevertheless, the use of a single RQ value could be involved in the incubat misleading because of the diversity of copepod diets and therefore of solution to the problerr the type of substrate being used during respiration. marine copepods. This i Nowadays, oxygen concentrations may be analysed with great pre- the use of animals or p cision by a modified Winkler method (Williams and Jenkinson 1982) method for the assessmc or by the polarographic technique (Teal 1971), which is less precise principles seem to be irr but allows a continuous record of oxygen saturation to be made. Two when comparing the fac other methods, discussed by Packard et al. (1984), have hardly been the shortest time of in used for the respiration of marine copepods. These are the Cartesian when using animals at h diver method, described by Klekowski (1971), which measures varia- l between respiration and tions in oxygen partial pressure; and microcalorimetry, which deter- i in metabolism (e.g. the e mines heat production, a parameter directly proportional to respira- tions (e.g. bacterial acti tion (Lasserre 1984; Pamatmat 1984). temperature. This last i can be measured simult Excretion. Copepods are known to excrete ammonium and phosphate, I because, apart from the both compounds being analysed routinely in chemical oceanography. similar. The soluble organic matter released by planktonic animals, however, i is analysed less frequently. The organic molecules may be mineralized The chemical forms of by UV irradiation in the presence of hydrogen peroxide (Armstrong j' and Tibbitts 1968) with a high efficiency, or by the Kjeldahl method i The main two compou for nitrogen (Holm-Hansen 1968) and persulphate oxidation for ammonium and phosph phosphorus (Menzel and Corwin 1965). The organic fraction is infer- I nitrogen and phosphor1 red from the difference between total nitrogen and inorganic nitrogen. i ments. The importance Amino acids, amines and ammonia can be analysed by the ninhydrin the biochemical process I I ! 1

! I ! Copepods The release of soluble end products of metabolism 119 ts released by copepods reaction of Moore and Stein (1948). Urea can be measured using seawater. This leads to the urease technique of McCarthy (1970) or the method of Newell, e of labelling by I%, as Adortgan and Cundy ( 1967). I) in experiments with Radioactive methods have included the use of 3?p for phosphorus a known and constant excretion by C.finmarchicus (Marshall and Orr 1961) and by Peters the measured, labelled and Lean (1973) for the freshwater copepod Diaptomus spp.; Copping vide (see Conover and and Lorenzen (1980) used in studies with C. bacficus and "N was used for mixed microzooplankton by Caperon, Schell, Hirota, and 'ate is to measure the Laws ( 1979), Glibert ( 1982), and Paasche and Kristiansen ( 1982). . The consumed oxygen The radioactive method is precise but is concerned with animals that in quotient (RQ) which feed on labelled food, which usually is phytoplankton, i.e. excretion by and that of consumed animals feeding on labelled detritus or microzooplankton is rarely rates are degraded, but measured with this technique. It also requires rather long experi- or proteins. As yet, no ments so that the animals' excretion products can reach a constant le for marine copepods specific activity (Conover and Francis 1973). ich earlier (Corner and This brief survey of the analytical methods and the difficulties $e RQ value could be involved in the incubation technique shows that there is no simple 3 diets and therefore of solution to the problem of measuring respiration and excretion by *ation. marine copepods. This is also.true of other methods that depend upon nalysed with great pre- the use of animals or plants under enclosed conditions (e.g. the8'% ns and Jenkinson 1982) method for the assessment of primary production). However, several ), which is less precise principles seem to be important: (1) the use of the same methodology ration to be made. Two when comparing the factors influencing metabolic rates; (2) selecting 984) , have hardly been the shortest time of incubation or the continuous flow technique? rhese are the Cartesian when using animals at high concentrations; (3) determining,the ratios which measures varia- between respiration and excretion in order to detect a possible change lorimetry, which deter- in metabolism (e.g. the effect ofstarvation) or in environmental condi- iroportional to respira- tions (e.g. bacterial activity, uptake of nutrients by plants) at a given temperature. This last point assumes that respiration and excretion can be measured simultaneously, but should pose no serious problem ionium and phosphate? because, apart from the actual analysis, much of the methodology is hemical oceanography. similar. onic animals, however, iles may be mineralized The chemical forms of soluble compounds excreted n peroxide (Armstrong iy the Kjeldahl method The main two compounds excreted by copepods are known to be sulphate oxidation' for ammopium and phosphate, although the release of organic forms of lrganic fraction is infer- nitrogen and phosphorus have been found during incubation experi- md inorganic nitrogen. ments. The importance of this organic excretion, its composition and tlysed by the ninhydrin the biochemical processes involved, however, are not yet clear. 120 The Biological Chemistry of Marine Copepods The release

Nitrogen excretion 70 ~ Copepods are generally known as ammonotelic animals, ammonium being produced from the oxidative deamination of glutamate through GDH (glutamate dehydrogenase). The dominance of ammonium g c excretion has been confirmed in most of the studies in which inorganic 5 60- and total nitrogen release have been simultaneously measured I’ 2 (Table 3.1). However, the importance of organic nitrogen excretion was the subject of a controversy between Corner and Newell (1967), who 50 - observed a low percentage for 24 h incubations of Calanus spp., and Johannes and Webb (1965) who obtained a higher percentage of organic excretion in short incubations with mixed plankton (see Corner and Davies 1971). The first explanation for such differences was concerned with the methodology used: thus, Corner and Newell argued that organic compounds resulted from abnormally high ani- Fig. 3.3. Increase in thc mal concentrations in the experimental containers used for short-term with that of the weight-] incubations. Johannes and Webb, however, claimed that bacterial eastern tropical Atlantic activity was significant in long incubations and would mineralize part of the organic nitrogen released. Neither explanation is entirely satisfactory, for very low levels of organic nitrogen excretion were found Figure 3.3 from Le B later for Calanus spp. using short incubations (4 h instead of 24 h) by view. Thus, there is a Corner et al. (1976); on the other hand, a high percentage of organic ammonium excreted excretion was found during long-term incubations (24 h) of mixed planktonic populatior! zooplankton by Le Borgne (1973, 1977). zooplankton other tk Even if the experimental conditions and temperature can influence organic nitrogen exc the amount of organic nitrogen excreted, such results indicate that the results with mixed PO nature of the animal under study also affects the type of excretion. tion than those with c come from studies c nitrogen fraction of t Table 3.1. Ammonium excretion as a percentage of total nitrogen excretion. amphipod Phronima st ~~ fusiformis and Thalia a Species Ammonium/total N Reference per cent for the ctenoF Calanus spp. 74.7 Corner and Newell (1 967) Roger (1982) has prl Calanus spp. 78.3 Butler et al. (1969) ocean: 42.5 per cent o: Calailus spp. feeding 86.0 Corner et al. (1972) for sergestids, 30.0 pe: on Biddulphia cells Calanus feeding as >90.0 Corner et al. (1976) cent for amphipods ai a carnivore Results by Copping Acartia clausi 84.0 Mayzaud (19736) organic carbon is g Calanus chilenris 76.1 Dagg et al. (1980) released. Part of this ( Centropages brachiatus 47.4 Dagg et al. (1980) Eucalanus inermis 73.5 Dagg et al. (1980) pounds, such as urea i of total nitrogen excr Copepods The release of soluble end products of metabolism 121

:c animals, ammonium ’Oi n of glutamate through inance of ammonium iies in which inorganic iltaneously measured yen excretion was the I Newell (1967), who s of Calanus spp., and higher percentage of mixed plankton (see I, Id,,,,+1O0 n for such differences Weight80 percentage90 of copepods (%) s, Corner and Newell abnormally high ani- Fig. 3.3. Increase in the inorganic fraction of nitrogen excretion (at 17°C) rs used for short-term ivith that of the weight-percentage of copepods in mixed zooplankton of the :aimed that bacterial eastern tropical Atlantic Ocean (from Le Borgne 19826, fig. 2). vould mineralize part ation is entirely satis- excretion were found Figure 3.3 from Le Borgne (1982a) presents data in support of this h instead of 24 h) by view. Thus, there is a positive correlation between the percentage of )ercentage of organic ammonium excreted and the relative biomass of copepods in the ons (24 h) of mixed planktonic population. In other words, the greater the proportion of zooplankton other than copepods, the greater the importance of :rature can influence organic nitrogen excretion. This observation would ’explain why ults indicate that the results with mixed populations generally show a higher,organic frac- le type of excretion. tion than those with copepods alone. Further confirmation of this has come from studies demonstrating the importance of the organic nitrogen fraction of taxa other than copepods: 49 per cent for the al nitrogen excretion. amphipod Phronima sedentaria, 30 per cent for the thaliaceans Saba f fusij5ormis and Thalia democratica (Mayzaud and Dallot 1973), and 54 eference per cent for the ctenophore hlnemiopsis leidyi (Kremer 1976). Similarly, orner and Newell (1967) Roger (1 982) has provided results for the tropical Atlantic open- utler et al. (1969) ocean: 42.5 per cent of organic nitrogen for euphausiids, 30.5 per cent xner et al. (1972) for sergestids, 30.0 per cent for peneids, 33 per cent for carids, 25 per cent for amphipods and 38.0 per cent for fish. )mer et al. (1976) Results by Copping and Lorenzen (1980) show that the excretion of ayzaud (19736) 0rganic”carbon is greater than the amounts of carbon dioxide tgg et al. (1 980) released. Part of this organic carbon is associated with nitrogen com- tgg et al. (1980) pounds, such as urea and amino acids. Urea can represent 10 per cent .gg et al. (1980) of total nitrogen excreted by Calanus spp. (Corner et al. 1976), and 122 The Biological Chemistry of Marine Copepods The rele 1 could be regulated by arginase (Bidigare 1983). It follows that urea point is important; excretion would depend on the food composition, particularly its level sent dietary phosp of arginine. On the other hand, amino acids could arise from a simple yet been made of th diffusion through the cellular membrane (Webb and Johannes 1967) fraction although t and are released as ‘spurts) by E.pileatus (Gardner and Paffenhöfer et al. 1963). 1982; Fig. 3.2). In the latter study, the importance of the discontinu- In spite of a lack:j ous amino acid excretion compared with that of total nitrogen could the organic compo1 not be assessed. As the ‘spurts)of amino acid excretion were not more high values are nc frequent at the start of the experiment, when the animals had just temperature and SI been fed, amino acids are not likely to have been lost during mastica- the relative import; tion or defaecation, two processes taking place during the first hour after feeding. Factors influencing Phosphorus excretion Respiration and ex( The first measurements of the liberation of inorganic phosphorus by or excreted per tim1 zooplankton were made by Cooper (1935) and Gardiner (1937), and hourly body loss (( those of organic phosphorus using mixed zooplankton by Pomeroy, those of the release Matthews, and Min (1963). Organic phosphorus as a fraction of total body loss daily are 1 phosphorus excretion is 74.4 per cent for Oiihona similis, 68.3 and 66.5 NH,-N excretion 1 per cent for C-II-C-IV and C-V-C-VI of A. tonsa respectively, and vington (1972) (41 66.8 per cent for Pseudocalanus minutus (Hargrave and Geen 1968). When no antibiotics were added to the experimental water, the proportions were lower, indicating a preferential organic phosphorus uptake by bacteria. Such a result conflicts with those of Peters and Lean (1973) who found that the freshwater copepod, Diaptomus spp., would release 90 per cent of 92Pduring short incubations and only 60 instance, on a g per cent during 10-h incubations, the decrease being caused by a preferential uptake of PO,-P by epibionts in general and bacteria in particular. A similar decrease in the inorganic fraction with time of Diel variations in ox incubation was observed by Le Borgne (1979) using mixed plankton. vertical migrations, a He regarded the effect as due to starvation. Temperature does not (Childress 1977) or I seem to influence the relative proportions of organic and inorganic Gerber 1979). There phosphorus excreted (Le Borgne 1982a), but food levels have this tions of respiration r effect with Culanus spp. (Butler et al. 1970). During a seasonal survey, Marshall and Orr ( these latter workers showed organic phosphorus excretion could Calanus hyperboreus; Be reach 72 per cent of total phosphorus during the spring bloom, but C.finmarchicus, P. mint was less at other seasons. To account for the high amounts of organic Corner (1968) for C. phosphorus released, Butler et al. suggested that the organic fraction longa and Pareuchaeta rí represented unwanted phosphorus material that was unassimilated A. clausi and C. helgola with other dietary constituents and was subsequently excreted rates are concerned, tl unchanged. In terms of calculating food-conversion efficiency this over and Corner (19( e Copepods . The release of soluble end products of metabolism 123 '83). It follows that urea point is important; for the organic phosphorus released could repre- tion, particularly its level sent dietary phosphorus that has not been assimilated. No study has could arise from a simple yet been made of the chemical composition of the organic phosphorus .ebb and Johannes 1967) fraction although this question was raised over 20 years ago (Pomeroy 2ardner and Paffenhöfer et al. 1963). rtance of the discontinu- In spite of a lack of information about the chemical composition of .t of total nitrogen could the organic compounds released by copepods, clearly the occasional excretion were not more high values are not simply the result of methodology. Food level, en the animals had just temperature and species composition may all be factors influencing )een lost during mastica- the relative importance of the organic fraction. Ice during the first hour

Factors influencing copepod respiration and excretion rates Respiration and excretion rates represent amounts of matter respired lorganic phosphorus by .d Gardiner (1937)) and or excreted per time and body weight unit. When a value for daily or hourly body loss (or uptake) is required, the weight units must be oplankton by Pomeroy, rus as a fraction of total those of the released (or consumed) element. Examples of values for body loss daily are those of Ikeda and Mitchell (1982) (2.0 per cent for ma similis, 68.3 and 66.5 . tonsa respectively, and NH+-N excretion by Calanus propinquus) and Corner, Head, and Kil- ;rave and Geen 1968)- vington (1972) (41.2 per cent for phosphorus released by C. helgolan- mental water, the prop- ' dicus). These two values are not the extremes of the range found in the al organic phosphorus literature, but they illustrate the limitations of a table that would ith those of Peters and present daily losses of body C, N, P, without taking certain key factors opepod, Diaptomus spp., into account. These are now reviewed. ncubations and only 60 Variations in metabolic rates are both temporal and spatial. For ase being caused by a instance, on a geographical scale, individuals of T. s&~lifera and general and bacteria in C. Qpicus display higher rates in the English Channel than they do in ic fraction with time of the Mediterranean Sea (LeRuyet-Person, Razouls, and Razouls 1975). using mixed plankton. Diel variations in oxygen consumption, which are often linked with Temperature does not vertical migrations, are shown by the bathypelagic copepod G. princeps organic and inorganic (Childress 1977) or U. vulgaris living in Enewetak atoll (Gerber and t food levels have this Gerber 1979). There have been many studies of the seasonal varia- iring a seasonal survey, tions of respiration rates: Gauld and Raymont (1953) for A. clausi; horus excretion could Marshall and Orr (1958) for C.finmarchicus; Conover (1962) for the spring bloom, but Calanus hyperboreus; Berner ( 1962) for T. longicornis; Anraku (1964) for C.finmarchicus, minutus, clausi and Labidocera aestiva; Conover and igh amounts of organic P, A. iat the organic fraction Corner (1968) for C. &perboreus (see Fig. 3.4) , C.finmarchicus, Metridia hat was unassimilated longa and Pareuchaeta norvegica; Gaudy (1973) for C. typicus, T. stylifeera, subsequently excreted A. clausi and C. helgolandicus. As far as seasonal variations in excretion iversion efficiency this rates are concerned, the only studies with copepods are those of Con- over and Corner (1968) for ammonium excretion and Butler et al. 124 The Biological Chemistry of Marine Copepods The releasc 20 A (1970) for total nitrc C.finmarchicus and C. 15 - - all studies of seaso: region and none with The three main fa( namely temperature, considered first. 0th 1 saturatian, salinity o may prevail in certai ture and food level B poikilotherms and th same individual. Or account for the var 1.o _1 stages, and between Tem perat ure Excretion or respirati temperature up to a I habitat. An example and temperature (T) t CI three rates, those of rl tion, with temperatui The shape of the h 1O0 workers (e.g. McLart 1 erally used is where A and B are c( the velocity of chemic is associated with thi tion faculty of poikilo, The link between M- Fig. 3.5. Three cases Fig. 3.4. Seasonal variations in rates of oxygen uptake (A) (in plitres (1) Q,, is steady and 02/mg/day) and ammonium release (B) (in pgN/mg/day) and of O:N excretion rates do not atomic ratios (C) for Calanus hyperboreus (from Conover and Corner 1968, fig. 1). total nitrogen excretic (2) QI,,is steady but of the rate with tempe the individual to temi between 5 and 15°C I Copepods The release of soluble end products of metabolism 125

(1970) for total nitrogen and phosphorus released by a mixture of 1 C.finmarchicus and C. helgolandicus. It should be emphasized that so far all studies of seasonal variations have dealt with the temperate region and none with tropical areas, where seasonal upwellings occur. The three main factors influencing respiration and excretion rates, namely temperature, amount of food and individual body weight are considered first. Other factors, such as hydrostatic pressure, oxygen saturation, salinity or light, which generally have far less effect, but may prevail in certain circumstances, are dealt with later. Tempera- ture and food level have a general effect on metabolic activity of poikilotherms and their influence may explain rate variations for the same individual. On the other hand, individual body weight can account for the variations found between various developmental stages, and between species.

Temperature Excretion or respiration rates of poikilotherms are a direct function of temperature up to a maximum depending on species, generation and habitat. An example of this relation between metabolic activity (M) C and temperature (T) is given for A. clausi in Fig. 3.5. Variations of three rates, those of respiration, ammonium and total nitrogen excretion, with temperature (5-20°C) are represented. The shape of the M-T relationship has been studied by numerous workers (e.g. McLaren 1963; Mayzaud 1973a) and the formula generally used is it1 = A*B7 where A and B are coefficients derived from Arrhenius's law linking the velocity of chemical reactions with temperature. The Qloconcept is associated with this model as Qlo= BIo, and describes the adapta- i. tion faculty of poikilotherms to temperature effect over a given range. The link between M-T curves and Qlois illustrated for A. clausi in IF~MIA~M Fig. 3.5. Three cases may be observed: uptake (A) (in @litres (1) Q,ois steady and close to unity, which means that respiration or i/mg/day) and of O:N excretion rates do not change with temperature. This happens for the lover and Corner 1968, total nitrogen excretion rate between 15 and 20°C (Fig. 3.5). (2) Ql0is steady but greater than unity, indicating a regular increase of the rate with temperature and demonstrating a good adaptation of ,the individual to temperature. This is the case for the respiration rate between 5 and 15°C (Fig. 3.5). 126 The Biological Chemistry of Marine Copepods The relea

temperature increas therefore, is concer: experimental tempe: The QI"value of a metabolic rate is dc example, Ql0for re (Fig. 3.5), which me excretion will be inf temperature changes by Ikeda and Hing F temperature ranging into account, the a\ ammonium excretior I 5 10 15 20 rate the effect of ter ingested food. Never 5- temperature on diffei 4- mixed plankton stuc 3- The same observatio ture on metabolic IC 2- e (Vidal 1980). In addition to the 1- +---y--+\i body weight also infl 5 10 15 20 and Whitledge (1982 Temperature "C Temperature "C Fig. 3.5. Metabolism-temperature (M-T) curves and Qlo values for boreal species to ten respiration or total nitrogen excretion by Acurtiu cluusi (from Mayzaud excretion being hig€ 1973~).Respiration rate in pulitres 02/mg (dry weight)/day; NH3-N excretion weight compared wit rate in pgNH3-N/mg (dry weight)/day; total nitrogen excretion rate in pg erature adaptations NT/mg (dry weight)/day. stronger than previ changes in the lipid (3) Qlois high or less than unity, meaning that the animal does not comparisons of rates control its metabolism, either because it is not an eurythermal species 1982). The role of li or because the temperature is too high. the QI,for phosphate It should be noted that description (3) of the Qlovalue does not take related to the body d into account the time to adapt to temperature changes, during which is not (Ikeda et al. l! there may be a temporary and important increase of the rate, such as Qlois also influen the overshoot found by Halcrow (1963), followed by an acclimatiza- Thus A. tonsa, a spec tion period and, eventually, the appearance of genetic variants Qlo(Conover 1956). (Newel1 and Branch 1980). Thus a planktonic species may not be Moroccan upwelling adapted to the sudden variations in temperature met during its verti- play greater Ql0vali cal migrations, but will be able to cope with a more or less regular mamma spp. (ChamF Copepods The release of soluble end products of metabolism 127 temperature increase during summer. Account (3) of Qltlvariations, therefore, is concerned with individuals that have acclimatized to experimental temperature. The Q,, value of a given species changes depending upon how the metabolic rate is defined, and with environmental conditions. For -+ example, Qlofor respiration is not the same as QI0for excretion I I (Fig. 3.5)) which means that the 0:N ratio relating respiration and 10 15 20 excretion will be influenced by temperature. This can be caused by temperature changes affecting different metabolic processes as shown by Ikeda and Hing Fay ( 1981) for Calanidae living in various regions of temperature ranging between O and 30°C. When body weight is taken into account, the average Q,, was 2.18 for respiration and 2.58 for ammonium excretion. In this case, however, it is not possible to separate the effect of temperature from that of variations in the type of ingested food. Nevertheless, it should be noted that varying effects of temperature on different metabolic processes have been observed with mixed plankton studied at two different temperatures (Table 3.2). The same observation has also been made for the influence of temperature on metabolic losses, assimilation and production of C. paczficus (Nidal 1980). In addition to the metabolic process considered, the definition of -+- _L I body weight also influences Ql0(Ra0 and Bullock 1954). Thus Vidal 10 15 20 i and Whitledge (1982) emphasize the role of lipids in the adaptation of Temperature "C boreal species to temperature, the Q,,, for respiration or ammonium :s and QI,, values for excretion being higher when the rates are expressed per unit dry z clausi (from iMayzaud l weight compared with lipid-free dry weight. 'This suggests that temp- it)/day; NH,-N excretion )gen eicretion rate in ,ug erature adaptations for the metabolic rate of crustaceans may be stronger than previously reported, and indicates that neglecting changes in the lipid content of animals . . . may lead to erroneous tt the animal does not comparisons of rates and levels of metabolism' (Vidal and Whitledge Ln eurythermal species 1982). The role of lipids during thermal changes could explain why the Q,ofor phosphate excretion (which is linked to lipid catabolism) is l,,,value does not take related to the body dry weight whereas that for ammonium excretion :banges, during which is not (Ikeda et al. 1982~). Ise of the rate, such as QI, is also influenced by feeding habit if respiration is involved. red by an acclimatiza- Thus A. tonsa, a species more carnivorous than A. clausi, has a higher : of genetic variants Q,,, (Gonover 1956). Similarly, species considered as carnivores in the c species may not be Moroccan upwelling, such as Anomalocera Patersonì or T. stylifra, dis- .e met during its verti- play greater Qlovalues than the herbivores C. helgolandicus or Pleuro- I more or less regular mamma spp. (Champalbert and Gaudy 1972). The reason for such Table 3.2. QI0 values of mesozooplankton respiration and excretion for several stations and temperature ranges.*

Temperature range (OC)

17-27 17-27 20-28 16-23 16-22 17-22 17-22 17-23 17-28 17-28

Respiration 2.06 1.42 2.75 3.71 2.48 1.86 2.36 2.79 2.35 2.41 Excretion NlOkll 1.80 1.96 2.39 2.64 1.92 1.37 2.62 2.39 1.91 2.58 NH4-N 2.36 1.30 2.34 3.25 2.59 2.03 3.23 3.00 2.36 2.83 P,,I,l 1.74 1.85 2.78 2.54 1.95 1.52 2.13 2.97 1.76 2.37 P0,P 1.89 1.82 1.26 3.48 1.81 1.43 2.24 2.91 2.21 1.92

' Data from Le Borgne (1982~)for eastern tropical Atlantic. -. -

AN O0 -I i -7The release of soluble end products of metabolism 129 FEB. MARCH 1 1

JULY 2o 1

.-.-----*

1°L Nov. I 1

I I I I, I 10 14 18 22 10 14 18 22 Fig. 3.6. Seasonal variations in respiration rate-temperature curves of Centropages typicus (from Gaudy 1973). Respiration rate in plitres 02/mg. Temperature in range 10-22°C.

differences is not clear, but might reflect the kind of substrate oxidized at various temperatures. If this is so, then differences in the O:N and 0:P ratios would appear with temperature changes. This aspect of the problem, however, was not studied by Champalbert and Gaudy ( 1972). Adaptation to-- the environmental temperature, as reflected by the QI0 value, undergoes changes linkedtdseasonal variations that mod- ify both the M-T curves, and hence QIo values and metabolic rates I. (Bullock 1955). Thus, each season presents a different relationship between temperature and respiration rate for C. ppicus (Fig. 3.6), T. splifera, A. clausi or C. helgolandicus (Gaudy 1973). Temperature would not be the sole cause of such variations, for distinct copepod generations have different abilities to adapt to temperature (Conover 1959; Bzner 1962; Gaudy 1973). Q,, values may also display geographical variations. For example, the respiratory QloofA. clausi shows * a steady value over a wider temperature range in the Mediterranean than in the Atlantic, which corresponds to different thermal condi- 130 The Biological Chemistry of Marine Copepods The rek tions in the two sea areas (Mayzaud 1973a). Similarly, Gaudy (1975) tion, because their reports different Qlovalues for the same species living in the western for egg production Mediterranean Sea and in the Ionian Sea, areas of different tempera- The effect of star tures. This study covered many species of copepods and took into metabolism is to C: account variations in dry body weight. of substrate oxidií minimal, or stand; Food level is a function of boc The effect of food level on rates of copepod metabolism has been nitrogen and phos] recognized, fed animals having been found to have higher rates than which also decrea2 controls remaining unfed (see review by Ikeda 1976). Intermediate be reduced. Reduc values are observed with animals with plenty of food and those observe'd by many starved for several days or weeks. Indeed, Conover and Lalli (1974) tatus; Nival et al. 1 regard food level as the main cause of variations in respiration rates, chicus and A. clausi although other workers found that temperature was a more important Ikeda (1970) for C factor (Ikeda 1970; Le Borgne 1982~).Possibly food level becomes for the species citc more critical for phytophagous animals in temperate regions where australis, Ikeda ( 19 marked variations occur in the phytoplankton biomass. , tions in lipid con The relationship between rates of respiration or ammonium starvation on met; excretion and the amount of chlorophyll in the environmeniwas shown weights have alsc during a sesonal survey by Conover and Corner (1968) using C. hyper- introduced by chs boreus (Fig. 3.4), C. finmarchicus, M. longa and P. norvegica, highest rates ship between bas being found during the spring bloom. A similar relationship for total Lalli 1974). Howe nitrogen or phosphorus excretion and levei-of plant food was also temperatures depl observed by Butler et al. (1970) for a mixture of C. helgolandicus and lipid reserves. Dar C.finmarchicus studied throughout the year. In fact, however, such a rate are very vari relationship may be indirect because the amount of food available may be steady aft1 influences the rate of ingestion, which in turn influences the rate of all; or it may be s release of metabolic end products. This has been shown for be defined. Simil2 ammonium and phosphate excretion by M. paczfica feeding on algal ammonium and cultures (Takahashi and Ikeda 1975) and for nitrogen excretion by indicates that a st C. helgolandicus feeding on an animal diet, i.e. nauplii of the barnacle a parameter as w Elminius modestus (Corner et al. 1976). As mentioned elsewhere, the I.' Body weight influence of food level on metabolic rates will differ accordinpu to the type of metabolism studied, the efficiency of food utilization being The inverse relatioi dependent on the kind of substrate oxidized. Miller and Landry I of metabolism (M) (1984), for example, report that the rate of ammonium excretion by confirmed (e.g. Co: female C. paczficus remains constant while the ingestion rate increases Fernández 1978; V with food level, thus indicating an increase in gross growth efficiency (i.e. the proportion of ingested food invested in growth, Ki).It is not certain, however, that the same result would be obtained with excre- where k is log k', k' tion of organic nitrogen or respiration, instead of ammonium excre- and b its slope. Wh Copepods 7The release of soluble end products of metabolism 131 milarly, Gaudy ( 1975) tion, because their experiment dealt with the utilization of the ration :s living in the western for egg production, a special case of food utilization. is of different tempera- The effect of starvation on rates of release of soluble end products of )pepods and took into metabolism is to cause a reduction of these, depending upon the type of substrate oxidized (body proteins, lipid reserves). The basal, or minimal, or standard metabolic rate, which characterizes starvation, is a function of body weight. Thus, if the decrease in oxygen uptake or metabolism has been nitrogen and phosphorus release is expressed in terms of body weight, have higher rates than which also decreases during starvation, then the starvation effect will a 1976). Intermediate be reduced. Reductions in body weight during starvation have been ty of food and those observed by many workers (Omori 1970, and Ikeda 1971a, for C. cris- lover and Lalli (1974) tatus; Nival et al. 1974, for T. stylifeera; lMayzaud 1976, for C.finmar- ns in respiration rates, chicus and A. clausi). Losses of body proteins have been described by was a more important Ikeda (1970) for C. cristatus, Nival et al. (1974) and Mayzaud (1976) lly food level becomes for the species cited above and Ikeda and Skjoldal (1980) for Acartia nperate regions where australis. Ikeda (1970) and Mayzaud (1976) have also observed reduc- biomass. tions in lipid content. It seems likely, therefore, that the effect of .ation or ammonium starvation on metabolic rates has been underestimated because body ivironment was shown weights have also been reduced. In addition to the complications . (1968) using C. hyper- introduced by changes in body weight, there is no clearcut relation- noruegica , highest rates ship between basal metabolic rate and temperature (Conover and , r relationship for total Lalli 1974). However, it seems to be reached more quickly at higher f plant food was also temperatures depending upon the rate under study and the levels of of C. helgolandicus and lipid reserves. Data for the time required to reach the b,asal metabolic fact, however, such a rate are very variable (see on Table 3.3). Thus, the respiration rate )unt of food available may be steady after 15 h, 3 days or never steady; it may not change at influences the rate of all; or it may be so variable that no standard rate of metabolism can ias been shown for be defined. Similar observations have been made in terms of rates of cifica feeding on algal ammonium and phosphate excretion. Such great diversity of data nitrogen excretion by indicates that a standard rate of metabolism is not so straightforward iauplii of the barnacle a parameter as was once thought. tioned elsewhere, the Wer according to the Body weight food utilization being The inverse relationship between the release rate of the end products . Miller and Landry of metabolism (M)and copepod body weight (w)has been generally mmonium excretion by confirmed (e.g. Conover 1960; Ikeda 1970, 1974, 1979; Gaudy 1975; ingestion rate increases Fernández 1978; Vidal 1980). The relationship is defined by gross growth efficiency hf = k*Wb n growth, K,). It is not be obtained with excre- where k is log k’, k’ being the M-axis intecept of log M = k’ + b log W .d of ammonium excre- and b its slope. When hf is the amount of matter respired,or excreted

The release of soluble end products of metabolism 133

per individual and unit of time, b generally lies between 0.66 and 1.OO, thus following the surface law of von Bertalanffy (195 1). According to Banse (1982) the average is close to 0.75. However, when .LI refers to dry body weight, body carbon, or body nitrogen, the slope becomes -(k - 1). The two coefficients, k and 6,and their variations, have been 2- i well studied because of their use in modelling. For example, Ikeda and Motoda (1978), and Ikeda, Carleton, Mitchell, and Dixon (19826) used the two coefficients k and b in order to infer respiration rates from body weights in the Kuro-shio current and the Australian Great Barrier Reef, respectively. The slope, 6, was thought to vary with temperature and this was observed by Ikeda (1970, 1974) for the rates of respiration and ammonium excretion by many planktonic species, including copepods and other taxa. But the influence of temperature on b has not been confirmed by Vidal and Whitledge (1982), who could not observe this effect with copepods of the Southern Bering Sea and the Mid-Atlantic l I Bight (Fig. 3.764. They concluded that temperature affected k but not b, and could find no relationship between b and temperature in the data of Ikeda for crustaceans. It should be kept in mind that an indirect temperature effect on metabolic rates could be caused by I body weight, for species in cold waters are known to be larger than those of the warmer regions and to have slower rates of development. Thus, Gaudy (1975) showed that the same species collected from two

! different areas in the Mediterranean had two distinct respiration rates at a given temperature. He interpreted this as an effect of temperature I on body weight, and logically it only seems to apply to stenothermic species undergoing no vertical migrations and avoiding the warm surface layer (e.g. Pareuchaeta gracilis, Chiridius poppe;, Arietellus setosus and Euchirella messinensis). By contrast, species with significant vertical migrations (e.g. P. abdominalis and E. marina) had no regional variations in body weight and therefore no differences in respiration rate because there was no temperature effect. Gaudy (1975), incidentally,

J. found that temperature influences the slope b between 10 and 24°C. Besides temperature, food level may influence the relationship be- , tween M and W,although the data of Vidal and Whitledge (1982) for rates of respiration and ammonium excretion provide no evidence of this effect. Probably, as shown by Paffenhöfer and Gardner (1984) for the ammonium excretion of different developmental stages of E. pilehtus, the rate is only affected when the food available falls below a critical level. These workers also pointed out that food level may influence body weight so that metabolic rates would decrease when expressed in terms of this. 134 The Biological Chemistry of Marine Copepods 1 The rek Haq 1967). This is W c tons and use more E C herbivorous specie: .- c neutral buoyancy.

a, ing for food. This W Lm vores was not, hov respiration rates c ö n.c because he includc fore, body weight I in Champalbert an tion rates of cope] small number of si belfaviour of a par kind of problem e: A copepod feedì ably have a lower tion rate than whe of two copepod SI 1- 1- 600.1L 1 10 100 10 100 1000 10000/~g //- 1000 *10 O00 /1g carnivorous, mighi Body dry weight (W) Body dry weight (W) considered. This j Fig. 3.7. Two kinds of relationship between metabolic rates (-44) and the Procházková (198 body dry'weight (MI): Left, variable slope and 124-axis intercepts for the plankton: ammoni excretion of phosphate (u) and ammonium (b) during the warm and the cold herbivores, but n season (Ikeda et al. 1982~).Right, with ihe same slope but variable 124-axis rates. Another exa intercepts for respiration (c) and ammonium excretion (d) (Vidal and Calanus spp. feedir Whitledge 1982). (I), Subtropical species; (2), boreal species. Ammonium the barnacle Elmin excretion rate (b) in ng N/animal/h and in (d) pg N/animal/day; phosphate (same temperaturi excretion rate (u) in ng P/animal/h; oxygen.consumption rate (c) in plitres O,/animal/day . C-V and females). its daily nitrogen 1 Such complex relationships between body weight, temperature and Biddulphia and, at food level make it advisable to use a multivariate model for metabolic depending on pre rates, instead of the A4 = k*JV*model (Conover 1978). It should be cent). The assim: borne in mind that k and b coefficients vary with the weight unit and the type of metabolism under study. Thus, Ikeda et al. (1982~)have given two distinct M = f(w> relationships for ammonium and phos- ler, but the data st: phate excretions, with the temperature effects as discussed earlier solely as a herbivore (Fig. 3.7). Similarly, Vidal and Whitledge (1982) present two rela- could be misleading tionships with identical slopes b, but different intercepts k' on the M-axis for rates of respiration and ammonium excretion (Fig. 3.7). Other factors Finally, the respiration rates of carnivorous species are higher than Available oxygen. A d those of herbivores of the same weight (Raymont 1959; Conover 1960; respiration rate. The I )pepods I The release of soluble end products of metabolism 135 'i Haq 1967). This is possibly because the former have heavier exoskele- II tons and use more energy to keep themselves up in the water, whereas herbivorous species have a higher lipid content and display positive or i neutral buoyancy. Carnivores also behave more actively while hunt- I ing for food. This difference between so-called herbivores and carnivores was not, however, observed by Ikeda (1970) in his study of the respiration rates of various zooplankton species. Possibly this was because he included organisms of differing water content and, therefore, body weight (e.g. crustaceans, molluscs, jelly-like animals). I I l 100 1000 1oooo/~g Champalbert and Gaudy (1972) interpret their data for the respiration rates of copepods in the Moroccan upwelling as reflecting the small number of strict herbivores in their study. Defining the feeding behaviour of a particular species of copepod is, in fact, crucial in this kind of problem especially as most of the species are omnivores. t A copepod feeding on plants and ingesting less protein will probably have a lower rate of ammonium excretion and a higher respira- \ tion rate than when it feeds on animal diets. Accordingly, comparison

I I I of two copepod species, one supposedly herbivorous and the other 100 1000 10 000,llg carnivorous, might be misleading if only one metabolic end product is Iry weight (W) considered. This is obvious from the data of Blaika, Brandl, and lic rates (:M) and the Procházková ( 1982) comparing the excretion rates of freshwater xis intercepts for the plankton: ammonium is released more readily by carnivores than by he warm and the cold herbivores, but no difference is observed for phosphate excretion e but variable ill-axis t rates. Another example is provided by Corner et al. (1972, 1976) for :tion (d) (Vidal and Calanus spp. feeding on Biddzilphia dells (phytoplankton) or nauplii of I species. Ammonium the barnacle Elminius modestus (zooplankton) under similar conditions nimal/day; phosphate :ion rate (c) in plitres (same temperature, 10°C; same incubation period, 4 h; same stages, C-V and females). On average, Calanus spp. released 26.6 per cent of its daily nitrogen ration per day as soluble nitrogen when feeding on ht, temperature and BidduQhia and, at least, 27.2 per cent when feeding on the nauplii, model for metabolic depending on prey density (the maximal percentage was 129 per 1978). It should be cent). The assimilation efficiencies were different for herbivorous

the weight unit and J: (34.1 per cent) and carnivorous (79.5-99.9 per cent) feeding, so the i et al. (1982a) have actual difference in rates of nitrogen excretion were somewhat smal- imonium and phos- l ler, but the data still support the view that regarding Calanus spp. 1s discussed earlier solely as a herbivore when considering its rate of ammonium excretion ) present two rela- could be misleading. ntercepts k' on the * :cretion (Fig. 3.7). Other factors sies are highkr than Available oxygen. A decrease in available oxygen causes a decrease in 959; Conover 1960; respiration rate. The precise relationship between the two varies with n

136 The Biological Chemistry of Marine Copepods I The rele species, being either linear or only occurring above a limiting-step very low salinities ( (Prosser 1961). The first type has been observed by Ikeda (1977a) for the metabolic rates A. tonsa; the other by Marshall, Nicholls, and Orr (1935) for C.finmar- have been studied. chicus, and by Alcaraz (1974), who used a Michaelis-Menten relation- Light. Artificial ligh ship between respiration rate and the partial pressure of oxygen for by 31 per cent abo the shrimp Palaemonetes varians. increase is 36 per The response of an animal towards low levels of available oxygen (Mayzaud 1971). depends upon its habitat and level of activity. Thus, a bathypelagic described for ammc species like G. princeps can tolerate the very low oxygen concentrations (1977), who regard ( 1O- 13 mm Hg at 5- 10°C) characteristic of its habitat during daytime tion from intense (Childress, 1977). Similarly, Eucalanus inermis and E. subtenuis can live however, no rate in in total anoxia during 12 h in the Peru upwelling and Calanus chilensis (e.g. Conover I or Centropages brachiatus at 0.2-0.8 ml of oxygen/litre (Boyd, Smith, and in dim light and dr Cowles 1980). In fjords of British Columbia the limiting oxygen con- Two conclusion centration for zooplankton respiration, as measured as electron- copepod respiratio transport system activity, is 5 pg-at. oxygen/litre or 0.06 ml/litre metabolic Ï-ates are (Devo1 1981). But the lethal oxygen concentration is not as low for the graphical or diel v; majority of the pelagic species that do not meet such drastic anoxic levels of available conditions. Ikeda (1977a) defines the percentage of oxygen+saturation metabolic rates shc at which the respiration rate of animals decreases to 50 per cent of the temperature, body rate at saturation as the P jo index. He concludes that the animals most tors, such as oxyge sensitive to under-saturation are those with high respiration rates that be considered. Secc are accustomed to oxygen-rich environments. in terms of one foi Hydrostatic pressure. Species with extensive vertical migrations are automatically appl affected by pressure and their sensitivity is a function of their habitat. kind of metabolic e For instance, MacDonald, Gilchrist, and Teal (1972) report from a strate oxidized. A study of locomotive activity that the hyponeustonic species A. patersoni follows. is more sensitive than the bathypelagic Negacalanus longicornis, Euaugaptilus magnus, P. gracilis and Pleuromamma robusta. The combined The ratios of oxygc effects of hydrostatic pressure and temperature have been demonstrated for G. princeps by Childress (1977) who found that both phosphorus releasc factors tended to reduce the oxygen-consumption rate at the lower Differences betwee. temperatures and higher pressures found at the greater depths or any other chemi' inhabited by the animals during daytime. pared or when the Salinity. Marine copepods, especially in the open ocean, are adapted to any one species is a constant salinity and generally react to any accidental decrease in efficiencies with w this by increasing their respiration rate. They thus have rather low dietary constituent powers of osmoregulation (Marshall 1973). However, euryhaline the substrate beir species living in estuaries or lagoons, such as A. clausi in the Ebrie depends on the pre lagoon of the Ivory Coast (Le Borgne and Dufour 1979) can stand copepod. Thus, fed The release of soluble end products of metabolism very low salinities (down to 4 parts per thousand). The effect of this on the metabolic rates of euryhaline copepods, however, does not seem to have been studied. Light. Artificial light causes the respiration rate ofA. clausi to increase by 31 per cent above that in darkness. For ammonium excretion the increase is 36 per cent and for total nitrogen excretion 27 per cent (Mayzaud 1971). This stimulating effect of light has also been described for ammonium and phosphate excretion rates by Fernández (1977), who regarded such increases as resulting from an escape reaction from intense light. Above a certain threshold of light energy, however, no rate increases are observed and this is why certain workers (e.g. Conover 1956) did not find any difference in metabolic rates in dim light and darkness. Two conclusions may be drawn regarding factors influencing copepod respiration and excretion rates. First, those influencing metabolic Ï-ates are interdependent so that an interpretation of geographical or diel variations simply in terms of either temperature or levels of available food is inadequate. Any model incorporating metabolic rates should take into account three main factors, namely temperature, body weight and food level. Eventually secondary factors, such as oxygen availability, pressure and light may also have to be considered. Secondly, coeficients which are assessed or measured in terms of one form of metabolic rate, such as respiration, do not automatically apply to others, such as excretion. This is because the kind of metabolic end products released depends on the type of substrate oxidized. A more detailed consideration of this aspect now follows.

The ratios of oxygen uptake to the amounts of nitrogen and phosphorus released Differences between the release rates of carbon, nitrogen, phosphorus or any other chemical element appear when different species are compared or when the influence of a particular environmental factor on any one species is considered. Such differences can arise because the efficiencies with which the animals use their food varies from one dietary constituent to another. A second source of difference is that of the substrate being used during catabolic processes. The latter depends on the prey being ingested and on the nutritional status of the copepod. Thus, fed animals degrade dietary carbohydrates and lipids

138 The Biological Chemistry of Marine Copepods I The re through the normal processes of glycolysis and P-oxidation, respec- Because the t€ tively. Proteins are brokwn down to amino acids which then undergo composition of tl oxidative deamination and transamination, the resulting metabolites significance and ( then entering the tricarboxylic cycle. Starved animals, however, uncertain, it is degrade body proteins into amino acids, which then undergo oxida- carbohydrates an tive deamination and the carbon skeletons are subsequently used in As far as marin gluconeogenesis. The amounts of phosphorus and nitrogen released ratio were those ( that correspond to a given amount of respired oxygen are different in was 13.5, less thai the two cases and this appears in the O:NH, (ratio of oxygen uptake: of Redfield et al. ( ammonium excretion), O:PO, (ratio of oxygen uptake: phosphate tion and respirati excretion) and NH,:PO, ratios. and Corner (1968 The first calculation of such ratios for zooplankton was made by ’ratio for C.Ig’perbc Harris (1959), and this early study was followed by those of many before the spring others, some of whom also considered excretions of total nitrogen and after, when anim; phosphorus (NT and PT). following summe (1968) for this SF The O:N ratio will be now used Complete oxidation of the particulate organic matter (POM) in the O:N ratio (Table sea, the average C:N:P elemental composition of which’is 106: 16: 1, needs 276 atoms of oxygen (Redfield, Ketchum, and Richards 1963). The oxidized subsiral Therefore, the oxidation of one nitrogen requires 276: 16 = 17.25 rich in carbohydrz oxygen atoms, a value of the O:N ratio which would apply to ing this period, thl copepods feeding on POM of average composition. If the oxidized quent decrease o substrate has a different composition from the average POM in the (1968) as the res1 sea, and if it consists of lipids and carbohydrates with low nitrogen Omnivorous and content, the O:N ratio will be greater than 17.25, and will eventually are not so depenc tend towards infinity if there is no nitrogen. The other extreme case is display lower and a substrate consisting entirely of protein, for which the O:N ratio has species. A similar I a minimal value, depending upon the carbon and nitrogen composi- as deduced from tion of the protein metabolized. For example, for a protein of 16 per mouth parts, has ; cent nitrogen and 45 per cent carbon, Conover and Corner (1968) The use of a pro calculated an 0:N ratio of 8.2. For other percentages of carbon and to the starvation e nitrogen in proteins, values of O:N ratios were 6.8 (Ikeda 19776)) less physiological reaci than 7 (Snow and Williams 1971) and even 4, for the body proteins of variable. The 0:N: C.finmarchicus (Mayzaud 1976). tion at 13°C when As yet, the form of excreted nitrogen in the O:N ratio has not been (Nival et al. 1974). dealt with. When the nitrogenous end product is ammonium, O:N is Paracalanus parvus, the O:NH, ratio. When part of the nitrogen released is organic, how- plumchrus than for 1 ever, the O:N ratio is less than O:NH, because N, is greater than study on the varia NH,. The problem of the origin of the organic nitrogen release is content, found tha therefore a major one: is it excretion of an end product of metabolism sporadically, protl or a secretion? (See p. 119). According to May: Zopepods The release of soluble end products of metabolism 139 d ß-oxidation, respec- Because the theoretical minimal 0:N ratio as a function of the Is which then undergo composition of the proteins degraded is dificult to assess and the : resulting metabolites significance and origin of the organic fraction of excreted nitrogen is :d animals, however, uncertain, it is hard to 'know the relative proportions of lipids, i then undergo oxida- carbohydrates and proteins used during catabolic processes. subsequently used in As far as marine copepods are concerned, the first values of the 0:N and nitrogen released ratio were those of Corner, Cowey, and Marshall (1965). The mean Ixygen are different in was 13.5, less than the theoretical value of 17 calculated from the data atio of oxygen uptake: of Redfield et al. (1963). Later, seasonal variations of nitrogen excre- :n uptake: phosphate tion and respiration and the O:NH, ratio were studied by Conover and Corner (1968) using temperate and boreal species. The minimum lankton was made by ratio for C. hyperboreus (Fig. 3.4), a phytophagous species, is observed red by those of many before the spring bloom, and reaches its maximal value immediately s of total nitrogen and after, when animals are rich in lipid. Afterwards, it decreases till the following summer. The main conclusions of Conover and Corner (1968) for this species and C.finmarchicus, M. longa, and P. norvegica will be now used to describe the principal causes of variations in the matter (POM) in the 0:N ratio (Table 3.4). of which is 106:16:1, , and Richards 1963). The oxidized substrate. During the spring bloom the food of herbivores is uires 276:16 =I725 rich in carbohydrate and their O:NH, lies between 20 and 30. Follow- iich would apply to ing this period, the use of lipid leads to the highest ratios. The subse- ition. If the oxidized quent decrease of O:NH, is interpreted by Conover and Corner average POM in the (1968) as the result of the utilization of dietary or body proteins. tes with low nitrogen Omnivorous and carnivorous species (e.g. hí, longa and P. norvegica) 5, and will eventually I are not so dependent on the phytoplankton bloom and, therefore, other extreme case is display lower and less variable O:NH, ratios than do the herbivorous ich the 0:N ratio has species. A similar relationship between O:NH, and feeding behaviour, nd nitrogen composi- as deduced from the amy1ase:trypsin ratio or the anatomy of the ir a protein of 16 per mouth parts, has also been found by Gaudy and Boucher ( 1983). r and Corner (1968) The use of a protein substrate at the end of winter can be compared ntages of carbon and to the starvation effect that has been studied by many workers. The ..8 (Ikeda 1977~))less physiological reactions of copepods towards starvation appear to be r the body proteins of variable. The O:NH, ratio of T.stylifeera decreases after a 4-day starvation at 13°C when the animal has to use its own proteins to survive :N ratio has not been (Nival et al. 1974). A similar conclusion is drawn by Ikeda ( 1977c) for s ammonium, 0:N is Paracalanus parvus, although the ratio is higher for starved Calanus ased is organic, how- plumchrzks than for fed individuals. Finally, Mayzaud (1976), from his e N, is greater than study on the variations of O:NH, and that of the lipid and protein c nitrogen release is content, found that C.finmarchicus uses lipids during starvation and, -oduct of metabolism sporadically, proteins, whereas A. clausi primarily uses proteins. According to Mayzaud (19736) such differences among species indi- Table 3.4. O:NH.,, O:PO,, and NH,,:PO,+atomic ratios For copepods. (Mean values given, with range in parentheses).

spccics O:NI-14 O:P04 NH.,:PO., Arca / season Reference

Pleuroniatnina 214 14.5 Bermuda Napora (1963) .w)fiias in Kremer (I 975) Calntius 13.5 (9.8-15.6) Clyde Sca area, spring Corner el al. (1965) jii~i~imG/U'CII.I. (feinde) Calaiu~~s 11-54 Gulf of Maine, Conover and Corner Jinmarchicus seasonal variations ( 1968) Calanus 10-150 hjperboreus Melridia 11-32 loriga Pai euchaela 13-54 tioruegica Cetrfropages 8.2 /&icus Euchirella 29.3 roslrata Pleuromanima 17.3 robusla Rliiticalaiius 27.3 11asuks Calanus 12.2" Clyde sea area, Butler el al. ( 1969) fintnarcliictu (10.0-14.0) spring to autumn and Calanus fielsolaiidicus mixed small 11.5" Clyde sea area, autumn copepods Calanus spp. 11.1" Clyde sea area, Butler el al. (1970) (10.4-1 1.8) annual cycle

-, Cnlaiius 16.5'' Corner el al. (1972) fielgolandicus (13.2-20.7) C111nnuscris falus 5.7 (3.9-9.9) 110 (76-127) 19 (12-24) Northcrn North Pacific Taguchi and Ishii (1 972) Calaiius /)1uiiicliriis 6.8 (5.8-9.2) 89 (76-124) 13 (12-20) Acartia clausi 1.61 Villefranche (starved animals) Mayzaud (19736) Teinora splgera 15 Upwelling off Morocco Nival el al. (1974) Calatuus 58.9 Nova Scotia (I-day starvation) Mayzaud (1976) finmarchicus Pnramlmiii r sn o En /UUllJlU Rhincalanus 27.3 nasutus Calanus 2.2* Clyde sea area, Butler et al. (1969) jumarchicus 10.0- 4.0) spring to autumn and Calanus helgolaridicus mixed small 1.5” Clyde sea area, autumn copepods Calanus spp. 11.1” Clyde sea area, Butler et al. (1970) (10.4-11.8) annual cycle

Calanus 16.5* Corner et al. (1972) helgolandicus (13.2-20.7) Calanus cristatus 5.7 (3.9-9.9) 110 (76-127) 19 (12-24) Northern North Pacific Taguchi and Ishii L (1972) Calanus plumchrus 6.8 (5.8-9.2) 89 (76-124) 13 (12-20) Acartia clausi 1.61 Villefranche (starved animals) klayzaud (1973b) Temora splifera 15 Upwelling off Morocco Nival et al. (1974) Calanus 58.9 Nova Scotia (1-day starvation) Mayzaud (1976) jìmnarchicus Paracalanus 30 250 4 Saanich Inlet (start of Ikeda (19776) parvus experiment on starvation) Calanus 20 450 10 plumchrus Acartia clausi 16.8-2 1.3 Barcelona (study of Fernández (1978) temperature effect) Calanus 23.3-30.4 helgolandicus and C.finmarchicus Temora 18.2-28.8 120-294 6.6-14.0 splfera Centropages 18.6-19.5 138-191 7.37-10.6 gpicus Pleuroinamina 27.1-35.8 329 9.52 gracilis Anomalocera 39.7-44.7 patersoni Clausocalanus 15.0-22.5 arcuicoriiis Calanus robustior 13.1- 18.4 129-2 17 6.73-33.1 Gaussia princeps 21.7 Midwater species Quetin et al. (1980) Calanus 10.8 (7.5-13.9) Antarctic Ocean Ikeda and Hing Fay propinguuJ (1981) Table 3.4. Cont.

Spccics O:NH, OTO, NHjPO, Area/season Reference

Ikcda and Hing Fay Calanus aculiis 119.0 (69.1-190.2) (1981) Calatius 13.9 138 10.0 /iro/)inquiis Ikeda and Mitchell ( 1982) Melridia 12.8 168 13.1 gerlacliei Eucalanus 6.35 Great Barrier Reef Ikeda el al. (I 982a) sii bcrassus (4.35-8.48) (warm and cool season) Calanus pauper 8.8 hocalanus gibber 8.6 Temora turbinala 7.6 Centro/)ages 6.6 Jicrcatus Calano/)ia 6.7 ellif)lica Labidacera spp. 7.6 Acartia paczjica 3.9 Tortanus gracilis 5.5 Corycaeus spp. 5.7 Eucalanus 4..4 Indian Ocean equatorial Gaudy and Boucher allenualus 9 area (1983) Calauus 3.66 29.6 8. I robustior Euchirella spp. 8.64 57.1 5.9 Euchaetn spinosa 17.2 84.4 9.2

Euchaela marina 12.8 64.3 5.4 Eucliaela acuta 28.6 296.9 10.4 Undeuchaeta 8.7 59.6 6.9 intermedin Undeucliaela 10.6 60.3 5.7 inajor Chirundina 26.0 59.0 2.3 sl re e t si Pleurornnniina 2.7 36.4 17 4 Labidocera spp. 7.6 Acartia pacifica 3.9 Tortanus gracilis 5.5 Corycaeus spp. 5.7 Eucalanus 4.4 Indian Ocean equatorial Gaudy and Boucher attenuatus area (1983) Calanus 3.66 29.6 8.1 robustior Euchirella spp. 8.64 57.1 5.9 Euchaetn spinosa 17.2 84.4 9.2

Euchaeta marina 12.8 64.3 5.4 Euchaeta acuta 28.6 296.9 10.4 Undeuchaeta L 8.7 59.6 6.9 intermedia Undeuchaeta 10.6 60.3 5.7 major Chirundina 26.0 59.0 2.3 streetsi Pleuromamma 2.7 36.4 13.4 x$hias Pleuromanima 12.6 223 17.8 abdominalis Arietellus 11.1 410 37 plumger Pontella spp. 5.41 51.0 7.9 Pontella 11.17 76.5 6.9 atlantica Pontella fera 6.0 1 73.28 12.35 Oncaea venusta 5.89 261.89 44.46 Candacia 6.59 36.16 6.32 paclpdactyla Scolecithrix 9.37 110.30 11.87 bradyi Undirrula darwini 11.29 12.41 6.97 Temora discaudata 13.96 75.07 4.99

* Denotes N:P ratio between total nitrogen and phosphorus excretion. 144 The Biological Chemistry of Marine Copepods i The relea cate variable adaptations to the effects of starvation and, as a conse- molecules, the oxid quence, variable sensitivities to the low winter food levels in temperate environment. When and boreal regions. the total phosphoru I I 143, an 0:P ratio cl Physiological state. The second cause of variation of the O:NH4ratio is (seeLeBorgne 1973) stage of development. Thus gravid females of C. hj9erboreus have lower sed earlier, reaches i O:NH, ratios than non-gravid ones, probably because of the 'greater 1970). reliance on protein metabolism in the absence of any appreciable lipid i (2) Part of the phc reserve' (Conover and Corner 1968). Similarly, these authors found a animal (Corner an( positive correlation between O:NH, and the individual size, which would then reflect t they link with the existence of lipid reserves. However, such a rela- overestimated. tionship was not found for other species by Ikeda and Mitchell (1982) . Accordingly, the or by Vidal and Whitledge (1982). rehult from the kin Temperature. Temperature seems to have no influence on the O:NH, utilization. Few stuc ratio for C. hjlperboereus between 2 and 8"C, although Conover and for the O:N ratio, tl Corner (1968) found a high variability in their data. The same con- behaviour and the clusion concerning the lack of a temperature effect is drawn by Gaudy study by Ikeda (197 and Boucher (1983) and appears in the results in Fernández (1978) that of starved anin with several copepod species. However, a thermal effect on O:NH, arises as to whether may be deduced from Q,,, values for T. stylifeera, C. hdgolandicus, more important for A. clausi and C. typicus found by Nival et al. (1974): between 13 and tein (poorer in pho 23"C, the Q,, for respiration was higher than for ammonium excre- animals is greater. 1 tion, thus- implying increasing O:NH, ratios with temperature. release in his experi Possibly, these conflicting results originate from species differences, I There does not a] adaptation to temperature depending upon the degree of use of .body weight (Ikeda stored lipids (Conover and Corner 1968). temperature on O: (1982a) for mixed zc (e.g. Fernández 197 The 0:P ratio ture, which is causc For particulate organic matter in the sea, an average of 276 atoms of compared with that oxygen are needed to oxidize one atom of phosphorus (Redfield et al. methodological arti 1963). However, other values for the 0:P ratio will be obtained effect of a natural pr depending upon the composition of the substrate catabolized. For animals. According instance, Ikeda (19774 calculated theoretical 0:P ratios from the teria1 activity is re mean phosphorus and carbon compositions of carbohydrates, lipids during experiments and proteins as oc;, 263 and 383 respectively. Apart from Ikeda's and total phosphorL results for C. plumchrus and several others (Table 3.4), however, most effect could be imF of the values for O:PO, fall below the 276 theoretical ratio, as pointed peratures, animals c out by Corner and Cowey (1968). Two hypotheses may explain this. starved. Indeed, su (1) There may be an incomplete oxidation through catabolic proces- clear from the resull ses so that part of the phosphorus is being released as organic starved between 6 al Copepods The release of soluble end products of metabolism 145

/ation and, as a conse- molecules, the oxidation of which will require more oxygen in the òod levels in temperate environment. When this organic fraction accounts for about one-half the total phosphorus released and the O:PO, ratio is approximately 143, an 0:P ratio close to the theoretical value (276) can be obtained n of the O:NH, ratio is (seeLeBorgne 1973).Thecontributionoftheorganicfraction, as discus- '. hyperboreus have lower sed earlier, reaches as much as 72 per cent for Culanus spp. (Butleret al. because of the 'greater 1970). if any appreciable lipid , these authors found a (2) Part of the phosphorus released may not be assimilated by the individual size, which animal (Corner and Cowey 1968) and the low observed 0:P ratio However, such a rela- would then reflect the fact that true phosphorus excretion had been ja and Mitchell ( 1982) overestimated. Accordingly, the variations in the 0:P ratio shown in Table 3.4 result from the kind of substrate oxidized and the efficiency of its ifluence on the O:NH, utilization. Few studies, however, have dealt with such variations. As ilthough Conover and for the 0:N ratio, the composition of the diet and, therefore, feeding r data. The same con- behaviour and the amount of food are likely to influence 0:P. The rect is drawn by Gaudy study by Ikeda (19774 shows that O:PO, of fed C. plumchrus is les than :s in Fernández (1978) that of starved animals, but this is not so for P. parvus. The question :rmal effect on O:NH, arises as to whether the excretion of organic phosphorus is relatively vlifera , C. helgolandicus, more important for fed individuals, or whether the proportion of pro- 974): between 13 and tein (poorer in phosphorus than lipids) being degraded by starved for ammonium excre- animals is greater. Ikeda (1977c), however, did not measure organic os with temperature. release in his experiments. )m species differences, There does not appear to be any relation between O:PO, and dry the degree of use of .body weight (Ikeda and Mitchell 1982). However, the influence of temperature on O:PO, and O:P, has been shown by Le Borgne (1982a) for mixed zooplankton (Table 3.2) and by the results of others (e.g. Fernández 1978, for copepods). The 0:P increase with temperature, which is caused by a slower increase in phosphorus excretion vrerage of 276 atoms of compared with that of respiration, could be the consequence of some phorus (Redfield et al. methodological artifact (bacterial activity or starvation) or to the atio will be obtained effect of a natural process specific to the metabolism of poikilothermic trate catabolized. For animals. According to Le Borgne (1982a)) it does not seem that bac- 0:P ratios from the terial activity is responsible because the observed increase occurs ' carbohydrates, lipids during experiments using antibiotics and because both the inorganic Apart from Ikeda's . and total phosphorus excretions are affected. However, the starvation le 3.4)) however, most effect çould be important since it is more marked at higher tem- etica1 ratio, as pointed peratures, animals degrading a substrate poorer in phosphorus when :ses may explain this. starved. Indeed, such an increase in O:PO, for starved animals is ]ugh catabolic proces- clear from the results of Ikeda and Skjoldal (1980) for Acartia australis ; released as organic starved between 6 and 49 h. Consequently, it is hard to know whether I* , I l, 146 The Biological Chemistry of Marine Copepods The re1 it is due to the effect of starvation during incubation experiments or if not appear to be a it actually happens in the natural environment, particularly when on NH,:PO, (Gau animals are facing temperature variations during their diel mig- found an increase rations. increasing less th; et al. (1982~)foun The N:P ratio and the cool (24.5 Provided the chemical composition of the sÜbstrate oxidized during Great Barrier Ree catabolic processes is known, a theoretical N:P ratio may be cal- The use of N:P rat culated. According to Ikeda (1977c), it would be 0.63 for lipid and 56 for protein. However, utilization efficiencies of ingested nitrogen and The observed diffe phosphorus have to be taken into account because they are likely to be ton and their excre different (Harris and Riley 1956). For zooplankton, the atomic ratio ‘led Ketchum (196 N:P is 24.6, greater than the value for phytoplankton (15.7). Accor- cent for zooplanktc dingly, for herbivorous feeding, the N:P ratio of excretion products subsequently ex ter should be less than 15.7. Such a value was found by Harris (1959): he argument, detailec quotes an excretion NH,:PO, atomic ratio of 7.0 (9.8 according to (1978), is as follou Butler et al, 1969, who recalculated the mean). Table 3.4 shows that If ANand A ,are this holds for most NH,:PO, values, in spite of the different types of assimilated, T, ani food used by copepods. Carnivorous copepods represent an interes- l/v, and W, the ql ting case because, if they have a body N:P ratio close to that of relations can be WI herbivores, the N:P ratio for their excretion products should be greater than the body N:P ratio of herbivores. This, however, is not shown by the data in Table 3.4 where Euchaetidae, strict carnivores, Assimilated quantit do not have NH,:PO, excretion ratios higher than those of other (i.e. the ration R),h copepods. Organic excretion may account for this, but was not D, and D,: measured in most instances. Thus, the ratio NT:PT for excretion could be different from NH,:PO,. The N:P excretion ratio is fairly constant compared with nitrogen and phosphorus excretion rates. Thus, during an annual cycle, the Ifa, is the N:P ra atomic ratio N:P for total N and P excreted by Calanus spp. is minimal excretion (Tx:Tp),L in winter (9.8), when plant food is virtually absent, and maximal eficiency ratio (ON during the spring bloom (13.0). This gives a ratio of only 0.75 between minimum and maximum. Moreover, organic phosphorus excretion, (1) and (2) + which is high during spring, may account for part of the seasonal variation in N:P ratio, which stresses the importance of measuring 1 Thui both inorganic and organic excretions. Besides the feeding status of Calanus spp., which appears to be a major factor, Butler et al. (1970) As the net growth found no differences in N:P ratios for CV, females and males as far as total excretion was concerned. Also, their table 6 shows a lower ratio for starved animals in five cases out of six, an observation similar to that concerning the low winter value referred to earlier. There does Copepods The release of soluble end products of metabolism 147 mion experiments or if not appear to be any thermal effect on NT:PT (Le Borgne 1982u), nor ent, particularly when on NH,:PO, (Gaudy and Boucher 1983), although Le Borgne ( 198%) luring their diel mig- found an increase in the latter ratio with temperature, PO, excretion increasing less than NH, excretion. This would explain why Ikeda et al. (19824 found variations in NH,:PO, between the warm (28.6) and the cool (24.5) season for species of copepod from the Australian istrate oxidized during Great Barrier Reef. \$:P ratio may be cal- be 0.63 for lipid and 56 The use of N:P ratios for the assessment of net growth efficiency f ingested nitrogen and The observed differences between N:P ratios for particles, zooplank- use they are likely to be ton and their excretion products (Harris and Riley 1956; Harris 1959) ikton, the atomic ratio led Ketchum (1962) to calculate a net growth efficiency of c. 50 per dankton (15.7). Accor- cent for zooplankton in Long Island Sound. Ketchum’s approach was ) of excretion products subsequently extended by Butler et al. (1969) for Calanus spp. The id by Harris (1959): he argument, detailed by Corner and Davis (1971) and Le Borgne f 7.0 (9.8 according to (1978), is as follows: 1. Table 3.4 shows that If ANand A ,are the quantities of dietary nitrogen and phosphorus if the different types of assimilated, T, and Tpthe quantities lost through metabolism, and s represent an interes- W, and W,the quantities laid down as production, the following ratio close to that of relations can be written: n products should be = Wx and = . This, however, is not As Ts+ A, TP+ kV, tidae, strict carnivores, Assimilated quantities, A, and A,, are equal to the ingested quantities :r than those of other (i.e. the ration R),R, and R,, corrected for the assimilation efficiency, for this, but was not D, and D,: r: P-r for excretion could A, = DgR, = TN f CVN (1) smpared with nitrogen A, = Dp‘Rp = Tp + Wp (2) g an annual cycle, the If a, is the N:P ratio for ingestion (R,:R,), a? that of the copepod Calanus spp. is minimal excretion (Tx:Tp),u3 the body ratio (Wx:Wp),and a4 the assimilation absent, and maximal efficiency ratio (D,:LI ,) , then: :io of only 0.75 between phosphorus excretion, (1) and (2) + ala,DpRp= u3Wp + a2Tp= ula,6Vp + ala4Tp )r part of the seasonal portance of measuring Thus tb’p = Tp(~2- U~U,) (ala4 - US)-’ (3) :s the feeding status of :or, Butler et al. (1970) As thelnet growth efficiency for phosphorous, K?,, is rles and males as far as e 6 shows a lower ratio observation similar to -I to .earlier. There does then (3) and (4) + K2,, = (ala4- u?) (a3 - a,) . 148 The Biological Chemistry of Marine Copepods The relea

Thus the ratio between production and assimilation, K,, may be (2) It is rapid and assessed from N:P ratios for the prey, the excretion, the body contents (3) It applies to mi: and the assimilation efficiencies. Similarly, it can be shown that K,, of a given species or -9, j for nitrogen is defined by: treated as a single b l phytoplankton, may K?,N = a3+K,,P/a]a4 are: The gross growth efficiency, KI,which is the ratio between produc- i (1) The choice ofa tion and ingeshon is equal to D,*K,,, for phos- I in the case of Calanu phorus and D,.K,2, for nitrogen. Using a percentage assimilation certain size-class (e. of 62 per cent for N and 69 per cent for P (i.e. a4= 0.90), the N:P ratio ton, detritus, microz for phytoplankton, that of excretion and body contents of Calanus spp. (the latter taken as equal to the production N:P), Butler et al. (1969) (2) The ratio betwc calculated and as 28.3 per cent and 33.1 per cent, respec- difficult to determint tively. Net growth efficiency, K,,p is 41 per cent and K2,N53.5 per cent. lets; such measurem In other words, Calanus spp. uses 41 per cent of its assimilated phos- easily break during ( phorus and 53.5 per cent of assimilated nitrogen for growth. These (3) Finally, the exter

results agree with those of Corner et al. (1967), from a separate I excreted is a major , approach, which supports the use of the N:P ratios method. As noted represents the end p by Butler et al (1969) net growth efficiencies were here concêrned with when secretions or L the whole period of Calanus spp. growth, but excluding egg production. with the excreted prc Since that time, this kind of calculation has been used only once for copepods, namely by Le Borgne and Dufour (1979) for the quasi- The importance of tl monospecific plankton of the Ebrie Lagoon in the Ivory Coast, with excreted by copepoc C-IV, C-V and adults ofA. clausi. Using a4 = 1, they found average values of K,,p of 37.2 per cent and IY,,~of 61.3 per cent for warm In addition to knowir (26-30°C) and rich waters (more than 4 mg chlorophyll a/m3). processes, the study c As literature values for body N:P ratios of planktonic carnivores the importance of CO (chaetognaths, polychaetes, siphonophores, hydromedusae and ecosystem. This is USI ctenophores) are greater than those of herbivorous animals of which consists o (copepods, some euphausiids, mysids, tunicates), Le Borgne (1978) copepods alone is to 1 suggested that the N:P ratio method could be applied to the higher should be determine trophic level. However, N:P ratios for the excretion and body contents obtaining their contri of carnivorous copepods do not seem to have been determined. Differ- seldom been done, ent C:N ratios have also been observed for planktonic prey and pre- (Table 3.5) together i dators, so the method could also be used with C:N ratios, provided the ton samples. As they complete release of carbon compounds is known (i.e. respiration and most-cases, a fair ide; organic excretion). tion can be obtained. The advantages of the N:P ratios method, as discussed by Le The importance of copepo Borgne (19824 and Mann et al. (1984)) are as follows: duced by respiration (1) It enables the production rate, a difficult parameter, to be marine environment. obtained from K, and the excretion rate. organisms may have Zopepods The release of soluble end products of metabolism 149 irnilation, K2, may be (2) It is rapid and allows a multiplicity of measurements. ion, the body contents (3) It applies to mixed populations, which means that various stages an be shown that of a given species or several species of the same trophic status may be treated as a single biomass and that all particulate diets, not simply phytoplankton, may be used as representing the diet. The drawbacks are: ratio between produc- 3,.K,,, for phos- (1) The choice of a, as the N:P ratio of the prey in assessing K?;thus, rcentage assimilation in the case of Calanus spp., is it all the particulate material or only a = 0.90), the N:P ratio certain size-class (e.g. 5p-50pm) or a certain fraction (phytoplank- intents of Calanus spp. ton, detritus, microzooplankton) that is used as a food? '), Butler et a¿. (1969) (2) The ratio between the assimilation eficiencies of N and P is 33.1 per cent, respec- difficult to determine and is based on measurements with faecal pel- tnd K,,N53.5 per cent. lets; such measurements are subject to error because the pellets can Fits assimilated phos- easily break during collection and release their contents, en for growth. These (3) Finally, the extents to which organic nitrogen and phosphorus are 57), from a separate excreted is a major problem for when the' measured excretion truly 50s method. As noted represents the end products of metabolism, a2is equal to N,P,, but e here concerned with when secretions or unassimilated compounds are analysed together uding egg production. with the excreted products, this no longer holds. :en used only once for (1979) for the quasi- The importance of the amounts of the products respired and he Ivory Coast, with excreted by copepods in the marine environment , they found average 3 per cent for warm In addition to knowing the kind of substrate oxidized during catabolic ilorophyll a/m3). processes, the study of respiration and excretion can be used to assess danktonic carnivores the importance of copepods in the transfer of organic matter in the hydromedusae and ecosystem. This is usually done for mixed zooplankton, only a fraction ierbivorous animals of which consists of copepods. However, if the part played by j), Le Borgne (1978) copepods alone is to be assessed, their excretion and respiration rates ipplied to the higher should be determined and then multiplied by their biomass, thus on and body contents obtaining their contribution to the zooplankton as a whole. As this has n determined. Differ- seldom been done, data for mixed zooplankton are presented ktonic prey and pre- (Table 3.5) together with the contributions of copepods to the plank- ratios, provided the ton samples. As they account for at least one-half of the biomass in . (i.e. respiration and most cases, a fair idea of their role in nutrient and oxygen consumption cambe obtained. as discussed by Le The importance of copepods in oxygen consumption. The carbon dioxide pro- 3llows: duced by respiration is not rate limiting for photosynthesis in the .t parameter, to be marine environment. However, oxygen consumption by heterotrophic organisms may have a significant effect on natural levels of oxygen. Table 3.5. Pcrcentage contributions of zooplankton excretion to phytoplankton nutrient requirement. (Copepods as a proportion of the total zooplankton is shown when stated).

Copepods as proportion of the total Excretion/phytoplankton Reference Sea area and season zooplankton (per cent) requirement (per cent)

-- ~ Aminoiiiuin excretion Long Island Sound 71.4,-100 of ind. 77 Harris (1959) (Oct.-June) Narragansett Bay: Martin (1 968) spring 2.5 autumn 181.7 Villefranche 80 Acartia clausi 23-43 Mayzaud (1971) Colombia river mouth 90 Jawed (1973) Ocean off Oregon 36 North Pacific gyre 40-50* Eppley et a¿. (1 973) Atlantic equator: Le Borgne (1977) March 64-83 (weight) 57 95 of animals 17 July N.W. Africa upwelling * 44 Smith and Whitledge (1977) Beaufort estuary Smith (1978) (Sept.-April) 1O0 6” Kuro-Shio Ikeda and Motoda (summer-win ter) 56-65 of animals 1 1-44 (1 978)

Narragansett Bay Vargo (1979) (annual cycle) 4.4 Peru upwelling 20 (weight); adults 5 Dagg et a¿. (1980) N.W. Africa upwelling 1.5-5 1 Fernández (1981) Australian Great Barrier Reef 52-59.5 of animals 9.0-29.2 Ikeda et al. (19826)

Urea excretion 1n Inn* ivorrn raciiic gyre 'ku--Ju "pp'cy Cl Ul. (1JI.I) Atlantic equator: Le Borgne (1977) March 64-83 (weight) 57 July 95 of animals 17 N. W. Africa upwelling 44 Smith and Whitledge (1977) Beaufort estuary Smith (1978) (Sept.-April) 1O0 6* Kuro-shio Ikeda ancl Motoda (summer-win ter) 56-65 of animals 1 1-44 (1978)

Narragansett Bay Vargo (1979) (annual cycle) 4.4 Peru'upwelling 20 (weight); adults 5 Dagg e! al. (1980) N.W. Africa upwelling 1.5-5 1 Fernández (1981) Australian Great Barrier Reef 52-59.5 of animals 9.0-29.2 Ikeda et al. (19826)

Urea excretion North Pacific gyre 40- 100* Eppley et al. (1973) Atlantic equator 2.2 Le B,orgne (1977) Beaufort estuary 8* Smith (1978) (Sept-April)

Phosphate excretion Narragansett Bay; Martin (1968) spring 16.9 autumn 200 North Pacific gyre 110-140* Eppley et al. (1973) N.W. Mediterranean; spring 2 Nival el al. (1975) Atlantic equator Le Borgne (1977) March 52.0 July 26.7 N. W. Africa upwelling 2-74 Fernández (1981) Australian Great Barrier Reef 6.6-25.6 Ikeda e! al. (19826)

Phytoplankton nutrient uptake. 152 The' Biological Chemistry of Marine Copepods The rele

This is particularly true for the deep oxygen minimum; although the nutrients is more c main heterotrophs are bacteria. In the upper layers of the oceans, For example, the p however, which are generally oxygen-saturated through photosynthe- spring bloom in tc tic processes and exchanges with the atmosphere, 'respiration of net northwest Africa (: zooplankton appears to be 1.5 to 2.5 orders ofmagnitude smaller than the Atlantic equatc the O, production implied by I4C productivity measurements' 1977). (Shulenberger and Reid 1981, for the 0-100 m layer of the mid- (2) The assessmen latitude Pacific). For the photic zone, net zooplankton appears to is often based on I'( consume as much oxygen as microzooplankton (<200 fim) according into nitrogen and p to King, Devol, and Packard (1978) and Ikeda (1979). According to some instances the King et al. (1 978), the ratio between zooplankton respiration, as ments with isotope: determined by its electron-transport system activity, and that of all Smith and Whitled! organisms collected on a fibre-glass filter (Gelman, type A) is equal to differ depending up 62.4 per cent. This percentage decreases at deeper levels, falling to (NH,-N, N03-N, ur only 6.1 per cent between 200 and 600 m, a result which shows the uptake is measurec minor role of zooplankton in the oxygen budget. Although such values bacteria, thus unde are likely to change from one oceanic area to another, it is probably plankton P needs. correct to conclude that oxygen consumption by copepods, which comprise only part of the total zooplankton, is negligible compared (3) Amounts excre with the oxygen production by phytoplankton in the photic layer and being studied. Thu: is low compared with the oxygen uptake by other heterotrophic organ- result from using la isms at greater depths. ture small animals tions of copepods ir determine. They are The importance of copepods in nutrient recycling and are overestima Excreted ammonium and phosphate may be taken up quickly by body weights of COI phytoplankton (see review by Corner and Davies 1971) and phos- mesozooplankton (: phate by bacteria. Organic molecules are utilized by bacteria and, in estimated when the some instances, by phytoplankton. This is so for urea (McCarthy animals such as cop 1971), amino acids (Stephens and North 1971) and phosphorus com- The percentages shc pounds (Corner and Davies 1971). The part played by zooplankton depending upon the organic excretion in bacterial assimilation seems not to have been Because of these v studied, mainly because the chemical forms of this material are not of zooplankton excr well known, the organic fraction usually being measured simply as the shown in Table 3.5, difference between total and inorganic excretions. However, numerous studies have compared excreted ammonium Conclusions and phosphate with phytoplankton needs (Table 3.5). The extent of primary production accounted for by zooplankton excretion lies Copepods generally between 2 and 200 per cent. There are several reasons for this and can survive hai explain why their me wide range: that, in spite of me (1) Young ecosystems, such as those in upwellings, are characterized knowledge of respirí by new production from nitrate, whereas production from recycled sources of variation, - ,ope pods The release of soluble end products of metabolism 153 inimum, although the nutrients is more characteristic of older and equ'ilibrated ecosystems. layers of the oceans, For example, the percentages shown in Table 3.5 are low during the through photosynthe- spring bloom in temperate areas (Martin 1968), in upwellings off :re, respiration of net northwest Äfrica (Smith and Whitledge 1977; Fernández 1981) and agiill.l !h. smaller than the Atlantic equatorial upwelling, which occurs in July (Le Borgne tivity ieasurements' 1977). m la- :r of the mid- (2) The assessment of phytoplankton nutritional needs is variable. It or ' acton appears to is often based on "C uptake values which are subsequently converted 10 ,um) according into nitrogen and phosphorus needs by using C:N and C:P ratios. In 79). According to some instances the calculation uses direct N and P uptake measure- 3n respiration, as ments with isotopes (e.g. Eppley, Renger, Venrick, and Mullin 1973; ity, and that of all Smith and Whitlkdge 1977; Smith 1978; Table 3.5) and these uptakes .îi, type A) is equal to differ depending upon whether only one compound or all compounds :eper levels, falling to (NH,-N, NO,-N, urea-N) are considered. Moreover, when phosphate :sult which shows the uptake is measured, it is concerned with both phytoplankton and Although such values bacteria, thus underestimating ratios between excretion and phyto- mother, it is probably plankton P needs. by copepods, which s negligible compared (3) Amounts excreted are a function of the zooplankton size-class n the photic layer and being studied. Thus an underestimate of zooplankton excretion may 'r heterotrophic organ- result from using large-meshed nets (e.g. >500 ,um), that do not capture small animals with high excretion rates. Moreover, the proportions of copepods in zooplankton populations are rather difficult to determine. They are usually based on numbers of animals (Table 3.5) ing and are overestimates in terms of biomass because the individual taken up quickly by body weights of copepods are often low compared with those of the vies 1971) and phos- mesozooplankton (>200 ,um). However, excretion may not be over- :d by bacteria and, in estimated when the percentages of individuals are used because small for urea (McCarthy animals such as copepods have higher excretion or respiration rates. and phosphorus com- The percentages shown in Table 3.5 lie between 50 and 100 per cent, layed by zooplankton depending upon the sea area concerned and the mesh-size of the nets. ms not to have been Because of these various complications, estimates of the importance ' this material are not of zooplankton excretion in supplying nutrients to phytoplankton, ieasured simply as the shown in Table 3.5, should be regarded as tentative. ns. excreted ammonium Conclusions )le 3.5). The extent of inkton excretion lies Copepods generally make up the bulk of marine zooplankton biomass ,eral reasons for this and can survive harsh experimental conditions. These two reasons explain why their metabolism has been the subject of extensive studies that, in spite of methodological uncertainties, have provided a fair ngs, are characterized knowledge of respiration and excretion. The influences of the main iuction from recycled sources of variation, i.e. temperature, food level and body weight, are 154 The Biological Chemistry of Marine Copepods The re known and can be jointly used in modelling to give an estimation of Boyd, C. M., Smi1 the role played by copepods in marine food-webs. copepods in the 1 One of the main developments in physiological studies on copepods 583-96. has been the’combining of measurements of the different end products Bullock, T. H. (1 95 of catabolism. Because their respective contributions depend upon the activity of poikili kind of substrate being oxidized, O:N, O:P, N:P ratios have given Butler, E. I., Cornc useful information about methodological questions, the interpretation and metabolism of rates and about seasonal variations and the efficiency of food util- of nitrogen and F United Kingdom, 4 ization. However, it seems that in terms of methodology the ‘state-of- Butler, E. I., Corne the-art’ needs further development, particularly as far as organic nit- and metabolism rogen and phosphorus excretions are concerned. Moreover, further phosphorus excrt advances could well exploit the use of O:N or 0:P ratios in the calcu- .\Biological Associati lation of growth efficiency, as has been done with N:P. Caperon, J., Schell tion rates in Kai References technique. Marint Champalbert, G. a Alcaraz, M. (1974). Respiración en crustáceos: Influencia de la concentra- copépodes de niv ción de oxigeno en el medio. investigación Pesquera, 38, 397-41 1. et canarienne. hí’ Anraku, M. (1964). Influence of the Cape God Canal on the hydrography Childress, J. J. 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