Role of Diatoms in Copepod Production: Good, Harmless Or Toxic?
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MARINE ECOLOGY PROGRESS SERIES Vol. 172: 305-308, 1998 Published October 22 ~ Mar Ecol Prog Ser COMMENT Role of diatoms in copepod production: good, harmless or toxic? Sigrun H. Jonasdottirl**,Thomas Kierboel, Kam W. ~ang*,Michael St. John1, Andre W. Visserl, Enric Saiz3, Hans G. am^ 'Danish Institule for Fisheries Research. Department of Marine and Coastal Ecology. Kavalergdrden 6. DK-2920 Charlonenlund, Denmark 'Department of Marine Sciences. University of Connecticut. Groton. Connecticut 06340-6097. USA 31nstitut de Cihncies del Mar. CSIC, P. Nacional sln. E-08039 Barcelona. Spain Nun~erousrecent studies have challenged the classi- nlalfornled nauplil than do, for example, dinoflagellate cal view that copepod production in the ocean is pri- and flagellate diets (Stattrup & Jensen 1990, Jonas- marily based on pelagic diatoms. Kleppel (1993), in dottir & Ki0rboe 1996, Ban et al. 1997).However, these particular, has argued that copepods in nature feed observations do not allow us to decide whether the mainly on other microplankters (dinoflagellates, cili- inadequacies of these diatom diets are due to toxicity ates) which prinlarily account for reproduction, and or nutntional insufficiency. Copepod eggs are typically that diatoms alone provide an insufficient diet for rich in lipids (Gatten et al. 1980) and essential fatty reproduction. More recently, several authors have acids are therefore potentially limiting nutritional argued that diatoms can be toxic or deleterious to components of diatom diets, as demonstrated by both copepods by preventing their eggs from hatching or laboratory studies (Jonasdottir & Kiarboe 1996) and causing the eggs to hatch into malformed, non-viable field observations (Pond et al. 1996). Deformities, nauplii (Ban et al. 1997 and references therein). While reduced egg production and hatchability are all well- there is now general consensus in the literature that established symptoms of essential fatty acid deficient some species of diatoms are nutritionally insufficient diets in other marine organisms, including crustaceans for copepod reproduction (Stattrup & Jensen 1990, (reviewed e.g. in Watanabe 1982, Harrison 1990). There Kleppel 1993, Jonasdottir & Kiarboe 1996), the idea are several other possibly limiting constituents (see that they are toxic remains controversial. Whether Harrison 1990). Potential toxic compounds, on the diatoms are toxic or nutritionally incomplete makes a other hand, have not yet been identified, with the major difference to the impact on pelagic food web exception of a few diatom species that have toxins dynamics and to our understanding of the basis for fish similar to those isolated from many dinoflagellates production in the oceans. Planktonic copepods account (Bates et al. 1993). for about 80% of the mesozooplankton biomass in the Probably the strongest evidence from feeding ocean and thus provide the most important link be- studies that diatoms can have deleterious effects on tween lower and higher trophic levels in the pelagic egg production and hatchability was provided by food web. Therefore, if toxic, the presence of diatom Chaudron et al. (1996).These authors found, in accor- blooms would reduce copepod population sizes, meso- dance with other studies, that negative effects on egg zooplankton secondary production and, hence, food hatching in Calanus helgolandicus only occurred after for planktivorous fishes. In this comment we examine several days of feeding on the examined diatoms. They the evidence for the toxicity hypothesis, argue that it also found that egg hatching success and the length of has not been rigorously tested in an ecologically mean- the time-lag both decreased with increasing concen- ingful way, and propose a possible avenue to follow. tration of diatoms and, hence, feeding and egg pro- Evidence. Several studies have clearly demonstrated duction. They considered this evidence that an in- that some diatom diets result in lower egg production hibitory compound was accumulating in the gonads of and/or hatching success and/or a higher frequency of the copepod. However, an alternative explanation is equally well supported by their observations, i.e. that the copepod exhausts its own storage of a limiting 0 Inter-Research 1998 h'esdle of full article not permitted 306 Mar Ecol Prog Ser 172: 305-308, 1998 nutritional component essential for hatching and that diatoms if one assumes that the diatoms provide the depletion is faster when egg production is high. stable food and that alternative food sources (ciliates, Using phytoplankton extracts, where the eggs are ex- dinoflagellates) complement the diet to make it nutri- posed to phytoplankton compounds at increasing ex- tionally complete. The field evidence may also, how- tract concentrations, has been applied to test for ever, be consistent with the idea that diatoms are toxic possible toxicity of diatoms. These experiments suggest (at least if eaten), but only if one assumes that the a toxic or deleterious effects of diatom extracts on em- copepods feed mainly or solely on the non-diatom bryonic development and hatching (Poulet et al. 1994, components of the microplankton. Ianora et al. 1995, Uye 1996).However, this effect is ev- How can we distinguish between a nutritionally ident only at concentrations that exceed those encoun- insufficient and a toxic phytoplankter? Because of the tered in nature by many orders of magnitude. In the ex- conceptual and practical problems with extract experi- periments of Jonasdottir & Kiurboe (1996),the negative ments, the experimental examination of whether a effects of diatom extracts on hatching disappeared particular phytoplankton species is toxic or nutrition- when oxygen was bubbled through the extract solu- ally insufficient has to come from feeding experiments. tions, suggesting that the effect was due to the anoxia Experiments in which a good (X) and a suspected poor that developed in this rich organic soup. More irnpor- (Y) (nutritionally insufficient or toxic) phytoplankter tant, however, is that such extract experiments are eco- are mixed (as in Schmidt & Jonasdottir 1997) have the logically irrelevant. This is because the extracted potential to provide the evidence. Generally, in the solutes may be very different from the solutes that leak simplest case, if Y is nutritionally insufficient, mixing out of or are exuded by a diatom (Dam & Tang 1998). with X would remove the 'deleterious' effects. On the Components of the food that are potentially toxic to egg other hand, if Y is toxic, mixing with X would not hatching are, by necessity, mediated by the female and, remove the effect. Consider an experiment in which thus, need to be examined by feeding experiments. species X and Y are given in varying proportions for a Likewise, potential toxic effects of diatom exudates constant total biomass concentration, less the saturat- need to be examined by applying exudates, not ex- ing concentration for egg production. The possible tracts. Similar criticism may apply to the experiments of responses are schematically presented in Fig. 1. A ref- Shaw et al. (1997) in which they found that high con- erence line is defined as the straight line connecting centrations of extracts of some diatoms acted to deter observations (egg production or hatching) at 100% Y feeding of copepods. Hay (1996, p. 107) characterized (point A) and 100% X (point B) and can be anywhere this type of extract experiments: 'Attempts to assess the between line 1 and line 2 in Fig. 1. Depending on food physiological effects of prey metabolltes on consumers quality or toxicity, observations for the mixed diets will by dissolving metabolites into water holding the con- fall on, above or below the reference line. sumer have minimal ecological relevance and should be discontinued. Such procedures may be no more re- alistic than suffocating humans in chocolate syrup and then concluding that chocolate is toxic.' Several field observations may throw light on the significance of diatoms as a potential basis of copepod nutrition, reproduction and growth. Firstly, it is a general observation in temperate waters that copepods respond positively to seasonal diatom blooms, both in terms of egg production (e.g.Dam et al. 1994, fiurboe & Nielsen 1994) and population increases (e.g. Krause & Thrams 1983). Secondly, dietary lipid tracer studies have repeatedly demonstrated that a diatom-based food chain leads to higher growth rate and better con- dition of fish larvae-which feed on copepods-com- pared to a dinoflagellate-based food chain (St. John & Fig. 1. Posslble outcomes in terms of egg production and egg hatching success of experiments in which a good (X) and a Lund 1996, Hansen 1998). Finally, the only field study suspected poor (Y) (nutritionally insufficient or toxic] phyto- that has simultaneously monitored egg hatching suc- plankton species are offered in different proporhons in mixed cess of copepods and phytoplankton species composi- suspensions to a copepod. Total biomass concentration is con- tion (Pond et al. 1996) found no negative relation stant and non-saturating for egg production. Data on line 1 Y as good as X; data in area 2: Y non-toxic but of lower nutri- between hatching success of Calanus helgolandicus tional value than X; data in area 3: Y is toxic, or Y is non-toxic eggs and abundance of diatoms. All the field evidence and of low nutritional value and the copepod exerts prey is, therefore, consistent with nutritional insufficiency of switching. Reference line and Line 2 explained in text Jonasdottir et a1 Role of dlatoms in copepod product~on 307 (1) Y is as good as X for production of viable eggs: that the experiments be repeated with species X re- Both egg production and hatching will fall on the ref- placed by another nutritionally complete and non-toxic erence line. If the clearance rates on the 2 species are species. the same, then the results would be independent of the In conclusion, then, neither field observations nor proportion of X and Y in the mixture and the reference laboratory expenments have so far provided conclu- line will be horizontal, i.e.