Organic and its role as a food source in chalk streams

Item Type book_section

Authors Ladle, M.

Publisher Freshwater Biological Association

Download date 29/09/2021 16:23:02

Link to Item http://hdl.handle.net/1834/22708 30 FIFTIETH ANNUAL REPORT REPORT OF THE DIRECTOR 31 REVIEW ARTICLES Food sources The relationships between food supply and rates of processing by the of chalk streams changes both with time and with position in the ORGANIC DETRITUS AND ITS ROLE AS A FOOD-SOURCE IN CHALK stream. Such changes are in accordance with the rates of input and STREAMS translocation of food materials, the population structure and abundance of consumers, and the influences of environmental conditions on feeding M. LADLE rates. These will now be considered in turn. Coarse particulate originates chiefly from the leaves, flowers and Introduction seeds of riverside trees, shrubs and herbs and from the remains of aquatic Organic detritus consists of the products of and and (Ladle & Bass 1981). Allochthonous input (from outside the (including droppings, cast skins etc.), and is often a stream) varies with the extent of tree canopy cover and can be as much as substantial part of the material carried by rivers. 400-500 g dry wt m-2 annually. The relative contribution of this material At the River Laboratory of the FBA, work on the role of organic detritus generally decreases with increasing stream width (Dawson 1976a). This in chalk streams falls into two categories. The first of these is concerned source shows marked increases in autumn and winter. The of with the mass movements of material carried by the flowing . The living Ranunculus penicillatus (water crowfoot) in chalk streams attains second deals with the processing of this material, particularly as food, by similar maxima of about 400 g dry wt m-2 (Ladle, Bass & Jenkins the animals of the river. 1972). Most of the losses of Ranunculus as large suspended material also The chalk stream (Westlake et al. 1972) included a speculative occur in the autumn and winter (Dawson 1976b, 1981). These inputs of annual budget of organic matter for a small chalk stream. At that time, large particles are available as food only when resting on the river bed or the annual net production by invertebrates was tentatively set at 130 g dry trapped by obstructions such as growing plants. 2 wt m~ , or about 0.3% of the throughput of fine organic detritus. A large Suspended fine particles of organic matter are present, in small quanti- question mark was assigned to these values. No consideration was given ties, in chalk-spring and borehole-source water (Baker & Farr 1977). to variation in populations of invertebrates or to the relationships between At first there is usually a rapid increase in concentrations as the water invertebrate production and detrital food material. This review deals passes downstream, although variations are great. Subsequently, down- with these topics. stream of the main groundwater inflows, there is a tendency for a decrease in mean concentration and in the frequency and amplitude of Feeding types variations. Most fine suspended matter is moved during winter floods, The available food in the form of detritus is considered to occur in three which represent only a small part of the annual timespan (Baker & Farr forms: fine particulate matter (<63 um, equivalent to the silt and clay 1977). fractions of sediment analysis) suspended in the water; fine particulate Deposits of fine particulate organic matter (80-85% less than 63 um matter in sediment deposits; and coarse particulate matter including dead diameter) are conspicuous in streambed sediments near the sources of leaves and aquatic plant fragments. The corresponding feeding types of chalk streams. Much of the larger material of between 125 um and invertebrates are suspension feeders, such as simuliid (blackfly) and 500 (um in diameter is clearly in the form of invertebrate faecal remains, chironomid (midge) larvae, fine-particle deposit feeders such as oligo- which are often characteristic of specific taxa (Ladle & Griffiths 1980). chaetes (worms), and coarse-particle feeders or shredders including Gam- Apart from local concentrations of organic matter, the proportion of the marus pulex (shrimp). Members of each type have been studied. These sediment which is inorganic increases downstream. The quantity of fine types are more or less equivalent to the functional groups of Anderson & organic deposits in the Bere Stream in summer and autumn was roughly Sedell (1979) and others. There are obvious limitations to such classifi- double the amount present in winter (Ladle 1972). Similar fluctuations cations. The same species of may span more than one feeding were noted in the Tadnoll Brook (Welton 1980). type or shift from one to another in different stages or seasons. In It should be noted that chalk-stream water contains dissolved organic addition to these detritus feeders there is a large group of grazing matter, with dissolved organic concentrations of 0.6-1.6 mg l-1 eating aufwuchs (organic films on surfaces). At times this material (Baker, Bartlett, Farr & Williams 1974). The importance of this material consists mainly of living algae. Some species of detritus-feeding inverte- as a food source is unknown. The throughput of dissolved organic matter brates may even be facultative carnivores. is one to three times that of suspended organic matter. Bacterial activity 32 FIFTIETH ANNUAL REPORT in the water column could not take up more than a small fraction of the dissolved organic matter present (Baker 1981). Consumer distribution The food is eaten by successions of species which can be broadly regarded as feeding on similar materials, each species being adapted to a particular reach of stream. Thus, of the fine suspension feeders, the blackflies Simulium spinosum and S. costatum inhabit the extreme upper reaches of chalk streams often within a few tens of metres of the source; S. latipes and Metacnephia amphora favour small intermittent streams (Ladle & Bass 1975); S. vernum, S. equinum, S. lineatum and S. erythrocephalum inhabit successive, overlapping downstream sections (Ladle & Esmat 1972; Ladle, Bass, Philpott & Jeffery 1977); while S. ornatum and S. angustipes are more or less ubiquitous. S. nolleri (syn. argyreatum) and S. posticatum (syn. austeni) have specialized habitat preferences associated with lake outflows and river impoundments respectively (Hansford & Ladle 1979; Hansford 1978). (Fig. 1). Fine particle deposit feeders and grazers, such as tubificid and lumbri- culid worms, show similar successions. From source to lower reaches of the River Frome the sequence is: Rhyacodrilus coccineus, Stylodrilus herin- gianus, Limnodrilus hoffmeisteri, Stylodrilus brachystylus (G. J. Bird pers. comm.), Psammoryelides barbatus and Tubifex ignotus (Ladle & Bird 1980). The shrimp Gammarus pulex, one of the major consumers of coarse particles, is widespread, but caddis, some of which eat similar material, are known to be longitudinally zoned. Similar distribution patterns are demonstrable in most other groups of chalk-stream invertebrates. Changes with time Changes with time include variations in the numbers and life stages of each species. The suspension feeder Simulium ornatum normally has four successive generations of larvae annually in the Bere Stream at Bere Heath. The annual cycles of this and other species are not constant and may differ in different regions of the river (Ladle, Bass, Philpott & Jeffery 1977). Many species of chalk stream insects including simuliids, chiron- omids and Ephemeroptera (mayflies) have several generations in a year. In summer the growth rates of these insects are comparatively high and metamorphosis frequently takes place at a smaller size than in the colder months (Ladle, Bass & Jenkins 1972; Welton, Ladle & Bass 1982). Oligochaete worms appear to have less well-defined life cycles. Charac- teristically, prolonged periods of reproductive activity are succeeded by several months in which only immature worms are present (Ladle 1971; Bird 1982). Lastly, the egg-bearing females of Gammarus pulex, a coarse-particle feeder, are present throughout the year, indicating that breeding is more or less continuous (Welton 1979). Longevity of this REPORT OF THE DIRECTOR 33

species ranged from 17-23 months for females and up to 30 months for males (Welton & Clarke 1980). Seasonal variations in discharge (flow) may modify the distribution and abundance of invertebrate detritus feeders. This phenomenon is clearly demonstrated in the case of the lumbriculid worm Stylodrilus, which is replaced by tubificids as the predominant form in the gravel bed of Bere Stream following flood disturbance (Ladle 1971).

Chalk stream invertebrate numbers and production Ladle (1972) gives examples of the distribution and densities of some major groups of invertebrates in chalk stream habitats. Eight major taxa exhibited population densities of between 1700 and 97 300 individuals m-2. In an account of a two-year study of Waterston Stream, a small 34 FIFTIETH ANNUAL REPORT REPORT OF THE DIRECTOR 35 headwater, Ladle & Bass (1981) tabulate 48 taxa which attained maximum between the fore and hind regions of the midgut in the nature of the densities greater than 1000 m-2. microbial content of the food. In contrast the results from grazing Production estimates for major groups of detritus feeders include a value organisms (e.g. Ephemerella ignita) and shredders (e.g. Gammarus pulex) of 5.5 g dry wt m-2 for Simuliidae and one of 12.9 g dry wt m-2 for were more variable, probably indicating a patchy intake (Fig. 2). Gammarus pulex (Ladle, Bass & Jenkins 1972; Welton 1979). Because of The actual gain of material from ingestion of is insufficient for the problems of interpreting life histories it is difficult to obtain meaningful the energetic requirements of most of the feeding organisms although some production figures for tubificid worms, but a series of biomass estimates nutrients may be supplied largely by bacteria. Of those species so far based on over 200 core samples from the Bere Stream over a period of 15 examined, only Chironomus appears likely to find bacteria an adequate months ranged from 8.5 to 12.5 g dry wt m-2. source of food. Much more work is needed before the sources of food which are actually used by important chalk stream detritivores can be Feeding rates and digestion resolved. Estimates of detritus ingestion rates have been obtained by a variety of field and laboratory methods. The results of all approaches are difficult Conclusion to evaluate because interference with either food or conditions of feeding There is no evidence to suggest that the invertebrate production gues- are liable to affect ingestion rates. Suspension feeders are comparatively stimate (130 g m-2 year-1) given by Westlake et al. (1972) was much in easy to study in the field by means of simple marking techniques involving error. In view of the enormous variation between times and places, it a brief introduction of charcoal powder to the flowing water (Ladle, Bass would require a massive research effort to make effective comparisons of & Jenkins 1972). There is little risk of misinterpretation because the measurements are made on animals feeding essentially on natural concen- trations of suspended particles. Deposit feeders and coarse-particle feed- ers are more difficult to study but the application of both field and laboratory methods provides the following ranges of maximum estimates of food ingested: Simuliidae 13-75 & dry wt m-2 day-1, Oligochaeta 43-162 g dry wt m-2 day-1 and Gammarus pulex 2-13 g dry wt m~2 day--1 (Ladle, Bass & Jenkins 1972; Ladle & Griffiths 1981; McCall & Fisher 1980; Welton 1979; Sutcliffe, Carrick & Willoughby 1981). Little is known of digestive processes in invertebrate detritivores al- though it is apparent that, at the very least, those elements of ingested food which require little or no digestion prior to assimilation, will be utilized. In theory, studies on the chemical composition of food together with information on the digestive capabilities of the animals, should provide conclusive answers regarding the quantitative role of detritus as food. In practice it is much more difficult to separate true detritus from the living organisms associated with it. Even the relatively large protozoa and the young stages of large invertebrates are not easily distinguished; total separation from the detritus therefore is difficult. Few studies on the utilization of the microbial component of organic detritus as food have been attempted, but a number of assumptions concerning the nature of ingested food can be made. Firstly, material ingested from suspension in the water is likely to be more homogeneous than that grazed from surfaces or fine sediment deposits, because of the well-mixed nature of the turbulent water. This is borne out by the studies of Baker & Bradnam (1976) and Bradnam (pers. comm.) who found that Simulium (a suspension feeder) showed a more consistent relationship 36 FIFTIETH ANNUAL REPORT REPORT OF THE DIRECTOR 37 even a few chalk-stream sites. Nonetheless it is only by such comparisons Ladle, M. & Bass, J. A. B. (1981). The ecology of a small chalk stream and its that we can assess how river communities are adapted in terms of their responses to drying during drought conditions. Arch. Hydrobiol. 90, 448-66. composition, development and production to exploit the available food Ladle, M., Bass, J. A. B. & Jenkins, R. (1972). Studies on production and food resources. consumption by larval Simuliidae (Diptera) of a chalk stream. Hydrobiologia, It is clear that the total quantity of detrital material processed by 39,429-48. invertebrate consumers is many times greater than the production of these Ladle, M., Bass, J. A. B., Philpott, F. R. & Jeffery, A. (1977). consumers. In fact the amount of detritus, associated algae, protozoa and Observations on the ecology of Simuliidae from the R. Frome, Dorset. Ecol. microorganisms ingested each year by chalk stream invertebrates may well Ent. 2,197-204. be similar to the annual input of autochthonous primary production plus Ladle, M. & Bird, G. J. (1980). 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