Ontogeny, diet shifts, and nutrient stoichiometry in fish

Most stoichiometric models do not consider the importance of ontogenetic changes in body nutrient composition and excretion rates. We quantified ontogenetic variation in stoichiometry and diet in gizzard shad, Dorosoma cepedianum , an omnivorous fish with a pronounced ontogenetic diet shift; and zebrafish, Danio rerio, grown in the lab with a constant diet. In both species, body stoichiometry varied considerably along the life cycle. Larval gizzard shad and zebrafish had higher molar C:P and N:P ratios than larger fish. Variation in body nutrient ratios was driven mainly by body P, which increased with size. Gizzard shad body calcium content was highly correlated with P content, indicating that ontogenetic P variation is associated with bone formation. Similar trends in body stoichiometry of zebrafish, grown under constant diet in the laboratory, suggest that ontogeny (e.g. bone formation) and not diet shift is the main factor affecting fish body stoichiometry in larval and juvenile stages. The N:P ratio of nutrient excretion also varied ontogenetically in gizzard shad, but the decline from larvae to juveniles appears to be largely associated with variation in the N:P of alternative food resources (zooplankton vs detritus) rather than by fish body N:P. Furthermore, the N:P ratio of larval gizzard shad excretion appears to be driven more by the N:P ratio at which individuals allocate nutrients to growth, more so than static body N:P, further illustrating the need to consider ontogenetic variation. Our results thus show that fish exhibit considerable ontogenetic variation in body stoichiometry, driven by an inherent increase in the relative allocation of P to bones, whereas ontogenetic variation in excretion N:P ratio of gizzard shad is driven more by variation in food N:P than by body N:P.     

Factors related to gizzard shad and the threadfin shad occurrence and abundance in Florida lakes

Gizzard shad Dorosoma cepedianum were collected in 23 and threadfin shad D. petenense were collected in 22 of the 60 Florida lakes sampled. Logistic regression equations were 94% effective for predicting gizzard shad occurrence from chlorophyll and lake surface area, and 84% effective for predicting threadfin shad occurrence from lake surface area and lake volume inhabited (PVI). Occurrence of both shad species was related positively to lake size. In lakes where gizzard shad or threadfin shad were collected, shad density and biomass of both shad species were related positively to chlorophyll. Gizzard shad populations were generally vulnerable to predation in lakes, with the per cent of gizzard shad ≤200mm L _T values exceeding 60% with few exceptions. Effects of gizzard shad and threadfin shad on fish community dynamics may be confined to relatively large (>100 ha) and fertile (chlorophyll >20–30μg l⁻¹) Florida lakes.     

Stoichiometry of nutrient excretion by fish: interspecific variation in a hypereutrophic lake

This study investigates how nutrient cycling rates and ratios vary among fish species, with a particular focus on comparing an ecologically dominant detritivore (gizzard shad) to other fishes in a productive lake. We also examined how nutrient cycling rates are mediated by body size (as predicted by allometry theory), and how variation in nutrient cycling is related to body and food nutrient contents (according to predictions of ecological stoichiometry). As predicted by allometry, per capita nitrogen and phosphorus excretion rates increased and mass-specific excretion rates decreased, with increasing mass. Body phosphorus content was correlated with body mass only in one species, bluegill. Contrary to stoichiometric predictions, there was no relationship between body P and mass-normalized P excretion rate, or between body N:P and excreted N:P, when all individuals of all species were considered.
However, at the species level, we observed some support for a body nutrient content effect on excretion as predicted by stoichiometry theory. For example, gizzard shad had lower body P (high body N:P) and also excreted P at higher rates (lower N:P) than bluegill, which had high body P (lower body N:P). We applied the Sterner (1990) homeostatic stoichiometry model to the two most common species in the study – gizzard shad and bluegill and found that food N:P had a greater effect than consumer body N:P on excreted N:P. This indicates that, in terms of variation among these species, nutrient excretion may be more of a function of food nutrient content than the nutrient content of the consumer. These results suggest that stoichiometry can provide a framework for variation among species in nutrient cycling and for evaluating the ecosystem consequences of biodiversity loss.     

Differential effects of elevated nutrient and sediment inputs on survival,growth and biomass of a common larval fish species (Dorosoma cepedianum)

1. Elevated allochthonous inputs of nutrients and sediments to aquatic ecosystems are associated with eutrophication and sedimentation. Reservoirs receive substantial subsidies of nutrients and sediments from catchments due to their large catchment : lake area ratios. We examined the effect of elevated subsidies of sediments and/or dissolved nutrients on the success (survival, growth, biomass and condition factor) of larval gizzard shad (Dorosoma cepedianum), a widespread and dominant omnivorous fish in reservoir ecosystems. 2. We simulated allochthonous agricultural subsides by manipulating dissolved nutrients and sediment inputs in a 2 × 2 factorial design in experimental mesocosms. We predicted that larval fish success would be greater under elevated nutrients. However, we propose two alternative hypotheses with respect to the overall effect of allochthonous sediment inputs. If sediment inputs negatively affect larval gizzard feeding success, larval success would be highest when only nutrients are added and lowest when only sediments are added (+N > +N+S ≥ C > +S). If high turbidity enhances larval foraging activity (due to greater contrast between prey and background), we predict that larval success would be highest when both subsidy types (nutrients and sediment) are elevated, intermediate when either nutrients or sediments are added and the lowest when no subsidies are added (+N+S > +N ≥ +S > C). 3. Our results indicate that elevated nutrient and sediment conditions enhanced larval gizzard shad biomass, but the overall nutrient addition effect was greater than the sediment addition effect (+N ～ +N+S > +S > C). We observed differential effects of nutrient and sediment inputs on larval survival, growth and condition factors. 4. The enhancement of fish biomass in elevated nutrients (+N, +N+S) relative to control conditions was associated with improved gizzard shad survival and not greater growth. The enhancement of fish biomass in the elevated sediment treatment (+S) relative to the control conditions was caused by an increase in survival that more than compensated for a negative effect of sediment addition on growth. 5. Our findings support the recommendation that reservoir management practices must consider the links between land use practices and food web dynamics. Our results suggest that reduction of subsidies of nutrients and sediments to productive reservoirs would decrease survival of larval gizzard shad due to lower food availability.     

Nutritional importance of detritivory in the growth and condition of gizzard shad in an Ohio reservoir

Synopsis Gizzard shad,Dorosoma cepedianum, in Acton Lake, Ohio, ingested foods of varying nutritional quality during the 1981–1983 growing seasons. Adult (ages 2–4) fish fed on a mixed diet (ORG>30%; C:N<7:1) of zooplankton and organic detritus in early summer, and on detrital materials (ORG<16%; C:N>11:1) during the remainder of the growing season. Age 0 (<35 mm standard length) fish ingested only detrital materials. The nutritional quality (ORG = 10 – 20%; C:N<11:1) of these foods displayed little seasonal variation, but was higher than that of organic detritus taken by adult fish in late summer and autumn. Growth and condition of gizzard shad were poor when the diet consisted of detrital materials; however, age 4 fish (1983) grew rapidly and condition improved when zooplankton were consumed. These results suggest that ingestion of poor-quality detritus can reduce the growth and condition of gizzard shad in Acton Lake, whereas the seasonal inclusion of high-quality zooplankton in the diet can result in rapid growth and improved condition.     

General empirical models for predicting the release of nutrients by fish,with a comparison between detritivores and non‐detritivores

1. We derived models of nutrient release [nitrogen (N) and phosphorus (P)] by fish based on studies that directly measured the release rates from 56 species across a broad range of fish mass, feeding histories and temperature. 2. We developed four separate models of nutrient release from multiple regression analysis: detritivore release rates of N and P, and non‐detritivore release rates of N and P. 3. Fish mass explained most of the variance (78–92%) in release rates. 4. Our predicted rates of release of P by fish (g ha^?1 day^?1) were similar to observed rates in the literature from other lakes. 5. The influence of a shift in diet (planktivory to detritivory) by a single species (gizzard shad, Dorosoma cepedianum, a facultative detritivore) on nutrient release rates was estimated. During periods of detritivory, gizzard shad accounted for on average 39% (<1–96%) of all nutrients released by the fish assemblage, and increased total fish assemblage release rates on average by 59% (<1–331%) compared to when gizzard shad were modelled as planktivores. 6. These models provide a rapid means for predicting the release of nutrients by fish assemblages and may facilitate more comprehensive comparisons of nutrient cycling by fish with other internal pathways.     

Seasonal and interannual variation in nutrient fluxes from tributary inputs, consumer recycling and algal growth in a eutrophic river impoundment

We measured tributary inputs, algal nutrient demand and excretion rates of consumers (gizzard shad and zooplankton) at a eutrophic river impoundment. During two summers with contrasting flow regimes, tributary inputs accounted for 38% (1998) and 3% (1999) of algal N demand and 95% (1998) and 17% (1999) of algal P demand. Gizzard shad contributions averaged 14% and 20% of algal demand for N whereas P contributions were 31% and 58% (1998, 1999; respectively). Zooplankton recycling accounted for a comparable fraction of algal P demand (47%) but a larger fraction of N demand (43%) because their excretia were N rich (N:P = 13:1) compared to fish (7:1). Nutrient release by one of the consumers (gizzard shad) was compared with tributary loading over a nine-year period to assess inter-annual variation in their relative importance. Historical records of inflow chemistry, discharge and gizzard shad biomass showed that variation in tributary inputs was the primary determinant of seasonal and inter-annual variation in nutrient loading. Consumer-derived nutrients were important in late-summer and during years when tributary inputs were low. We propose a conceptual model in which primary production is regulated by external nutrient loading and consumer recycling acts to stabilize and sustain production during periods of diminished external inputs.     

The importance of nutrient supply by fish excretion and watershed streams to a eutrophic lake varies with temporal scale over 19 years

Animals transform and translocate nutrients at ecologically relevant rates, contributing to eutrophication in aquatic ecosystems by mobilizing otherwise unavailable nutrients. Yet we know little about how animal-mediated nutrient cycling compares with external abiotic nutrient sources over long periods (years–decades) and at multiple timescales. To address this, we conducted a 19-year study in a eutrophic reservoir examining nitrogen (N) and phosphorus (P) inputs from watershed streams versus excretion by an abundant fish (gizzard shad, Dorosoma cepedianum) at weekly, monthly and seasonal timescales. Over the entire time period, watershed N and P loading was 33- and 3-fold greater than fish N and P excretion, respectively. However, fish N excretion exceeded watershed nutrient loading in 36% of weeks and 43% of months, and fish P excretion in 68% of weeks and 58% of months during the growing season. Fish excretion had lower temporal variability in both supply rate and N:P ratio than watershed loading. Fish excretion also supplied nutrients at a much lower molar N:P ratio than the watershed (mean of daily N:P supply ratios were 15 and 723, respectively). In eutrophic lakes with high fish biomass, fish excretion can strongly influence algal biomass and community composition. Eutrophication management efforts should consider removal of benthivorous fish, like gizzard shad, in addition to other watershed management practices to improve water quality. Future climate change will modulate the interplay between fish- and watershed-mediated nutrient dynamics by altering the geographic distribution of detritivorous fish and the frequency and severity of storm and drought events.     

Impact of the removal of gizzard shad (Dorosoma cepedianum) on nutrient cycles in Lake Apopka,Florida

1. The St. Johns River Water Management District removed over 5.4 million kg of gizzard shad (Dorosoma cepedianum) from Lake Apopka, FL during 1993–2005, as a means of reducing lake phosphorus and phytoplankton concentrations and improving water clarity. Other steps included reduction of external nutrient inputs and operation of a treatment wetland. We measured nutrient excretion by Lake Apopka gizzard shad to quantify the nutrient effect of this biomanipulation. 2. Both N and P excretion were significantly affected by fish body mass and temperature. Larger fish had lower mass‐specific rates of excretion than smaller fish. 3. High water temperature increased P excretion to a much greater extent than N, resulting in a low N : P of nutrient excretion in midsummer. The N : P of excretion was lower than has been observed in other systems, probably because of higher water temperature. 4. Removal of gizzard shad >200 g prevented the annual release of 45 800 kg N year^?1 (3.46 kg N ha^?1 year^?1) and 7700 kg P year^?1 (0.62 kg P ha^?1 year^?1) on average. The actual impact on the P cycle varied substantially from year to year (range 7900–78 800 kg N year^?1; 1200–14 800 kg P year^?1), primarily because of fluctuations in the catch. 5. On an annual basis, the P directly removed in fish tissues was similar to that removed by the treatment wetland. The P excretion prevented by the removal of fish was approximately 20% of the reduction in external P loading achieved during 1993–2005. 6. In the short term, most of the P demand of planktonic primary producers is met through recycling of P, which greatly exceeds external P loading. Depending on population biomass, phosphorus excretion by the resident gizzard shad population was similar in magnitude to the P release by diffusive flux from the sediments.     

Piscivore enhancement effects on food webs depend on planktivore body size and species composition in replicated whole lake experiments

Recent syntheses of trophic cascade and biomanipulation research have suggested that the effects of piscivores on planktivorous fish populations are reduced, when planktivores are capable of outgrowing predator gape limitation and in systems with complex food web interactions. These hypotheses, however, have not been tested in long-term, whole-lake, experiments where processes such as fish recruitment and compensatory food web responses may be important. We conducted a replicated whole-lake experiment to test for the effects of supplemental piscivore introductions on food webs of eutrophic lakes dominated by deep-bodied planktivores. Responses to piscivore enhancement were compared between lakes differing in food web structure due to the presence of omnivorous gizzard shad (Dorosoma cepedianum). A significant decrease in the relative abundance of juvenile planktivorous fish, and an increase in total benthic macroinvertebrate density was observed in lakes containing mainly bluegills (Lepomus machrochirus). In contrast, lakes containing gizzard shad exhibited no significant responses to piscivore manipulation. Our results support the hypothesis that food webs in lakes dominated by deep-bodied planktivorous fish species respond weakly to piscivore enhancement. In addition, our findings support the hypothesis that cascading trophic interactions are weaker in lake ecosystems with more complex food web interactions such as those containing gizzard shad.     

Lake trophic state and the limnological effects of omnivorous fish

Ecologists have hypothesized that planktivorous fish have greater effects on the plankton and water quality of oligotrophic lakes than eutrophic lakes. We tested this hypothesis in a tank-mesocosm experiment of factorial design in which five biomass levels of filter-feeding omnivorous gizzard shad (Dorosoma cepedianum) were cross-classified with two levels of lake trophic state achieved by filling tank-mesocosms with water and plankton transported by truck from two lakes with different trophic states. The presence of gizzard shad significantly increased total phosphorus, primary productivity, chlorophyll, and particulate phosphorus (PP) 2–20 and 20–200 μm and significantly decreased Secchi depth, cladocerans, copepods and PP > 200 μm. The effects of gizzard shad on chlorophyll, Secchi depth, cladocerans, copepods and PP 2–20 and > 200 μm were dependent on lake trophic state and most intense in the eutrophic lake system. This experiment suggests that filter-feeding omnivorous fish interact synergistically with trophic state so that the limnological effects of omnivorous fish become more intense with increased eutrophication.     

Sediment processing by gizzard shad, Dorosoma cepedianum (Lesueur), in Acton Lake, Ohio, U.S.A.

Gizzard shad are primarily detritivorous in Acton Lake, a 253-ha impoundment in southwestern Ohio, U.S.A. To determine the magnitude of sediment utilization by the gizzard shad population in Acton Lake. I used data on population density and age structure, daily ration, and feeding selectivity in estimating the mass of sediments processed by shad daily from April through November. At densities of 4595–10 645 fish ha⁻¹(wet weight biomass = 90–121 kgha ¹), gizzard shad could process 3.8–23.0 kg of dry sediments ha⁻¹ day ¹. On average throughout the growing season, gizzard shad could process a dry mass of sediments each day equivalent to 13% of shad wet weight biomass. Because of the high rate of sedimentation (> 700 kg dry sediment ha⁻¹ day⁻¹) in Acton Lake, gizzard shad can process < 4% of the freshly deposited sediments each day, and therefore are likely to have little effect on benthic community dynamics in the system.     

Detritus Quality Controls Macrophyte Decomposition under Different Nutrient Concentrations in a Eutrophic Shallow Lake, North China

Macrophyte decomposition is important for carbon and nutrient cycling in lake ecosystems. Currently, little is known about how this process responds to detritus quality and water nutrient conditions in eutrophic shallow lakes in which incomplete decomposition of detritus accelerates the lake terrestrialization process. In this study, we investigated the effects of detritus quality and water nutrient concentrations on macrophyte decomposition in Lake Baiyangdian, China, by analyzing the decomposition of three major aquatic plants at three sites with different pollution intensities (low, medium, and high pollution sites). Detritus quality refers to detritus nutrient contents as well as C∶N, C∶P, and N∶P mass ratios in this study. Effects of detritus mixtures were tested by combining pairs of representative macrophytes at ratios of 75∶25, 50∶50 and 25∶75 (mass basis). The results indicate that the influence of species types on decomposition was stronger than that of site conditions. Correlation analysis showed that mass losses at the end of the experimental period were significantly controlled by initial detritus chemistry, especially by the initial phosphorus (P) content, carbon to nitrogen (C∶N), and carbon to phosphorus (C∶P) mass ratios in the detritus. The decomposition processes were also influenced by water chemistry. The NO(3)-N and NH(4)-N concentrations in the lake water retarded detritus mass loss at the low and high pollution sites, respectively. Net P mineralization in detritus was observed at all sites and detritus P release at the high pollution site was slower than at the other two sites. Nonadditive effects of mixtures tended to be species specific due to the different nutrient contents in each species. Results suggest that the nonadditive effects varied significantly among different sites, indicating that interactions between the detritus quality in species mixtures and site water chemistry may be another driver controlling decomposition in eutrophic shallow lakes.     

Influence of gizzard shad on phytoplankton size and primary productivity in mesocosms and earthen ponds in the southeastern U.S.

Gizzard shad (Dorosoma cepedianum), a filter feeding omnivore, can consume phytoplankton, zooplankton and detritus and is a common prey fish in U.S. water bodies. Because of their feeding habits and abundance, shad have the potential to affect primary productivity (and hence water quality) directly through phytoplankton grazing and indirectly through zooplankton grazing and nutrient recycling. To test the ability of shad to influence primary productivity, we conducted a 16-day enclosure study (in 2.36-m³ mesocosms) and a 3-year whole-pond manipulation in 2–5 ha earthen ponds. In the mesocosm experiment, shad reduced zooplankton density and indirectly enhanced chlorophyll a concentration, primary productivity, and photosynthetic efficiency (assimilation number). While shad did not affect total phytoplankton density in the mesocosms, the density of large phytoplankton was directly reduced with shad. Results from the pond study were not consistent as predicted. There were few changes in the zooplankton and phytoplankton communities in ponds with versus ponds without gizzard shad. One apparent difference from systems in which previous work had been conducted was the presence of high densities of a potential competitor (i.e., larval bluegill) in our ponds. We suggest that the presence of these extremely high larval bluegill densities (20–350 larval bluegill m^–3; 3–700 times higher density than that of larval gizzard shad) led to the lack of differences between ponds with versus ponds without gizzard shad. That is, the influence of gizzard shad on zooplankton or phytoplankton was less than the influence of abundant bluegill larvae. Differences in systems across regions must be incorporated into our understanding of factors affecting trophic interactions in aquatic systems if we are to be able to manage these systems for both water quality and fisheries.     

Effects of gizzard shad on benthic communities in reservoirs

Effects of gizzard shad Dorosoma cepedianum on benthic communities in a large southern reservoir (Lake Texoma, U.S.A.) were examined during two field enclosure and exclosure experiments in which enclosures were stocked at high and low densities in 1998 and 1999, respectively. In both years, chironomid abundance significantly increased in treatments that excluded large fishes from foraging on sediments. Mean abundance of chironomids and ostracods were significantly higher ( P  < 0·05) in exclosures than enclosures stocked with gizzard shad at 1140–1210 kg ha⁻¹. In 1999, benthic invertebrate abundances did not differ ( P  > 0·08) between exclosure and enclosures stocked at 175–213 kg ha⁻¹. Per cent organic matter, algal abundance and abundance of other macroinvertebrates in sediments did not differ significantly among treatments in either year. Although chironomid abundance was reduced in gizzard shad enclosures in 1998, food habits from this and other studies showed that adult gizzard shad in Lake Texoma only consumed detritus and algae. It is likely that high sedimentation rates in Lake Texoma limit the ability of gizzard shad to regulate algae and detritus in benthic sediments. Thus, it is concluded that disturbance of benthic sediments by gizzard shad caused the observed reduction in chironomid abundance, rather than through consumption or competition for resources.     

Atmospheric nitrogen-deposition may intensify phosphorus limitation of shallow epilithic periphyton in unproductive lakes

1. We tested whether increasing atmospheric nitrogen (N) deposition along a north–south gradient intensifies epilithic phosphorus (P) limitation in oligotrophic Swedish lakes from the north to the south. We examined the epilithic community at a shallow depth from seven northern and six southern Swedish lakes, and also compared the results with a lake located geographically between the two groups. We determined lake nutrient state, epilithic nutrient ratios and epilithic algal composition, as well as grazer N : P ratios, grazer-epilithon N : P imbalance, and N : P cycling ratios.
2. Epilithic communities appear to be generally more N-limited in the northern lakes and more P-limited in the southern lakes. Lake water total N (Tot-N) and epilithic N : P ratios were lower in northern than in southern lakes and the proportion of N₂-fixing cyanobacteria was higher in northern than in southern lakes.
3. Gastropod grazers had lower N : P imbalances and cycled less N relative to P in northern than in southern lakes.
4. Atmospheric N-deposition showed a strong positive correlation with lake water Tot-N and a much weaker positive correlation with epilithon N : P ratios. Atmospheric N-deposition also correlated negatively with the proportion of N₂-fixing cyanobacteria.
5. There are indications that increased atmospheric N-deposition towards the south might intensify P-limitation of epilithic algae and invertebrate grazers, although more studies are needed to show the strength and generality of our findings.     

Nutrient stoichiometry of linked catchment‐lake systems along a gradient of land use

1. Catchments export nutrients to aquatic ecosystems at rates and ratios that are strongly influenced by land use practices, and within aquatic ecosystems nutrients can be processed, retained, lost to the atmosphere, or exported downstream. The stoichiometry of carbon and nutrients can influence ecosystem services such as water quality, nutrient limitation, biodiversity, eutrophication and the sequestration of nutrients and carbon in sediments. However, we know little about how nutrient stoichiometry varies along the pathway from terrestrial landscapes through aquatic systems. 2. We studied the stoichiometry of nitrogen and phosphorus exported by three catchments of contrasting land use (forest versus agriculture) and in the water column and sediments of downstream reservoirs. We also related stoichiometry to phytoplankton nutrient limitation and the abundance of heterocystous cyanobacteria. 3. The total N : P of stream exports varied greatly among catchments and was 18, 54 and 140 (molar) in the forested, mixed‐use and agricultural catchment, respectively. Total N : P in the mixed layers of the lakes was less variable but ordered similarly: 35, 52 132 in the forested, mixed‐use and agricultural lake, respectively. In contrast, there was little variation among systems in the C : N and C : P ratios of catchment exports or in reservoir seston. 4. Phytoplankton in the forested lake were consistently N limited, those in the agricultural lake were consistently P limited, and those in the mixed‐use lake shifted seasonally from P‐ to N limitation, reflecting N : P supply ratios. Total phytoplankton and cyanobacteria biomass were highest in the agricultural lake, but heterocystous (potentially N fixing) cyanobacteria were most abundant in the forested lake, corresponding to low N : P ratios. 5. Despite large differences in catchment export and water column N : P ratios, the N : P of sediment burial (integrated over several decades) was very low and remarkably similar (4.3–7.3) across reservoirs. N and P budgets constructed for the agricultural reservoir suggested that denitrification could be a major loss of N, and may help explain the relatively low N : P of buried sediment. 6. Our results show congruence between the catchment export N : P, reservoir N : P, phytoplankton N versus P limitation and the dominance of heterocystous cyanobacteria. However, the N : P stoichiometry of sediments retained in the lakes was relatively insensitive to catchment stoichiometry, suggesting that a common set of biogeochemical processes constrains sediment N : P across lakes of contrasting catchment land use.     

Detritus and nutrient dynamics in the shore zone of lakes: a review

The importance of detritus varies greatly among shore zones of lakes, but in a large majority of these regions detrital pathways prevail. Aside from a great spatial and seasonal variability, macrophytes and bottom sediments appear to be dominant stores of nutrients in these habitats. Macrophytes hold a central position in nutrient cycling in the shore-littoral lake zones. They are the main source of autochthonous detritus as they prevail in the total biomass of littoral organisms, and they are only rarely available as direct food of consumers. Various processes and interactions determine the role of macrophytes in nutrients dynamics. These are: the intensity of nutrient uptake and translocation, release of nutrients by healthy plants and from decomposing plants, exchange of elements between macrophytes and their periphyton, as well as interception of seston by macrophyte stands. Particular plant species differ in their time of dying and susceptibility to decomposition. The changes in decomposing material (size structure of particles and nutrient content) mean that detritus in various stages of decomposition differs in its role in trophic dynamics of shore-littoral lake zones. Several types of shore regions as regards detritus sources and retention level are discussed.     

Influence of food‐web structure on the biodegradability of lake sediment

1. Sediment plays a key role in internal nutrient cycling and eutrophication in lakes. However, studies focusing on the efficiency of the biomanipulation techniques for improving the control of primary producers have rarely examined the effects of changes in food‐web structure on the sediment biochemical composition and biodegradability. 2. In a 1‐year experiment conducted in large replicated mesocosms, we tested how the absence or presence of a zooplanktivorous fish (roach, Rutilus rutilus) affected the elemental composition and the potential biodegradability of recently deposited sediment in a eutrophic system. The potential biodegradability of these sediments was assessed in laboratory microcosms by measuring the production of CO₂ during 44‐day incubations. 3. The potential biodegradability of recently deposited sediment from the fish treatment was 60% higher than that from the fishless treatment. This higher biodegradability was corroborated by a higher annual loss of sediment in fish enclosures (36%) than in fishless ones (16%). Annual losses of carbon, nitrogen and organic phosphorous were higher for sediment from fish enclosures. 4. Carbon and nitrogen contents of sediment were higher for the fish treatment. In contrast, the sediment C/N ratio, one of the proxies used to estimate sediment biodegradability, did not differ between treatments. No relationship was observed between elemental composition of sediment and its potential biodegradability. This latter appeared to be more probably dependent on the biochemical composition of the sediment and especially on the content of labile compounds such as proteins, sugars and polyunsaturated fatty acids. The use of sterols as biomarkers revealed an important degradation by microorganisms of 1‐year‐old sediment from both fish and fishless treatments. 5. Our results revealed that fish biomanipulations might favour clear water states not only through a stronger top–down control on phytoplankton but also through a lower biodegradability of sediment reducing internal nutrient cycling.     

The impact of substrate and lake trophy on the biomass and nutrient status of benthic algae

Algal biomass, C:N:P (carbon:nitrogen:phosphorus) ratios and APA (biomass specific alkaline phosphatase activity) were measured in benthic algal communities on living substrates (mussels and macrophytes) and on rocks and stones (epilithon) in three lakes of different trophy. Benthic algal communities on living substrates had lower C:N:P ratios than epilithon, whereas algal biomass was highest on rocks and stones. Benthic algal biomass increased with the trophic level of a lake despite an increase of C:N:P ratios in the benthic community. The differences in C:N:P ratios and algal biomass between lakes of different trophy were higher on inert substrates than on macrophytes and mussels, probably because algae on living substrates could compensate a poor nutrient supply from lake water with substrate nutrients. However, the substrate was not, as expected, the most important nutrient supply in the oligotrophic lake, but in the eutrophic lake. Therefore, differences between inert and living substrates in a single lake were highest in the eutrophic lake. APA values of the oligotrophic lake were very high especially for benthic algae on stones, indicating an ability of the community to take up nutrients from organic sources. In conclusion, living substrates were an important nutrient source for benthic algae and the importance of this nutrient supply did not decrease with increasing lake trophy.