Continental-scale patterns of nutrient and fish effects on shallow lakes: introduction to a pan-European mesocosm experiment

1. Shallow lake ecosystems are normally dominated by submerged and emergent plants. Biological stabilising mechanisms help preserve this dominance. The systems may switch to dominance by phytoplankton, however, with loss of submerged plants. This process usually takes place against a background of increasing nutrient loadings but also requires additional switch mechanisms, which damage the plants or interfere with their stabilising mechanisms. 2. The extent to which the details or even major features of this general model may change with geographical location are not clear. Manipulation of the fish community (biomanipulation) has often been used to clear the water of algae and restore the aquatic plants in northerly locations, but it is again not clear whether this is equally appropriate at lower latitudes. 3. Eleven parallel experiments (collectively the International Mesocosm Experiment, IME) were carried out in six lakes in Finland, Sweden, England, the Netherlands and Spain in 1998 and 1999 to investigate the between‐year and large‐scale spatial variation in relationships between nutrient loading and zooplanktivorous fish on submerged plant and plankton communities in shallow lakes. 4. Comparability of experiments in different locations was achieved to a high degree. Cross‐laboratory comparisons of chemical analyses revealed some systematic differences between laboratories. These are unlikely to lead to major misinterpretations. 5. Nutrient addition, overall, had its greatest effect on water chemistry then substantial effects on phytoplankton and zooplankton. Fish addition had its major effect on zooplankton and did not systematically change the water chemistry. There was no trend in the relative importance of fish effects with latitude, but nutrient addition affected more variables with decreasing latitude. 6. The relative importance of top‐down and bottom‐up influences on the plankton differed in different locations and between years at the same location. The outcome of the experiments in different years was more predictable with decreasing latitude and this was attributed to more variable weather at higher latitudes that created more variable starting conditions for the experiments.     

Responses of phytoplankton to fish predation and nutrient loading in shallow lakes: a pan-European mesocosm experiment

1. The impacts of nutrients (phosphorus and nitrogen) and planktivorous fish on phytoplankton composition and biomass were studied in six shallow, macrophyte‐dominated lakes across Europe using mesocosm experiments. 2. Phytoplankton biomass was more influenced by nutrients than by densities of planktivorous fish. Nutrient addition resulted in increased algal biomass at all locations. In some experiments, a decrease was noted at the highest nutrient loadings, corresponding to added concentrations of 1 mg L^?1 P and 10 mg L^?1 N. 3. Chlorophyll a was a more precise parameter to quantify phytoplankton biomass than algal biovolume, with lower within‐treatment variability. 4. Higher densities of planktivorous fish shifted phytoplankton composition toward smaller algae (GALD < 50 μm). High nutrient loadings selected in favour of chlorophytes and cyanobacteria, while biovolumes of diatoms and dinophytes decreased. High temperatures also may increase the contribution of cyanobacteria to total phytoplankton biovolume in shallow lakes.     

Response of zooplankton to nutrient enrichment and fish in shallow lakes: a pan-European mesocosm experiment

1. Responses of zooplankton to nutrient enrichment and fish predation were studied in 1998 and 1999 by carrying out parallel mesocosm experiments in six lakes across Europe. 2. Zooplankton community structure, biomass and responses to nutrient and fish manipulation showed geographical and year‐to‐year differences. Fish had a greater influence than nutrients in regulating zooplankton biomass and especially the relative abundances of different functional groups of zooplankton. When fish reduced the biomass of large crustaceans, there was a complementary increase in the biomasses of smaller crustacean species and rotifers. 3. High abundance of submerged macrophytes provided refuge for zooplankton against fish predation but this refuge effect differed notably in magnitude among sites. 4. Large crustacean grazers (Daphnia, Diaphanosoma, Sida and Simocephalus) were crucial in controlling algal biomass, while smaller crustacean grazers and rotifers were of minor importance. Large grazers were able to control phytoplankton biomass even under hypereutrophic conditions (up to 1600 μg TP L^?1) when grazer biomass was high (>80–90 μg dry mass L^?1) or accounted for >30% of the grazer community. 5. The littoral zooplankton community was less resistant to change following nutrient enrichment in southern Spain, at high temperatures (close to 30 °C), than at lower temperatures (17–23 °C) characterising the other sites. This lower resistance was because of a greater importance of nutrients than zooplankton in controlling algal biomass. 6. Apart from the reduced role of large crustacean grazers at the lowest latitude, no consistent geographical patterns were observed in the responses of zooplankton communities to nutrient and fish manipulation.     

The influence of nutrient loading,climate and water depth on nitrogen and phosphorus loss in shallow lakes: a pan-European mesocosm experiment

Coarse woody debris (CWD) represents a relatively stable habitat in many lakes with forested shorelines providing a living place for a wide range of species. The spatial complexity of CWD is recognized as an important factor promoting the abundance, diversity and productivity of littoral biota, mainly by providing shelters and moderating predator–prey interactions. However, little is as yet known on the response of different species to various levels of CWD complexity and the effects of the spatial arrangement of CWD on the connectivity between littoral populations. It is also unclear how CWD decay, which modifies the surface complexity of wood and the quality of food, affects the diversity of wood-associated species and trophic interactions. Further research is also needed to recognize the contribution of littoral wood to carbon sequestration and nutrient fluxes, considering factors affecting the CWD decay rate, such as wood species and environmental conditions. CWD resources are systematically depleted by shoreline development which leads to disruptions in the functioning of lake ecosystems. Attempts at restoring CWD habitat provided ambiguous effects on littoral species and therefore better understanding of the role of CWD in lake ecosystems is crucial to the development of successful restoration projects and effective management programmes.     

Cascading trophic effects in pampean shallow lakes: results of a mesocosm experiment using two coexisting fish species with different feeding strategies

Planktivorous fish, both visual predators and filter feeders, enhance eutrophication processes in lakes. In pampean shallow lakes several planktivorous species may coexist but often two species dominate: silverside (Odontesthes bonariensis), a visual planktivorous fish when young adult, and sabalito (Cyphocharax voga), an omnivorous filter feeder. To assess the relative strength of the cascading trophic effects of the two species, a mesocosm experiment was conducted using different proportions of both species as treatments. Differences were found in water transparency, phytoplankton biomass, crustacean abundance, individual size and biomass. Our results suggest that visual predators intensify eutrophication effects more strongly than filter feeders do.     

Mesocosm experiments on nutrient and fish effects on shallow lake food webs in a Mediterranean climate

1. Nutrient and fish manipulations in mesocosms were carried out on food‐web interactions in a Mediterranean shallow lake in south‐east Spain. Nutrients controlled biomass of phytoplankton and periphyton, while zooplankton, regulated by planktivorous fish, influenced the relative percentages of the dominant phytoplankton species. 2. Phytoplankton species diversity decreased with increasing nutrient concentration and planktivorous fish density. Cyanobacteria grew well in both turbid and clear‐water states. 3. Planktivorous fish increased concentrations of soluble reactive phosphorus (SRP). Larger zooplankters (mostly Ceriodaphnia and copepods) were significantly reduced when fish were present, whereas rotifers increased, after fish removal of cyclopoid predators and other filter feeders (cladocerans, nauplii). The greatest biomass and diversity of zooplankton was found at intermediate nutrient levels, in mesocosms without fish and in the presence of macrophytes. 4. Water level decrease improved underwater light conditions and favoured macrophyte persistence. Submerged macrophytes (Chara spp.) outcompeted algae up to an experimental nutrient loading equivalent to added concentrations of 0.06 mg L^?1 PO₄‐P and 0.6 mg L^?1 NO₃‐N, above which an exponential increase in periphyton biomass and algal turbidity caused characean biomass to decline. 5. Declining water levels during summer favoured plant‐associated rotifer species and chroococcal cyanobacteria. High densities of chroococcal cyanobacteria were related to intermediate nutrient enrichment and the presence of small zooplankton taxa, while filamentous cyanobacteria were relatively more abundant in fishless mesocosms, in which Crustacea were more abundant, and favoured by dim underwater light. 6. Benthic macroinvertebrates increased significantly at intermediate nutrient levels but there was no relationship with planktivorous fish density. 7. The thresholds of nutrient loading and in‐lake P required to avoid a turbid state and maintain submerged macrophytes were lower than those reported from temperate shallow lakes. Mediterranean shallow lakes may remain turbid with little control of zooplankton on algal biomass, as observed in tropical and subtropical lakes. Nutrient loading control and macrophyte conservation appear to be especially important in these systems to maintain high water quality.     

Fish and nutrient enrichment effects on rotifers in a Mediterranean shallow lake: a mesocosm experiment

A mesocosm experiment was conducted “in situ” in a Chara dominated shallow lake near Valencia (Spain) to study top–down and bottom–up effects on rotifers by means of nutrient and fish additions. Both processes were important in determining rotifer abundance, biomass and diversity. A total of 36 mesocoms were established with triplicate treatment combinations of three fish levels (from no fish to 45 individuals of Gambusia holbrooki males) and four nutrient enrichment levels (from no additions to 10 mg l⁻¹ nitrate-N and 1 mg l⁻¹ phosphate-P). The main effect was a notable increase of planktonic and plant associated rotifers densities with fish. Rotifers benefited from mosquitofish predation on microcrustaceans and chironomids. The results showed a marked negative relationship between rotifer and cyclopoid abundances, indicating the importance of the predatory pressure of cyclopoids on rotifers. Effects on rotifer diversity were also evident, in general rotifer diversity decreased with nutrients and increased with fish. The effects of nutrients analysed at species level showed two contrasting density responses: an increase or a decrease with nutrients, which levelled off at high nutrient concentrations. High-level nutrient additions (from 5 mg l⁻¹ nitrate-N and 0.5 mg l⁻¹ phosphate-P) induced a switch to a turbid state with macrophyte disappearance. Most planktonic rotifer species, as well as plant associated ones, diminished when the turbid state was well established, especially in the mesocosms without fish. In the turbid mesocosms, relative abundance of plant-associated rotifers (as a whole) was higher than that of planktonic rotifers. The changes in rotifer species composition after the switch from a clear to a turbid water state are also described. Species of the genus Anuraeopsis, Trichocerca and Hexarthra, dominant in the clear water state, practically disappeared in the turbid water state, in which Proalides tentaculatus and Lecane nana were the main species. Guest editors: S. S. S. Sarma, R. D. Gulati, R. L. Wallace, S. Nandini, H. J. Dumont & R. Rico-Martínez Advances in Rotifer Research     

Response of epiphytic algae to nutrient loading and fish density in a shallow lake: a mesocosm experiment

Studies on the effect of eutrophication on the ecology of shallow lakes, usually pay scant attention to changes within the epiphytic algal community, though the contribution of this to the ecosystem dynamics is transcendental. In order to test the influence of nutrient loadings and fish densities in the structure of algal epiphyton in a shallow lake, an experiment was performed using in situ mesocosms. Nutrient additions were related to significant decreases in the total epiphyton biovolume and that of bacillariophyceans and zygnematophyceans, but with increases in the abundance of cyanobacteria. The different response of algal groups at the higher nutrient concentrations (increases or decreases in their abundance and/or biovolume) can be related to their ecophysiological constraints such as different resistance to toxicant ammonium accumulation. Plant-associated macroinvertebrates numbers were positively correlated with total numbers of epiphyton. The presence of planktivorous fish enhanced the abundance and biovolume of all algal groups, except cyanobacteria. Fishes enhanced the abundance of plant-associated animals and of total epiphyton. Fish indirect effects (e.g., nutrients release) and their dietary particularities were among the factors that together with nutrients influenced epiphyton growth. The role of indirect effects of fishes and the importance of their dietary particularities are stressed as key factors to understand the processes controlling epiphyton ecology and the food web structure of shallow lakes. Handling editor: D. Ryder.     

Water level and fish-mediated cascading effects on the microbial community in eutrophic warm shallow lakes: a mesocosm experiment

Information on the effects of water level changes on microbial planktonic communities in lakes is limited but vital for understanding ecosystem dynamics in Mediterranean lakes subjected to major intra- and inter-annual variations in water level. We performed an in situ mesocosm experiment in an eutrophic Turkish lake at two different depths crossed with presence/absence of fish in order to explore the effects of water level variations and the role of top-down regulation at contrasting depths. Strong effects of fish were found on zooplankton, weakening through the food chain to ciliates, HNF and bacterioplankton, whereas the effect of water level variations was overall modest. Presence of fish resulted in lower biomass of zooplankton and higher biomasses of phytoplankton, ciliates and total plankton. The cascading effects of fish were strongest in the shallow mesocosms as evidenced by a lower zooplankton contribution to total plankton biomass and lower zooplankton:ciliate and HNF:bacteria biomass ratios. Our results suggest that a lowering of the water level in warm shallow lakes will enhance the contribution of bacteria, HNF and ciliates to the plankton biomass, likely due to increased density of submerged macrophytes (less phytoplankton); this effect will, however, be less pronounced in the presence of fish.     

Zooplankton response to shading effects of free-floating plants in shallow warm temperate lakes: a field mesocosm experiment

Dense mats of free floating plants (FFP) often produce severe underwater light attenuation and strong oxygen depletion in the water column. In this study, we experimentally assessed the zooplankton response to artificial shading using field mesocosms. During 30 days, we simulated three different light scenarios by mimicking the persistence, absence, and fluctuation of FFP typically encountered in vegetated shallow subtropical lakes. We used dark meshes to simulate the abiotic effects engineered by FFP. Both in the permanently covered and fluctuating situations, anoxia impaired zooplankton development. Anoxia constituted a major driving force in shaping the zooplankton response, whereas the feeding resource availability (phytoplankton) seemed to play a minor role; no top down effect on phytoplankton occurred in anoxic situations. In the fluctuating cover regime (periodic darkness and anoxia), the temporal variation of nanophytoplankton was not affected by zooplankton; once again oxygen availability seemed the main force shaping the zooplankton dynamics. Either periodical or permanent shading, associated to anoxic conditions, impaired the success of small herbivores. Large herbivores and microphytoplankton were negatively affected only under persistent shade and anoxia. In contrast, when neither light nor oxygen limitation occurred, such as in the scenario without shading, top-down control occurred. This study highlights the importance that the oxygen dynamics driven by the presence of FFP exert on the structure and dynamics of zooplankton assemblages and on the top down cascading effects on phytoplankton in warm temperate or subtropical shallow lakes.     

Resuspension of algal cells by benthivorous fish boosts phytoplankton biomass and alters community structure in shallow lakes

1. Positive effects of fish on algal biomass have variously been attributed to cascading top‐down effects and to nutrient enrichment by fish excretion. 2. Here, we used a combination of field and laboratory approaches to test an additional hypothesis, namely that the physical resuspension of settled algal cells by fish enhances algal biomass and alters community composition. 3. A multi‐lake survey showed that phytoplankton biomass and the fraction of motile algae increased with the concentration of inorganic suspended solids. This correlation could not be explained by wind‐induced resuspension because of the small size of the lakes. 4. In an enclosure experiment, chlorophyll‐a concentration, phytoplankton abundance and inorganic suspended solids increased significantly in the presence of Cyprinus carpio (common carp), but only if the fish had access to the sediment. No such effects were seen when a net prevented carp reaching the sediment. 5. The effects of enhanced nutrients and reduced zooplankton grazing as a result of fish feeding could not (fully) explain these observations, suggesting that the resuspension by carp of settled algae from a surface film on the sediment was the major factor in the outcome of the experiment. 6. An increase in diatoms and green algae (organisms with a relatively large sedimentation velocity) only in enclosures where carp could reach the sediment supported this view. 7. Several lines of evidence indicate that fish‐induced resuspension of algal cells from the sediment is an important mechanism that affects phytoplankton biomass and community composition in shallow lakes.     

The influence of water level on macrophyte growth and trophic interactions in eutrophic Mediterranean shallow lakes: a mesocosm experiment with and without fish

1. Water‐level fluctuations are typical of lakes located in the semi‐arid Mediterranean region, which is characterised by warm rainy winters and hot dry summers. Ongoing climate change may exacerbate fluctuations and lead to more severe episodes of drought, so information on the effects of water level on the functioning of lake ecosystems in such regions is crucial. 2. In eutrophic Lake Eymir, Turkey, we conducted a 4‐month (summer) field experiment using cylindrical 0.8‐m‐ (low‐water‐level) and 1.6‐m‐deep (high‐water‐level) mesocosms (kept open to the sediment and atmosphere). Fish (tench, Tinca tinca, and bleak, Alburnus escherichii) were added to half of the mesocosms, while the rest were kept fishless. Ten shoots of Potamogeton pectinatus were transplanted to each mesocosm. 3. Sampling for physicochemical variables, chlorophyll a (chl‐a), zooplankton and per cent plant volume inhabited (PVI%) by macrophytes was conducted weekly during the first 5 weeks, and subsequently biweekly. Macrophytes were harvested on the last sampling date. During the course of the experiment, the water level decreased by 0.41 ± 0.06 m. 4. Throughout the experiment, fish affected zooplankton abundance (?), nutrient concentrations (+), chl‐a (+) and water clarity (?) most strongly in the low‐water‐level mesocosms and the zooplankton community shifted towards dominance of small‐sized forms. The fishless mesocosms had a higher zooplankton/phytoplankton ratio, suggesting higher grazing. 5. Greatest macrophyte growth was observed in the low‐water‐level fishless mesocosms. However, despite high nutrient concentrations and low water clarity, macrophytes were also abundant in the fish mesocosms and particularly increased following a water‐level decrease from midsummer onwards. Macrophyte growth was poor in the high‐water‐level mesocosms, even in the fishless ones with high water clarity. This was ascribed to extensive periphyton development reducing light availability for the macrophytes. 6. Our results indicate that a reduction in water level during summer may help maintain the growth of macrophytes in Mediterranean eutrophic shallow lakes, despite a strong negative effect of fish predation on water clarity. It is therefore probable that an expected negative effect of global climate change on water clarity because of eutrophication and enhanced top‐down control of fish may be, at least partly, counteracted by reduced water level, provided that physical disturbance is not severe.     

Effects of water-level fluctuations on the littoral benthic fish community in lakes: a mesocosm experiment

Changes in the water level of lakes, either natural or man-made,are important environmental perturbations for eulittoral benthicfish communities. In outdoor mesocosm experiments, we testedthe effects of decreasing shelter availability due to autumnlake-level decrease on the behavior and the growth of two littoralbenthic dwellers, the juvenile burbot and the stone loach. Thetwo species showed significantly different changes in behaviorwhen shelter availability decreased. Burbot built up a distincthierarchy when shelter became sparse, with larger fish beingsignificantly more successful in competing for suitable shelterthan smaller conspecifics. The hierarchy, however, ceased whenshelter availability decreased below a certain level. The largestfish then increasingly abandoned shelter use, while smallerfish persisted with their sheltering behavior. Stone loach,in contrast, showed no hierarchical order or size-related shelteruse. Only burbot showed a significant relationship between theability to occupy the preferred shelter and the somatic growth.These two species provide an example of different strategiesto deal with environmental perturbations like lake-level decrease,with the stenoecious, dominant strategy of the burbot and theeuryoecious, evasive strategy of stone loach. The results demonstratethe importance of conducting behavioral studies under as naturalconditions as possible.     

变温对鱼类抗氧化防御影响的Meta分析

检索中国期刊全文数据库(1994.1～2009.8)、万方数据库(1980.1～2009.8)、维普数据库(1989.1～2009.8),以及Scopus(1960.1～2009.8)、Elsevier(52009.8)、SpringerLink(52009.8)和Blackwell(52009.8)数据库,系统收集涉及温度变化导致鱼类组织超氧化物歧化酶(SOD)、过氧化氢酶(CAT)、谷胱甘肽过氧化物酶(GPX)、谷胱甘肽还原酶(GR)、还原型谷胱甘肽(GSH)、氧化型谷胱甘肽(GSSG)和丙二醛(MDA)变化的相关研究,对纳入23篇文献进行了数据提取和Meta分析,并系统评价变温对鱼类抗氧化防御的影响。除GSSG由于研究数太少不能分析外,Meta分析显示:升温显著提高SOD(标准化均数差SMD=1.0,95% CI=0.4～1.7,P=0.001)和GPX(SMD=0.4,95% CI=0.1～0.7,P=0.005)活力,降温显著下调GPX(SMD=-0.9,95% CI=-1.7～-0.1,P=0.025)和GR(SMD=-1.6,95% CI=-2.5～-0.8,P<0.001)活力。升降温对CAT活力和GSH均无显著影响(P>0.05),但都会显著增加MDA水平(SMD=1.2～1.4,P<0.006)。不同鱼类、组织和测定方法不是引起研究异质性的主要因素,但试验设计的变温幅度是产生SOD、CAT和MDA研究间异质性的主要因子,实验开始温度也会引起GSH研究间的异质性。     

Effects of contrasting omnivorous fish on submerged macrophyte biomass in temperate lakes: a mesocosm experiment

1. Freshwater fish can affect aquatic vegetation directly by consuming macrophytes or indirectly by changing water quality. However, most fish in the temperate climate zone have an omnivorous diet. The impact of fish as aquatic herbivores in temperate climates therefore remains unclear and depends on their dietary flexibility. 2. We tested the effects of a flexible omnivore and an herbivore on aquatic vegetation by comparing the effects of rudd (Scardinius erythrophthalmus, the most herbivorous fish in temperate climates) with grass carp (Ctenopharyngodon idella) in a mesocosm pond study. Exclosures distinguished herbivorous effects of fish on submerged macrophytes from indirect effects through changes in water quality, whereas stable isotope food‐web analysis provided information on fish diets. 3. We hypothesised that rudd, with its flexible diet and preference for animal food items, would only indirectly affect macrophytes, whereas grass carp, with its inflexible herbivorous diet, would directly affect macrophyte biomass. 4. Only grass carp significantly reduced macrophyte biomass through consumption. Rudd had no effect. Food‐web analysis indicated that rudd predominantly consumed animal prey, whereas grass carp included more plants in their diet, although they also consumed animal prey. Grass carp significantly affected water quality, resulting in lowered pH and increased N‐NH₄ concentrations, whereas more periphyton growth was observed in the presence of rudd. However, the indirect non‐herbivorous effects of both fish species had no effect on macrophyte biomass. 5. Both fish species should be considered as omnivores. Despite the fact that rudd is the most herbivorous fish in the western European climate zone, its effect on submerged macrophyte biomass is not substantial at natural densities and current temperatures.     

Response of fish and plankton to nutrient loading reduction in eight shallow Danish lakes with special emphasis on seasonal dynamics

1. For 13 years the response of the plankton and fish community to a decline in external phosphorus loading was studied in eight lakes with a mean depth <5 m. We conducted chi‐square analyses of sign of slope (positive or negative) of bimonthly averages of plankton variables for the eight lakes versus time. For fish, we compared results from two periods, i.e. 1989–1994 versus 1994–2001 as less data were available. 2. Fish community structure tended to respond to the lowered concentration of total phosphorus (TP), although not all changes were significant. While catch per unit effort (multi‐mesh sized gill nets) of cyprinids (especially bream, Abramis brama and roach, Rutilus rutilus) was highest in the first 5‐year period, the quantitative importance particularly of perch (Perca fluviatilis), pike (Esox lucius) and rudd (Scardinius erythropthalmus), a littoral species, increased significantly after 1994. 3. No changes occurred in zooplankton biomass, except for an increase in November and December. Biomass of small cladocerans, however, declined during summer and autumn, and the proportion of Daphnia to cladoceran biomass also increased. Average body weight of Daphnia and that of all cladocerans increased. The proportion of calanoids among copepods decreased in summer and the average body weight of cyclopoids and calanoids decreased during summer and autumn/early winter. 4. Total biovolume of phytoplankton declined significantly in March to June and tended to decline in November and December as well, while no significant changes were observed during summer and autumn. Non‐heterocystous cyanobacteria showed a decreasing trend during summer and autumn, while heterocystous cyanobacteria increased significantly in late summer. An increase in late summer was also evident for cryptophytes and chrysophytes, while diatoms tended to decline during most seasons. 5. We conclude that phytoplankton, and probably also fish, responded rapidly to reduced loading, whereas the effect on zooplankton was less pronounced. However, increases in body weight of cladocerans and the zooplankton to phytoplankton biomass ratio during summer indicate reduced top‐down control on zooplankton and enhanced grazing on phytoplankton. This conclusion is supported by a tendency for fish biomass to decline and a shift towards greater dominance by piscivores and, thus, an increased likelihood of predator control of zooplanktivorous cyprinids.     

Two mesocosm experiments investigating the control of summer phytoplankton growth in a small shallow lake

1. Mesocosm experiments were carried out to examine the relative importance of top down (fish predation) and bottom up (nutrient addition) controls on phytoplankton abundance in a small shallow lake, Little Mere, U.K., in 1998 and 1999. These experiments were part of a series at six sites across Europe. 2. In the 1998 experiment, top‐down processes (through grazing of large Cladocera) were important in determining phytoplankton biomass. The lack of plant refugia for zooplankton was probably important in causing an increasing chlorophyll a concentration even at intermediate fish density. Little Mere normally has abundant macrophytes but they failed to develop substantially during both years. Bottom‐up control was not important in 1998, most probably because of high background nutrient concentrations, as a result of nutrient release from the sediments. 3. In 1999 neither top‐down nor bottom‐up processes were significant in determining phytoplankton biomass. Large cladoceran grazers were absent even in the fish‐free enclosures, probably because dominance of cyanobacteria and high phytoplankton biomass made feeding conditions unsuitable. As in 1998, bottom‐up control of phytoplankton was not important, owing to background nutrient concentrations that were even higher in 1999 than in 1998, perhaps because of the warmer, sunnier weather. 4. The differing outcomes of the two experiments in the same lake with similar experimental designs highlight the importance of starting conditions. These conditions in turn depended on overall weather conditions prior to the experiments.     

Effects of warming and nutrients on sediment community respiration in shallow lakes: an outdoor mesocosm experiment

1. Climate warming is expected to change respiration in shallow lakes but to an extent that depends on nutrient state. 2. We measured sediment respiration (SR) over the season in the dark on intact sediment cores taken from a series of flow‐through, heated and unheated, nutrient‐enriched and unenriched mesocosms. The natural seasonal temperature cycle ranged from 2 to 20 °C in the unheated mesocosms. In the heated mesocosms, the temperature was raised 4–6 °C above ambient temperatures, depending on season, following the A2 climate change scenario downscaled to the local position of the mesocosms, but enlarged by 50%. We further measured ecosystem respiration (ER) in the mesocosms based on semi‐continuous oxygen measurements. 3. SR changed over the season and was approximately ten times higher in summer than in winter. SR showed no clear response to warming in the nutrient‐enriched treatment, while it increased with warming in the unenriched mesocosms which also had lower fish densities. 4. ER was not affected by artificial warming or nutrient enrichment, but it was ten times higher in summer than in winter. 5. SR contributed 24–32% to ER. The SR:ER ratio was generally stimulated by warming and was higher in winter than in summer, especially in the nutrient‐enriched mesocosms. 6. Our results indicate that climate warming may lead to higher SR, especially in clear, macrophyte‐dominated systems. Moreover, the contribution of SR to ER will increase with higher temperatures, but decrease as the winters get shorter.     

Effect of larval fish and nutrient enrichment on plankton dynamics in experimental ponds

The hypotheses that larval fish density may potentially affect phytoplankton abundance through regulating zooplankton community structure, and that fish effect may also depend on nutrient levels were tested experimentally in ponds with three densities of larval walleye, Stizostedion vitreum (0, 25, and 50 fish m^–3), and two fertilizer types (inorganic vs organic fertilizer). A significant negative relationship between larval fish density and large zooplankton abundance was observed despite fertilizer types. Larval walleye significantly reduced the abundances of Daphnia, Bosmina, and Diaptomus but enhanced the abundance of various rotifer species (Brachionus, Polyarthra, and Keratella). When fish predation was excluded, Daphnia became dominant, but Daphnia grazing did not significantly suppress blue-green algae. Clearly, larval fish can be an important regulator for zooplankton community. Algal composition and abundance were affected more by fertilizer type than by fish density. Inorganic fertilizer with a high N:P ratio (20:1) enhanced blue-green algal blooms, while organic fertilizer with a lower N:P ratio (10:1) suppressed the abundance of blue-green algae. This result may be attributed to the high density of blue-green algae at the beginning of the experiment and the fertilizer type. Our data suggest that continuous release of nutrients from suspended organic fertilizer at a low rate may discourage the development of blue-green algae. Nutrient inputs at a low N:P ratio do not necessarily result in the dominance of blue-green algae.     

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.