Does stocking of filter-feeding fish for production have a cascading effect on zooplankton and ecological state? A study of fourteen (sub)tropical Chinese reservoirs with contrasting nutrient concentrations

Stocking of filter-feeding fish is a common tool used in Chinese reservoirs to increase fish production because of low natural recruitment. Whether such stocking has important negative effects on zooplankton with cascading effects on phytoplankton is debated. We compared the zooplankton communities in fourteen reservoirs with different nutrient concentrations and fish densities. Both chlorophyll a (Chla) and fish catch were positively related with total phosphorus (TP), whereas zooplankton biomass did not show a similar relationship with TP. Zooplankton seemed to be influenced by fish as high fish catches coincided with a low proportion of calanoids of the total copepod biomass, a high proportion of rotifers of the total zooplankton biomass, a low zooplankton:phytoplankton biomass ratio, and the absence of Daphnia irrespective of TP concentration. Both zooplankton biomass and most of the zooplankton:phytoplankton biomass ratios were among the lowest reported in the literature for the nutrient range studied. Furthermore, the Chla:TP ratio was higher than what is typically observed in temperate lakes. We conclude that top-down control of zooplankton is of key importance in reservoirs in South China where frequent stocking of filter-feeding fish seems to contribute to poor water quality in the form of higher algal biomass and reduced clarity.     

武汉东湖水生植物生态学研究 Ⅱ.后湖水生植被动态与水体性质

报道１９９１－１９９３年武汉东湖子湖后湖区水处植被和水体理化性质的调查结果。上前后湖水生植物较东湖其它湖区丰富,共记载了４７种,３年间水生植物生物量和水生植被分布面积变动剧烈。１９９１年水生植被分布面积约为该湖区面积的８．４％;１９９２年高达３８１ｈｍ＾２,覆盖面积达８０％;而１９９３年水生植被面积不到１％。经分析,认为目前影响后湖水生植被的扩要因子是草食性鱼类,只要停止放养     

Omnivory by Planktivores Stabilizes Plankton Dynamics, but May Either Promote or Reduce Algal Biomass

Classical models of phytoplankton–zooplankton interaction show that with nutrient enrichment such systems may abruptly shift from limit cycles to stable phytoplankton domination due to zooplankton predation by planktivorous fish. Such models assume that planktivorous fish eat only zooplankton, but there are various species of filter-feeding fish that may also feed on phytoplankton. Here, we extend these classical models to systematically explore the effects of omnivory by planktivorous fish. Our analysis indicates that if fish forage on phytoplankton in addition to zooplankton, the alternative attractors predicted by the classical models disappear for all realistic parameter settings, even if omnivorous fish have a strong preference for zooplankton. Our model also shows that the level of fish biomass above which zooplankton collapse should be higher when fish are omnivorous than when fish are zooplanktivorous. We also used the model to explore the potential effects of the now increasingly common practice of stocking lakes with filter-feeding fish to control cyanobacteria. Because omnivorous filter-feeding fish forage on phytoplankton as well as on the main grazers of phytoplankton, the net effect of such fish on the phytoplankton biomass is not obvious. Our model suggests that there may be a unimodal relationship between the biomass of omnivorous filter-feeding fish and the biomass of phytoplankton. This implies that to manage for reductions in phytoplankton biomass, heavy stocking or strong reduction of such fish is best.     

Experimental impacts of fish on small and large cladocerans under eutrophic conditions

The impacts of fish and nutrient enrichment on the biomass of small and large cladoceran species were studied in mesocosms under eutrophic conditions. Fish shifted the ratio between small and large species to favour small cladocerans compared to control mesocosms, while nutrient enrichment favoured large cladocerans. Both fish vital activity and nutrient enrichment increased total and blue-green algae concentrations. While chlorophyll concentrations increased in response to fish and nutrient enrichment, there were no significant effects of fish on total cladoceran biomass and the relative proportion of large species due to high variability among replicates within treatments. However, fish and nutrient enrichment affected average total cladoceran biomass and the biomass of large species compared to control conditions.     

Restoration of shallow lakes by nutrient control and biomanipulation—the successful strategy varies with lake size and climate

Major efforts have been made world-wide to improve the ecological quality of shallow lakes by reducing external nutrient loading. These have often resulted in lower in-lake total phosphorus (TP) and decreased chlorophyll a levels in surface water, reduced phytoplankton biomass and higher Secchi depth. Internal loading delays recovery, but in north temperate lakes a new equilibrium with respect to TP often is reached after <10–15 years. In comparison, the response time to reduced nitrogen (N) loading is typically <5 years. Also increased top-down control may be important. Fish biomass often declines, and the percentage of piscivores, the zooplankton:phytoplankton biomass ratio, the contribution of Daphnia to zooplankton biomass and the cladoceran size all tend to increase. This holds for both small and relatively large lakes, for example, the largest lake in Denmark (40 km²), shallow Lake Arresø, has responded relatively rapidly to a ca. 76% loading reduction arising from nutrient reduction and top-down control. Some lakes, however, have proven resistant to loading reductions. To accelerate recovery several physico-chemical and biological restoration methods have been developed for north temperate lakes and used with varying degrees of success. Biological measures, such as selective removal of planktivorous fish, stocking of piscivorous fish and implantation or protection of submerged plants, often are cheap versus traditional physico-chemical methods and are therefore attractive. However, their long-term effectiveness is uncertain. It is argued that additional measures beyond loading reduction are less cost-efficient and often not needed in very large lakes. Although fewer data are available on tropical lakes these seem to respond to external loading reductions, an example being Lake Paranoá, Brazil (38 km²). However, differences in biological interactions between cold temperate versus warm temperate-subtropical-tropical lakes make transfer of existing biological restoration methods to warm lakes difficult. Warm lakes often have prolonged growth seasons with a higher risk of long-lasting algal blooms and dense floating plant communities, smaller fish, higher aggregation of fish in vegetation (leading to loss of zooplankton refuge), more annual fish cohorts, more omnivorous feeding by fish and less specialist piscivory. The trophic structures of warm lakes vary markedly, depending on precipitation, continental or coastal regions locations, lake age and temperature. Unfortunately, little is known about trophic dynamics and the role of fish in warm lakes. Since many warm lakes suffer from eutrophication, new insights are needed into trophic interactions and potential lake restoration methods, especially since eutrophication is expected to increase in the future owing to economic development and global warming.     

Community resistance and change to nutrient enrichment and fish manipulation in a vegetated lake littoral

1. High biomass of macrophytes is considered important in the maintenance of a clear‐water state in shallow eutrophic lakes. Therefore, rehabilitation and protection of aquatic vegetation is crucial to the management of shallow lakes. 2. We conducted field mesocosm experiments in 1998 and 1999 to study community responses in the plant‐dominated littoral zone of a lake to nutrient enrichment at different fish densities. We aimed to find the threshold fish biomass for the different nutrient enrichment levels below which large herbivorous zooplankton escapes control by fish. The experiments took place in the littoral of Lake Vesijärvi in southern Finland and were part of a series of parallel studies carried out jointly at six sites across Europe. 3. In 1998, when macrophyte growth was poor, a clear‐water state with low phytoplankton biomass occurred only in unenriched mesocosms without fish or with low fish biomass (4 g fresh mass m^?2). Both nutrient enrichment and high fish biomass (20 g fresh mass m^?2) provoked a turbid water state with high planktonic and periphytic algal biomass. The zooplankton community was dominated by rotifers and failed to control the biomass of algae in nutrient enriched mesocosms. The littoral community thus had low buffer capacity against nutrient enrichment. 4. In 1999, macrophytes, especially free‐floating Lemna trisulca L., grew well and the zooplankton community was dominated by filter‐feeding cladocerans. The buffer capacity of the littoral community against nutrient enrichment was high; a clear‐water state with low phytoplankton biomass prevailed even under the highest nutrient enrichment. High grazing rates by cladocerans, together with reduced light penetration into the water caused by L. trisulca, were apparently the main mechanisms behind the low algal biomass. 5. Effects of fish manipulations were less pronounced than effects of nutrient enrichment. In 1999, clearance rates of cladocerans were similar in fish‐free and low‐fish treatments but decreased in the high‐fish treatment. This suggests that the threshold fish biomass was between the low‐ and high‐fish treatments. In 1998, such a threshold was found only between fish‐free and low‐fish treatments. 6. The pronounced difference in the observed responses to nutrient enrichment and fish additions in two successive years suggests that under similar nutrient conditions and fish feeding pressure either clear or turbid water may result depending on the initial community structure and on weather.     

背角无齿蚌(Anodonta woodiana)对浅水系统底栖和浮游藻类竞争关系的影响

底栖藻类和浮游藻类之间的竞争关系对浅水生态系统的结构、功能具有重要的影响,双壳类可通过滤食控制浮游藻类,从而改变底栖藻类与浮游藻类之间的竞争结果。论文通过比较放养背角无齿蚌(Anodonta woodiana)(蚌处理组)与不放养背角无齿蚌(对照组)系统中底栖藻类、浮游藻类的生物量和优势种等的变化,研究了滤食性双壳类对底栖藻类和浮游藻类间竞争的影响。结果表明,背角无齿蚌可显著降低浮游藻类生物量,提高水体透明度和沉积物表面光照条件,从而显著提高底栖藻类的生物量;背角无齿蚌也改变了浮游藻类的优势种,使优势种由蓝藻转变成硅藻。因此,滤食性双壳类有利于促进浅水生态系统从混水态向清水态转变,本研究结果对富营养化浅水湖泊修复与管理具有一定的参考意义。     

Evaluation of Restoration Measures in a Shallow Lake through a Comparison of Present Day Zooplankton Communities with Historical Samples

Many shallow lakes have lost a large part of their ecological value during the past decades. Human-induced factors such as eutrophication and inappropriate fish stock management are generally the main causes for this loss. To restore such degraded habitats, several measures are taken, typically involving a reduction of nutrient loading and interventions in the aquatic food web functioning (biomanipulation). In this study, we report on the joint effects of a series of restoration measures in a shallow lake (Lake Kraenepoel, Belgium, 22 ha) and evaluate these effects via three different criteria. The first criterion is that the target condition, being a clearwater phase with submerged macrophytes, was successfully achieved and persisted for a period of at least 5 years after restoration. Second, we detected a substantial change in community structure of cladoceran zooplankton and an associated increase in species richness and conservation value following restoration measures. Finally, we observed that the general structure of the present day cladoceran zooplankton community resembles well that of the preeutrophication period (1929–1931). Current species richness, however, tends to be lower than in the reference period, and some rare species are still lacking. It is conceivable that, when submerged macrophytes develop further, a subset of specialist species may reappear. Overall, the use of historical habitat-specific samples offers a major opportunity for evaluating restoration success in great detail. Community structures may directly be compared, the gain or loss of specific species can accurately be documented, and more insights in the observed patterns be obtained.     

Experimental study of trophic cascade effect of silver carp (Hypophthalmichthys molitrixon) in a subtropical lake, Lake Donghu: on plankton community and underlying mechanisms of changes of crustacean community

An enclosure experiment was carried out to test trophic cascade effect of filter-feeding fish on the ecosystem: growth of crustacean zooplankton, and possible mechanism of changes of crustacean community structure. Four fish biomass levels were set as follows: 0, 116, 176 and 316 g m^-2, and lake water (containing ca. 190 g m^-2 of filter-feeding fishes) was comparatively monitored. Nutrient levels were high in all treatments during the experiment. Lowest algal biomass were measured in fishless treatment. Algal biomass decreased during days 21–56 as a function of fish biomass in treatments of low (LF), medium (MF) and high (HF) fish biomass. Crustaceans biomass decreased with increasing fish biomass. Small-bodied cladocerans, Moina micrura, Diaphanosoma brachyurum and Scapholeberis kingii survived when fish biomass was high whilst, large-bodied cladocerans Daphnia spp. and the cyclopoids Theromcyclops taihokuensis, T. brevifuratus, Mescyclops notius and Cyclops vicinus were abundant only in NF enclosures. Evasive calanoid Sinodiaptomus sarsi was significantly enhanced in LF, but decreased significantly with further increase of fish biomass. Demographic data indicated that M. micrura was well developed in all treatments. Our study indicates that algal biomass might be controlled by silver carp biomass in eutrophic environment. Changes of crustacean community are probably affected by the age of the first generation of species. Species with short generation time were dominant and species with long generation time survived less with high fish biomass. Evasive calanoids hardly developed in treatments with high fish biomass because of the (bottle neck) effect of nauplii. Species abundance were positively related to fish predation avoidance. Other than direct predation, zooplankton might also be suppressed by filter-feeding fish via competition.     

The effect of river restoration on fish,macroinvertebrates and aquatic macrophytes: A meta-analysis

An increasing number of rivers have been restored over the past decades and several studies investigated the effect on biota. The published monitoring results have already been summarized in narrative reviews but there are few quantitative reviews and a comprehensive meta-analysis on different organism groups and factors influencing restoration effect is missing. We compiled monitoring results and information on catchment, river and project characteristics from peer-reviewed literature and unpublished databases to (i) quantify the effect of restoration measures on fish, macroinvertebrates and macrophytes, and (ii) identify predictors which influence restoration effect. Results indicated significant effects of restoration on all three organism groups, especially of widening projects on macrophyte richness/diversity, instream measures on fish and macroinvertebrates, and higher effects on abundance/biomass compared to richness/diversity. Restoration effect was most strongly affected by agricultural land use, river width and project age. Effects were smaller but restoration generally still increased richness/diversity and abundance/biomass in agricultural catchments. Since land use is a proxy for different pressures, the underlying causal relationships have to be investigated in more detail. Project age was the most important factor but had non-linear and even negative effects on restoration outcome, indicating that restoration effects may vanish over time. The meta-analysis indicated that river managers in general can expect an increase of richness/diversity and abundance/biomass of all three organism groups investigated, especially of macrophytes in widening projects and of fish and macroinvertebrates if instream measures are applied. However, variability was high, stressing the need for adaptive management approaches. Furthermore, the large but non-linear and different (even negative) effects of project age stressed the need for long-time monitoring to better understand the trajectories of change caused by restoration measures and to identify sustainable measures. The meta-analysis was restricted to metrics commonly reported in literature and future studies would greatly benefit from authorities and scientists reporting original monitoring data, which would allow to use functional metrics to investigate the effect of restoration measures and to infer causal relationships.     

Ecological restoration in eutrophic Lake Wuli: A large enclosure experiment

A large-scale enclosure experiment for lake restoration was carried out in Lake Wuli, a northern bay of shallow and eutrophic Lake Taihu in China. The large enclosure with an area of 10 ha was set up in the littoral zone and was bordered by waterproof fabric which did not cover the sediments. Multiple approaches were used and included fish removal, piscivorous fish stocking, shoreline reconstruction, aquatic macrophyte planting, benthic macro-animal stocking, and silver carp cultivation in pens for reduction of cyanobacteria. The results showed that the coverage of aquatic macrophytes increased from 0% to 45.7%. Mean concentrations of TN and TP inside the enclosure from May 2004 to May 2008 were 22.2% and 26.0% of those outside, respectively. Secchi depth was 0.40 m outside the enclosures and 0.75 m inside. However, responses of phytoplankton to the restoration project lagged behind improvement of water quality and reestablishment of aquatic plants. The phytoplankton biomass gradually decreased after the third year of the restoration. Stocking piscivorous fish and planting submerged macrophytes could not increase zooplankton biomass and enhance graze pressure on phytoplankton, most likely due to high omnivorous fish density and lower nutrition inside the enclosure. Higher grazing pressure of zooplankton on phytoplankton was observed in May and October every year. Zooplankton to phytoplankton biomass ratios were significantly negatively correlated with phytoplankton biomass outside (r = −0.440, p < 0.01) and inside the enclosure (r = −0.336, p < 0.05) from February 2004 to March 2007. Therefore, phytoplankton biomass inside and outside the enclosure was lower in May and October. Higher grazing pressure of zooplankton on phytoplankton in spring may result in occurrence of the clear-water phase that facilitated growth of submerged macrophytes in the littoral in Lake Wuli, and a clear-water state and improved water quality would likely be sustained throughout the year after reestablishment of submerged macrophytes.     

Effects of a filter-feeding fish [silver carp, Hypophthalmichthys molitrix (Val.)] on phyto- and zooplankton in a mesotrophic reservoir: results from an enclosure experiment

SUMMARY 1. Silver carp, Hypophthalmichthys molitrix (Val.), feeds on both phyto- and zooplankton and has been used in lake biomanipulation studies to suppress algal biomass. Because reports on the effects of silver carp on lake food webs have been contradictory, we conducted an enclosure experiment to test how a moderate biomass of the fish (10 g wet weight m⁻³) affects phytoplankton and crustacean zooplankton in a mesotrophic temperate reservoir.
2. Phytoplankton biomass <30 μm and particulate organic carbon (POC) <30 μm were significantly higher in enclosures with silver carp than in enclosures without fish, whereas Secchi depth was lower. Total copepod biomass declined strongly in both treatments during the experiment, but it was significantly higher in fish-free enclosures. Daphnid biomass was also consistently higher in enclosures without fish, although this effect was not significant. However, the presence of fish led to a fast and significant decrease in the size at maturity of Daphnia galeata Sars. Thus, the moderate biomass of silver carp had a stronger negative effect on cladoceran zooplankton than on phytoplankton.
3. Based on these results and those of previous studies, we conclude that silver carp should be used for biomanipulation only if the primary aim is to reduce nuisance blooms of large phytoplankton species (e.g. cyanobacteria) that cannot be effectively controlled by large herbivorous zooplankton. Therefore, stocking of silver carp appears to be most appropriate in tropical lakes that are highly productive and naturally lack large cladoceran zooplankton.     

A 2-year experimental study on nutrient and predator influences on food web constituents in a shallow lake of north-west Spain

1. A 2‐year study was carried out on the roles of nutrients and fish in determining the plankton communities of a shallow lake in north‐west Spain. Outcomes were different each year depending on the initial conditions, especially of macrophyte biomass. In 1998 estimated initial ‘per cent water volume inhabited’ (PVI) by submerged macrophytes was about 35%. Phytoplankton biomass estimated as chlorophyll a was strongly controlled by fish, whereas effects of nutrient enrichment were not significant. In 1999 estimated PVI was 80%, no fish effect was observed on phytoplankton biomass, but nutrients had significant effects. Water temperatures were higher in 1998 than in 1999. 2. In the 1998 experiment, cladoceran populations were controlled by fish and cyanobacteria were the dominant phytoplankton group. There were no differences between effects of low (4 g fresh mass m^?2) and high (20 g fresh mass m^?2) fish density on total zooplankton biomass, but zooplankton biomass was higher in the absence of fish. With the high plant density in 1999, fish failed to control any group of the zooplankton community. 3. Total biovolume of phytoplankton strongly decreased with increased nutrient concentrations in 1998, although chlorophyll a concentrations did not significantly change. At higher nutrient concentrations, flagellate algae became more abundant with likely growth rates that could have overcompensated cladoceran feeding rates. This change in phytoplankton community composition may have been because of increases in the DIN : SRP ratio. Both chlorophyll a concentration and total phytoplankton biovolume increased significantly with nutrients in the 1999 experiment. 4. A strong decline of submerged macrophytes was observed in both years as nutrients increased, resulting in shading by periphyton. This shading effect could account for the plant decline despite lower water turbidity at the very high nutrient levels in 1998.     

Long term study on the influence of eutrophication, restoration and biomanipulation on the structure and development of phytoplankton communities in Feldberger Haussee (Baltic Lake District, Germany)

Feldberger Haussee provides a classic example of eutrophication history of hardwater lakes in the Baltic Lake District (Germany) and of changes in their algal flora during the 20th century. The lake originally was regarded as slightly eutrophic. A process of drastic eutrophication from the 1950s until the end of the 1970s caused mass developments of blue-green and green algae. A restoration program was started in the 1980s to improve the water quality of the lake using both diversion of sewage outside the catchment area, and biomanipulation by altering the fish community. This restoration program led to positive changes in the lake ecosystem. Direct effects of biomanipulation resulted in an increase of herbivorous zooplankton, a decrease of phytoplankton biomass, and an increase of water transparency. The recovery of Feldberger Haussee also may have been indirectly enhanced by an increase in nutrient sedimentation as a consequence of intensified calcite precipitation, decrease in phosphorus remobilization due to a pH-decrease, increased NIP-ratio, and recolonization of the littoral zone by macrophytes. This paper concentrates on the long term development of the phytoplankton community as a response to changes in the food web structure as well as to alterations in the chemical environment of the algae. Both are reflected in four major stages passed by the algal assemblage between 1980 and 1994: (1) From 1980-summer 1985 dense green algal populations were found indicating similar conditions as in the 1970s during the period of maximum eutrophication. (2) A diverse phytoplankton community during summer 1985–1989 showed the first effects of a recovery. (3) From 1990–1992 the phytoplankton was characterized by ungrazeable filamentous blue-green algae first of all as a response to increased herbivory of zooplankton on edible species and to increasing N/P-ratios. (4) Finally, the algal species diversity increased in 1993 and 1994 whereas the phytoplankton biomass decreased showing the success of the combined restoration measures.     

Invasive Crayfish Threaten the Development of Submerged Macrophytes in Lake Restoration

Submerged macrophytes enhance water transparency and aquatic biodiversity in shallow water ecosystems. Therefore, the return of submerged macrophytes is the target of many lake restoration projects. However, at present, north-western European aquatic ecosystems are increasingly invaded by omnivorous exotic crayfish. We hypothesize that invasive crayfish pose a novel constraint on the regeneration of submerged macrophytes in restored lakes and may jeopardize restoration efforts. We experimentally investigated whether the invasive crayfish (Procambarus clarkii Girard) affects submerged macrophyte development in a Dutch peat lake where these crayfish are expanding rapidly. Seemingly favourable abiotic conditions for macrophyte growth existed in two 0.5 ha lake enclosures, which provided shelter and reduced turbidity, and in one lake enclosure iron was added to reduce internal nutrient loading, but macrophytes did not emerge. We transplanted three submerged macrophyte species in a full factorial exclosure experiment, where we separated the effect of crayfish from large vertebrates using different mesh sizes combined with a caging treatment stocked with crayfish only. The three transplanted macrophytes grew rapidly when protected from grazing in both lake enclosures, demonstrating that abiotic conditions for growth were suitable. Crayfish strongly reduced biomass and survival of all three macrophyte species while waterfowl and fish had no additive effects. Gut contents showed that crayfish were mostly carnivorous, but also consumed macrophytes. We show that P. clarkii strongly inhibit macrophyte development once favourable abiotic conditions for macrophyte growth are restored. Therefore, expansion of invasive crayfish poses a novel threat to the restoration of shallow water bodies in north-western Europe. Prevention of introduction and spread of crayfish is urgent, as management of invasive crayfish populations is very difficult.     

营养盐加富和鱼类添加对浮游植物群落演替和多样性的影响

浮游植物是水体生态系统敞水区最重要的初级生产者,其组成与多样性反映了群落的结构类型和存在状态。通过围隔实验,模拟水库春季发生的营养盐加富和鱼类放养的干扰,分析在这两种干扰下的浮游植物群落演替过程中优势种和稀有种的变化,并通过以丰度与生物量为变量的香农和辛普森多样性指数的计算,分析浮游植物群落演替过程中的多样性变化特征。结果表明,营养盐加富干扰下的浮游植物群落的优势种变化和演替更为明显,营养盐加富与鱼类添加对浮游植物群落多样性变化的影响符合中度干扰理论。在优势种优势度变化较大的浮游植物群落演替过程中,多样性指数与浮游植物生物量有较高的负相关性。在浮游植物群落演替过程中,香农和辛普森多样性指数的变化趋势基本一致,采用丰度与生物量为变量的两种多样性指数的计算结果对实验系统中浮游植物群落多样性的分析结果没有明显的影响。     

Fish community assemblages changed but biomass remained similar after lake restoration by biomanipulation in a Chinese tropical eutrophic lake

While the effects of lake restoration by fish manipulation are well-studied in the temperate zone, comparatively little information is available on this issue from tropical lakes. It may be expected that fish removal leads to faster recovery of the fish stock here than in temperate lakes due to more frequent and earlier reproduction, which may, in turn, delay positive effects of the restoration. We studied the community composition, feeding type and abundance of fish in three basins of a tropical shallow lake: one unrestored basin (UR) and two basins restored by fish manipulation and transplantation of submerged macrophytes. While omni-benthivorous fish dominated the biomass in the restored basins 3 and 5 years after restoration, planktivores were most abundant in the UR, although total fish biomass remained similar. One-way analyses of similarities based on fish species presence/absence, abundance and biomass data revealed significant differences in fish community composition among the restored basins and UR, and redundancy analyses further indicated that submerged macrophytes were a key driver behind this difference. Our results indicate that active implantation of submerged macrophytes to stabilise the fish community is a tool to consider when planning lake restoration by biomanipulation in the tropics.     

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-mediated indirect effects in a littoral food web

The strength of the direct effect by scraping cladocerans and the indirect effect of nutrient regeneration by filtering herbivorous cladocerans on periphyton growth was investigated in a littoral food web. Ten enclosures were erected in a lake in an area with artificial vegetation. Five enclosures were stocked with juvenile perch ( Perca fluviatilis ) and five lacked fish. In addition, a reference area in the artificial vegetation was sampled. The mesh size of the net surrounding the cages was chosen to allow an inflow of phytoplankton into the cages from the surrounding water. The periphyton and filtering herbivorous cladoceran biomasses were highest in the fish-free treatment. There was no difference in phytoplankton biomass between treatments despite the large difference in filtering herbivorous cladoceran biomass, suggesting that the inflow of phytoplankton into enclosures completely compensated for the loss due to filtering. The reference area and the enclosure with fish showed the same patterns in developments with respect to filtering herbivorous cladocerans and periphyton. Scraping cladoceran biomass was higher in the fish-free treatment resulting in a positive correlation between scraping cladoceran and periphyton biomass. Our results suggest that the positive indirect effect of filtering herbivorous cladoceran nutrient regeneration on periphyton was stronger than the negative direct effect of grazing by scraping cladocerans on periphyton in this semi-open system, and that pelagic production by phytoplankton may foster periphyton growth in the littoral habitat via filtering herbivorous cladocerans. Furthermore, heterogeneity within trophic levels involving primary producers of different growth forms such as phytoplankton and periphyton may enhance the potential for compensatory responses via nutrient recycling.     

Modelling nutrient cycles in relation to food web structure in a biomanipulated shallow lake

The modelPCLAKE describes the phosphorus and nitrogen cycles within a shallow lake ecosystem, including the sediment and a simplified biological food web. All components are modelled in a generalized way rather than a very detailed one. This model has been applied to Lake Zwemlust, a small biomanipulated lake in The Netherlands. Formerly, this highly eutrophic lake was dominated by cyanobacteria and devoid of macrophytes. Biomanipulation was carried out in 1987 by pumping-out of the water, removal of all fish, and refilling of the lake with seepage water. The lake was restocked with some rudd, pike, zooplankton and seedlings of macrophytes, and then monitored up to 1992. Macrophytes developed rather quickly and reached their maximum biomass during the six-years period in 1989. Despite the continuously high nutrient (N and P) loading, algal biomass remained low due to nitrogen limitation, caused by competition with the macrophytes. From 1990 onwards, the macrophytes declined again and a species shift occurred, following an increase of herbivorous birds on the lake and the development of herbivorous fishes.Model simulations grossly reproduced the observed developments in Lake Zwemlust before and after the biomanipulation measures. The existence of multiple steady states at the same trophic state and the possible shift between them could be simulated well. This study also demonstrates the interrelation between system structure and the distribution and cycling of nutrients. It is concluded, that within general boundary conditions set by the trophic state of the system, the food web structure determines the actual nutrient flows and the occurrence of nutrient limitations of the primary producers. It is shown that both aspects can be integrated in one mathematical model. The long-term stability of the macrophyte dominance in the lake is discussed.