Fish manipulation as a lake restoration tool in shallow,eutrophic, temperate lakes 2: threshold levels,long-term stability and conclusions

In order to evaluate short-term and long-term effects of fish manipulation in shallow, eutrophic lakes, empirical studies on relationships between lake water concentration of total phosphorus (P) and the occurrence of phytoplankton, submerged macrophytes and fish in Danish lakes are combined with results from three whole-lake fish manipulation experiments. After removal of less than 80 per cent of the planktivorous fish stock a short-term trophic cascade was obtained in the nutrient regimes, where large cyanobacteria were not strongly dominant and persistent. In shallow Danish lakes cyanobacteria were the most often dominating phytoplankton class in the P-range between 200 and 1 000μg P l⁻¹. Long-term effects are suggested to be closely related to the ability of the lake to establish a permanent and wide distribution of submerged macrophytes and to create self-perpetuating increases in the ratio of piscivorous to planktivorous fish. The maximum depth at which submerged macrophytes occurred, decreased exponentially with increasing P concentration. Submerged macrophytes were absent in lakes>10 ha and with P levels above 250–300μg P l⁻¹, but still abundant in some lakes<3 ha at 650μg P l⁻¹. Lakes with high cover of submerged macrophytes showed higher transparencies than lakes with low cover aboveca. 50μg P l⁻¹. These results support the alternative stable state hypothesis (clear or turbid water stages). Planktivorous fish>10 cm numerically contributed more than 80 per cent of the total planktivorous and piscivorous fish (>10 cm) in the pelagical of lakes with concentrations above 100μg P l⁻¹. Below this threshold level the proportion of planktivores decreased markedly toca. 50 per cent at 22μg P l⁻¹. The extent of the shift in depth colonization of submerged macrophytes and fish stock composition in the three whole-lake fish manipulations follows closely the predictions from the relationships derived from the empirical study. We conclude that a long-term effect of a reduction in the density of planktivorous fish can be expected only when the external phosphorus loading is reduced to below 0.5–2.0 g m⁻² y⁻¹. This loading is equivalent to an in-lake summer concentration below 80–150μg P l⁻¹. Furthermore, fish manipulation as a restoration tool seems most efficient in shallow lakes.     

First attempt to apply whole-lake food-web manipulation on a large scale in The Netherlands

Lake Breukeleveen (180 ha, mean depth 1.45 m), a compartment of the eutrophic Loosdrecht lakes system, was selected to study the effects of whole-lake foodweb manipulation on a large scale. In Lake Loosdrecht (dominated by filamentous cyanobacteria), due to water management measures taken from 1970–1984 (sewerage systems, dephosphorization) the external P load has been reduced from 1.2 g m⁻² y⁻¹ to 0.35 g m⁻² y⁻¹. The water transparency (Secchi-depthca. 30 cm), however, has not improved. The aim of the food-web manipulation in Lake Breukeleveen was not only to improve the light climate of the lake, but also to study if the successfull effects observed in small lakes (a few ha) can be upscaled. In March 1989 the standing crop of planktivorous and bentivorous fish populations was reduced by intensive fishery, fromca. 150 kg ha⁻¹ toca. 57 kg ha⁻¹. The lake was made unaccessible to fish migrating from the other lakes and it was stocked with large-sized daphnids and 0⁺ pike. However, water transparency did not increase in the following summer and autumn 1989, which is in contrast with great improvement in the light conditions previously observed in smaller lakes. The main explanations for the negative outcome in Lake Breukeleveen are: 1) the rapid increase of the planktivorous fish biomass and carnivorous cladocerans, predating on the zooplankton community; 2) suppression of the large daphnids by the high concentrations of filamentous cyanobacteria; 3) high turbidity of the lake due to resuspension of bottom material induced by wind, unlike in smaller lakes, and thus inability of submerged macrophytes to develop and to stabilize the ecosystem.     

Macrophyte-related shifts in the nitrogen and phosphorus contents of the different trophic levels in a biomanipulated shallow lake

Lake Zwemlust, a small highly eutrophic lake, was biomanipulated without reducing the external nutrient loading, and the effects were studied for four years. In this paper we pay special attention to the shifts in relative distribution of nitrogen and phosphorus in the different trophic levels and to the changes in growth limitation of the autotrophs.Despite of the high external nutrient loads to the lake (ca 2.4 g P m^–2 y^–1 and 9.6 g N m^–2 y^–1), the effects of biomanipulation on the lake ecosystem were pronounced. Before biomanipulation no submerged vegetation was present in the lake and P and N were stored in the phytoplankton (44% N, 47% P), fish (33% N, 9% P) and in dissolved forms (23% N, 44% P). P and N contents in sediments were not determined. In the spring and summer following the biomanipulation (1987), zooplankton grazing controlled the phytoplankton biomass and about 90% of N and P were present in dissolved form in the water. From 1988 onwards submerged macrophyte stands continue to thrive, reducing the ammonium and nitrate concentrations in the water below detection levels. In July 1989 storage of N and P in the macrophytes reached 86% and 80%, respectively. Elodea nuttallii (Planchon) St.John, the dominant species in 1988 and 1989, acted as sink both for N and P during spring and early summer, withdrawing up to ca 60% of its N and P content from the sediment. At the end of the year only part of the N and P from the decayed macrophytes (ca 30% of N and 60% of P) was recovered in the water phase of the ecosystem (chiefly in dissolved forms). The rest remained in the sediment, although some N may have been released from the lake by denitrification.In summer 1990 only 30% of the N and P was found in the macrophytes (dominant species Ceratophyllum demersum L.), while ca 30% of N and P was again stored in phytoplankton and fish.     

The influence of plant-associated filter feeders on phytoplankton biomass: a mesocosm study

Low phytoplankton biomass usually occurs in the presence of submerged macrophytes, possibly because submerged macrophytes enhance top-down control of phytoplankton by offering a refuge for efficient grazers like Daphnia against fish predation. However, other field studies also suggest that submerged macrophytes suppress phytoplankton in the absence of Daphnia. In order to investigate these mechanisms further, we conducted an outdoor mesocosm experiment to study the effect of submerged macrophytes (Elodea nuttallii) on phytoplankton and zooplankton biomass. The experiment combined four nutrient addition levels (0, 10, 100, and 1000 μg P l⁻¹; N/P ratio: 16) with three macrophyte levels (no macrophytes, artificial macrophytes, and real macrophytes). We inoculated the tanks with species-rich inocula of phytoplankton and zooplankton but excluded fish or macro-invertebrates. Probably due to the lack of predators in the mesocosms, potential grazing rates of pelagic zooplankton (estimated from zooplankton biomass) did not differ between the macrophyte treatment combinations. Compared to the treatment combinations without macrophytes, lower phytoplankton biomass occurred in the treatment combinations with real macrophytes at all the nutrient addition levels and in those with artificial macrophytes at all the nutrient levels except the highest. Significantly, higher abundances of plant-associated filter feeders (Simocephalus vetulus and Ceriodaphnia spp.) occurred in the treatment combinations with real and artificial macrophytes. The estimated potential grazing rate of these plant-associated filter feeders indicated that these filter feeders could be responsible for the lower phytoplankton biomass in the presence of real and artificial macrophytes. Our results suggest that the plant-associated filter feeders may be significant grazers in vegetated shallow lakes.     

Food web manipulation in Lake Zwemlust: Positive and negative effects during the first two years

The hypertrophic Lake Zwemlust, a small water body used as a swimming pool, was characterized by algal blooms in summer, reducing the Secchi disk transparency to less than 0.3 m. Since in The Netherlands a Secchi disk transparency of 1 m is obligatory for swimming waters, corrective measures were called for to improve the light climate of the lake. In March, 1987, as an experiment, the lake was drained by pumping out the water to facilitate fish elimination. Planktivorous and benthivorous fish species, which were predominant, were removed by seine- and electro-fishing. After the lake had refilled by seepage it was restocked by a new simple fish community comprising pike (Esox lucius) and rudd (Scardinius erythrophthalmus) only. Stacks of willow twigs (Salix) and macrophytes (roots ofNuphar lutea and seedlings ofChara globularis) were introduced into the lake as spawning grounds and refuges for the pike against cannibalism and as shelter for the zooplankton. The effects of this food web manipulation on the light climate, phytoplankton, zooplankton, fish, macrophytes, macrofauna and on the nutrient concentrations were monitored during 1987 and 1988. In summer 1987, despite of high nutrient concentrations, the phytoplankton density was low, due to control by zooplankton, causing a Secchi disk transparency of 2.5 m, the maximum depth. Chlorophyll-a concentrations were low (<5 g Chl.l^–1), blooms of cyanobacteria did not occur and a shift from rotifers to cladocerans took place. In 1988, however, also some negative effects were noticed. Macrophytes and filamentous green algae reached a much higher biomass (50–60% cover of the lake bottom) than in 1987; some species, growing through the entire water column, interfered with the lake's recreational use. Associated with the macro-vegetation and possibly with the absence of larger cyprinids, the diet of which also comprises snails, a large scale development of the snail population, among themLymnaea peregra var.ovata took place. This species is known to act as an intermediate host of the bird-parasitizing trematodeTrichobilharzia ocellata, the cercariae of which cause an itching sensation at the spot of penetration of the human skin, accompanied by rash (schistosome dermatitis or swimmers' itch); in July, 1988, about 40% of the bathers complained about this itching. A positive effect of the macrophytes and filamentous green algae was the high uptake of nitrogen, resulting in a low nitrogen concentration in the lake and growth limitation of the phytoplankton population by nitrogen in the summer of 1988. In 1988 the cladocerans were abundant in April only; and unlike in 1987, in the summer of 1988 there was a shift from cladocerans to rotifers. Therefore, only in early spring (April) zooplankton grazing controlled phytoplankton growth and in summer nitrogen limitation was the major controlling factor, keeping chlorophyll-a concentrations low.     

Can macrophytes be useful in biomanipulation of lakes? The Lake Zwemlust example

Lake Zwemlust (area 1.5 ha, Z_m 1.5 m) has been the object of an extensive limnological study since its biomanipulation involving removal of planktivorous fish (bream) in March 1987 and emptying of the lake. In the subsequent summer period of 1987 the Secchi depth increased to the lake bottom (2.5 m), compared withca 30 cm in the earlier summers. The reaction of submerged macrophytes to improving under-water light climate was rapid. In summer 1987, besides the introducedChara globularis, 5 species of submerged macrophytes occurred and colonized 10% of the lake area. In 1988 and 1989 only quantitative changes were observed; new species did not appear, but the area colonized by macrophytes increased by 7 and 10 times, respectively.Elodea nuttallii was dominant among the macrophytes andMougeotia sp. among the filamentous green algae. Their abundance, contributed to transient N-limination of phytoplankton causing a persistent clear water phase in 1988 and 1989, unlike in 1987 when zooplankton grazing contributed chiefly to the water clarity. Laboratory bioassays on macrophytes confirmed nitrogen limitation.     

Biomanipulation of Lago di Candia (Northern Italy): a three-year experience of aquatic macrophyte management

A long-term experimental aquatic plant management programme has been in progress since 1985 in the eutrophic Lago di Candia (Northern Italy). 7.7 ha of water chestnut (Trapa natans L.) were harvested in 1986, 8.2 ha in 1987 and 11.0 ha in 1988, from August to October, removing 334, 290 and 418 tons of fresh plant material, respectively, from the lake. This annual harvesting of about 50% of the total water chestnut cover resulted in the removal from the lake of about 70 kg yr⁻¹ of phosphorus, a significant portion of the external annual loading originating mainly from run-off and precipitation. In addition, up to 38t yr⁻¹ of organic matter and 1t yr⁻¹ of nitrogen were removed. First results of the effect of the harvesting on Secchi-disc transparency, oxygen, total phosphorus, phytoplankton and zooplankton are discussed also in connection with a study of fish manipulation (Giussaniet al., 1990).     

Structural and grazing responses of zooplankton community to biomanipulation of some Dutch water bodies

Structure and grazing activities of crustacean zooplankton were compared in five lakes undergoing manipulation with several unmanipulated eutrophic (shallow) and mesotrophic (deep) lakes in The Netherlands. The biomanipulated lakes had lesser number of species and their abundance, both of rotifers and crustaceans, and had much larger mean animal size (3–11 μg C ind.⁻¹) than in the unmanipulated eutrophic lakes (0.65 μG C ind.⁻¹). WhereasD. hyalina (=D. galeata) andD. cucullata generally co-occurred in the unmanipulated lakes, in the manipulated lakes bothD. hyalina and other large-bodied daphnids,D. magna,D. pulex (=D. pulicaria), were the important grazers. In the biomanipulated lakes an increase in the individual crustacean size and of zooplankton mass were reflected in a decrease in seston concentration, higher Secchi-disc depth and a marked decrease in the share in phytoplankton biovolume of cyanobacteria. Biomass relationship between seston (150 μm) and zooplankton indicated a Monod type relationship, with an initial part of the curve in which the zooplankton responds linearly to the seston increase up to aboutca. 2 mg C l⁻¹, followed by a saturation of zooplankton mass (0.39 mg C l⁻¹) at 3–4 mg C l⁻¹ seston, and an inhibitory effect on zooplankton mass at seston levels>4 mg C l⁻¹. This latter is related to predominance in the seston of cyanobacteria. In the biomanipulated lakes, the zooplankton grazing rates often exceeded 100% d⁻¹, during the spring, and food levels generally dropped to <0.5 mg C l⁻¹. The computed specific clearance rate (SCR) of zooplankton of 1.9 l mg⁻¹ Zoop C is well within the range of SCR values (1.7–2.2 l mg⁻¹ Zoop C) from deep and mesotrophic waters, but about an order of magnitude higher than in the eutrophic lakes, with the food levels 10-fold higher. For 25% d⁻¹ clearance of lake seston between 35 and 60 ind. l⁻¹ are needed in the biomanipulated lakes against 1200–1300 ind. l⁻¹ in eutrophic lakes. Similarly, about 10 to 15 times more crustacean grazers are required to eliminate the daily primary production in the eutrophic lakes than in the biomanipulated lakes. These numbers are inversely related to the differences in animal size. The corresponding biomass values of zooplankton needed to clear the daily primary production in the eutrophic waters were 0.1–0.2 mg C l⁻¹ in the biomanipulated lakes, but about 0.45 mg C l⁻¹ in the unmanipulated eutrophic waters. Only if the water was kept persistently clear by zooplankton was there a balanced seston budget between the inputvia primary production and elimination by zooplankton. Mostly, however, the input exceeded the assimilatory removal by zooplankton, such that the estimated seston loss could be attributed to sedimentation and mineralization.     

Why biomanipulation can be effective in peaty lakes

The effects of fish stock reduction (biomanipulation) was studied in an 85 ha shallow peaty turbid lake. The lake cleared in a 4-week period in April–May 2004, which demonstrated that biomanipulation can be effective in peaty lakes. We demonstrated that it is possible to reduce the fish stock to <25 kg ha⁻¹ benthivorous fish and <15 kg ha⁻¹ planktivorous fish, sufficiently low to switch the lake from a turbid to a clear state. Knowledge of lake morphology, fish stock, fish behaviour, and a variety of fishing methods was necessary to achieve this goal. It is expected that continuation of fisheries to remove young of the year planktivorous species is needed for several years, until macrophytes provide sufficient cover for zooplankton and can compete with phytoplankton. Cladocerans developed strongly after fish removal. The clearing of the lake coincided with a sudden decrease of filamentous cyanobacteria and suspended detritus, and a strong increase of Bosmina. We assume that Bosmina was able to reduce filamentous prokaryotes and detritus. After the disappearance of the cyanobacteria, Bosmina disappeared too. After the clearing of the lake Daphnia dominated in zooplankton and apparently was able to keep phytoplankton levels low. In our case, wind resuspension did not prevent biomanipulation from being successful. No correlation between windspeed and turbidity was found, neither in an 85 ha nor in a 230 ha shallow peaty lake. Regression analysis showed that on average 50% of the amount of suspended detritus can be explained by resuspension by fish and 50% by phytoplankton decomposition. The main goal of this biomanipulation experiment, clear water and increased submerged plant cover in a shallow peaty lake, was reached.     

Impact of submerged macrophytes on fish-zooplanl phytoplankton interactions: large-scale enclosure experiments in a shallow eutrophic lake

1. The impact of changes in submerged macrophyte abundance on fish-zooplankton-phytoplankton interactions was studied in eighteen large-scale (100 m²) enclosures in a shallow eutrophic take. The submerged macrophytes comprised Potamategon pectinatus L., P. pusillus L. and Callitriche hermaphroditica L. while the fish fry stock comprised three-spined sticklebacks, Gasterosteus acuteatus L., and roach, Rutilus rutilus L. 2. In the absence of macrophytes zooplankton biomass was low and dominated by cyclopoid copepods regardless of fish density, while the phytoplankton biovolume was high (up to 38 mm³1) and dominated by small pennate diatoms and chlorococcales. When the lake volume infested by submerged macrophytes (PVI) exceeded 15–20% and the fish density was below a catch per unit effort (CPUE) of 10 (approx. 2 fry m^?2), planktonic cladoceran biomass was high and dominated by relatively large-sized specimens, while the phytoplankton biovolume was low and dominated by small fast-growing flagellates. At higher fish densities, zooplankton biomass and average biomass of cladocerans decreased and a shift to cyclopoids occurred, while phytoplankton biovolume increased markedly and became dominated by cyanophytes and dinoflagellates. 3. Stepwise multiple linear regressions on log-transformed data revealed that the biomass of Daphnia, Bosmina, Ceriodaphmia and Chydorus were all significantly positively related to PVI and negatively to the abundance of fish or PVI x fish. The average individual biomass of cladocerans was negatively related to fish, but unrelated to PVI. Calculated zooplankton grazing pressure on phytoplankton was positively related to PVI and negatively to PVI x fish. Accordingly the phytoplankton biovolume was negatively related to PVI and to PVI x zooplankton biomass. Cyanophytes and chryptophytes (% of biomass) were positively and Chlorococcales and diatoms negatively related to PVI, while cyanophytes and Chlorococcales were negatively related to PVI x zooplankton biomass. In contrast diatoms and cryptophytes were positively related to the zooplankton biomass or PVI x zooplankton. 4. The results suggest that fish predation has less impact on the zooplankton community in the more structured environment of macrophyte beds, particularly when the PVI exceeds 15–20%. They further suggest that the refuge capacity of macrophytes decreases markedly with increasing fish density (in our study above approximately 10 CPUE). Provided that the density of planktivorous fish is not high, even small improvements in submerged macrophyte abundance may have a substantial positive impact on the zooplankton, leading to a lower phytoplankton biovolume and higher water transparency. However, at high fish densities the refuge effect seems low and no major zooplankton mediated effects of enhanced growth of macrophytes are to be expected.     

Effectiveness of phytoplankton control by large-bodied and small-bodied zooplankton

Employingin situ enclosures containing inocula of the lake zooplankton (mainlyDaphnia galeata, Daphnia cucullata andBosmina spp.) from a moderately eutrophic Lake Ros (Northern Poland) or large-bodiedDaphina magna, the following observations on succession of phytoplankton were made: 1) whereasD. magna could control the density of all the photoplankton size classes, the lake zooplankton could not suppress the large-sized phytoplankters or net phytoplankton; 2) the lake zooplankton was able to control the density of small algae (< 50μm), but its effect on large algae may be opposite: a promotion of net phytoplankton growth by removing small-sized algae which can out-compete net phytoplankton for limited PO₄-P resources (<5μg P l⁻¹). Since efficiency of phytoplankton density control byD. magna decreased with an increase in net phytoplankton abundance, biomanipulation could not be successful without introducing or maintaining a high population of large-bodied cladoceran species before high densities of large algae would make the control of phytoplankton inefficient.     

Colonization and succession of submerged macrophytes in shallow Lake Væng during the first five years following fish manipulation

Colonization of submerged macrophytes and changes in species composition were studied in shallow Lake Væng during the first five years (1987–91) following fish manipulation in 1986–1988 and a resultant significant improvement in lake water transparency. No submerged macrophytes were present in the lake from 1981–1986, during which time the summer mean Secchi depth ranged from 0.6 and 0.8 m. From 1987 to 1990, Secchi depth increased from 0.9 m to 1.8 m and macrophyte coverage consequently increased (1 % of the lake area in 1987, 2% in 1988, 50% in 1989, 80% in 1990 and 90% in 1991). At the same time, the macrophytes became taller, and the weedbeds more dense. The macrophytes colonized from the exposed and deeper part of the lake towards the sheltered and more shallow part of the lake, a colonization pattern that was confirmed by transplantation experiments. The delay in colonization of the shallow parts may be caused by waterfowl grazing. The vegetation was initially dominated by Potamogeton crispus L., but there was a gradual change during 1988–1989 and Elodea canadensis Michx became exclusively dominant in 1990–1991.     

Zooplankton allochthony is spatially heterogeneous in a boreal lake

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Three years of experience in biomanipulating a small eutrophic lake: Lago di Candia (Northern Italy)

Results obtained from a step by step approach to the biomanipulation of a natural lacustrine environment (Lago di Candia, Northern Italy) are presented. Since the diversion of the municipal sewage of the small town of Candia, runoff and precipitation have been the sole contributors of nutrient to the lake. Fish population is mainly characterized by rudd (Scardinius erythrophthalmus) overstocking and by a low density of large-mouth-bass (Micropterus salmoides) and pike (Esox lucius). During 1986 about 12t of rudd (1–2 year old) were removed from the lake. Considering 1986 as ‘control year’, average Secchi disc transparency improved from 2.3 m in 1986 to 3.3 m in 1988; phytoplankton biovolume decreased from 114 to 58 mm³ l⁻¹ but zooplankton biovolume increased from 8 to 11.5 mm³ l⁻¹. The results achieved show that a grodual biomanipulation treatment can have a satisfactory outcome, and has the advantage of not producing catastrophic situations either in the biotic or in the abiotic compartments of the lake.     

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.     

Ecology of Dictyosphaerium pulchellum Wood (Chlorophyta, Chlorococcales) in a shallow, acid, forest lake

This study relates to the ecology of Dictyosphaerium pulchellum Wood in Delamere Lake in Cheshire, UK. Dictyosphaerium pulchellum is a cosmopolitan, green colonial phytoplankton species that occasionally forms dense, monospecific populations in lakes. Delamere Lake is a small, shallow, acid lake (mean pH, 4.5) with very high phytoplankton biomass (annual mean chlorophyll a, 290 μg l⁻¹) and devoid of any significant cladoceran population, the efficient grazers of phytoplankton. A predominantly unicellular form of D. pulchellum was the dominant species in Lake Delamere, and it comprised on average ca. 80% (maximum >99%) of the lake phytoplankton biovolume. Laboratory and lake experiments were conducted on this species showed that its pH tolerance varied between 2.4 and 10.7, and its optimum tolerance range between 3.3 and 8.5 depending on other environmental variables. Low pH was not responsible for the unicellular habit of this alga, but a very high nutrient regime could be an important factor. Bioassays revealed that in Delamere Lake this species was limited by nitrogen, but nitrogen did not hamper high growth in the lake. Dictyosphaerium pulchellum can persist at low light levels, tolerate CO₂-deficiency and can grow in polyhumic water with water colour around 300 mg Pt l⁻¹, but probably not in darker waters. The dominance of D. pulchellum in Delamere Lake is apparently due to a combination of several factors: its ability to tolerate both low pH and high turbidity, exploit high nutrient conditions, absence of effective grazing pressure by zooplankton and being a superior competitor.     

Biomanipulation by introduction of herbivorous zooplankton. A helpful shock for eutrophic lakes?

In 1988 and 1989Daphnia magna were introduced into the Gewerbepark Pond and into the University Pond, and also into a limnocorral (10 m diameter, 5 m deep) in Postfelden Reservoir. Limnological parameters were regularly monitored in all the three water bodies over three months or longer. The objective of the experiments was to induce a clear-water period in turbid eutrophic waters by direct manipulation of the zooplankton community. Introduction of four million individuals ofD. magna (10 ind. 1⁻¹) caused a long lasting clear-water period in the limnocorral. While Secchi-disc transparency in the reservoir decreased from 1.3 m to<1 m during summer, transparency increased to a stable level of about 4 m in the corral. Despite very low phytoplankton biomass,D. magna was still abundant. The experiment was terminated after three months because of oxygen depletion in the corral. In the Gewerbepark Pond 3.3 million individuals ofD. magna (0.6 ind. 1⁻¹) were introduced at the beginning of August. The daphnids soon increased 60-fold in their density causing a persistently high water transparency and low phytoplankton biomass up to November. Ammonia concentration rose steeply, whereas that of nitrate and oxygen decreased. Subsequently in summer, concentrations of chlorophyll as well as of inorganic phosphorus and nitrogen were extremely low. Instead of the phytoplankton,Elodea canadensis grew enormously and covered 12% of the pond surface. The biomanipulation experiment in University Pond failed, probably due to predation by carp.     

Changes in benthic macroinvertebrate abundance and lake isotope (C,N) signals following biomanipulation: an 18-year study in shallow Lake Vaeng,Denmark

Change in the abundance of benthic macroinvertebrates and the stable isotope composition (C, N) of benthic invertebrates and zooplankton in Lake Vaeng, Denmark, was investigated over an 18-year period following biomanipulation (removal of cyprinids). During the first nine years after biomanipulation, the lake was clear and submerged macrophytes were abundant; after this period, a shift occurred to low plant abundance and high turbidity. Two years after the biomanipulation, total density of benthic macroinvertebrates reached a maximum of 17042 (±2335 SE) individuals m⁻² and the density was overall higher when the lake was in a clear state. Redundancy analysis (RDA) suggested macrophyte abundance and total nitrogen (TN) concentration were the dominant structuring forces on the benthic macroinvertebrate assemblage. Stable isotope analysis revealed that δ¹³C of macroinvertebrates and zooplankton was markedly higher in years with high submerged macrophyte abundance than in years without macrophytes, most likely reflecting elevated δ¹³C of phytoplankton and periphyton mediated by a macrophyte-induced lowering of lake water CO₂ concentrations. We conclude that the strong relationship between macrophyte coverage and δ¹³C of macroinvertebrates and cladocerans may be useful in paleoecological studies of past changes in the dynamics of shallow lakes, as change in macrophyte abundance may be tracked by the δ¹³C of invertebrate remains in the sediment.     

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.     

Life histories ofNeomysis integer,and its copepod prey,Eurytemora affinis,in a eutrophic and brackish shallow lake

Data was collected on the population structure and fecundity of the mysidNeomysis integer and the calanoid copepodEurytemora affinis in Hickling Broad, a shallow and eutrophic brackish lake, over a two-year period 1988/89. Standing biomass and production rate estimates were made using estimates of size-specific dry weights and development times obtained from laboratory measurements, field observations and information found in the literature. Both mysid and copepod are capable of a rapid response to favourable conditions and have high rates of birth, growth and production.E. affinis reproduces throughout the year with an estimated annual (May 1988–May 1989) production of 20 g dry wt m⁻². Copepod standing biomass was less in 1989 compared with 1988 owing to an overall reduction in copepod body-size and a reduction of size at maturity. There was a suggestion from the data that this was caused by predation from a greater number of large (>9 mm body-length) mysids in 1989 compared with 1988.N. integer is highly seasonal in its growth with distinct peaks of recruitment in May and July. Annual (May 1988–May 1989) production of the mysid was estimated as 5.8 g dry wt m⁻². AlthoughE. affinis is the only available prey ofN. integer in the lake, the mysid population appears independent of changes in that of the copepod and probably avoids negative predator-prey feedback mechanisms owing to an ability to feed on epiphytic algae.