Evaluating the utility of stable isotope analyses of archived freshwater sample materials

We evaluated the potential utility of stable isotope analysis of tissues commonly archived by aquatic biologists. Previous studies with chemically preserved samples have shown contradictory results, which present an obstacle for the use of archived sample materials. We tested the effects of ethanol and formalin preservation on zooplankton and of ethanol on benthic macroinvertebrate δ¹³C and δ¹⁵N values. We found that neither formalin nor ethanol had a significant effect on δ¹³C and δ¹⁵N values of preserved zooplankton. Nor did ethanol significantly affect δ¹³C or δ¹⁵N values of macroinvertebrates. However, ethanol preservation slightly, but significantly decreased C:N ratios of both zooplankton and macroinvertebrates, probably reflecting some extraction of lipids. Overall, the effects of preservatives on δ¹³C and δ¹⁵N values that we observed were minor. We also compared δ¹³C and δ¹⁵N values analysed from roach scales and perch operculum bones with those analysed from muscle tissue. Decalcification of scales and operculum bones only slightly improved our comparison to muscle tissue δ¹³C and δ¹⁵N values. Decalcified scales had slightly higher δ¹³C and lower δ¹⁵N values. Similarly, decalcified operculum bones showed slightly increased δ¹³C and decreased δ¹⁵N values to those for fish muscle. Our results confirm that scales and operculum bones can provide a suitable proxy for fish muscle in isotope studies with minor correction. We conclude that various archived sample materials can indeed be used with confidence for historical reconstructions of freshwater food webs by stable isotope analysis. Handling editor: K. Martens     

Trophic polymorphism in bluegill sunfish (<Emphasis Type="Italic">Lepomis macrochirus</Emphasis>) introduced into Lake Biwa: evidence from stable isotope analysis

Trophic polymorphism was recently reported in introduced bluegill (Lepomis macrochirus) in Lake Biwa, Japan, where three morphs are specialized in benthic invertebrates (benthivorous type), submerged aquatic plants (herbivorous type), and zooplankton (planktivorous type). We evaluated the long-term effects of food resource utilization by these trophic morphs using stable isotope ratios, δ¹⁵N and δ¹³C. A significant difference in δ¹⁵N was found between the benthivorous and planktivorous types. The planktivorous type had the higher δ¹⁵N value, which corresponded with the value expected from its prey, zooplankton. The lower δ¹⁵N value of the benthivorous type would be derived from the lower δ¹⁵N values of benthic prey organisms compared to zooplankton. These results support previous findings that the benthivorous and planktivorous types have different food resource utilization. In contrast, the δ¹⁵N and δ¹³C values of the herbivorous type were distinctly different from the expected values, indicating that this type was unlikely to utilize aquatic plants substantially, contradicting the results of the dietary analysis.     

Effects of grazing by fish and waterfowl on the biomass and species composition of submerged macrophytes

Biomanipulation improved water transparency of Lake Zwemlust (The Netherlands) drastically. Before biomanipulation no submerged vegetation was present in the lake, but in summer 1987, directly after the measure, submerged macrophyte stands developed following a clear-water phase caused by high zooplankton grazing in spring. During the summers of 1988 and 1989 Elodea nuttallii was the most dominant species and reached a high biomass, but in the summers of 1990 and 1991 Ceratophyllum demersum became dominant. The total macrophyte biomass decreased in 1990 and 1991. In 1992 and 1993 C. demersum and E. nuttallii were nearly absent and Potamogeton berchtholdii became the dominant species, declining to very low abundance during late summer. Successively algal blooms appeared in autumn of those years reaching chlorophyll-a concentrations between 60–130 µg l^–1. However, in experimental cages placed on the lake bottom, serving as exclosures for larger fish and birds, E. nuttallii still reached a high abundance during 1992 and 1993. Herbivory by coots (Fulica atra) in autumn/winter, and by rudd (Scardinius erythrophthalmus) in summer, most probably caused the decrease in total abundance of macrophytes and the shift in species composition.     

Stable isotope analysis of the origins of zooplankton carbon in lakes of differing trophic state

Carbon stable isotope analysis was carried out on zooplankton from 24 United Kingdom lakes to examine the hypothesis that zooplankton dependence on allochthonous sources of organic carbon declines with increasing lake trophy. Stable isotope analysis was also carried out on particulate and dissolved organic matter (POM and DOM) and, in 11 of the lakes, of phytoplankton isolates. In 21 of the 24 lakes, the zooplankton were depleted in ¹³C relative to bulk POM, consistent with previous reports. δ¹³C for POM showed relatively little variation between lakes compared to high variation in values for DOM and phytoplankton. δ¹³C values for phytoplankton and POM converged with increasing lake trophy, consistent with the expected greater contribution of autochthonous production to the total organic matter pool in eutrophic lakes. The difference between δ¹³C for zooplankton and that for POM was also greatest in oligotrophic lakes and reduced in mesotrophic lakes, in accordance with the hypothesis that increasing lake trophic state leads to greater dependence of zooplankton on phytoplankton production. However, the difference increased again in hypertrophic lakes, where higher δ¹³C values for POM may have been due to greater inputs of ¹³C-enriched organic matter from the littoral zone. The very wide variation in phytoplankton δ¹³C between lakes of all trophic categories made it difficult to detect robust patterns in the variation in δ¹³C for zooplankton. Received: 2 November 1998 / Accepted: 3 December 1999     

Can artificial plant beds be used to enhance macroinvertebrate food resources for perch (<Emphasis Type="Italic">Perca fluviatilis</Emphasis> L.) during the initial phase of lake restoration by cyprinid removal?

Restoration of shallow lakes to a clear-water state, often characterized by high submerged macrophyte cover and a high proportion of piscivores such as perch, Perca fluviatilis L., frequently involves removal of a large proportion of the zoobenthivorous fish, such as bream, Abramis brama L., and roach, Rutilus rutilus L. (i.e. biomanipulation). However, establishment of submerged macrophytes is often delayed following fish removal. This is unfortunate because plant beds typically host high densities of the macroinvertebrates constituting the diet of small perch and thus help perch to go through the bottleneck from feeding on macroinvertebrates to feeding on fish. Establishment of artificial plant beds may be a useful tool to enhance macroinvertebrate population growth and thus food resources for small perch until the natural plants have established. To investigate this restoration option, we studied during two growing seasons (June–October) the composition and abundance of the macroinvertebrate community in artificial plant beds installed in shallow Lake Væng (Denmark) comprising the initial phase of a biomanipulation effort by fish removal. Lake areas with artificial plant beds exhibited substantially higher macroinvertebrate densities than the lake bottom. This suggests that artificial plant beds may be used as feeding grounds for small perch, similarly to the well-known refuge effect for zooplankton against fish predation. In this way, artificial plant beds could help maintain a clear-water state during the transient period when natural submerged vegetation is not yet established in the lake.     

Changes in trophic linkages to shortfin eels (<Emphasis Type="Italic">Anguilla australis</Emphasis>) since the collapse of submerged macrophytes in Lake Ellesmere,New Zealand

Lake Ellesmere (Te Waihora) is a nationally important coastal brackish lake in New Zealand, however degradation in water quality and loss of submerged macrophytes over past decades have raised concerns in regards to the declining status of the lake’s commercial and customary fisheries, predominantly targeted at shortfin eels (Anguilla australis). We investigated foodweb dynamics and trophic linkages to shortfin eels in Lake Ellesmere using a combination of abundance assessments, dietary studies, and stable isotope analyses. Data from our study are compared with historical data sets on benthic invertebrate community composition and shortfin eel diets to trace changes in the trophic linkages to top predators that have occurred since the late 1960s. Stable isotope analyses indicate that the foodweb is predominantly driven by epipelic and phytoplankton derived carbon sources, although it was difficult to discriminate between these two carbon pools because of wind-driven resuspension of lake sediments. Comparison of our survey results with historical data sets indicates a clear shift in benthic biota from being dominated by phytofaunal species such as Potamopyrgus antipodarum (comprising 90% of total invertebrate biomass) during the 1960s, to now being almost entirely comprised of subterranean species such as Chironomus zealandicus and oligochaetes (together comprising 82% of total invertebrate biomass). This shift in benthic communities has resulted in significant changes in the size-specific diet of juvenile shortfin eels (<400 mm) from those reported for Lake Ellesmere during the mid 1970s, with Chironomus larvae now comprising 65% of the diets of juvenile eels, whereas historically P. antipodarum was the dominant food item (>30% of total biomass). This shift towards foraging on smaller sediment-dwelling species could have implications for juvenile eel bioenergetics, and may help explain why juvenile shortfin growth rates have significantly decreased in past decades. Juvenile shortfins now appear to switch to foraging on preyfish (mainly common bullies, Gobiomorphus cotidianus) at a smaller size (≈400 mm) than historically recorded (>500 mm). Dietary and stable isotope signatures indicated that small shortfins (100–299 mm) have considerable overlap in trophic position (δ¹³C = −20.4‰, δ¹⁵N = 13.6‰) with common bullies (δ¹³C = −20.5‰, δ¹⁵N = 13.7‰), the dominant fish in Lake Ellesmere (92% of total abundance CPUE), potentially indicating that these two species may directly compete for food resources. These findings again highlighted the importance of C. zealandicus in sustaining the fish populations of the lake. Handling editor: S. Declerck     

Biogenic methane contributes to the food web of a large,shallow lake

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Sympatric variability of isotopic baselines influences modeling of fish trophic patterns

Stable isotope signatures of freshwater snails and mussels have been established as a convenient baseline measurement at the primary consumer level for food-web coupling studies. We measured δ¹⁵N and δ¹³C of primary consumers, including mussels (Anodonta woodiana, Cristaria plicata, and Unio douglasiae), snails (Bellamya aeruginosa and Hippeutis sp.), and zooplankton from the same habitat within a shallow eutrophic lake. Primary consumers showed positive relationship between δ¹⁵N and δ¹³C, indicating a linkage between planktonic and benthic habitats in this system. The variation in isotope ratios was higher in short-lived primary consumers (zooplankton) compared with the long-lived primary consumers (mussels and snails), suggesting limited availability of short-lived primary consumers as isotopic baselines in aquatic food-web assessment. Significant differences in isotope ratios were also found among three species of mussels, and when using these mussels separately as pelagic baselines to calculate trophic position and contribution of planktonic and benthic sources of fishes, bias and even misestimates were observed. This finding suggests that caution is needed when multiple primary consumers coexist in the same habitat, and it is important to assess potential effects of different baselines used.     

Sources of organic matter and methylmercury in littoral macroinvertebrates: a stable isotope approach

The main objective of this study was to assess organic matter (OM) and methylmercury (MeHg) sources for freshwater littoral macroinvertebrate primary consumers. The carbon and nitrogen stable isotope ratios (δ¹³C, δ¹⁵N) of sources (epiphytes, macrophytes, suspended particulate matter _SPM) and of macroinvertebrate consumers were measured in a fluvial lake with extensive macrophyte beds (emergent and submerged). To determine the relative contribution of each OM source to macroinvertebrate diets we used the IsoSource model that examines all possible combinations of solutions for each source. Total and MeHg concentrations of consumers were also measured. Results show that epiphytes and macrophytes are dominant in the diet of macroinvertebrates, especially in early summer (July). In mid-summer (August), SPM constitutes a non-negligible OM source to the primary consumers. Hg concentrations were higher in epiphytes than in the other OM sources. The proportion of epiphytes in macroinvertebrate diet was positively correlated with the percentage of MeHg in their tissues. There was no relationship between SPM assimilation and Hg concentration in macroinvertebrate consumers. These results suggest that epiphytes and macrophytes constitute the main pathway of Hg bioaccumulation in littoral food webs.

Major changes in CO<Subscript>2</Subscript> efflux when shallow lakes shift from a turbid to a clear water state

Lakes can be sources or sinks of carbon, depending on local conditions. Recent studies have shown that the CO₂ efflux increases when lakes recover from eutrophication, mainly as a result of a reduction in phytoplankton biomass, leading to less uptake of CO₂ by producers. We hypothesised that lake restoration by removal of coarse fish (biomanipulation) or invasion of mussels would have a similar effect. We studied 14–22 year time series of five temperate Danish lakes and found profound effects on the calculated CO₂ efflux of major shifts in ecosystem structure. In two lakes, where limited colonisation of submerged macrophytes occurred after biomanipulation or invasion of zebra mussels (Dreissena polymorpha), the efflux increased significantly with decreasing phytoplankton chlorophyll a. In three lakes with major interannual variation in macrophyte abundance, the efflux declined with increasing macrophyte abundance in two of the lakes, while no relation to macrophytes or chlorophyll a was found in the third lake, likely due to high groundwater input to this lake. We conclude that clearing water through invasive mussels or lake restoration by biomanipulation may increase the CO₂ efflux from lakes. However, if submerged macrophytes establish and form dense beds, the CO₂ efflux may decline again.     

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.     

Spatial changes in carbon and nitrogen stable isotopes of the plankton food web in a saline lake ecosystem

We investigated spatial changes in the isotope ratios of the plankton food web in Lake Chany, Siberia, Russia, especially at an estuarine transition zone of the lake. The δ¹³C values of particulate organic matter (POM) varied among the sampling sites, and increased with increasing pH of the lake water. This may reflect a shift by phytoplankton from using CO₂ to using bicarbonate for photosynthesis with increasing pH. The δ¹³C values of zooplankton community also changed at each site along with those of the POM. This was indicative of carbon isotope changes of plankton food webs between the stations along an environmental gradient.     

Benthic-pelagic trophic interactions within the fish assemblage in the western Bering Sea shelf area according to stomach content analysis and ratios of C and N stable isotopes

The composition, abundance, diet and trophic status of zooplankton, bottom invertebrates, fish and nekton were analyzed based on the data collected by the staff of the TINRO-Center during complex bottom trawl catches on the Bering Sea shelf in the fall of 2004. The stomach contents of mass fish species were analyzed and the nitrogen and carbon isotopic composition of 36 mass species of plankton, benthos, nekton and nektobenthos, which together make up the basis of pelagic and bottom communities, was determined. It was found that zooplankton noticeably differ from benthic invertebrates in carbon isotopic composition: δ¹³C values in zooplankton varied from −20.3‰ to −17.9‰; in benthos—from −17.5‰to −13.0‰; and in fish—from −19.2‰ (juvenile walleye pollock) to −15.3‰ (saffron cod). The levels of ¹³C isotope in the tissues of fish depended mostly on the share of pelagic or benthic animals in their diet. δ¹⁵N values in the studied species ranged from 8.6‰ (in sea urchins) to 17.2‰ (in large Pacific cods), which corresponds to a trophic level of 2.8. Obviously the δ¹⁵N values reflect the degree of predation and generally show the ratio of primary, secondary and tertiary consumers in a fish’s diet. Trophic interactions manifest a high degree of interdependence between benthic and pelagic communities (even without taking into account such lower components of the food web as phytoplankton, bacteria, and protozoa) occurring in most nektonic species that depend on both bottom and pelagic food.     

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.     

Macroinvertebrate food web structure in a floodplain lake of the Bolivian Amazon

Two stable isotopes δ¹³C and δ¹⁵N were used to identify the energy sources and trophic relationships of the main freshwater macroinvertebrates in a floodplain lake of the Beni River (Bolivian Amazonia). Four energy sources (seston, bottom sediment, periphyton, and aquatic macrophytes) and macroinvertebrate communities were collected during three periods of the river hydrological cycle. Macroinvertebrates showed greater temporal variation in isotope values than their food sources. Six trophic chains were identified: four were based on seston, periphyton, C₃ macrophytes, and bottom sediments, and the last two chains on a combination of two carbon sources. One mixed seston and periphyton sources during the wet season while the other mixed periphyton and macrophytes sources during the wet and dry seasons. Periphyton was the most important energy source supporting the highest number of trophic levels and consumers. The macrophytic contribution was only significant during the dry season. Bottom sediments constituted a marginal energy source. As each season is associated with different physical and chemical conditions, processes organizing macroinvertebrate food web structure in the Beni floodplain seem strongly linked to hydrological seasonality.     

Zooplankton allochthony is spatially heterogeneous in a boreal lake

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Differences in nitrogen and carbon stable isotopes between planktonic and benthic microalgae

We compiled published data on the nitrogen and carbon stable isotope ratios of phytoplankton and benthic microalgae from lentic systems and explored the primary factors determining the isotope values among systems. Also, we investigated seasonal changes in nitrogen stable isotope ratios of phytoplankton and benthic microalgae in the strongly acidic lake, Lake Katanuma, which has only two dominant species, Pinnularia acidojaponica as a benthic diatom and Chlamydomonas acidophila, a planktonic green alga. From the published dataset, it may be concluded that δ¹³C of benthic diatoms were more enriched than those of phytoplankton at the same sites, although the nitrogen isotope of phytoplankton and benthic microalgae were similar. This differences in δ¹³C between benthic microalgae and phytoplankton could be explained by the boundary layer effect. On the other hand, nitrogen isotope values of both benthic microalgae and phytoplankton were primarily controlled by the same environmental factor, and boundary layer effects are not the primary factor determining the nitrogen isotope values of microalgae. Also, we showed temporal dynamics in nitrogen isotopes of benthic and planktonic microalgae species in Lake Katanuma, and the trends of nitrogen isotopes are similar between benthic and planktonic microalgae, as concluded from the published dataset.     

Benthic macroinvertebrates associated with four species of macrophytes

Benthic macroinvertebrates associated with four species of macrophytes (Nymphoides peltata, Ceratophyllum demersum, Polygonum amphibium and Carex sp.) were investigated during two growing seasons (2001 and 2002) in the slow-flowing Čonakut Channel in the Kopački rit Nature Park in Croatia. A total of 31 macroinvertebrate taxa were found. C. demersum, a submerged plant with dissected leaves, supported the highest macroinvertebrate abundance, almost seven times more than N. peltata, a floating plant with undissected leaves, which harboured the lowest abundance during the research period. Chironomidae larvae (50–83%) and Oligochaeta (14–46%) were the most abundant groups recorded on all macrophyte species. Water-level fluctuation, because of its influence on the appearance and growth of aquatic vegetation, and the trophic state of water within the macrophyte stands seemed to be the main factors which affected the taxonomic composition and abundance of macroinvertebrates.     

Phytoplankton biomass in Lake Xolotlán (Managua): Its seasonal and horizontal distribution

Some well-documented studies on restoring eutrophic lake systems in The Netherlands by fish stock management have been evaluated with the emphasis on the role of macrophytes. Furthermore, the factors determining the light climate for submerged macrophytes in a large shallow eutrophic lake (Lake Veluwe) have been assessed and the potential success of biomanipulation in large scale projects is discussed. Today relatively little attention has been paid to macrophyte management although the importance of macrophytes in lake restoration has been recognized regularly. The biomanipulation strategy was successful in small scale projects. In a large scale project, however, wind-induced resuspension may largely determine the underwater light climate through attenuation by the water column and periphytic layer. Therefore, restoration of relatively large waterbodies by fish stock management only is expected not to lead to any noteworthy improvement of the light climate for submerged macrophytes. Additional measures aimed at reducing wind-induced resuspension of sediment particles and reestablishing of the macrophyte stands are required for successful biomanipulation strategies. Water quality managers should pay more attention to macrophyte stands in biomanipulation projects because macrophytes enhance a more stable and diverse ecosystem. Restoration objectives and the methods of their achievement must be carefully planned since an abundant submerged macrophyte vegetation may have undesirable effects as well.     

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.