Top-down effects of underyearling fish on a phytoplankton community

1. The influence of underyearling (0+) fish on planktonic algal biomass and composition was examined during June–August in eight large enclosures (40 m³). Averaged for the entire period, Daphnia densities, transparency and phosphate concentrations were significantly higher ( P < 0.05) and chlorophyll a values significantly lower in fishless controls than in enclosures with fish (bleak ( Alburnus alburnus ), roach ( Rutilus rutilus ) or perch ( Perca fluviatilis )).
2. Enclosures without fish demonstrated a more heterogeneous algal community structure than enclosures with fish. Desmidiales and dinoflagellates were more abundant in controls, whereas green algae dominated the phytoplankton community in enclosures with fish. Larger grazing-resistant algae occurred most frequently in enclosures without fish.
3. The importance of cascading trophic interactions was demonstrated even under strong eutrophic conditions. The strongest effects on Daphnia densities, phytoplankton biomass and nutrients were observed in enclosures with perch, with weaker effects in enclosures with cyprinids. Differences in initial fish size and species accounted for the top-down effects observed. The results of this study emphasize the importance of 0+ perch as a key predator in structuring the plankton community in lakes.     

Top-down effects of underyearling fish on a phytoplankton community

1. The influence of underyearling (0+) fish on planktonic algal biomass and composition was examined during June–August in eight large enclosures (40 m³). Averaged for the entire period, Daphnia densities, transparency and phosphate concentrations were significantly higher ( P < 0.05) and chlorophyll a values significantly lower in fishless controls than in enclosures with fish (bleak ( Alburnus alburnus ), roach ( Rutilus rutilus ) or perch ( Perca fluviatilis )).
2. Enclosures without fish demonstrated a more heterogeneous algal community structure than enclosures with fish. Desmidiales and dinoflagellates were more abundant in controls, whereas green algae dominated the phytoplankton community in enclosures with fish. Larger grazing-resistant algae occurred most frequently in enclosures without fish.
3. The importance of cascading trophic interactions was demonstrated even under strong eutrophic conditions. The strongest effects on Daphnia densities, phytoplankton biomass and nutrients were observed in enclosures with perch, with weaker effects in enclosures with cyprinids. Differences in initial fish size and species accounted for the top-down effects observed. The results of this study emphasize the importance of 0+ perch as a key predator in structuring the plankton community in lakes.     

Long-term pattern of alternative stable states in two shallow eutrophic lakes

• 1 Lake Tåkern and Lake Krankesjön, two moderately eutrophic, shallow lakes in southern Sweden, have during the past few decades shifted several times between a clear-water state with abundant submerged vegetation and a turbid state with high phytoplankton densities.
• 2 Between 1985 and 1991, Lake Takern was in a clear state, whereas Lake Krankesjon shifted from a turbid to a clear state. During this shift, the area covered by submerged macrophytes expanded, followed by an increase in water transparency, plant-associated macroinvertebrates, and piscivorous fish. Nutrient concentrations, phytoplankton biomass and abundance of planktonic cladocerans decreased.
• 3 In both lakes, water level fluctuations were the most common factor causing shifts, affecting submerged macrophytes either through changes in light availability or through catastrophic events such as dry-out or mechanical damage by ice movement.
• 4 Our data give further support for the existence of two alternative stable states in shallow lakes maintained by self-stabilizing feedback mechanisms.

     

Changes in phytoplankton,benthic invertebrate and fish assemblages of boreal lakes following invasion by Gonyostomum semen

1. During recent decades, Gonyostomum semen populations have spread in northern temperate regions forming dense blooms that may dominate the phytoplankton assemblage for extended periods. In this study, we investigate the effects of G. semen blooms in boreal brown water lakes with special emphasis on phytoplankton, fish and benthic invertebrate assemblages using data from 10 boreal lakes sampled annually over a 10‐year period. 2. Significant differences in phytoplankton and benthic invertebrate assemblages were found between lakes with high (3.01 mm³ L^?1; >80% phytoplankton biomass) and lakes with low G. semen biomass (0.03 mm³ L^?1; <5% phytoplankton biomass). In particular, high G. semen lakes had lower biomass of smaller, edible phytoplankton and a higher abundance and biomass of benthic invertebrates, especially Chaoborus flavicans, and perch than low G. semen lakes. 3. The length distribution of fish also suggested a tendency towards large and older fish and a lower recruitment success in high G. semen lakes, as denoted by lower abundances of fish shorter than 10 cm and higher biomass and abundance of fish longer than 15 cm in high G. semen lakes. 4. This study shows that high G. semen lakes are characterised by less edible phytoplankton, dominance by a few species and enhanced benthic secondary and fish production. Hence, the conjecture that high biomasses of G. semen create a bottleneck in the energy transfer to higher trophic levels seems less likely in boreal lakes.     

Periphyton-macroinvertebrate interactions in light and fish manipulated enclosures in a clear and a turbid shallow lake

In a clear and a turbid freshwater lake the biomasses of phytoplankton, periphytic algae and periphytonassociated macrograzers were followed in enclosures with and without fish (Rutilus rutilus) and four light levels (100%, 55%, 7% and < 1% of incoming light), respectively. Fish and light affected the biomass of primary producers and the benthic grazers in both lakes. The biomass of primary producers was generally higher in the turbid than the clear lake, and in both lakes fish positively affected the biomass, while shading reduced it. Total biomass of benthic grazing invertebrates was higher in the clear than in the turbid lake and the lakes were dominated by snails and chironomids + ostracods, respectively. While light had no effect on the biomass of grazers in the clear lake, snail breeding was delayed in the most shaded enclosures and presence of fish reduced the number of snails and the total biomass of grazers. In the turbid lake ostracod abundance was not influenced by light, but was higher in fish-free enclosures. Density of chironomids correlated positively with periphyton biomass in summer, while fish had no effect. Generally, light-mediated regulation of primary producers was stronger in the turbid than in the clear lake, but the regulation did not nambiguously influence the primary consumers. However, regulation by fish of the benthic grazer community was stronger in the clear than in the turbid lake, and in both lakes strong top-down effects on periphyton were seen. The results indicate that if present-day climate in Denmark in the future is found in coastal areas at higher latitudes, the effect of lower light during winter in such areas will be highest in clear lakes, with typically lower fish biomass and higher invertebrate grazer density.     

Diet of Arctic charr (Salvelinus alpinus (L.)) and brown trout (Salmo trutta L.) in sympatry in two high altitude alpine lakes

Lake Pisses and Lake Labarre are two oligotrophic high altitude alpine lakes that have sympatric populations of Arctic charr and brown trout. These two lakes have similar morphometric, physical and chemical characteristics. The zooplanktonic and benthic fauna show little diversity. But the density of benthos (Chironomidae) and zooplankton is higher in Lake Pisses. The fish fauna of Lake Pisses is slightly more abundant than that of Lake Labarre, althought in both lakes fish density is low. A study of the diet of the two species revealed differences. In Lake Pisses, where the food supply is better, Arctic charr takes exclusively pelagic and benthic prey, whereas in Lake Labarre it also takes exogenous prey and thus comes into competition with trout. Length and body weight growth rates for Arctic charr are higher in Lake Pisses than in Lake Labarre. For trout, maximum length recorded was in Lake Pisses. The results show that the abundance of Chironomidae favours coexistence of the two species in Lake Pisses and confirm that, in the face of shortage of food, Arctic charr is better adapted than trout. This revised version was published online in July 2006 with corrections to the Cover Date.     

Community structure and diel migration of zooplankton in shallow brackish lakes: role of salinity and predators

Diel horizontal migration (DHM), where zooplankton moves towards macrophytes during daytime to avoid planktivorous fish, has been reported as a common migration pattern of zooplankton in shallow temperate freshwater lakes. However, in shallow eutrophic brackish lakes, macrophytes seem not to have the same refuge effect, as these lakes may remain turbid even at relatively high macrophyte abundances. To investigate the extent to which macrophytes serve as a refuge for zooplankton at different salinities, we introduced artificial plants mimicking submerged macrophytes in the littoral zone of four shallow lakes, with salinities ranging from almost freshwater (0.3) to oligohaline waters (3.8). Furthermore, we examined the effects of different salinities on the community structure. Diel samples of zooplankton were taken from artificial plants, from areas where macrophytes had been removed (intermediate areas) and, in two of the lakes, also in open water. Fish and macroinvertebrates were sampled amongst the artificial plants and in intermediate areas to investigate their influence on zooplankton migration. Our results indicated that diel vertical migration (DVM) was the most frequent migration pattern of zooplankton groups, suggesting that submerged macrophytes were a poor refuge against predation at all salinities under study. Presumably, this pattern was the result of the relatively high densities of small planktivorous fish and macroinvertebrate predators within the submerged plants. In addition, we found major differences in the composition of zooplankton, fish and macroinvertebrate communities at the different salinities and species richness and diversity of zooplankton decreased with increasing salinity. At low salinities both planktonic/free-swimming and benthic/plant-associated cladocerans occurred, whilst only benthic ones occurred at the highest salinity. The low zooplankton biomass and overall smaller-bodied zooplankton specimens may result in a lower grazing capacity on phytoplankton, and enhance the turbid state in nutrient rich shallow brackish lakes.     

Winter ecology of shallow lakes: strongest effect of fish on water clarity at high nutrient levels

While the structuring role of fish in lakes is well studied for the summer season in North temperate lakes, little is known about their role in winter when fish activity and light irradiance potentially are lower. This is unfortunate as the progressing climate change may have strong effects on lake winter temperature and possibly on trophic dynamics too. We conducted an enclosure experiment with and without the presence of fish throughout winter in two shallow lakes with contrasting phosphorus concentrations. In hypertrophic Lake Søbygård, absence of fish led to higher biomass of zooplankton, higher grazing potential (zooplankton:phytoplankton ratio) and, accordingly, lower biomass of phytoplankton and chlorophyll a (Chl a), while the concentrations of total nitrogen (TN), total phosphorus (TP), oxygen and pH decreased. The average size of egg-bearing Daphnia and Bosmina and the minimum size of egg-bearing specimens of the two genera rose. In the less eutrophic Lake Stigsholm, zooplankton and their grazing potential were also markedly affected by fish. However, the decrease in Chl a was slight, and phytoplankton biovolume, pH and the oxygen concentration were not affected. TN was higher when fish were absent. Our results indicate that: (i) there is a notable effect of fish on zooplankton community structure and size during winter in both eutrophic and hypertrophic North temperate lakes, (ii) Chl a can be high in winter in such lakes, despite low light irradiance, if fish are abundant, and (iii) the cascading effects on phytoplankton and nutrients in winter may be more pronounced in hypertrophic lakes. Climate warming supposedly leading to reduced winter mortality and dominance of small fish may enhance the risk of turbid state conditions in nutrient-enriched shallow lakes, not only during the summer season, but also during winter.     

Effects of nutrients and grazers on periphyton phosphorus in lake enclosures

SUMMARY. 1. Periphyton. measured as particulate phosphorus (PP) and expressed as periphyton PP, growing on vertically oriented substrata (polyvinyl impregnated nylon) under different nutrient loadings, light intensities (exposures), and grazer communities was examined in eight large enclosures (750 m3) where nutrients (N and P) and planktivorous fish (1+yellow perch) were added in a 2x2 factorial design.
2. During the first 3 weeks of the experiment (25 June to 15 July), there was a significantly higher accumulation of phosphorus into periphyton (periphyton PP) with fertilization, but fish addition had no effect. During the fourth to seventh weeks (16 July to 12 August), addition of fish was associated with lower abundance of amphipods and chironomids and higher concentration of periphyton PP. In the enclosures without fish, these invertebrates were over 25 times more abundant, and periphyton PP decreased substantially compared to the June-July period. Fertilization increased periphyton PP only at high exposures in the enclosures with fish.
3. Exposure had a significant effect on periphyton PP. In the enclosures with fish, high abundance of nanoplankton reduced water transparency, and periphyton PP was lower in the deeper waters which may have been due to limitation by low light. Lower periphyton PP was also observed at the surface on sunny sides of enclosures without fish, and therefore with high water transparency. This pattern may have been due to inhibitory effects of high light intensity.
4. Periphyton communities in the enclosures with fish had higher uptake rates for planktonic phosphorus, and lower rates of phosphorus release, suggesting that periphyton with high phosphorus demand may have high internal cycling of assimilated phosphorus.     

Impact of fish predation on cladoceran body weight distribution and zooplankton grazing in lakes during winter

1. It is well accepted that fish, if abundant, can have a major impact on the zooplankton community structure during summer, which, particularly in eutrophic lakes, may cascade to phytoplankton and ultimately influence water clarity. Fish predation affects mean size of cladocerans and the zooplankton grazing pressure on phytoplankton. Little is, however, known about the role of fish during winter. 2. We analysed data from 34 lakes studied for 8–9 years divided into three seasons: summer, autumn/spring and winter, and four lake classes: all lakes, shallow lakes without submerged plants, shallow lakes with submerged plants and deep lakes. We recorded how body weight of Daphnia and then cladocerans varied among the three seasons. For all lake types there was a significant positive correlation in the mean body weight of Daphnia and all cladocerans between the different seasons, and only in lakes with macrophytes did the slope differ significantly from one (winter versus summer for Daphnia). 3. These results suggest that the fish predation pressure during autumn/spring and winter is as high as during summer, and maybe even higher during winter in macrophyte‐rich lakes. It could be argued that the winter zooplankton community structure resembles that of the summer community because of low specimen turnover during winter mediated by low fecundity, which, in turn, reflects food shortage, low temperatures and low winter hatching from resting eggs. However, we found frequent major changes in mean body weight of Daphnia and cladocerans in three fish‐biomanipulated lakes during the winter season. 4. The seasonal pattern of zooplankton : phytoplankton biomass ratio showed no correlation between summer and winter for shallow lakes with abundant vegetation or for deep lakes. For the shallow lakes, the ratio was substantially higher during summer than in winter and autumn/spring, suggesting a higher zooplankton grazing potential during summer, while the ratio was often higher in winter in deep lakes. Direct and indirect effects of macrophytes, and internal P loading and mixing, all varying over the season, might weaken the fish signal on this ratio. 5. Overall, our data indicate that release of fish predation may have strong cascading effects on zooplankton grazing on phytoplankton and water clarity in temperate, coastal situated eutrophic lakes, not only during summer but also during winter.     

Predation-driven dynamics of zooplankton and phytoplankton communities in a whole-lake experiment

Summary 1. Species compositions of zooplankton and phytoplankton were followed in Tuesday Lake before and after experimental manipulation of its fish populations (addition of piscivorous largemouth bass, removal of planktivorous minnows). Plankton dynamics were compared to those of adjacent, unmanipulated Paul Lake, where piscivorous fish have been dominant historically. 2. Indices of similarity for the zooplankton communities in the two lakes in 1984 prior to the manipulation were low; however, following the manipulation in spring, 1985, similarity of the zooplankton in the two lakes rose considerably and remained high throughout 1986. This was the result of an increase in Tuesday Lake of previously rare large-bodied cladocerans (Daphnia pulex, Holopedium gibberum) which were the dominants in Paul Lake, and the disappearance in Tuesday Lake of the dominant small-bodied copepod Tropocyclops prasinus, a minor component of the Paul Lake zooplankton. These observations are consistent with prior observations of the effects of size-selective predation on zooplankton communities. 3. Phytoplankton communities also responded strongly to the manipulation, with similarity indices for the two lakes rising from low levels in 1984 to high levels of similarity in 1985 and 1986, reflecting the decrease of formerly dominant Tuesday Lake taxa which were unimportant in Paul Lake and the appearance or increase in Tuesday Lake of several taxa characteristic of the Paul Lake phytoplankton assemblage. these results clearly show that food web structure can have pronounced effects on community composition at all levels of the food web, and that, just as zooplankton communities are structured by sizeselective predation, phytoplankton communities are structured by herbivory. These observations may provide some insight into factors governing the complex distributions of phytoplankton species among various lakes.A contribution from the University of Notre Dame Environmental Research Center, funded by NSF grants BSR-83-08918 and BSR-86-06271     

Benthos as the basis for arctic lake food webs

Plankton have traditionally been viewed as the basis for limnetic food webs, with zooplankton acting as a gateway for energy passing between phytoplanktonic primary producers and fish. Often, benthic production has been considered to be important primarily in shallow systems or as a subsidy to planktonic food web pathways. Stable isotope food web analyses of two arctic lakes (NE14 and I minus) in the Toolik Lake region of Alaska indicate that benthos are the primary source of carbon for adults of all species of benthic and pelagic fish present. We found no effect of turbidity, which may suppress benthic algae by shading, on food web structure. Even though Secchi transparency varied from 10.2 m in NE14 to 0.55–2.6 m in I minus, food webs in both lakes were based upon benthos, had four trophic levels, and culminated with omnivorous lake trout. We suggest that the importance of benthos in the food webs of these lakes is due to their extreme oligotrophy, resulting in planktonic resources that are insufficient for the support of planktivorous consumers.     

Benthic community response (primarily Chironomidae) to nutrient enrichment and alkalinization in shallow,soft water humic lakes

A comparison of the benthic fauna found in two shallow lakes in the New Jersey Pinelands (USA) illustrated the impact of elevated pH and nutrients caused by residential and agricultural disturbance on a naturally acidic, poorly buffered aquatic environment. Detailed community analysis suggested that change in community composition was a better indicator of response to disturbance than biological diversity indices. Chironomidae (insecta) were the predominant components of the benthic macroinvertebrate assemblage of both undisturbed Oswego Lake (low pH, low nutrients) and Nescochague Lake (fluctuating pH, elevated nutrients). The genera Procladius, Tribelos, and Pagastiella dominated Oswego Lake, where as Zalutschia zalutschicola, Procladius, Dicrotendipes, and Tanytarsus dominated Nescochague Lake. Glyptotendipes was a common and unique member of the Nescochague Lake assemblage. Cluster analysis indicated that the chemical differences between lakes were the most important community determinants, although, within each lake, depth and substrate affected the local communities. Oswego Lake exhibited a depauperate nonchironomid benthic fauna typical of low nutrient, acid lakes. In turn, Nescochague Lake exhibited an enriched nonchironomid fauna including mollusks and planaria which were not found in Oswego Lake.     

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.     

Plankton communities of three Central Florida lakes

A study of planktonic primary productivity and community structure was carried out on three lakes of varying morphometric and chemical features, but which were within a single watershed in Central Florida. Primary productivity in these lakes was evaluated by means of in situ light and dark bottle determinations as well as by calculations based upon chlorophyll and light data. Lake Mize, a deep sinkhole lake containing highly colored waters of low pH, proved to be a very unproductive lake, while Biven's Arm, a shallow-basin lake with alkaline waters and Newnan's lake, a shallow-basin like with highly colored waters both showed relatively high productivities. Phytoplankton components of the two productive lakes were quite similar to one another in being composed of bloom-forming blue-green and green algae, while the predominant forms in Lake Mize were chrysophytes, dinoflagellates and a diatom. Zooplanktonic forms were more uniform among the lakes than were the phytoplankton, though population levels reflected relative productivities among the lakes. Rotifers predominated in the Lake Mize plankton while cladocerans were relatively inabundant. This was opposite to the condition in Biven's Arm and Newnan's Lake. It is suggested that the constant presence of larval Chaoborus in the water column in Lake Mize may be partially responsible for the paucity of cladocera.     

鲤对浅水湖泊水质影响的围隔实验研究

鲤是我国浅水湖泊常见鱼类,鲤可以通过扰动底泥、牧食浮游动物、排泄营养盐等途径影响湖泊水质。为了研究鲤对浅水湖泊水质的影响,论文于2009年6月23日至8月18日在暨南大学明湖进行原位围隔实验。实验分为三个处理组(三组围隔):(1)在围隔中放养鲤(放鱼组);(2)在围隔底部放置塑料隔网,在隔网上部空间放养鲤(放鱼放网组);(3)围隔中不放隔网也不放鲤(空白对照组)。实验结果表明:鲤可以显著增加水体悬浮物、总氮、总磷和叶绿素a含量,显著降低了水体透明度。而鲤是否扰动底泥对水体悬浮物、总氮、总磷和叶绿素a含量等指标的影响不显著。 因此,该研究中鲤对浅水湖泊的影响可能主要是通过排泄等途径实现的,鲤扰动底泥对水质的影响不大。     

Biomanipulation and its feasibility for water quality management in shallow eutrophic water bodies in The Netherlands

Biomanipulation as a tool for lake restoration is discussed mainly using literature data. It is based on the exploitation of the interactions both within and between the trophic levels in an aquatic ecosystem. Important among the interactions are: competition for light and nutrients between aquatic macrophytes and phytoplankton and among different phytoplankton species; grazing by planktonic and benthic filter feeders; and size-selective predation by fish. In several case studies biomanipulation has proved to be successful in restorating mildly eutrophic small waterbodies. However, for long-term stability of the restored ecosystems supplementary measures like reducing the external nutrient loadings are needed. The feasibility of the different biomanipulation measures to improve the water quality in shallow Dutch lakes is discussed. Preliminary results on biomanipulation experiments in enclosures withOscillatoria agardhii and the benthic filter feederDreissena polymorpha are given.     

浅水湖泊表层沉积物记录的枝角类群落空间分布特征

浅水湖泊污染负荷能力较低,对环境变化敏感,过度开发会导致生态功能明显退化甚至发生稳态转变.本研究通过提取云南18个浅水湖泊的表层沉积物枝角类信息,识别枝角类群落的空间分布特征并甄别其群落构建的环境驱动因子.结果表明:表层沉积物枝角类群落的地理分布存在显著的空间差异,滇西北的枝角类群落以底栖物种占绝对优势,而随着营养水平的增加,滇东南湖泊枝角类优势物种由底栖型向浮游型转变.统计分析表明,海拔和水体总磷水平是驱动群落分布空间差异的关键环境因子,分别独立解释了枝角类群落变化的22.0%和7.7%,反映了海拔梯度及其指示的气候环境过程是控制枝角类群落空间分布的重要因子.同时枝角类群落随营养水平的变化出现差异性的结构特征,可能指示了流域开发、污染物输入、水生植被变化等人类活动扰动的综合影响.同时,海拔和总磷水平表现出显著的相互作用并解释了枝角类群落变化的26.3%,指示了云南地区的人类活动强度随着海拔梯度呈现显著的空间差异,并通过营养盐输入等过程调控了枝角类群落的结构特征.     

Impacts of introduced brown and rainbow trout on benthic invertebrate communities in shallow New Zealand lakes

1. Brown and rainbow trout have been introduced to many inland waters in New Zealand, but research on the impacts on native communities has focused mainly on streams. The purpose of this study was to compare the benthic communities of trout and troutless lakes. Based on previous studies in North America and Europe, we predicted that the benthic biomass, and especially the abundance of large invertebrates, would be lower in lakes with trout as compared to those without. We surveyed the invertebrate fauna of 43 shallow, high‐elevation lakes (26 with and 17 without trout) in four geographic clusters on the central South Island and then conducted a detailed quantitative study of invertebrate biomass and community structure in 12 of these lakes. 2. Benthic community composition and diversity of lakes with and without trout were nearly identical and biomass was as high or higher in the lakes with as without trout. There was no evidence that trout have caused local extinctions of benthic invertebrates. Although the proportional abundance of large‐bodied aquatic was slightly lower in lakes with than without trout, the abundance of several groups of large‐bodied benthic taxa (dragonflies, caddisflies and water bugs) did not differ. 3. Our findings are in contrast to those in North American and Europe where trout introductions into previously troutless lakes have led to declines in the abundance of benthic invertebrates, especially large‐bodied taxa. We propose that the modest effects of trout in New Zealand could be explained by (i) the high areal extent of submergent vegetation that acts as a benthic refuge, (ii) low intensity of trout predation on benthic communities and/or (iii) characteristics of the benthic invertebrates that make them relatively invulnerable to fish predation. 4. Regardless of the relative importance of these hypotheses, our results emphasise that the same invertebrates occurred in all of the lakes, regardless of size, elevation and presence of trout, suggesting habitat generalists dominate the benthic fauna in shallow New Zealand lakes.     

The benthic communities of the saline lakes Abijata and Shala (Ethiopia)

Lake Abijata lies in a shallow depression (maximum depth 8–9.5 m); the water is green with phytoplankton and it supports large fish and bird communities. Lake Shala lies in a deep caldera (maximum depth reputedly 260 m); phytoplankton is sparse and fish and bird communities scanty.Lakes Abijata and Shala, sampled in January, 1985, had conductivities of 14 000 and 21 000 microSiemens cm^-1 at 25 °C respectively, mainly due to high sodium, carbonate and chloride ions. Calcium concentrations are very low.The benthic fauna was studied with an Ekman grab to a depth of 8.5 m in Abijata and 15.5 m in Shala and was found to be dense in both lakes but varying greatly in composition at different depths. In Abijata the benthos consisted mainly of Ostracoda and Chironomidae, and in Shala mainly of Tubificidae, Ostracoda and Chironomidae. There were very few Nematoda. No true halophilic species were found but the community consisted of euryhaline forms found also in non-saline waters. Predatory invertebrates were absent and many of the dominant species, notably of the Chironomidae, were different from those of non-saline lakes nearby.