Enclosures study of trophic level interactions in the mesotrophic part of Lake Balaton

Enclosures, open to the bottom sediments and to the atmosphere, containing about 17 m³ of lake water in the mesotrophic area of Lake Balaton, were used to elucidate the role of the benthivorous fish bream (Ambramis brama L.) in the lake during 1984–1986.Throughout the whole period water was less transparent in the enclosure containing fish, which strongly influenced the concentrations of suspended solids and chlorophyll a.Both phytoplankton biomass and production readily responded to nutrient increase in the enclosure with fish. In 1985 diatoms were replaced by cyanobacteria whereas in 1986, at a lower fish stocking, a shift in algal structure towards chlorophytes was observed.Egested organic substances and the resuspension of sediment particles by fish increased bacterial production.     

The Importance of Higher Trophic Level in the Process of Eutrophication in Enclosure

Enclosures open to the sediments and the atmosphere were used to elucidate the response of algae and bacteria to the nutrient enrichment from fish in the mesotrophic area of Lake Balaton. Active transformation between the forms of nitrogen was observed especially in the enclosure with fish. Both nitrogen and phosphorus values were influenced by fish. Changes in nutrient dynamics, composition and biomass of algae and bacterial production were measured. It has been shown that benthivorous fish with a biomass illustrative of the most eutrophic part of the lake were able to generate high bacterial production rate and a strong outburst of blue-greens.     

Short-term and long-term effects of zooplanktivorous fish removal in a shallow lake: a synthesis of 15 years of data from Lake Zwemlust

SUMMARY 1. Removal of zooplanktivorous fish (mainly bream) in 1987 from a shallow eutrophic lake in the Netherlands, Lake Zwemlust, resulted in a quick switch from a turbid state with cyanobacteria blooms to a clear state dominated by macrophytes.
2. The clear state was not stable in the long term, however, because of high nutrient loadings.
3. In 1999, another removal of zooplanktivorous fish (mainly rudd) had similar effects as in 1987, although macrophytes returned more slowly.
4. In the years directly following both interventions there was a 'transition period' of very clear water with high densities of zooplanktonic grazers in the absence of macrophytes; low oxygen concentrations indicate that during those years primary production was low relative to heterotrophic activity.
5. The transition period appears to provide the light climate necessary for the return of macrophytes.
6. Reduction of nutrient loading is necessary to improve water quality in Lake Zwemlust in the long term. In the short term, repeated fish stock reduction is a reasonable management strategy to keep Lake Zwemlust clear.     

The Role of Fish Communities in Water Quality Management of a Large Shallow Lake

Management measures of Lake Balaton such as wetland reconstruction at the main inflow to the lake along with the “unplanned” commercial fishery led to great changes in the density and biomass of fish populations. There was no significant difference in CPUE data between the two, eastern and western, basins. Biomass of total fish stock in Lake Balaton has decreased substantially, 2–3 times between 1991–1999, and ranges between 120–194 kg ha⁻¹. Bottom‐up effects are more important than the top‐down effects due to the impact of internal nutrient load. Changes in the biomass and thus the activity of omnivorous fish in the lake lowered the intensity of various indirect effects and feedback mechanisms causing changes in the nutrient metabolism of the lake. Intensified fishery effort in Lake Balaton did not result in an increased stock of piscivores. The ratio of piscivores and omnivores remained at 5% during the whole study period. Despite this low piscivores to omnivores ratio, the water quality has improved in all basins.     

Effects of light and microcrustacean prey on growth and investment in carnivory in Utricularia vulgaris

SUMMARY 1. In a 5-week enclosure experiment, we studied the effects of light (ambient light, low light) and prey availability (no prey, prey added) on growth and investment in carnivory in Utricularia vulgaris .
2. Investment in carnivory, measured as the proportion of biomass allocated to bladders, was strongly affected by our manipulations of light intensity and prey density. In the treatment with high prey density the light reduction decreased the investment in bladders from 25% to zero. The effect of prey density on investment in bladders was negative. Because prey addition increased the concentration of nutrients, especially phosphorus, we propose that the effect of the prey treatment on investment reflected altered nutrient concentrations.
3. Availability of prey increased growth and apical biomass of Utricularia . As Utricularia had very few bladders in some treatments we suggest that the effect was due to a combination of live prey trapped and increased nutrient availability from dead prey.
4. Abundance of periphyton on Utricularia and on the enclosure walls was highest in the treatments with high prey density where nutrient concentrations were highest. Thus we interpret the response of periphyton as primarily reflecting nutrient availability.     

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。     

Effects of physical refuges on fish–plankton interactions

1.Refuges that reduce fish-induced mortality of zooplankton are considered to be key factors in controlling phytoplankton growth in lake ecosystems. In order to better understand the role of physical refuges for zooplankton on zooplanktivorous fish-plankton relationships, an enclosure experiment was run in a mesotrophic lake. Even-link systems (zooplankton and phytoplankton) and odd-link systems (zooplanktivorous fish, zooplankton and phytoplankton) were established. We also established an odd-link system with a physical refuge for zooplankton where fish predation was limited in the upper half of the enclosure.
2.Fish negatively affected density and mean body length of herbivorous zooplankton and total zooplankton, filtering rates with some intermediate effects in the presence of the refuge. A clear refuge effect was observed for the dominant herbivore, Ceriodaphnia . On the other hand, the refuge seemed to increase the vulnerability of those taxa that aggregated in upper layers of the water column. Grazing was thus reduced in both odd-link systems.
3.The lack of significant correlation between nutrient availability and phytoplankton biomass in enclosures suggested a top-down control of algal growth in our experimental systems. In both odd-link systems ('fish' and 'refuge') phytoplankton biomass was significantly enhanced, and transparency was reduced in comparison with the even-link system.     

A simple model of the eco-hydrodynamics of the epilimnion of Lake Tanganyika

1. The ecosystem response of Lake Tanganyika was studied using a four-component, nutrient–phytoplankton–zooplankton–detritus, phosphorus-based ecosystem model coupled to a nonlinear, reduced-gravity, circulation model. The ecosystem model, an improved version of the earlier eco-hydrodynamics model developed for Lake Tanganyika, was used to estimate the annual primary production of Lake Tanganyika and its spatial and temporal variability. The simulations were driven with the National Centres for Environmental Protection (NCEP) records for winds and solar radiation forcing.
2. The simulated annual cycles of the four ecosystem variables and the daily net primary production were compared with the observations. The comparison showed that simulations reproduced realistically the general features of the annual cycles of epilimnial phosphate, net primary production and plankton dynamics.
3. The climatic simulations for the years 1970–2006 yielded a daily averaged integrated upper layer net production ranging from 0.11 to 1.78 g C m⁻² day⁻¹ and daily averaged chlorophyll- a (chl- a ) from 0.16 to 4.3 mg m⁻³. Although the nutrient concentrations in the epilimnion during the strong wind years were high, the net production was low, which is partly because of the greater vertical mixing, produced by strong winds, exposing the phytoplankton to low light conditions in deeper waters. The simulated annual net production and chl- a agreed quite well with observed production available in the literature.
4. We envisage using this model to predict the future scenarios of primary productivity in the lake.     

The influence of upstream predation on the expression of fish effects in downstream patches

1. Enclosures were installed in a fishless stream and divided transversely into upstream and downstream sections. Downstream sections were further divided longitudinally, and one of the downstream sections in each enclosure was stocked with juvenile coho salmon ( Oncorhynchus kisutch ), and the other side was left as a fishless control. Densities of juvenile coho were then manipulated in upper sections of enclosures to determine the effect of upstream predation intensity on fish predation effects in downstream sections.
2. Significantly fewer mayflies (primarily Ameletus sp.) were observed grazing on unglazed ceramic tiles in lower enclosure sections with fish present. There was no detectable effect of fish density in upstream enclosure sections on the number of mayflies observed grazing on ceramic tiles in lower sections of the enclosures.
3. There was a significant positive effect of both fish presence and upstream density on chlorophyll a concentrations on ceramic tiles in lower enclosure sections, but not on chlorophyll a on natural gravel substratum.
4. Behavioural experiments with mayflies and coho in streamside troughs suggest that Ameletus sp. responds primarily to mechanical rather than chemical cues from coho parr.     

The effect of grazing and nutrient supply on periphyton associated bacteria

The effects of nutrient additions and grazing by macro-invertebrates on periphyton-associated algae and bacteria were studied by performing an enclosure experiment on three occasions from early spring to summer at mesotrophic Lake Erken and V?dd?, at the Swedish Baltic coast. There were significant interactions between nutrient additions and grazing on bacterial biomass and specific activity in Lake Erken. Thus, the importance of either bottom-up or top-down effects could not be singled out. Bacterial biomass increased with enrichment only in the absence of grazers. Grazer presence tended to increase bacterial biomass in ambient nutrient conditions, but to decrease bacterial biomass under enrichment. For specific activity the positive response to enrichment was restricted to grazer presence. Hence, grazing by macro-invertebrates may have an indirect positive effect on bacterial activity by enhancing nutrient conditions through their feeding activities and/or fecal pellets production. In addition, we found a significant relationship between bacterial production and chlorophyll a at both sites. This relationship weakened in the presence of macro-invertebrates. Thus, the importance of internal nutrient regeneration by bacteria and algae decreased, possibly due to increased nutrient availability, in the presence of macro-invertebrate grazers.     

Alga consumption of four dominant planktonic crustaceans in Lake Balaton (Hungary)

Between 1981–83 the gut contents ofDaphnia galeata, D. cucullata, Eudiaptomus gracilis, andCyclops vicinus were examined with light and scanning electron microscope to obtain information on the feeding of these species in Lake Balaton. The twoDaphnia species feed mainly on abioseston, and it is assumed that their primary nutrient source was organic matter adsorbed onto the surfaces of the abioseston granules plus bacteria and detritus.E. gracilis feeds on algae, showing a preference for green algae and diatoms.C. vicinus is also a prodigious consumer of algae in Lake Balaton, utilizing the whole size spectrum of phytoplankton. Concerning the trophic relationships between phytoplankton and zooplankton in Lake Balaton, that between diatoms and bothE. gracilis andC. vicinus is the most conspicouos. Convincing evidence for an extensive utilization of blue-green algae was not found. Though there is no firm evidence yet, it is likely that theDaphnia are dependent on organic matter adsorbed on the abioseston.     

Changes in the plankton community following introduction of filter-feeding planktivorous fish

1. We conducted enclosure experiments in a shallow eutrophic lake, in which a biomass gradient of the filter-feeding planktivore, silver carp, Hypophthalmichthys molitrix Valenciennes, was created, and subsequent community changes in both zooplankton and phytoplankton were examined.
2. During a summer experiment, a bloom of Anabaena flos-aquae developed (≈ 8000 cells mL⁻¹) solely in an enclosure without silver carp. Concurrent with, or slightly preceding the Anabaena bloom, the number of rotifer species and their abundance increased from seven to twelve species (1700–14 400 organisms L⁻¹) after the bloom in this fish-free enclosure. Protozoans and bacteria were generally insensitive to the gradient of silver carp biomass.
3. During an autumn experiment, on the other hand, large herbivorous crustaceans were more efficient than silver carp in suppressing the algae, partly because the lower water temperature (≈ 24 °C) inhibited active feeding of this warm-water fish and also formation of algal colonies. Heterotrophic nanoflagellate and bacterial densities were also influenced negatively by the crustaceans.
4. Correspondence analysis (CA) was applied to the weekly community data of zooplankton and phytoplankton. A major effect detected in the zooplankton community was the presence/absence of silver carp rather than the biomass of silver carp, whereas that in the phytoplankton community was the fish biomass before the Anabaena bloom, but shifted to the presence/absence of the fish after the bloom.     

Role of phosphorus and nitrogen for bacteria and phytopankton development in a large shallow lake

Nutrient (P and N) enrichment experiments in small enclosures (20 l) were carried out to determine P and/or N limitation of bacterioplankton in Lake Võrtsjärv. The specific interest of the study was to test if it is possible to detect nutrient `physiological' or growth (rate) limitation of bacterioplankton and competition for nutrients (N and P) with phytoplankton in generally nutrient rich lake. Thymidine and leucine incorporation; leucine aminopeptidase, -D-glucosidase and alkaline phosphatase activity, total count of bacteria, chlorophyll a concentration and primary production as well as the concentrations of different chemical forms of N and P were followed during 4–5 days of the experiment. To address the question of the interactions between nutrients, bacterio- and phytoplankton, experimental and seasonal data sets were included in the analyses. Phosphorus (P) had a positive effect on bacterioplankton in enclosure experiments in June 1997; no effects of nutrients were found in September 1996, while in May 1996, P affected mainly the phytoplankton. On the seasonal scale, the development of bacterioplankton was connected to primary production, total phosphorus and temperature. In enrichment experiments, bacterioplankton was mainly related with primary productivity but the possible importance of bacterial grazers could be presumed. Thus, no evidence was found for nutrient growth limitation and/or competition for N and/or P, rather bacterioplankton depended on organic food supply originating from phytoplankton.     

Impacts of bleak (Alburnus alburnus) and roach (Rutilus rutilus) on water quality,sedimentation and internal nutrient loading

Lake Vesijärvi, southern Finland, suffered sewere eutrophication by sewage effluent from the city of Lahti during the 1960's and the early 1970's. The municipal sewage loading was diverted from the lake in 1976 and the lake started to recover. However, in the 1980's blue-green algal blooms increased again and the recovery of the lake faded. Enclosure experiments demonstrated that high roach (Rutilus rutilus) biomass is one of the key factors in the fading recovery of the lake. In this study, the influence of roach and another cyprinid fish species (bleak, Alburnus alburnus) to planktonic algal productivity and biomass in Lake Vesijärvi was examined. Enclosure experiments in the field showed the impacts of planktivorous bleak on water quality; in an enclosure with a density of 1 fish m^–2 average daily algal production (1370 mg C m^–2) and chlorophyll-a concentration (50–90 µg 1^–1) were more than twice that in an enclosure without fish. Laboratory experiments showed that the availability of planktonic food affects the foraging behaviour of roach and consequently the internal nutrient loading from the sediment into the water. Roach caused the highest phosphorus loading and turbidity when there was no zooplanktonic food available in the water. The possible interactions between planktivorous and omnivorous fish species are discussed.     

Ecological restoration in eutrophic Lake Wuli: A large enclosure experiment

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

Dynamics and ecological significance of daily internal load of phosphorus in shallow Lake Balaton, Hungary

1. As supported by field data, turbidity recorded by light scattering sensors could reliably be converted into concentration of suspended particulate matter (SPM) and coefficient of vertical light attenuation (K_d) in Lake Balaton. 2. Autocorrelation analysis revealed that proper determination of SPM concentration and K_d required daily sampling. To approximate daily rate of resuspension, 15 min or more frequent measurements were needed. Thus, routine monitoring provides very little insight into environmental variability of shallow lakes as habitats for phytoplankton. 3. The internal P load was estimated from daily rate of resuspension and P desorption capacity of sediments. The latter was assumed to be proportionate to the potentially mobile inorganic P content of SPM. A comparison with net primary production and nutrient status of phytoplankton showed that the proposed method of estimating time series of internal P load captured seasonal trends. 4. The daily rate of resuspension was high whereas that of internal P load was low in Lake Balaton relative to other shallow lakes. The latter reflects favourable behaviour of the calcite‐rich sediments. As a consequence, carrying capacity of Basin 1 of Lake Balaton was P‐determined. 5. The timing of external and internal loads was radically different. While the former showed mostly seasonal changes, large pulses characterised the latter. As a consequence, internal load may supply more P to phytoplankton growth during the critical summer months than external load. However, the relative importance of these sources may show substantial interannual variability. 6. Large resuspension events often followed each other during periods of 10–15 days. It has been shown that disturbances in this frequency range are of key importance in maintaining the diversity of phytoplankton. We propose that resuspension can be perceived not only as a disturbance factor but also as a factor that periodically relaxes nutrient stress. The former feature may dominate the instantaneous effect, whereas the latter may determine the persistent effect of resuspension on succession of phytoplankton.     

Population dynamics and food of bleak (Alburnus alburnus L.) in the littoral zone of Lake Balaton,Hungary

Intra- and interspecific effects together with changing environmental conditions regulate the dynamics of the fragile bleak populations in Lake Balaton. Size- and age-structures, length-weight relationships and mortality were studied on 1493 individuals and growth history were determined using scales from 703 specimens. The population growth rate was slow and the basic population parameters showed area variations along the longitudinal axis of the lake. Gut contents were analyzed in 224 individuals. The bleak stocks are trophically strongly related to the littoral zooplankton and benthos; and they play a significant energy-mediating role in the food-webs between the periphytic communities and fish assemblages in the littoral zone of Lake Balaton.     

Spatially heterogeneous trends in nearshore and offshore chlorophyll a concentrations in lakes Michigan and Huron (1998–2013)

1. Nutrient abatement programmes have been successfully implemented around the globe to reduce nutrient loading into aquatic ecosystems. Concurrently, the worldwide spread of invasive filter feeders, such as dreissenid mussels, may alter nutrient dynamics in invaded systems by sequestering nutrients away from pelagic zones and reducing primary production in offshore areas. Such is the case in the Laurentian Great Lakes of North America, where decades of nutrient abatement and the establishment of dreissenid mussels have seemingly resulted in more oligotrophic conditions and altered spatial patterns of nutrient availability and primary production.
2. Recent studies have focused on whole lake trends in primary production despite spatial differences in tributary inputs, bathymetry, and other environmental conditions that can affect primary production in nearshore areas. Thus, we hypothesised that trends in nearshore chlorophyll concentrations in different areas may diverge in a manner consistent with spatial differences in nutrient input. To evaluate these differences in surface chlorophyll responses, we assessed temporal trends in four different areas of Lake Michigan and three different areas of Lake Huron.
3. We hypothesised that in lakes Huron and Michigan, nearshore zones have experienced slower declines of chlorophyll concentrations relative to offshore zones. To assess this hypothesis, we estimated temporal trends of surface water chlorophyll concentrations (a proxy for primary production) from satellite imagery from 1998 to 2013. We calculated average surface chlorophyll concentrations for 10-m depth intervals ranging from the shore (0–10 m) to offshore (>90 m) during representative months of May, July, and September. We then analysed these data to determine if long-term trends in surface chlorophyll varied by season, proximity to the shoreline, and water depth.
4. The rates of annual change in chlorophyll concentrations in nearshore areas were markedly different to offshore trends in both lakes. Chlorophyll concentrations declined overtime in offshore areas, but nearshore chlorophyll concentrations were either stable (in May) or increased (in July and September) throughout the time series.
5. Differences in chlorophyll concentrations among areas were prominent in Lake Michigan. While differences between the northern and southern basin have been previously documented, trends in chlorophyll concentrations also differed between the eastern and western sides of Lake Michigan. Despite similar bathymetry and geographic features to Lake Michigan, regional trends were not observed in Lake Huron.
6. The results of this study are generally consistent with the nearshore shunt hypothesis, which predicts that dreissenid filtering, nutrient re-suspension, and continued nutrient loading from tributaries can cause an increase in primary production in nearshore areas during periods of offshore oligotrophication. Thus, the localised effects of nutrient abatement programmes in a given lake will be influenced by complex interactions between lake bathymetry and the presence of non-native filter feeding organisms.

     

Operation of the Kis-Balaton reservoir: evaluation of nutrient removal rates

As one of the major measures for controlling the man-made eutrophication of Lake Balaton, the Hidvég reservoir of 20 km² surface area was built near the mouth of River Zala, draining half the watershed of the lake, and representing the largest nutrient source for the lake. The reservoir, as the first element of the expected total system of 70 km² surface area (Kis-Balaton Control System), started to operate in June 1985, aiming at removing nutrients primarily through sedimentation, adsorption and uptake by macrophytes.Detailed investigations began with the operation. These cover the observation of upstream and downstream nutrient loads and the water quality in the reservoir, the study of major phosphorus removal processes, and analysis of the nitrogen cycle and of the behaviour of phytoplankton, zooplankton, fish and macrophytes. The research programme is completed by the evaluation of observations (including the use of phosphorus budget models), with special emphasis on future operation modes of the reservoir.The nutrient removal efficiencies in the reservoir came up to expectations. The removal rates for suspended solids, total-P, soluble reactive-P and nitrate-N exceeded 50 % in the first full year of operation (1986). As a result of reservoir operation, nutrient loads in the western basin of Lake Balaton have been significantly reduced. However, the improvement in water quality can be expected only with a lag time due to the internal P load of the basin.     

Control mechanisms of arctic lake ecosystems: a limnocorral experiment

To assess the potential impact of human exploitation on arctic lakes and to determine how these eco systems are regulated we initated a limnocorral experiment in Toolik Lake, Alaska, in the summer of 1983. The limnocorrals were 5 m in diameter and from 5–6 m in depth and were open to the sediments. In 1983 four limnocorrals were deployed in an isolated bay of Toolik Lake within a cross-classified treatment regime of high and low inorganic nitrogen and phosphorus additions and high and low free swimming fish additions. The objective of the nutrient addition was to stimulate phytoplankton growth and determine the extent to which increased plant production was passed through pelagic and benthic food chains. The objective of the fish addition was to determine the impact of fish predation on large-bodied zooplankton, especially the zooplanktivorous copepod Heterocope, then to study the effect of altered Heterocope densities on small-bodied zooplankton species population dynamics. In 1984 two more limnocorrals were deployed, one a low fish, 1 × nutrient addition treatment and the other a no fish, no nutrient treatment. The fish manipulation was changed to confining several fish in cages with the cages held in corrals for varying lengths of time. The addition of inorganic nitrogen and phosphorus dramatically increased phytoplankton productivity. This increase in algal biomass and production greatly altered the light environment and water quality in the nutrient treated limnocorrals. The secchi disk depth in the nutrient treated limnocorrals declined each summer reaching as low as 1 m in 1985. Both oxygen content and pH increased in the nutrient treatment corrals. Corrals not receiving nutrient additions remained near lake concentrations for most water quality parameters. While phytoplankton biomass was stimulated in 1983 phytoplankton growth was not sufficient to draw down all the nitrogen and phosphorus added and these nutrients reached high levels in the last half of the summer. In 1984 phosphorus remained above 20 μg in the nutrient-treated corrals but ammonia dropped to reference levels by day 25. In 1985 both nutrient concentrations rapidly declined to reference levels. Most pelagic components responded to the nutrient additions. Microbial production was stimulated in the nutrient treated limnocorrals and bacterial population sizes built up to nearly 8–10 times those of the reference corrals. However, microheterotrophs soon increased in abundance and apparently grazed down bacteria to reference levels. Phytoplankton population density, as estimated by chlorophyll a determinations, increased dramatically with nutrient addition such that each year the phytoplankton densities were higher than before. Primary productivity was also stimulated and appeared not to be light limited even when phytoplankton densities rose to high levels. In the first two years of the experiment zooplankton densities were little altered by the increased phytoplankton densities. However, by 1985 daphnid densities were quite a bit higher in the high nutrient addition limnocorrals. The benthic community and sediment response was much less affected by nutrient addition. Overall sediment respiration increased in the nutrient treated corrals but underlying sediments seemed little affected. Decomposition of Carex litter was likewise little affected by nutrient addition. Benthic invertebrates were also little impacted by the nutrient addition and increased sedimentation of phytoplankton. However, the response of benthic invertebrates is difficult to assess fully in the current experiment because chironomids, a prominent component of the benthic community, failed to recruit into the limnocorrals and the corrals physically shifted during ice-out in the spring of 1984 disturbing the sediment in several corrals. The fish additions in 1983 of free swimming grayling essentially eliminated large bodied zooplankton, especially Heterocope septentrionalis, from all four limnocorrals. In subsequent summers Heterocope were not so dramatically preyed upon but generally were found in higher densities in the low or no fish treatments. However, either when Heterocope were eliminated in 1983 or were in rough inverse proportion to fish density, altered Heterocope abundance had no obvious affect on small-bodied zooplankton abundance. The fish treatment apparently influenced the zooplankton response to high nutrient addition in 1985. In the high nutrient limnocorrals daphnid populations became very abundant, but in the high fish treatment the daphnid responding was the small-bodied D. longiremis while in the low fish treatment the daphnid responding was the large-bodied D. middendorffiana. Thus we have considerable evidence for bottom up control of phytoplankton density and production. This increased production ultimately, but not for two years, stimulated zooplankton density increases. Increased nutrients had little effect on the benthos or sediments. Fish manipulations influenced large-bodied zooplankton but had little effect on small-bodied zooplankton. Because grayling are predominantly plankton feeders in lakes, no fish effect on benthic invertebrates was expected. Limnocorrals thus seem good systems to study nutrient-phytoplankton interactions. They are not as suitable for benthic invertebrate studies and fish manipulations may be difficult. Most other limnocorral studies were of brief duration; however, in the present study the limnocorrals seemed to perform well over a three year period.