An artificial neural network approach for studying phytoplankton succession

Artificial neural networks are used to model phytoplankton succession and gain insight into the relative strengths of bottom-up and top-down forces shaping seasonal patterns in phytoplankton biomass and community composition. Model comparisons indicate that patterns in chlorophyll aconcentrations response instantaneously to patterns in nutrient concentrations (phosphorous (P), nitrite and nitrate (NO₂/NO₃–N) and ammonium (NH₄–H) concentrations) and zooplankton biomass (daphnid cladocera and copepoda biomass); whereas lagged responses in an index of algal community composition are evident. A randomization approach to neural networks is employed to reveal individual and interacting contributions of nutrient concentrations and zooplankton biomass to predictions of phytoplankton biomass and community composition. The results show that patterns in chlorophyll aconcentrations are directly associated with P, NO₂/NO₃–N and daphnid cladocera biomass, as well as related to interactions between daphnid cladocera biomass, and NO₂/NO₃–N and P. Similarly, patterns in phytoplankton community composition are associated with NO₂/NO₃–N and daphnid cladocera biomass; however show contrasting patterns in nutrient– zooplankton and zooplankton–zooplankton interactions. Together, the results provide correlative evidence for the importance of nutrient limitation, zooplankton grazing and nutrient regeneration in shaping phytoplankton community dynamics. This study shows that artificial neural networks can provide a powerful tool for studying phytoplankton succession by aiding in the quantification and interpretation of the individual and interacting contributions of nutrient limitation and zooplankton herbivory on phytoplankton biomass and community composition under natural conditions.     

Influence of gizzard shad on phytoplankton size and primary productivity in mesocosms and earthen ponds in the southeastern U.S.

Gizzard shad (Dorosoma cepedianum), a filter feeding omnivore, can consume phytoplankton, zooplankton and detritus and is a common prey fish in U.S. water bodies. Because of their feeding habits and abundance, shad have the potential to affect primary productivity (and hence water quality) directly through phytoplankton grazing and indirectly through zooplankton grazing and nutrient recycling. To test the ability of shad to influence primary productivity, we conducted a 16-day enclosure study (in 2.36-m³ mesocosms) and a 3-year whole-pond manipulation in 2–5 ha earthen ponds. In the mesocosm experiment, shad reduced zooplankton density and indirectly enhanced chlorophyll a concentration, primary productivity, and photosynthetic efficiency (assimilation number). While shad did not affect total phytoplankton density in the mesocosms, the density of large phytoplankton was directly reduced with shad. Results from the pond study were not consistent as predicted. There were few changes in the zooplankton and phytoplankton communities in ponds with versus ponds without gizzard shad. One apparent difference from systems in which previous work had been conducted was the presence of high densities of a potential competitor (i.e., larval bluegill) in our ponds. We suggest that the presence of these extremely high larval bluegill densities (20–350 larval bluegill m^–3; 3–700 times higher density than that of larval gizzard shad) led to the lack of differences between ponds with versus ponds without gizzard shad. That is, the influence of gizzard shad on zooplankton or phytoplankton was less than the influence of abundant bluegill larvae. Differences in systems across regions must be incorporated into our understanding of factors affecting trophic interactions in aquatic systems if we are to be able to manage these systems for both water quality and fisheries.     

Effects of zooplankton on nutrient availability and seston C : N : P stoichiometry in inshore waters of Lake Biwa, Japan

Forty-eight-hour experimental manipulations of zooplankton biomass were performed to examine the potential effects of zooplankton on nutrient availability and phytoplankton biomass (as measured by seston concentration) and C : N : P stoichiometry in eutrophic nearshore waters of Lake Biwa, Japan. Increasing zooplankton, both mixed-species communities and Daphnia alone, consistently reduced seston concentration, indicating that nearshore phytoplankton were generally edible. The zooplankton clearance rates of inshore phytoplankton were similar to rates measured previously for offshore phytoplankton. Increased zooplankton biomass led to increased concentrations of nutrients (NH₄-N, soluble reactive phosphorus [SRP]). Net release rates were higher than those found in previous measurements made offshore, reflecting the nutrient-rich nature of inshore seston. Zooplankton nutrient recycling consistently decreased TIN : SRP ratios (TIN = NH₄ + NO₃ + NO₂). This effect probably resulted from the low N : P ratios of nearshore seston, which were lower than those commonly found in crustacean zooplankton and thus resulted in low retention efficiency of P (relative to N) by the zooplankton. Thus, zooplankton grazing inshore may ameliorate algal blooms due to direct consumption but tends to create nutrient supply conditions with low N : P, potentially favoring cyanobacteria. In comparison with previous findings for offshore, it appears that potential zooplankton effects on phytoplankton and nutrient dynamics differ qualitatively in inshore and offshore regions of Lake Biwa. Received: September 4, 2000 / Accepted: January 23, 2001     

Nutrient limitation of phytoplankton communities in Subarctic lakes and ponds in Wapusk National Park, Canada

We determined the limiting nutrient of phytoplankton in 21 lakes and ponds in Wapusk National Park, Canada, using nutrient enrichment bioassays to assess the response of natural phytoplankton communities to nitrogen and phosphorus additions. The goal was to determine whether these Subarctic lakes and ponds were nutrient (N or P) limited, and to improve the ability to predict future impacts of increased nutrient loading associated with climate change. We found that 38% of lakes were not limited by nitrogen or phosphorus, 26% were co-limited by N and P, 26% were P-limited and 13% were N-limited. TN/TP, DIN/TP and NO₃ ⁻/TP ratios from each lake were compared to the Redfield ratio to predict the limiting nutrient; however, these predictors only agreed with 29% of the bioassay results, suggesting that nutrient ratios do not provide a true measure of nutrient limitation within this region. The N-limited lakes had significantly different phytoplankton community composition with more chrysophytes and Anabaena sp. compared to all other lakes. N and P limitation of phytoplankton communities within Wapusk National Park lakes and ponds suggests that increased phytoplankton biomass may result in response to increased nutrient loading associated with environmental change.     

Effect of young-of-the-year walleye (Percidae: Stizostedion vitreum) on plankton dynamics and water quality in ponds

To understand the impact of young-of-the-year (YOY) fish on food web dynamics and water quality, we stocked larval walleye (9 mm TL) (Stizostedion vitreum) in six experimental ponds using two fish densities (10 and 50 fish m^–3) with three replicates. At high fish density, the average abundances of cladocerans and copepods and the Secchi depth were lower whereas abundances of rotifers and algae, gross primary productivity (GPP), pH and total phosphorus concentration were higher than at low fish density. Fish impact on bacterial abundance, dissolved oxygen, nitrogen and phosphorus concentrations, however, was not significant. The within treatment measurements of all variables except GPP were significantly different over time. Our results indicate that YOY walleye predation at high density can affect plankton community by reducing large zooplankton biomass and water clarity, and increasing phytoplankton abundance. The impact of YOY piscivorous fish on plankton should be considered when biomanipulation is applied for improvement of water quality.     

Impact of mass mortality of a mosquito fish,Gambusia affinison the ecology of a fresh water eutrophic lake (Lake Naini Tal,India)

Between 28th March and 4th April, 2000 a fungal infection killed >80% of the most abundant planktivorous fish, Gambusia affinis in Lake Naini Tal, Uttaranchal, India. In response to this mortality, planktonic communities and some eutrophication-related parameters viz., primary productivity, phosphate–phosphorus, nitrate–nitrogen and transparency of the water, were considerably changed. Total zooplankton number more than doubled, phytoplankton number reduced nearly to half, primary production and phosphate-phosphorus was dramatically reduced, while nitrate–nitrogen and water clarity increased. The phytoplankton decline was caused by increased zooplankton grazing (top-down control) rather than phosphorus deficiency (bottom-up control). After 3 months, Gambusia and planktonic communities and nutrient levels reverted back almost to their pre-mortality state. Thus removal of G. affinis could improve water quality of Lake Naini Tal.     

Changes in trophic state and aquatic communities in high Arctic ponds in response to increasing goose populations

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Macrophytes moderate the taxonomic and functional composition of phytoplankton assemblages during a nutrient loading experiment

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The effects of reduced phosphorus and nitrogen loading on phytoplankton in Mondsee,Austria

Following restoration measures (ring canalization, treatment plant with phosphorus precipitation), phosphorus loading declined step-wise from 26.21 t year^–1 to 9.18 t year^–1 during the period 1979–1984 while P-retention increased from 48% to 78%. Phosphorus loading was poorely correlated with precipitation. Inorganic nitrogen load, largely NO₃– N, did not decline but was significantly correlated with precipitation (r = 0.95) throughout the investigation period (1978–1989).Total phosphorus loading reached acceptable levels in 1984 when compared to critical loading calculated according to Vollenweider (1976). Phosphorus input to the lake has remained at these levels in recent years.Average annual chlorophyll-a concentrations and biovolume of phytoplankton in the top 20 m layer of the lake decreased, in correspondence with the respective phosphorus concentrations, from eutrophic to mesotrophic levels. The decline was accompanied by a drastic reduction of blue-green algal populations, and especially of Oscillatoria rubescens D.C..     

Interactions between sediment and water in a shallow and hypertrophic lake: a study on phytoplankton collapses in Lake Søbygård,Denmark

Short-term changes in phytoplankton and zooplankton biomass have occurred 1–3 times every summer for the past 5 years in the shallow and hypertrophic Lake Søbygård, Denmark. These changes markedly affected lake water characteristics as well as the sediment/water interaction. Thus during a collapse of the phytoplankton biomass in 1985, lasting for about 2 weeks, the lake water became almost anoxic, followed by rapid increase in nitrogen and phosphorus at rates of 100–400 mg N M^–2 day^–1 and 100–200 mg P m^–1 day^–1. Average external loading during this period was about 350 mg N m^–2 day^–1 and 5 mg P m^–2 day^–1, respectively.Due to high phytoplankton biomass and subsequently a high sedimentation and recycling of nutrients, gross release rates of phosphorus and nitrogen were several times higher than net release rates. The net summer sediment release of phosphorus was usually about 40 mg P m^–2 day^–1, corresponding to a 2–3 fold increase in the net phosphorus release during the collapse. The nitrogen and phosphorus increase during the collapse is considered to be due primarily to a decreased sedimentation because of low algal biomass. The nutrient interactions between sediment and lake water during phytoplankton collapse, therefore, were changed from being dominated by both a large input and a large sedimentation of nutrients to a dominance of only a large input. Nitrogen was derived from both the inlet and sediment, whereas phosphorus was preferentially derived from the sediment. Different temperature levels may be a main reason for the different release rates from year to year.     

Nutrient enrichment and zooplankton effects on the phytoplankton community in microcosms from El Andino reservoir (Venezuela)

To quantify the effects of nutrient enrichment (N and P) and zooplankton grazing on the phytoplankton community structure of El Andino reservoir (Venezuela), in situ microcosms were installed for 6–7 days. Microcosms consisted of polyethylene bags (42 cm × 71 cm, non-cylindrical shaped) filled with 10 l of filtered epilimnetic water. Experiments were carried out on a monthly basis from January to December 1993. The lack/addition of nutrients was cross-classified with the absence/presence of zooplankton, resulting in an experimental design of four treatment levels: (1) no nutrient addition, zooplankton absent (C); (2) nutrient addition (150 NH₄Cl mol ml^–1 and 10 KH₂PO₄ mol ml^–1; 1 ml per l of sample), zooplankton absent (N); (3) no nutrient addition, zooplankton present (collected from the reservoir water column using a 6-m vertical tow with a 80-m plankton net) (Z); and (4) nutrient addition (as in [2]), zooplankton present (as in [3]) (NZ). Treatments were triplicated, and samples were collected at the start and end of each experiment. Significant differences between treatments were determined using a two-way ANOVA at p<0.05. Nutrient enrichment caused an increase in phytoplankton biomass, with the increase of all algal groups, except Pyrrhophyta. In spite of this, relative proportions of Cyanobacteria decreased in most cases. Chlorophyta and Bacillariophyta increased, probably due to their greater competitive abilities for phosphorus. After enrichment, Scenedesmus was the dominant species from January to June, while from July to December, Dactylococcopsis and Lyngbya dominated in the enriched microcosms. Zooplankton affected the phytoplankton community in microcoms through grazing and nutrient (mainly P) regeneration. Cladocerans (Ceriodaphnia cornuta, Moina micrura and Diaphanosoma sp.) mainly grazed on diatoms, although particulate material was present in almost all the gut contents analyzed. Particulate material probably consisted of micro-algae, detritus, bacteria, triturated algae and mineral particles. Ostracoda mainly fed on Peridinium and particulate material, whereas Thermocyclops sp. and rotifers (Brachionus spp. and Keratella spp.) mainly ingested particulate material. On the other hand, zooplankton excretion caused a slight increase in phytoplankton biomass and P concentrations in microcosms with the animals present. The effects of nutrient and zooplankton did not interact in most cases. Experimental results suggest that, at the initial stages of a eutrophication process, phytoplankton could increase their abundance and biomass, but might not change its community structure. Since there was a strong correlation between phosphorus and chlorophyll-a (bottom-up control), it is suggested that eutrophication could be avoided by controlling P input to the reservoir.     

Phytoplankton and zooplankton community structure after nutrient additions to the oligotrophic Lake Hecklan,Sweden

Lake Hecklan, in central Sweden, was fertilized with phosphorus and nitrogen during thermal stratification (late May-early Oct) 1984–1987. The nutrient additions were relatively small and raised the total phosphorus concentrations from 6 to 10 µg l^–1. The working hypothesis was that this moderate increase in the phosphorus concentration could increase the phytoplankton biomass without adverse changes in the planktonic community structure. The fertilization increased the phytoplankton biomass from 0.1 to a maximum of 2 mm³ l^–1. Chrysophyceae and Cryptophyceae dominated throughout the experimental period. Thus, the phytoplankton composition remained typical for a Swedish forest lake and provided a potential for increased zooplankton growth. An increased growth of zooplankton was indicated by increased biomass of Cladocera and Copepoda in 1984 and 1985, and by increased fecundity of herbivorous zooplankton.     

A mass balance evaluation of the ecological significance of historical nitrogen fluxes in Lake Kinneret

Lake Kinneret (LK) is a monomictic lake that has undergone significant biological and chemical changes over the last three decades of the twentieth century. The transition between the 1970s and the 1980s attracted a lot of scientific attention as it was marked by significant changes in the ecology of the lake. In the early 1980s, phytoplankton biomass increased, apparently in response to an increase in the external soluble reactive phosphorus (SRP) load. This period was marked by a rise in hypolimnetic levels of ammonium (NH₄) and SRP as well as surface water dissolved oxygen (DO) and pH. Cconcomitantly, in surface waters in winter levels of NH₄ increased and NO₃ decreased. In this study interrelationships amongst these observations were examined with a mass balance modelling approach, including simulation of individual nutrient sources and sinks, focusing on nitrogen fluxes in winter. The step-like rise in phytoplankton biomass in 1981 may have been triggered by the increase in winter external loads of SRP, as P is likely to be the growth-limiting nutrient during this season. The additional P load led to a sequence of changes including greater summer phytoplankton biomass, followed by enhanced sedimentation of organic matter. Furthermore, higher organic matter mineralization fluxes within the hypolimnion resulted in elevated levels of NH₄ and SRP in this layer through the 1980s, with a feedback to productivity in the trophogenic zone following seasonal destratification in early winter. In an apparent transition period (late 1970s to early 1980s), an increase in the modelled rate of nitrate (NO₃) production occurred via nitrification together with increased uptake of the additional nitrate by phytoplankton. These results are consistent with increased phytoplankton abundance and elevated levels of surface water NH₄ and DO during this period. Through this period the increase in phytoplankton uptake of NO₃ predominated over the increase in nitrification, and NO₃ concentrations in the 1980s were reduced compared with the previous decade, with increased partitioning of N in biomass and NH₄.     

Contemporary state of the zooplankton in Lake Peipsi

L. Peipsi is one of the richest fish lakes in Europe. Planktivorous smelt dominates in the fish fauna. The abundance of zooplankton fluctuates between 43 600–2241 500 ind m^–3, with the average 974 000 ind m^–3, biomass ranges from 0,09–3,69 g m^–3, with the average 1,86 g m^–3. Since the 1960s the abundance of rotifers has risen considerably while the mean zooplankter weight (B/N) has decreased from 0.005 mg to 0.004 mg. Zooplankton production (herbivores 20.6, predators 1.8, whole zooplankton community 22.4 g C m^–2 per period between May and October) can be considered high. Predatory zooplankton eats on an average 50% of the production of herbivorous zooplankton; about 50% of the whole zooplankton production (P_{Filt + Pred}) reaches fishes. The production of herbivorous zooplankton constitutes 10.1% of primary production. This ratio indicates a direct relationship between zoo- and phytoplankton in the food chain; the detrital food chain seems of little importance. About 6% of phytoplankton energy reaches fishes. The transformation of energy in the food web is efficient. On the basis of zooplankton L. Peipsi can be considered a moderately eutrophic or meso-eutrophic lake.     

Effect of larval fish and nutrient enrichment on plankton dynamics in experimental ponds

The hypotheses that larval fish density may potentially affect phytoplankton abundance through regulating zooplankton community structure, and that fish effect may also depend on nutrient levels were tested experimentally in ponds with three densities of larval walleye, Stizostedion vitreum (0, 25, and 50 fish m^–3), and two fertilizer types (inorganic vs organic fertilizer). A significant negative relationship between larval fish density and large zooplankton abundance was observed despite fertilizer types. Larval walleye significantly reduced the abundances of Daphnia, Bosmina, and Diaptomus but enhanced the abundance of various rotifer species (Brachionus, Polyarthra, and Keratella). When fish predation was excluded, Daphnia became dominant, but Daphnia grazing did not significantly suppress blue-green algae. Clearly, larval fish can be an important regulator for zooplankton community. Algal composition and abundance were affected more by fertilizer type than by fish density. Inorganic fertilizer with a high N:P ratio (20:1) enhanced blue-green algal blooms, while organic fertilizer with a lower N:P ratio (10:1) suppressed the abundance of blue-green algae. This result may be attributed to the high density of blue-green algae at the beginning of the experiment and the fertilizer type. Our data suggest that continuous release of nutrients from suspended organic fertilizer at a low rate may discourage the development of blue-green algae. Nutrient inputs at a low N:P ratio do not necessarily result in the dominance of blue-green algae.     

Plankton composition and water quality in a pond of spring origin in Turkey

Phyto/zooplankton composition, chlorophyll a, and some water quality parameters were investigated in a spring-originated pond in Central Anatolia between February 2001 and January 2002. Water temperature, pH, dissolved oxygen, Secchi depth, total and calcium hardness, nitrate-nitrogen, nitrite-nitrogen, ammonia-nitrogen, total phosphorus, and soluble reactive phosphorus levels were analyzed. A total of 49 species belonging to Bacillariophyceae, Chlorophyceae, Cyanophyceae, Cryptophyceae, and Dinophyceae were identified. The highest phytoplankton abundance was found in August, whereas the lowest was determined in January. Phytoplankton abundance increased from February to August and declined in the following months. The Bacillariophyceae were dominant in the phytoplankton community. A total of 21 species of Rotifera, 2 species of Cladocera, and 1 genus of Copepoda were found. The zooplankton community was dominated by Rotifera. The highest abundance of zooplankton was recorded in July and the lowest value in November. The annual mean concentration of chlorophyll a was measured as 1.90 μg l⁻¹. In spite of these eutrophic levels (mean values of total phosphorus and nitrate-nitrogen: 0.069 mg P l⁻¹ and 0.68 mg N l⁻¹), phytoplankton cannot grow satisfactorily because of the short water retention time (0.6 day⁻¹). The shallowness of the pond together with the low phytoplankton biomass and the high concentrations of nutrients are discussed.     

The influence of Procambarus clarkii (Cambaridae,Decapoda) on water quality and sediment characteristics in a Spanish floodplain wetland

This study describes the effects of the American red swamp crayfish, Procambarus clarkii Girard, on water quality and sediment characteristics in the Spanish floodplain wetland, Las Tablas de Daimiel National Park. Our short term enclosure study during a summer drawdown revealed that crayfish acted as a nutrient pump that transformed and translocated sediment bound nutrients to the water column. Water quality impoverishment was mainly due to the increase of dissolved inorganic nutrients (soluble reactive phosphorus and ammonia), and a significant increase of total suspended solids occurred likely as a result of crayfish associated bioturbation. At the same time, crayfish reduced the content of organic matter in the sediment and we observed a slight increase of total sediment phosphorus and nitrogen content as a result of crayfish benthic activity. P. clarkii effects, in terms of internal nutrient loading (229.91 mg TP m^–2 d^–1), were shown to be important on a local scale, indicating the significance of internal nutrient supply to water column primary producers particularly under low external supply (summer). Extrapolations to the whole ecosystem, however, revealed a negligible crayfish contribution (0.06%) to total internal nutrient loading (0.035 mg TP m^–2 d^–1). Hence, crayfish spatial heterogeneity patterns are important in global and local matter fluxes and nutrient cycles in wetlands.     

Compositional and functional consequences of environmental change in Belgian farmland ponds

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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.