Structural and functional characteristics of epiphyton and epipelon in relation to their distribution in Lake Vechten

Epiphyton and epipelon were quantitatively collected, respectively, from the submerged macrophytes and the sandy lake bottom of Lake Vechten (The Netherlands). On a weight basis, epiphyton was maximal in autumn and epipelon in summer. In winter the chemical composition of epiphyton and epipelon was similar. In summer the epiphyton had on a unit weight basis more organic matter and carbonate, and had per unit organic matter a higher algal number, nitrogen and energy content than the epipelon. Algae predominating the epiphyton were filamentous greens and pennate diatoms; those in the epipelon were pennate diatoms and blue-green algae. In both cases, species known to frequent the phytoplankton were abundant. The diatoms were quantified using paper chromatographic pigment analyses. Both the epiphyton and the epipelon exhibited maximal photosynthesis in mid summer. That light was generally the limiting factor was evident from periphyton developed on artificial substrates. This periphyton differed widely in its composition from that on the natural substrates, mainly because the latter collected much more sedimenting matter.In dense Ceratophyllum stands light was severely attenuated and the significant gradients in oxygen and pH were caused by the differences with depth in the proportions of photosynthesis and respiration. The oxygen content and pH at the bottom decreased owing to epipelic respiration. The epiphytic composition depended greatly on the degree of light attenuation. The epiphytic and epipelic respiration, except during part of the early summer, exceeded photosynthesis on a 24 h basis; this included the macrophytic photosynthesis during the time the vegetation was maximally developed. During the growing season import of organic matter, i.e. deposited seston, greatly exceeded that due to the photosynthetic production. After the summer maximum, the epipelon decreased faster than predicted from its oxygen exchange. It was concluded that sedimentation and resuspension determined mainly the changes in epiphyton and epipelon. Especially when covered with vegetation, the lower littoral of Lake Vechten plays a large part in the aerobic decomposition of sestonic organic matter.     

Implications of rapid changes in chlorophyll-<Emphasis Type="Italic">a</Emphasis> of plankton,epipelon, and epiphyton in a Pampean shallow lake: an interpretation in terms of a conceptual model

Biomass assessments of algae in wetlands usually include only the phytoplankton community without considering the contribution of other algal associations to total algal biomass. This omission prevents an accurate evaluation of the phytoplankton community as an integral part of the total ecosystem. In the present work, the biomass contributions (expressed as chlorophyll-a content per m² of lake) of phytoplankton, epiphyton on both submerged and emergent macrophytes, and epipelon were measured in Lacombe Lake, Argentina, for the purpose of (1) establishing the relative importance of the phytoplankton and (2) evaluating the entire contribution of algal biomass within the context of the Goldsborough & Robinson conceptual model. Our sampling was carried out monthly for a year in sites representative of different conditions with respect to water depth and type of macrophytes. Physicochemical analyses of water were performed following standard methods. Plankton was collected in a five-level profile at deeper stations and in subsurface samples at the shallow one. Samples of sediment obtained with corers were collected for epipelon sampling and segments of plants were cut at different levels, so as to obtain the epiphytes by scraping. Pigment was extracted with aqueous acetone and calculations were made by means of the Lorenzen equation. According to the Goldsborough & Robinson model, a Lake State developed here during the winter (phytoplankton maxima: 150 mg chlorophyll-a per m²). Then, through the subsequent growth of the submerged macrophytes, an Open State was observed, characterized by a maximum epiphyton biomass (at 3,502 mg chlorophyll-a per m²) along with lower levels of phytoplankton biomass. The epiphytic algae on the emergent macrophytes were always present but attained only relatively low biomass values (maximum: 120 mg of chlorophyll-a per m² in February). The epipelon biomass varied between 50 and 252 mg chlorophyll-a per m², registering a considerable contribution of settled algae from the water column (phytoplankton). This study contributes to our knowledge of wetland dynamics through its assessment of the rapid changes in the relative contributions of both planktonic and attached algae to the total algal biomass within the context of specific environmental factors. Guest editors: U. M. Azeiteiro, I. Jenkinson & M. J. Pereira Plankton Studies     

Development of submerged macrophyte and epiphyton in a flow-through system: Assessment and modelling predictions in interconnected reservoirs

Every approach to lake restoration requires the reestablishment of submerged macrophytes. However, macrophyte overgrowth in shallow lakes may lead to deterioration and a consequent necessity for restoration treatments. We assumed that a major threat to the increased trophic level in the Jankovac flow-through system arises from the sediment, where the accumulation of deciduous leaf litter and decayed macrophyte fragments could generate anoxic conditions. The integrated Water Quality Model (WQM) and the Submerged Aquatic Vegetation Model (SAVM) were combined in the Jankovac Model (JanM) and applied during the vegetated season in 2008 and 2014, with the aim to offer a possible approach to the maintenance of good water quality. The impacts of flow velocity and epiphyton growth on submerged macrophyte coverage and biomass were simulated. Biocenotic analyses suggested that epiphyton growth was more extensive in 2014 in comparison to 2008. The results of JanM indicated that increased flow velocities enhanced macrophyte growth and dissolved oxygen concentrations concurrently with the decline of epiphyton biomass. Furthermore, results suggested that epiphyton growth rate of 0.4 d⁻¹ maintained macrophyte coverage and biomass at a satisfactory level of 70% reservoir coverage. Considering the proposed scenarios hydraulic treatment could be applied to regulate submerged macrophytes in shallow reservoirs, as an efficient and less invasive approach than sediment removal, especially in sensitive karst areas.     

COMPARISON OF IN SITU PHOTOSYNTHETIC ACTIVITY OF EPIPHYTIC,EPIPELIC AND PLANKTONIC ALGAL COMMUNITIES IN AN OLIGOTROPHIC LAKE,SOUTHERN FINLAND1

The photosynthetic activity of different algal communities at the outer edge of an Equisetum fluviatile L. stand in an oligotrophic lake (Pääjärvi, in southern Finland) was investigated. Production by the algal communities was measured simultaneously using a modified ¹⁴C-method, and the results were related to the volume of algae and the available irradiance. The relative production rate (P/B quotient) of phytoplankton was ca. 3 × that of epiphyton and ca. 20 × that of epipelon. Epiphyton productivity remained almost constant although the algal volume varied greatly, suggesting that the surface layer of the algal community was mainly responsible for the photosynthetic activity. In the littoral area (at 1 m depth) primary production/m² of lake surface by phytoplankton, epiphyton and epipelon was similar but in the littoriprofundal area (2–4 m) phytoplankton production was twice that of epipelon. Primary productivity of epiphyton and epipelon/m² of substratum was about equal to phytoplankton productivity/m³ of water at the same irradiance. This relation provided a means of estimating the relative contributions of the different algal communities to the total algal production in the lake.     

Mesocosm experiments on nutrient and fish effects on shallow lake food webs in a Mediterranean climate

1. Nutrient and fish manipulations in mesocosms were carried out on food‐web interactions in a Mediterranean shallow lake in south‐east Spain. Nutrients controlled biomass of phytoplankton and periphyton, while zooplankton, regulated by planktivorous fish, influenced the relative percentages of the dominant phytoplankton species. 2. Phytoplankton species diversity decreased with increasing nutrient concentration and planktivorous fish density. Cyanobacteria grew well in both turbid and clear‐water states. 3. Planktivorous fish increased concentrations of soluble reactive phosphorus (SRP). Larger zooplankters (mostly Ceriodaphnia and copepods) were significantly reduced when fish were present, whereas rotifers increased, after fish removal of cyclopoid predators and other filter feeders (cladocerans, nauplii). The greatest biomass and diversity of zooplankton was found at intermediate nutrient levels, in mesocosms without fish and in the presence of macrophytes. 4. Water level decrease improved underwater light conditions and favoured macrophyte persistence. Submerged macrophytes (Chara spp.) outcompeted algae up to an experimental nutrient loading equivalent to added concentrations of 0.06 mg L^?1 PO₄‐P and 0.6 mg L^?1 NO₃‐N, above which an exponential increase in periphyton biomass and algal turbidity caused characean biomass to decline. 5. Declining water levels during summer favoured plant‐associated rotifer species and chroococcal cyanobacteria. High densities of chroococcal cyanobacteria were related to intermediate nutrient enrichment and the presence of small zooplankton taxa, while filamentous cyanobacteria were relatively more abundant in fishless mesocosms, in which Crustacea were more abundant, and favoured by dim underwater light. 6. Benthic macroinvertebrates increased significantly at intermediate nutrient levels but there was no relationship with planktivorous fish density. 7. The thresholds of nutrient loading and in‐lake P required to avoid a turbid state and maintain submerged macrophytes were lower than those reported from temperate shallow lakes. Mediterranean shallow lakes may remain turbid with little control of zooplankton on algal biomass, as observed in tropical and subtropical lakes. Nutrient loading control and macrophyte conservation appear to be especially important in these systems to maintain high water quality.     

Seasonal succession of lotic epiphyton: effects of macrophyte cover and grazing by chironomid larvae

The epiphyton is a community of periphyton on the aquatic plants. The seasonal dynamics of biomass, production and community structure of epiphytic algae on Potamogeton perfoliatus was studied at the Moscow River reach with low flow velocity from May to October 2000. The relative importance of physical-chemical environmental variables and herbivorous pressure in structuring the epiphyton community was accessed using multivariate redundancy analysis. Algal composition was mainly determined by the macrophyte cover, suggestion the overriding importance of the underwater light availability in seasonal development of epiphyton. There was a significant positive relationship between biomass of attached diatom Cocconeis placentula and abundance of chironomid larvae (r = 0.57; p < 0.01). This indicates that increase in herbivorous pressure caused the shift in epiphyton community structure. However, the intensity of herbivorous impact on epiphyton was ultimately determined by light availability in macrophyte stands. At low irradiance level and consequent slow growth of algae the upper layer of epiphyton was seriously destroyed by chironomids that resulted in dominance of C. placentula. As irradiance level and epiphyton productivity increased chironomids were unable to prevent biomass accumulation of the upper layer thus inhibiting the growth of C. placentula. The high concentrations of nutrients and relatively stable discharge determined the crucial role of light regime in the regulation of trophic interactions between epiphyton and chironomid larvae.     

湿地中的藻类生态学研究进展

从湿地中藻类的种群结构、藻类在湿地中的功能、湿地中的藻类生产力及其影响因素等方面综述了天然湿地中的藻类生态学研究进展．湿地植物区系主要有附泥藻类、附植藻类、后周丛藻类和浮游植物4种类型,其中常见的是附泥藻类的硅藻、绿藻和蓝藻．藻类最显著的作用是作为湿地食物网中的初级生产者,也作为湿地环境污染的生物指示物．影响藻类生产力的因素有水力学因素、营养、温度、光、大型植物及草食动物和其它动物．未来对藻类的研究应侧重于湿地藻类生物多样性、藻类生物量、生产力、种群组成的环境控制及其相互关系,以及藻类作为水环境及湿地污染程度指标的研究,“基因治藻”也将是未来研究的新方向．     

西湖湖西浮游与着生藻类季节变化及相互关系

2014年11月至2015年8月调查了西湖湖西沿岸带浮游藻类和不同基质(植物、石块和底泥)上着生藻类的群落结构及季节变化, 分析了着生丝状藻与浮游丝状藻的相互关系以及它们与环境因子的相关性, 探讨湖西生态修复过程中季节性暴发的丝状藻水华的原因。结果表明浮游藻类和植物、底泥、石头上着生藻类中均以硅藻门种类数(分别占52.5%、60.4%、86.7%和72.7%)最多, 蓝藻门(分别占10.1%、8.9%、6.7%和15.2%)和绿藻门(分别占26.3%、19.8%、5.6%和10.6%)次之, 其他门类相对较少, 浮游藻类与着生藻类优势种季节差异较大。附植丝状藻密度显著高于附泥和附石丝状藻, 且狐尾藻上着生丝状藻密度与浮游丝状藻密度呈显著正相关, 表明狐尾藻着生丝状藻可能是浮游丝状藻较重要的来源之一, 该结果可为西湖丝状藻水华的控制提供一些参考。相关性分析表明, 着生藻类和丝状藻与各理化因子(水深、透明度、溶解氧、水温、pH、TN、SRP、TP等)均无显著相关性。     

Periphyton nutrient content,biomass and algal community on artificial substrate: response to experimental nutrient enrichment and the effect of its interruption in a tropical reservoir

Periphyton plays an important functional role in the retention of nutrients in aquatic ecosystems, especially phosphorus. We evaluated the effects of enrichment with N and P and the effect after 20 days of no additional N and P on periphyton on artificial substratum in open-bottom mesocosms. The aim was to jointly evaluate periphyton, phytoplankton and zooplankton in the presence of macrophytes. Experimental conditions simulated natural conditions and nutrient addition was based on the maximum concentration recorded in mesotrophic reservoir. Our hypothesis is that the periphyton is sensitive to the effects of N and P enrichment and its interruption, despite the positive response of phytoplankton and zooplankton. Two treatments were designed using open-bottom mesocosms (n = 3): control (no nutrient addition); NP+ (combined phosphorus and nitrogen addition). Sampling for the measurement of biotic and abiotic variables was performed, with 10 days of continuous enrichment, on the 3rd, 6th and 11th, and 20 days after enrichment had ended (31st day). Periphyton chlorophyll a, dry mass and algal density increased significantly with the addition of N and P and decreased 20 days after the interruption of the enrichment. The highest periphyton P content was found in the NP+ treatment. The enrichment had a positive effect on Chrysophyceae (Chromulina spp.) and rotifer (Polyarthra spp.) density and the interruption of enrichment favored Bacillariophyceae (Gomphonema sp.) and rotifers (Gastropus stylifer). Phytoplankton responded positively to enrichment. Along with the high macrophyte coverage over the experimental period, we evidenced the positive effect enrichment had on phytoplankton biomass and zooplankton abundance. Therefore, periphyton on artificial substrate was sensitive to effects of N and P enrichment and its interruption, responding promptly to changes in nutrient availability in a scenario of high competition and grazing.     

Phytoplankton and Epipelon Responses to Clear and Turbid Phases in a Seepage Lake (Buenos Aires,Argentina)

Annual changes in the algal density and concentrations of chlorophyll a, total phosphorus, and organic matter were analyzed in water and sediments at four sites characterized by the presence or absence of submerged and emergent macrophytes, during turbid‐ and clear‐water conditions to determine the contribution of the algal components of the plankton and the epipelon and to identify the most typical species in each community. Three states were recognized: one turbid and two clear, with different submerged macrophyte cover. The peaks of phytoplankton and epipelon occurred in the turbid phase, whereas the highest proportion of true epipelic algae in sediments was reached in the second clear phase. The Oscillatoriaceae dominated during the turbid phase in the water and throughout the entire year within the sediments. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of nutrients, submerged macrophytes and zooplankton grazing on phytoplankton biomass and diversity along a latitudinal gradient in Europe

In order to evaluate latitudinal differences in the relationship of phytoplankton biomass and diversity with environmental conditions in shallow lakes, we sampled 98 shallow lakes from three European regions: Denmark (DK), Belgium/The Netherlands (BNL) and southern Spain (SP). Phytoplankton biomass increased with total phosphorus (TP) concentrations and decreased with submerged macrophyte cover across the three regions. Generic richness was significantly negatively related to submerged macrophyte cover and related environmental variables. Zooplankton:phytoplankton biomass ratios were positively related to submerged macrophyte cover and negatively to phytoplankton generic richness in DK and BNL, suggesting that the low generic richness in lakes with submerged macrophytes was due to a higher zooplankton grazing pressure in these regions. In SP, phytoplankton generic richness was not influenced by zooplankton grazing pressure but related to conductivity. We observed no relationship between phytoplankton generic richness and TP concentration in any of the three regions. The three regions differed significantly with respect to mean local and regional generic richness, with BNL being more diverse than the other two regions. Our observations suggest that phytoplankton diversity in European shallow lakes is influenced by submerged macrophyte cover indirectly by modulating zooplankton grazing. This influence of submerged macrophytes and zooplankton grazing on phytoplankton diversity decreases from north to south.     

OPINION Manipulating lake community structure: where do we go from here?

SUMMARY. 1 More than 10 years experience with whole lake pelagic manipulation has suggested some general trends applicable to all freshwater pelagic communities and some specific trends related to lake depth.
2 Among the general trends is the observation that the trophic cascade is strongly damped. This means that changes in phytoplankton biomass can be assured only when the fish community is strongly manipulated.
3 Among the depth related trends is the observation that in shallow lakes, changes in fish community structure are more likely to have cascading impacts on phytoplankton than are changes in deep lakes.
4 In shallow lakes, fish removal frequently results in decreased turbidity which is associated with the development of dense macrophyte populations and significant reductions of algal standing stocks. The mechanisms involve: increased grazing by zooplankton, the removal of fish induced bioturbation and nutrient recycling, and direct and indirect macrophyte effects (shading, zooplankton refuges and competition for nutrients).
5 In shallow lakes, where planktivore biomass can be regulated and macrophyte development is acceptable, fish biomanipulalions are likely to result in reduced algal populations and improved water quality.
6 In deep lakes, where macrophytes are not as important, long-term effects of fish manipulations are strongly dependent upon the probability of non-grazable algal bloom development. This is determined by many factors (chemical, physical and grazer related) which modify the impact that grazers have on phytoplankton biomass.
7 In deep lakes, successful fish biomanipulations may only be effective when chemical and physical factors are altered to produce algal species compositions that permit strong top-down control of prey by predators.     

Effects of macrophyte architecture and leaf shape complexity on structural parameters of the epiphytic algal community in a Pampean stream

Habitat heterogeneity is one of the main factors determining distribution of organisms, and vegetation is of primary importance in shaping the structural environment in aquatic systems. The effect of macrophyte complexity on macroinvertebrates has been well researched; however, much remains to be revealed about the influence of complexity on epiphytic algae. Here, we used fractal dimension to study the effect of complexity at two scales, macrophyte architecture and leaf shape, on several parameters of the epiphytic algal community (number of individuals, biomass, taxon richness and diversity) in a Pampean stream. Four submerged macrophyte species with different complexities and associated algae were sampled in late spring, summer and autumn. Important differences were found in fractal dimension of the whole plant and leaves among macrophyte species. The particulate organic matter and chlorophyll a associated positively to leaf fractal dimension, but not to plant fractal dimension, partially supporting the hypothesis of a positive effect of macrophyte complexity on periphyton biomass. No association was found in fractal dimension with algal abundance, taxon richness or diversity. Complementary, a mesocosm experiment was performed with plastic imitations of different plant fractal dimensions. After four weeks, there were differences in chlorophyll a and autotrophy index between treatments that suggested a positive effect of complexity on autotrophic periphyton biomass. These results indicate that the well-known positive effect of macrophyte complexity on macroinvertebrates might be partially explained by a positive effect of complexity on periphyton biomass.     

Are zooplankton food resources poor in the vegetated littoral zone of shallow lakes?

1. The distribution of zooplankton in shallow lakes is negatively related to macrophyte density. However, the abundance of their food along density gradients of macrophytes is unknown. A common but untested assumption is that food quantity and quality for pelagic zooplankton is poor in the littoral zone owing to the deleterious influence of macrophytes on phytoplankton. 2. We tested this assumption with a combination of a field survey and laboratory experiments. We collected seston samples from the littoral and pelagic zones of four shallow temperate lakes and related food quantity (phytoplankton biovolume) and quality to macrophyte abundance (per cent volume infested). Seston food quality was assessed in three ways: N/C and P/C ratios, polyunsaturated fatty acid content and phytoplankton community composition. In the laboratory, we measured the growth and reproduction of Daphnia pulicaria on diets consisting of seston from the littoral and pelagic zones in one lake. 3. In our four study lakes, food quantity was not significantly influenced by macrophyte abundance, and food quality was generally high. Laboratory experiments showed increased juvenile growth, but no significant change in D. pulicaria reproduction, when feeding on littoral resources compared to pelagic resources. 4. Our results suggest that there is no nutritional cost to pelagic zooplankton inhabiting the littoral zone. Therefore, it is likely that other factors (e.g. predation, abiotic factors) are involved in determining zooplankton habitat use.     

Indirect effects of fish community structure on submerged vegetation in shallow,eutrophic lakes: an alternative mechanism

The loss of submerged macrophytes during eutrophication of shallow lakes is a commonly observed phenomenon. The proximate reason for this decline is a reduction of available light due to increasing phytoplankton and/or epiphyton biomass. Here we argue that the ultimate cause for the transition from a macrophyte-dominated state to a phytoplankton-dominated state is a change in fish community structure. A catastrophic disturbance event (e.g. winterkill) acting selectively on piscivores, cascades down food chains, eventually reducing macrophyte growth through shading by epiphyton, an effect that is reinforced by increasing phytoplankton biomass. The transition back from the phytoplankton to the macrophyte state depends on an increase in piscivore standing stock and a reduction of planktivores. A conceptual model of these mechanisms is presented and supported by literature data and preliminary observations from a field experiment.     

Indirect effect of different fish communities on nutrient chlorophyll relationship in shallow hypertrophic water quality reservoirs

Effects of different fish communities on the proportion of different nitrogen and phosphorous forms and the amount of phytoplankton (chlorophyll a) were examined in two consecutive years (1992–1993) in three Hungarian shallow water reservoirs (Cassette and outer reservoir of the Kis–Balaton Water Protection System, and Marcali reservoir). Possible interactions between nutrient concentrations and the amount of phytoplankton in these reservoirs were also examined. Considerable differences in the proportions of different nutrient forms were observed between the three test sites, which could be explained by the presence of different fish stocks in these reservoirs. In the Cassette, the fish biomass necessary for a water quality improvement was around 50 kg ha⁻¹. Phytoplankton biomass was controlled by the zooplankton, consequently chlorophyll a concentrations decreased considerably, while those of dissolved nutrients significantly increased. In the outer reservoir, phytoplankton was controlled bottom-up, since the 250 kg ha⁻¹ fish biomass was larger than the critical value due to the high proportion of planktivorous species. Chlorophyll a concentrations were high, and nutrients were mainly in particulate form (in algal cells). In the Marcali reservoir, the recently introduced silver carp population could not control fully the phytoplankton. The biomass of phytoplankton decreased only slightly, while its composition changed considerably. Although biomanipulation with silver carp is suitable for ceasing cyanobacterial blooms, reduction of the amount of planktivorous fish seems to be a more adequate method for increasing water transparency, rather than introduction of phytoplankton feeding fish.

     

Responses of phytoplankton to fish predation and nutrient loading in shallow lakes: a pan-European mesocosm experiment

1. The impacts of nutrients (phosphorus and nitrogen) and planktivorous fish on phytoplankton composition and biomass were studied in six shallow, macrophyte‐dominated lakes across Europe using mesocosm experiments. 2. Phytoplankton biomass was more influenced by nutrients than by densities of planktivorous fish. Nutrient addition resulted in increased algal biomass at all locations. In some experiments, a decrease was noted at the highest nutrient loadings, corresponding to added concentrations of 1 mg L^?1 P and 10 mg L^?1 N. 3. Chlorophyll a was a more precise parameter to quantify phytoplankton biomass than algal biovolume, with lower within‐treatment variability. 4. Higher densities of planktivorous fish shifted phytoplankton composition toward smaller algae (GALD < 50 μm). High nutrient loadings selected in favour of chlorophytes and cyanobacteria, while biovolumes of diatoms and dinophytes decreased. High temperatures also may increase the contribution of cyanobacteria to total phytoplankton biovolume in shallow lakes.     

A 2-year experimental study on nutrient and predator influences on food web constituents in a shallow lake of north-west Spain

1. A 2‐year study was carried out on the roles of nutrients and fish in determining the plankton communities of a shallow lake in north‐west Spain. Outcomes were different each year depending on the initial conditions, especially of macrophyte biomass. In 1998 estimated initial ‘per cent water volume inhabited’ (PVI) by submerged macrophytes was about 35%. Phytoplankton biomass estimated as chlorophyll a was strongly controlled by fish, whereas effects of nutrient enrichment were not significant. In 1999 estimated PVI was 80%, no fish effect was observed on phytoplankton biomass, but nutrients had significant effects. Water temperatures were higher in 1998 than in 1999. 2. In the 1998 experiment, cladoceran populations were controlled by fish and cyanobacteria were the dominant phytoplankton group. There were no differences between effects of low (4 g fresh mass m^?2) and high (20 g fresh mass m^?2) fish density on total zooplankton biomass, but zooplankton biomass was higher in the absence of fish. With the high plant density in 1999, fish failed to control any group of the zooplankton community. 3. Total biovolume of phytoplankton strongly decreased with increased nutrient concentrations in 1998, although chlorophyll a concentrations did not significantly change. At higher nutrient concentrations, flagellate algae became more abundant with likely growth rates that could have overcompensated cladoceran feeding rates. This change in phytoplankton community composition may have been because of increases in the DIN : SRP ratio. Both chlorophyll a concentration and total phytoplankton biovolume increased significantly with nutrients in the 1999 experiment. 4. A strong decline of submerged macrophytes was observed in both years as nutrients increased, resulting in shading by periphyton. This shading effect could account for the plant decline despite lower water turbidity at the very high nutrient levels in 1998.