Clear-water Period Research Articles

A 15-year Study of the Spatial Distribution of Rutilus rutilus and Perca fluviatilis in Late Summer in Two Shallow Lakes With Contrasting Trophic State

Roach (Rutilus rutilus) and perch (Perca fluviatilis) are dominant species in northern-temperate lakes of Europe, their relative importance depending on trophic state and habitat complexity. We studied the habitat distribution of roach and perch over a 15-year period in two Danish lakes, Lake Væng, undergoing major changes in water clarity and macrophyte coverage, and the permanent turbid Lake Søbygård. We used multi-mesh sized gill nets in 5-6 different sections of the lakes following the same program in all years. Both species were evenly distributed in the sections and among habitats, in Lake Væng during the turbid period and in Lake Søbygård during the whole study period. During the clear-water period in Lake Væng, however, the distribution of roach and perch was uneven and the density negatively correlated with macrophyte coverage and density, but the strength of the relationship differed between the two species and between small (≤ 8 cm for roach and <10 cm for perch) and larger fish. Our results suggest that water clarity and macrophyte density were of key importance in determining roach and perch distribution in these two shallow lakes.

Invasion of Ceratium furcoides in subtropical lakes in Uruguay: Environmental drivers and fish kill record during its bloom

The invasive freshwater dinoflagellate Ceratium furcoides is extending its distribution in South America with increasing environmental impacts associated with its bloom. We here report two events related to C. furcoides distribution expansion in Uruguay: (1) the main environmental drivers (physical and chemical factors, extreme wind events and zooplankton composition) of the first appearance and bloom of C. furcoides in 2012 in a subtropical eutrophic shallow lake (Lake Blanca, Uruguay); and (2) a fish kill event of Prochilodus lineatus observed during a bloom of C. furcoides in 2016 in a deep eutrophic lake (Puente de las Americas, Uruguay). The bloom of C. furcoides in Lake Blanca started in spring 2012 (December) after a clear water period with high phytoplankton species replacement after a cyanobacterial bloom of Raphidiopsis raciborskii. Extreme wind during this period may have initiated the bloom of C. furcoides by enhanced cysts resuspension from the sediments. High nutrient availability and low zooplankton grazing, further allowed C. furcoides to expand and reach over 96% of the phytoplankton biomass. In Lake Puente de las Americas, we registered the fish kill of the large-sized benthic Prochilodus lineatus during a bloom of C. furcoides. This bloom caused the oxygen depletion in the hypolimnion and the gills from these fish exhibited massive accumulations of C. furcoides cells compared to the ones collected in non-bloom conditions. Concurring to other studies our results suggest that the C. furcoides bloom likely caused the fish kill of P. lineatus by asphyxia. Our study is the earliest register of a bloom of C. furcoides in Uruguay (in 2012) and discusses the potential environmental effects of its bloom in subtropical lakes.

Dissolved oxygen deficits in a shallow eutrophic aquatic ecosystem (fishpond) – Sediment oxygen demand and water column respiration alternately drive the oxygen regime

Biological processes tend to dominate the oxygen regime of productive waters. However, in shallow aquatic ecosystems, it is unclear whether the oxygen regime is driven by oxygen production and consumption in the water column or by sediment oxygen demand (SOD). In managed eutrophic ecosystems, this question is especially important in the context of extreme daily oscillations of dissolved oxygen (DO) that could breach physiological limits of heterotrophic aerobic organisms. High-frequency measurement of DO, temperature, global radiation (Gl.Rad.), and pH in a 0.6 m deep, 22 ha eutrophic fishpond Rod (Czech Republic) shows that the oxygen regime depended on the ecosystem state. Over the clearwater period in the early season, the DO level reflected ecosystem heterotrophy with relatively low daily DO oscillations. However, during the summer phytoplankton bloom, the fishpond was primarily autotrophic with extreme DO fluctuation. During late summer, a collapse of the phytoplankton bloom and an associated shift towards heterotrophy and DO deficit frequently occur. In-situ mesocosm experiments in Rod fishpond were conducted throughout 2018 and 2019 growing seasons, to address the importance of SOD to the oxygen regime. We enclosed the water column in transparent and opaque/dark plastic cylinders open or closed to the sediment. The results show that the proportional contribution of SOD to total respiration decreased from 70 to 90% at low phytoplankton biomass (expressed as Chlorophyll-a (Chl-a) concentration) to approximately 10% at phytoplankton bloom. At night, the difference between the oxygen consumption in the cylinders with or without sediment was statistically significant, when the concentration of Chl-a was <100 μg·L−1. On the contrary, the difference was not significant when the concentration of Chl-a was >100 μg·L−1. This revealed that the impact of SOD is negligible at high phytoplankton biomass.

Species-specific schooling behaviour of fish in the freshwater pelagic habitat: an observational study.

Social living of animals is a broadly occurring phenomenon, although poorly studied in freshwater systems, fish schooling behaviour is an excellent example. The composition of fish schools, species-specific schooling tendencies and preferences of adult fish were studied in the pelagic habitat of the Římov Reservoir, Czech Republic. Video recordings captured over a total of 34 days (16 h per day) in the clear water period of three seasons were analysed. From four species identified as school-forming species - bream, bleak, roach and perch, 40% of the individuals observed formed schools of 3-36 individuals. Although conspecific schools prevailed, 20% of individuals formed heterospecific schools, except bleak that schooled strictly with conspecifics. Schools were composed of individuals of similar body size and life strategy. Heterospecific schools were significantly larger than conspecific schools and showed uneven proportion among species, that is, one species being more abundant when the school dimension increased. Probability of encounter in bleak was lowest and proved highest inclination for schooling. Gregarianism levels depended on species morphology and body size, with larger and morphologically advanced fish tending less to sociability. This indicates that the antipredator function of schooling behaviour is intensified with increasing vulnerability of the species.

Vertical distribution of epiphyton biomass and diversity in a shallow lake during contrasting ecosystem regimes

We investigated seasonal and depth variations in biomass and composition of bulrush-associated epiphyton in a shallow seepage lake in Argentina and examined whether differences existed in the vertical profiles between sites, whether the zonation with respect to depth changed according to different regimes of the ecosystem, and what conditions most greatly affected the epiphyton vertical distribution. We performed sample collection every 4 weeks for one year at both the centre and the periphery of the bulrush stand, sectioned the bulrush stems into segments to give a fixed 7-level vertical profile, and recorded the water physicochemical characteristics over the same profile. The epiphyton vertical variations were similar at both sampling locations.The epiphyton-biomass profiles underwent an abrupt change at a depth of 45–65cm during the clear-water period, while comparable values were recorded along the profiles during the turbid phase. The temperature had a positive effect during late spring–early summer that coincided with the transitional period and beginning of the clear period.The principal driver of the epiphyton-biomass vertical distribution is the light availability, followed by the exposure to water movements. The resident species with high contributions to the overall biovolume were related to the percentage of incident light, and particularly those community members that were predominant in the clear-water period.

Effect of environmental variables on epiphyton in a pampean lake with stable turbid- and clear-water states

In the present study, we examined the variation in composition and biomass of epi- phyton growing on the bulrush Schoenoplectus californicus in a pampean lake that passed through a turbid- and a clear-water period within an annual cycle. Monthly sampling was per- formed year-round at the center and periphery of a bulrush stand. We measured the physicochem- ical parameters in the water column using standardized field and laboratory methods. The epiphy- ton was sampled by sectioning bulrush stems into segments for pigment analysis, abundance measurements, and biovolume quantification. Two different types of communities were observed during the annual cycle. In the turbid-water period, epiphyton biomass and diversity remained low despite sufficient levels of phosphates. An abrupt increase in epiphyton biomass, along with structural changes in the community, occurred after an improvement in water transparency, sug- gesting that the epiphyton was strongly dependent on underwater light conditions. Differences in epiphyton composition and biomass between the center and the periphery of the bulrush stand were negligible. We conclude that the availability of colonizing surface is not a relevant driver for the colonization and growth of these epiphyton assemblages. The epiphyton in pampean lakes may remain stable and resilient for relatively long periods, even when confronted with envi - ronmental perturbations, and changes in these communities will only occur in relation to radical alterations in light and nutrient availability.

Cool winters versus mild winters: effects on spring plankton in Lake Peipsi

The influence of two winter periods with a different duration of the ice cover on Lake Peipsi (Estonia) on plankton and nutrient content was analysed. The winters of 2005 and 2006 were cold with ice duration of 140 days, whereas the winters of 2007 and 2008 were mild with about 50 and 15 days of ice duration, respectively. Total phosphorus (TP) concentration was lower, while silicon content and the nitrogen-phosphorus ratio (TN : TP) were markedly higher in the springs after the short winters of 2007 and 2008. The high Si concentration and TN : TP ratio persisted throughout the growing season of those years. Unicellular centric diatoms showed a sharp increase in April 2008, while the large filiform diatom Aulacoseira islandica dominated in the cool winters and after ice break-up. The high spring peak of diatoms was followed by their low biomass in summer. In May 2008, total zooplankton biomass, cladoceran biomass, and rotifer biomass were two times and that of copepods three times as high as in the Mays after the long-lasting ice cover. In the Junes after the mild winters the biomasses of total zooplankton and both crustacean groups (Cladocera, Copepoda) were about two times as high as the corresponding indicators for the Junes after the cool winters. The biomass of rotifers, on the contrary, was two times lower in the Junes after the warm winters because the numerous cold stenotherms Polyarthra dolichoptera and Synchaeta verrucosa had totally disappeared from zooplankton. The influence of ice duration on phyto- plankton is most likely indirect, acting through nutrients, and on zooplankton direct, acting through water temperatures. The springs after warm winters related positively to zooplankters' mean weight, zooplankton-phytoplankton biomass ratio, and the timing of the clear water period.

Initiation of the midsummer decline of Daphnia as related to predation, non-consumptive mortality and recruitment: a balance

The midsummer decline (MSD) of daphnids has been attributed to a range of factors related to either resource availability or predation. However, the relative magnitude and importance of each of these factors remained unknown. Therefore, we quantified simultaneously, but independently, both consumptive mortality due to predation and non-consumptive mortality of Daphnia galeata due to senescence, disease, food limitation as well as limiting physical or chemical factors between May and July 1998 in Bautzen Reservoir, Germany. These losses were balanced by the population gains by recruitment. The predation pressure exerted by age-0 fish, by older zooplanktivorous fish and by carnivorous zooplankton did not completely account for the observed mortality pattern of D. galeata that caused the MSD. Non-consumptive mortality in the field was calculated from recruitment patterns estimated from field samples and from age-specific survival probabilities of Daphnia offspring according to lifetable experiments. The results suggest a high non-consumptive mortality during the clear-water phase which is comparable to the magnitude of consumptive losses and may give a possible explanation for the remaining part of Daphnia mortality. By integrating all data concerning the initial abundance in early May as well as population gains and losses of daphnids, a balance sheet was drawn up for four years of investigation (1995-1998). The differences between the cumulated positive and negative book values corresponded well to the actually observed Daphnia abundance in Bautzen Reservoir at the transition period to an MSD of every year. Generally, the hypothesis is supported that an MSD should be expected if the period with bottom-up limitation during the clear-water phase overlaps strongly with top-down effects whereby in detail the following combination of conditions should be fulfilled: (i) a high biomass of age-0 percids in May/June, (ii) an early start of Daphnia consumption by carnivorous zooplankton, (iii) a quick increase in Daphnia density leading to the dominance of a peak cohort during the clear-water phase and (iv) a long lasting clear-water period characterized by reduced longevity and low recruitment of Daphnia. [KEYWORDS: AGE-0 PERCIDS; ROACH; CARNIVOROUS ZOOPLANKTON; POPULATION BUDGET; DAPHNIA; LONGEVITY]

The role of charophytes in increasing water transparency: a case study of two shallow lakes in Estonia

The hydrochemical regime and the biota in two lakes of Vooremaa landscape protection area, Central Estonia, were studied in 2000–2001 within the frames of the EC project ECOFRAME aimed to work out water quality criteria for shallow lakes in Europe. Lake Prossa is a macrophyte-dominated lake with an area of 33 ha and a mean depth of 2.2 m. Most of the bottom in this lake is covered by a thick mat of charophytes all the year round. Lake Kaiavere located at a distance of 10 km from the former is a much larger (250 ha, mean depth 2.8 m) plankton-dominated lake. Despite the different area and the Chara meadows in Lake Prossa, the nutrient dynamics were very similar in both lakes. The first vernal phytoplankton peak was expressed in reduced Secchi depth in both lakes. After that peak, the water became clear in L. Prossa but remained turbid in Lake Kaiavere. In the Chara-lake the mean individual weight of zooplankton reached its maximum in both years in June. In the year 2000 it coincided with the peak of the zooplankton/phytoplankton ratio and with the minimum of soluble reactive phosphorus both contributing to the initiation of the clear-water period. Towards autumn, the individual mean weight of zooplankton decreased in the Chara-lake and remained smaller than in the plankton-dominated lake. Hence, zooplankton grazing could initiate the clear-water phase in the Chara-lake but other factors were needed for its maintenance. Another parameter that showed a clear difference between these two lakes was the carbonate alkalinity. It was rather stable or even increasing during spring in the plankton-dominated lake, while it decreased by nearly 50% between April and July in the Chara-lake. The calculated saturation level of calcite remained still lower in the Chara-lake showing that the co-precipitation of algae with autochthonous calcite could not explain the observed increase in water transparency above Chara meadows. The reduced sediment resuspension and the possible allelopathic influence of charophytes on phytoplankton remain the main explanations for the maitenance of the extensive clear-water period in the Chara-lake.

Phosphorus uptake by plankton and periphyton in relation to irradiance and phosphate availability in a subtropical lake (Lake Okeechobee, Florida, USA)

Phosphorus (P) uptake by plankton and periphyton was studied at the wholecommunity level, and a conceptual model was developed relating community P dynamics to irradiance and phosphate (PO 4 ) availability. Studies were done in the littoral wetland and pelagic-littoral ecotone regions of a shallow subtropical lake using 32 P-labelled inorganic (DIP) and organic (DOP) phosphorus as tracers, under laboratory conditions simulating ambient temperature, In the pelagic-littoral ecotone, where light attenuation and P availability are highly variable, P dynamics displayed a strong temporal pattern. During a turbid period when underwater irradiance was low, water column PO 4 was high, and community P uptake was very low. During a clear water period when underwater irradiance was higher, water column PO 4 was lower, and there was rapid uptake of both PO 4 and DOP by plankton. In the littoral region underwater irradiance always was high, PO 4 always was low, and there was active yearround uptake of PO 4 and DOP both by plankton and periphyton. We also observed adaptations to P deficiency at the community level, in terms of greater use of DOP as a P source when PO 4 availability declined.

Dissolved and particle-bound geosmin in a mesotrophic lake (lake Zürich): spatial and seasonal distribution and the effect of grazers

Depth profiles of geosmin, of which a dissolved and particle-bound fraction was distinguished, were analyzed monthly in a seasonal study in a stratified mesotrophic prealpine lake (Lake Zürich). Remarkable seasonal and spatial differences in the total amounts and ratios between both fractions were observed. Equally low concentrations of dissolved and particle-bound geosmin were found in the winter season during turnover conditions (maximum total geosmin 3.1 ng l −1). In the clear-water period the dissolved fraction of geosmin increased dramatically (93% of the total) whereas the particle-bound geosmin showed only minor changes. In the autumn and first part of the winter surface films contributed essentially to the particle-bound fraction of geosmin exhibiting maximum concentrations of 21 ng l −1. Possible producers of geosmin in Lake Zürich ( Aphanizomenon, Oscillatoria, Planktothrix) were cultivated and their content of geosmin was determined. Feeding experiments with Daphnia and Simocephalus simulating the phenomenon of the clear-water period clearly demonstrated that grazing can be regarded as a major mechanism for the liberation of geosmin from particles. The mean degradation rate in a microcosm experiment caused by pelagic bacteria was 3.1 ng l −1 d −1; this rate was high enough to explain the concentration differences between consecutive sampling dates.

Differential success of cyclopoid copepods in the pelagic zone of eutrophic lakes

Numerous investigations have shown that some species of cyclopoid copepods invaded the pelagic zone of lakes during the eutrophication, i.e. that eutrophication favours cyclopoids. Cyclops vicinus, for example, is very common in the pelagic zone of eutrophic lakes; Acanthocyclops robustus and Diacyclops bicuspidatus, however, are less successful. A review of population characteristics showed that C. vicinus displays extremely flexible life cycles, that many A. robustus populations contain an extremely low percentage of females relative to males and that in some D. bicuspidatus populations only juveniles invaded the pelagic zone whereas most adults remained benthic. Enclosure experiments revealed that the low percentage of females in A. robustus populations is caused by fish predation. Comparisons of body sizes and clutch sizes suggest that the combination of large egg sacs and low to intermediate female sizes makes A. robustus and probably D. bicuspidatus particularly vulnerable to fish predation so that their success in the pelagic zone of eutrophic lakes is reduced when fish predators are abundant. The present results on C. vicinus suggest that many eutrophic lakes provide conditions that enable this species to overcome the clear water period and which permit development of juveniles and reproduction of adults almost the year round.

Seasonal changes in the biochemistry of lake seston

1. The quantity of seston was measured and the elemental carbon, nitrogen and phosphorus (C, N, P) and biochemical composition (carbohydrate, protein, lipid) of the &lt; 53 μm size fraction in three temperate lakes during one year was analysed. The lakes differed in nutrient concentration and were characterized as oligotrophic, mesotrophic and eutrophic. Linear regression analyses defined associations between seston composition and either lake trophic status, depth or season.2. The concentration of particulate organic seston was greatest during spring and autumn and lowest during the clear water period in early summer. Seasonal patterns in seston elemental and biochemical percentage composition (quality) were observed to be independent of differences in seston quantity.3. Concentrations of seston C, N and P were high in most cases in the spring and autumn and low in summer. Concentrations of P were particularly high during late summer and early autumn in the metalimnion, perhaps because of recovery of P from anaerobic sediments and hypolimnetic waters. Because seston C and N did not increase as markedly as P, C : P and N : P ratios both declined in the autumn. Primary production was thought to be co‐limited by N and P in all three of these lakes; however, the data suggested that N might be more important as a major limiting nutrient in the eutrophic lake as the metalimnion increased in depth in late summer and autumn.4. Concentrations of protein, carbohydrate, polar lipid and triglyceride generally increased with lake type as expected (greatest in the eutrophic lake), but showed no relationship with water depth. As the year progressed, no significant changes were measured in protein and carbohydrate concentrations; however, the concentration of polar lipid decreased and triglyceride increased significantly with time of year.5. The biochemical composition of seston varied during the year and among lakes; for example, in Lake Waynewood the proportion of protein composing the seston (percentage protein by weight) varied from &lt; 10% to &gt; 40%. No statistically significant patterns in the percentage protein or carbohydrate were found. However, the proportion of seston comprised of triglyceride decreased with lake type and increased during the year; whereas the proportion of seston as polar lipid increased with lake type and decreased during the year. Triglyceride comprised most of the lipid. Both protein : lipid and protein : carbohydrate ratios tended to be greatest in summer and lowest in the spring and autumn.6. Relationships between samples and biochemical composition analysed by Canonical Correspondence Analysis (Canoco) indicated similar patterns in seasonal changes in seston biochemistry for the three lakes, with samples separated primarily by vectors for lake type (oligotrophic to eutrophic) and the percentage polar lipid (proportion of total lipid) and secondarily by vectors for date and water depth (epilimnion or metalimnion).7. These seasonal biochemical changes in the seston food base were compared with biochemical changes known to occur in algae grown under N‐or P‐limited conditions in the laboratory, and the resultant quality of this algal food for suspension‐feeding consumers (zooplankton). It was concluded that zooplankton were likely to be physiologically challenged by these distinct seasonal shifts in the quality of lake seston.

Phytoplankton pigments and adenosine triphosphate (ATP) in Lake Peipsi-Pihkva

The material for pigment analysis was collected 1–3 times a year from Lake Peipsi-Pihkva in 1983, 1987, 1988, 1991 and 1992–1995. Concentrations of chlorophyll a, b and c (Chla, Chlb, Chlc), pheopigment (Pheo) and adenosine triphosphate (ATP) were measured biweekly in 1985–1986. The mean of all Chla values was 20.2 mg m−1 (median 13.3 mg m−1) indicating the eutrophic state of the lake. Average Chlb, Chlc, Pheo and carotenoid (Car) contents were 3.7 mg m−3, 4.1 mg m−3, 3.0 mg m−3 and 4.8 mg m−3, respectively. The average Chlb/Chla ratio was 22.9%, Chlc/Chla 23.4%, Pheo/Chla 38%, Car/Chla 37% and ATP/Chla 3%, the medians being 14.3, 13.6, 17.5, 39.4 and 1.9%, respectively. The proportion of Chla in phytoplankton biomass was 0.41%, median 0.32%. There were no significant differences in temperature, oxygen concentration, Chla, and ATP between the surface and bottom water; the lake was polymictic during the vegetation period. The Chla concentration had its first peak in May followed by a decrease in June and July. In late summer Chla increased again achieving its seasonal maximum in late autumn. The ATP concentration was the highest during spring and early summer, decreasing drastically in autumn together with the decline of primary production. ATP/Chla was the highest during the “clear water period” in June and early July, which coincided also with the high proportion of Chla in phytoplankton biomass. The highest Chla occurred in November (average 37.2 mg m−3) when Secchi transparency was the lowest (1.05 m). Concentrations of Chlb, Chlc and carotenoids were the highest in August, that of Pheo in June. Concentrations of Chla and other pigments were the lowest in the northern part of Lake Peipsi (mean 14.7 mg m−3, median 12.5 mg m−3) and the highest in the southern part of Lake Pihkva (mean 47.9 mg m−3, median 16.3 mg m−3). An increase of Chla and decrease of Secchi depth could be noticed in 1983–1988, while in 1988–1994 the tendency was opposite.

A seasonal study of phosphorus deficiency in a eutrophic reservoir

SUMMARY Seston elemental composition (C, N, P) and the orthophosphate uptake rate constant, k(upt), were measured in the epilimnion of the eutrophic Římov Reservoir in the period March‐August 1989. Seasonal series of log C/P, log N/P and log [k(upt)] values were highly correlated (r2 in the range 0.84–0.93), but, based on literature‐derived criteria, k(upt) values indicated stronger P deficiency than did the C/P values. The results showed moderate to strong phosphorus deficiency in summer, moderate deficiency at the end of the spring peak of phytoplankton, and no deficiency early in the spring or during the spring clear‐water period.

Phosphorus Deficiency Diagnostics in the Eutrophic Rímov Reservoir

In order to assess the occurrence and the degree of phosphorus deficiency, the upper layer (0-2 m) of the eutrophic Římov Reservoir (Czechoslovakia) was sampled from March to October 1990 in one- to three-week intervals. Soluble reactive phosphorus and maximum possible orthophosphate (by Rigler's radiobioassay) concentrations, seston elemental stoichiometry (C/P; N/P), orthophosphate turnover time, and phosphorus deficiency index (= ratio of the primary production to the Vmax of the PO4 uptake) were estimated. Early in spring, during the spring clear-water period, and in the autumn, no indications of P-deficiency were found. Strong P-deficiency was detected in summer (June - September), moderate to strong deficiency also during the spring phytoplankton maximum. The concentration of SRP &amp;lt; 5 µg.1−1 indicates phosphorus deficiency in the Říimov reservoir.

Biomanipulation, will it work for your lake? A simple test for the assessment of chances for clear water, following drastic fish-stock reduction in shallow, eutrophic lakes

From the results of biomanipulation case studies in the Netherlands, a simple, test was derived for the assessment of chances for clear water, following fish stock reduction. The test is directed primarily to lake managers of shallow, eutrophic lakes in order to enhance the performance of new biomanipulation projects. A prerequisite for success is that at least 75% of the planktivorous and benthivorous fish are removed from the lake. The chance for a clear-water period after fish reduction is determined by the opportunities for large Daphnia species and by the impact of benthivorous fish and wind on sediment resuspension. A bloom of filamentous Cyanobacteria (> 100 000 ind./ml) and large numbers of mysid shrimp ( Neomysis spp. > 100 ind./m 2) in early spring may prevent the development of Daphnia and consequently a decrease in algal biomass. Submerged macrophytes play a key role in stabilizing the clear-water state. Long-term stability is expected at total phosphorus concentrations < 0.10 mg/1. By proceeding through questionnaires the lake manager can assess the chances for success of biomanipulation.

Effects of planktivorous fish mass mortality on the plankton community of Lake Mendota, Wisconsin: implications for biomanipulation

Massive mortality of planktivorous fish had a dramatic impact on plankton community dynamics of Lake Mendota, Wisconsin, USA. After fish mortality, the larger Daphnia pulicaria replaced the smaller Daphnia galeata mendotae, resulting in greater grazing pressure on phytoplankton. This was accompanied by a much longer spring clear-water period and lower summer phytoplankton biomass compared to years before the fish mortality. Analysis of historical data (from the mid-1970’s) showed that previous fluctuations in planktivorous abundance had similar effects on Daphnia abundance and species composition, and on spring phytoplankton biomass. However, the mid-1970’s fish fluctuations had no detectable effect on summer phytoplankton. Concentrations of phosphorus were much higher in the 1970’s (spring P 80–135 μg 1−1 ’) than in the 1980’s (spring P 19–36 μg 1−1) and it is possible that high P concentrations may reduce trophic cascade effects on summer phytoplankton communities. This suggests that the success of biomanipulation programs may be dependent on lake nutrient status.

Seasonal variations in the trophic structure of phyto-and zooplankton communities in lakes in different trophic states

A number of environmental factors are important in regulating the trophic structure of plankton. Increased nutrient supply results in an explosive development of small algae and herbivorous zooplankton. In the eutrophic lakes (Greifensee, Hallwilersee) several algal community collapses occur each year as a consequence of the grazing habit of the zooplankton, although only the grazing phase in May/June results in a ‘clear water period’. Vertical mixing influences the zooplankton community by reactivating the resting stages of predatory copepods. Predation phases can only be observed in lake Lucerne, where the hypolimnion ist deep and aerobic. Well balanced relationships between the different trophic levels are restricted to the phases with growth limiting conditions.