Restoration by biomanipulation in a small hypertrophic lake: first-year results

Biomanipulation was carried out in order to improve the water quality of the small hypertrophic Lake Zwemlust (1.5 ha; mean depth 1.5 m). In March 1987 the lake was drained to facilitate the elimination of fish. Fish populations were dominated by planktivorous and benthivorous species (total stock c. 1500 kg) and were collected by seine- and electro-fishing. The lake was subsequently re-stocked with 1500 northern pike fingerlings (Esox lucius L.) and a low density of adult rudd (Scardinius erythrophthalmus). The offspring of the rudd served as food for the predator pike. Stacks of Salix twigs, roots of Nuphar lutea and plantlets of Chara globularis were brought in as refuge and spawning grounds for the pike, as well as shelter for the zooplankton.The impact of this biomanipulation on the light penetration, phytoplankton density, macrophytes, zooplankton and fish communities and on nutrient concentrations was monitored from March 1987 onwards. This paper presents the results in the first year after biomanipulation.The abundance of phytoplankton in the first summer (1987) after this biomanipulation was very low, and consequently accompanied by increase of Secchi-disc transparency and drastic decline of chlorophyll a concentration.The submerged vegetation remained scarce, with only 5 % of the bottom covered by macrophytes at the end of the season.Zooplankters became more abundant and there was a shift from rotifers to cladocerans, comprised mainly of Daphnia and Bosmina species, the former including at least 3 species.The offspring of the stocked rudd was present in the lake from the end of August 1987. Only 19% of the stocked pike survived the first year.Bioassays and experiments with zooplankton community grazing showed that the grazing pressure imposed by the zooplankton community was able to keep chlorophyll a concentrations and algal abundance to low levels, even in the presence of very high concentrations of inorganic N and P. The total nutrient level increased after biomanipulation, probably due to increased release from the sediment by bioturbation, the biomass of chironomids being high.At the end of 1987 Lake Zwemlust was still in an unstable stage. A new fish population dominated by piscivores, intended to control the planktivorous and benthivorous fish, and the submerged macrophytes did not yet stabilize.     

The effects ofAnabaena flos-aquae inoculation,pH elevation,and N/P manipulation on the algal biomass and species composition of an acid lake

Three bag experiments were performed in Cheat Lake, West Virginia, an acid lake, to determine whether a blue-green alga,Anabaena, could be introduced coupled with a pH change to neutrality and nutrient addition.     

An assessment of the importance of emergent and floating-leaved macrophytes to trophic status in the Loosdrecht lakes (The Netherlands)

The potential importance of the six major emergent and floating-leaved macrophyte species in recycling of sediment phosphorus in the Loosdrecht lakes was studied. Representative plant samples were collected at the time of maximum biomass, and analysed for biomass and carbon, nitrogen and phosphorus contents. Species cover was determined by aerial photography.Total cover in the seven lakes studied ranged between 2 and 26 percent. For the four main species, biomass per unit area increased with lake trophic status. Consistent differences in C, N and P contents per unit biomass were not observed. Although cover values were small, significant amounts of C, N and P were contained in the macrophytes when compared with maximum sestonic content.Potential P loads from macrophyte decay were calculated. In Lake Loosdrecht, the P load represented 15 percent of current external P inputs. The potential importance of macrophyte decay to P recycling in the other lakes is greater.Decay of macrophyte species at the end of the growing season appears to affect autumnal nutrient and chlorophyll a levels in the water column of some lakes. The re-establishment of submerged species following lake restoration may increase the importance of this pathway in the lakes.     

Zebra mussels (Dreissena polymorpha): a new perspective for water quality management

In the evaluation of the role of lake restoration programmesin situ measurements of the filtration rate of the freshwater musselDreissena polymorpha have been performed in Lake Wolderwijd, The Netherlands. The filtration rate mainly depends on the suspended matter content of the water, and shows an inverse exponential relationship with this factor. The filtration activity is temperature indifferent between approx. 5 and 20 °C. At low temperatures the filtration rate drops abruptly, at high temperatures gradual inhibition occurs. The filtration rate shows a sigmoidal relation with the length of the mussel. The largestD. polymorpha have a diminished filtration rate compared to animals of smaller size. This might be a degenerative feature of the oldest mussels. In Lake Wolderwijd a population density of 675 per m² is required to compensate phytoplankton growth by grazing. Manipulation of the size of the population can be executed by adding suitable substrates for the mussel.     

Predominance of picoplankton and nanoplankton in eutrophic Calder Lake

A study was conducted to examine factors regulating the biomass of algal picoplankton in Calder Lake, a small eutrophic lake in southern New York state. A particular focus was a current paradigm which suggests that larger cells may dominate in nutrient-rich waters, while smaller cells may predominate only in oligotrophic waters. Over two years, phytoplankton biomass consisted predominantly (74% on average) of very small organisms; nanoplankton (<20 to 2 µm: 39%) and picoplankton (<2 µm to 0.2 µm: 35%), despite the presence of surface blooms of colonial cyanobacteria (Microcystis aeruginosa, Anabaena limnetica), and dense metalimnetic populations of the dinoflagellate Ceratium hirundinella. This dimictic system is characterized by relatively high levels of total P (max = 85, % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGabmiEayaara% aaaa!3702!\[\bar x\] = 9.7 µg P/L), inorganic P (max = 26, % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGabmiEayaara% aaaa!3702!\[\bar x\] = 4.5 µg P/L), and total inorganic N (max = 285, % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGabmiEayaara% aaaa!3702!\[\bar x\] = 85 µg P/L), but larger forms were rarely the most abundant. Unlike some marine systems, greater abundance of algal picoplankton was not associated with deeper strata (low light), or warmer temperatures. Data suggest that midsummer nutrient limitation, especially P-limitation, favors the development of pico- and nanoplankton in the limnetic zone of eutrophic lakes.     

The spring development of phytoplankton in Lake Erken: species composition,biomass, primary production and nutrient conditions — a review

Relative abundance and within-lake distributions of three fishes, northern redbelly dace (Phoxinus eos), finescale dace (Phoxinus neogaeus), and central mudminnow (Umbra limi), were examined using minnow traps in Tuesday Lake, a small bog lake in the Upper Peninsula, Michigan. For these species, catches in minnow traps placed at the perimeter of the lake were 21 to 52 times higher than catches in midlake traps. Variance: mean ratios of perimeter trap catches indicated that both dace species were highly aggregated while the distribution of mudminnows was less aggregated or random. Over an 11 day period during which all fish caught were removed from the lake, catch per unit effort (CPUE) of both dace species declined in response to fish removal. In contrast, CPUE for mudminnows was low initially, increased to an asymptote and then declined only in the last 5 days of the fish removal. The patterns of CPUE for mudminnows indicated that mudminnow trapability and/or activity was reduced in the presence of high densities of dace. The low abundance of dace in traps with many mudminnows suggested mudminnows avoided traps already containing dace. Throughout the removal period, CPUE provided an accurate index of dace abundance, whereas this was true for mudmnnows only after dace populations had been reduced drastically. Therefore, in any use of minnow traps to estimate populations, both spatial distributions and relative species abundance of small fishes must be taken into account.     

Impact of whitefish on an enclosure ecosystem in a shallow eutrophic lake: selective feeding of fish and predation effects on the zooplankton communities

Bag-type enclosures (75 m³) with bottom sheets and tube-type enclosures (105 m³) open to the bottom sediment were stocked with exotic whitefish (Coregonus lavaretus maraena) to study their predation effects on the plankton community. The fish fed mainly on adult chironomids during the period of their emergence (earlier part of the experimental period). Thereafter, the food preference was shifted to larvae of chironomids and crustacean zooplankters. The predation effects on the plankton community were not evident in the bag-type enclosures where zooplankton densities were consistently low. The fish reduced the crustacean populations composed ofBosmina fatalis, B. longirostris andCyclops vicinus in the tube-type enclosures where the prey density was high (above ca. 50 individuals 1⁻¹). The results suggested that the intensity of predation depended on the prey density. Rotifers increased in the fish enclosure, probably becauseCoregonus reduced the predation pressure byCyclops vicinus on rotifers and allowed the latter to increase. In the fish enclosures, no marked changes in species composition were observed. Zooplankton predated by the fish seemed to be distributed near the walls of the enclosures. Problems of enclosure experiments for examining the effects of fish predation on pelagic zooplankton communities are discussed.     

Hydrogeochemistry of Lake Gallocanta (Aragón,NE Spain)

Lake Gallocanta has undergone drastic changes during the last thirteen years. Water level changed from a high level (Z_max = 2 m) to total dryness in 1985. From 1986 to 1988 slow refilling occurred. The water volume fluctuations have been studied in relation to climatic variations recorded for that period. Variations in the major dissolved ions were related to water volume fluctuations from data at two different stages, one corresponding to the drying phase and another to the refilling phase. Mineralogical composition of the salts precipitated at different stages was examined by X-ray diffraction. Interstitial water and mineralogical composition of recent sediments were also studied along a transect through the lake.The water column decrease from 1977 to 1985 is related to decreasing annual rainfall (500-250 mm respectively). The refilling in 1986–1988 is due to high annual rainfall (537 mm). In addition to these fluctuations, seasonal changes of the water level between 20 and 50 cm occurred every year.Gallocanta is a Na-Mg-Cl-(SO₄) type lake. During the drying period a typical salt enrichment occurs with linear relationships between TDS, Cl, Na and K. Alkalinity is linearly correlated with Ca at relatively low salinities. As salinity increases a linear relationship between Ca and SO₄ is observed. Minerals formed from the brine are halite, bischofite, epsomite, hexahydrite, mirabilite, gypsum, aragonite, calcite and dolomite. The molar ratio Mg/Ca of the interstitial water changes from 1.5 along the shorelines, where calcite and aragonite precipitate, to 40 in the center of the lake. Sediment cores from the central part of the lake show aragonite in the top layers, magnesian calcite and low proportions of quartz and illite, while at 20 cm depth a high proportion of gypsum is present. In contrast, cores from the shore of the lake are mainly composed of low magnesium calcite in the top layers and low magnesium calcite together high magnesium calcite and dolomite between 30 and 70 cm depth. Gypsum deposits only occur in significant proportions at 80–100 cm depth.The refilling process showed relationships between volume and salt concentration following the Langbein model. The salt mass in solution decreased about 50% from the drying to the refilling phase. However, Mg content decreased about 70 % for the same period, suggesting a contribution of this element to the dolomite formation.     

The effects of planktivorous fishes on the plankton community in a eutrophic lake

Stocking silver carp, a phytoplankton feeder, and bighead carp, an omnivorous plankton feeder, into an eutrophic lake at high densities caused a dramatic change in the lake ecosystem.Microcystis, which had been dominant in summer and a main food of the fishes decreased markedly, and green algae smaller than 10 μm then became dominant. Consequently, chlorophyll-a per unit area decreased slightly, while the rate of production was higher than that in the previous years. As the total density of the fishes increased (from 0.09 to 0.11 fish m⁻²), the growth of silver carp was retarded, while that of bighead carp increased.Microcystis, was unable to become dominant due to increased grazing pressure by the fishes, and small green algae became dominant. The lake conditions thus became more favourable for zooplankters which selectively consumed small green algae, and accordingly, the production of zooplankton rose. Bighead carp consumed more animal food, which they assimilate at a higher rate than plant food, and grew better in spite of the fact that the fish density increased. The feeding rate of silver carp was greatly reduced because the green algae were too small to ingest, and the fish therefore grew poorly. Results of a computer simulation of a model consisting of five compartments, representing the blue-green algae, green algae, zooplankton, silver and bighead carp, support the food-web change observed in the lake.     

Seasonal coupling between phyto- and bacterioplankton in a sand pit lake (Créteil lake,France)

Phyto- and bacterioplankton biomass and activity were simultaneously measured during the course of one year in the shallow Créteil Lake (France).Phytoplankton was dominated, during the whole year, by small-sized organisms (10 to 25 µm). Bacteria were in a majority small coccoids (<0.3 µm). Phyto -and bacterioplankton abundances averaged respectively 3.3 × 10⁶ cells l^–1 and 6 × 10⁹ cells l^–1.The phasing of the activity and biomass periods suggest a close coupling between phyto- and bacterioplankton. There were two distinct periods of high activity and biomass. Maximal values were observed in summer but an early increase occurred also in winter. Low or undetectable phytoplankton excretion rates, when heterotrophic activity was maximum, indicated a bacterial uptake of up to 100% of the released algal products during the incubation period. Heterotrophic uptake measurements with both glucose and amino acids revealed a seasonal change of the substrates in the lake, glucose uptake being associated more with the maximum activity of the algae, while the amino acids uptake was relatively higher during their decline.The maximal photosynthetic rate averaged 21.5 mgC m^–3 h^–1 and mean Vmax values were 0.056 and 0.050 mgC m^–3 h^–1 respectively for glucose and amino acids uptake.