Geographical and genetic distances among zooplankton populations in a set of interconnected ponds: a plea for using GIS modelling of the effective geographical distance

In systems of interconnected ponds or lakes, the dispersal of zooplankton may be mediated by the active population component, with rivulets and overflows functioning as dispersal pathways. Using a landscape-based approach, we modelled the effective geographical distance among a set of interconnected ponds (De Maten, Genk, Belgium) in a Geographic Information System (GIS) environment. The first model (the Landscape Model; LM) corrects for the presence of direct connections among ponds and was based on the existing landscape structure (i.e. network of connecting elements among ponds, travelling distance and direction of the current). A second model (the Flow Rate Model; FRM) also incorporated field data on flow rates in the connecting elements as the driving force for the passive dispersal of the active zooplankton population component. Finally, the third model (the Dispersal Rate Model; DRM) incorporated field data on zooplankton dispersal rates. An analysis of the pattern of genetic differentiation among Daphnia ambigua populations inhabiting 10 ponds in the pond complex reveals that the effective geographical distance as modelled by the flow rate and the dispersal rate model provide a better approximation of the true rates of genetic exchange among populations than mere Euclidean geographical distances or the landscape model that takes into account solely the presence of physical connections.     

Effects of fertiliser and crayfish on plankton and nutrient dynamics in hardwater ponds

Although phosphorus fertilisation can improve productivity in most freshwater ponds, phosphate may become limiting in extremely hard water due to rapid precipitation with calcium. Hence we studied the characteristics of plankton and nutrient dynamics in water containing >400 mg CaCO₃ l^–1in pond and microcosm systems. The field experiment was conducted in eight earthen ponds involving two nutrient ratios (N:P = 1:1 and 20:1) with or without crayfish. Fertilisation significantly increased concentrations of NO₂–N and NO₃–N, but soluble reactive phosphorus was depleted to the level prior to fertilisation within 24 h. The laboratory test showed that after 6 h of fertilisation, 45% phosphorus was precipitated by calcium, 30% phosphorus was assimilated by phytoplankton and only 25% phosphorus remained in water column. The phytoplankton abundance in hardwater ponds was regulated by the abundance of zooplankton population rather than by either crayfish or fertilisation. The presence of crayfish only increased the concentration of total phosphorus. This study suggests that when phytoplankton production is required in crayfish ponds the maintenance of phytoplankton abundance will depend on the effective control of zooplankton rather than fertilisation. Due to the rapid precipitation of phosphorus by calcium in hard water ponds, more frequent phosphorus fertilisation is needed to enhance primary productivity.     

Effects of lindane and deltamethrin on zooplankton communities of experimental ponds

Two insecticides, lindane (321 µg l^–1) and deltamethrin (13 µg l^–1) were employed in a four mesocosm experiment (two ponds of 10 m³ and two of 16 m³) to asses the impact of water pollution by pesticides. Resistance of the different zooplankton species was variable and depended upon both the group and the insecticide concentration. No effect of lindane was observed on macrozooplancton such as Cladocera and Copepoda. In the deltamethrin-treated pond, all species of zooplankton were found dead a day after the treatment. The microzooplankton (Rotifera and copepod nauplii) were highly susceptible to both insecticides. Although the larvae of Chaoborus were present in the ponds after the treatments, their density decreased (less than 1 individual l^–1). The elimination of filter-feeding zooplankton by deltamethrin was followed by an increase of the concentration of chlorophyll a in the post-treatment period. Two months later the original zooplankton population recovered, with the addition of a new and dominant species: Ceriodaphnia reticulata.     

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.     

Zooplankton as a compound mineralising and synthesizing system: phosphorus excretion

Data on phosphate excretion rates of zooplankton are based on measurements using the pelagic crustacean zooplankton of Lake Vechten and laboratory-cultured Daphnia galeata. In case of Daphnia sp we measured the effects of feeding on P-rich algae and P-poor algae (Scenedesmus) as food on the P-excretion rates at 20°C. The excretion rates of the natural zooplankton community, irrespective of the influence of the factors mentioned, varied by an order of magnitude: 0.025–0.275µg PO₄-Pmg^–1C in zooplankton (C _zp ) h^–1. The temperature accounted for about half the observed variation in excretion rates. The mean excretion rates in the lake, computed for 20°C, varied between 0.141 and 0.260 µg Pmg^–1C _zp h^–1. Based on data of zooplankton biomass in the lake the P-regeneration rates by zooplankton covered between 22 and 239% of the P-demand of phytoplankton during the different months of the study period.In D. galeata, whereas the C/P ratios of the Scenedesmus used as food differed by a factor 5 in the experiments, the excretion rates differed by factor 3 only. Despite the higher P-excretion rates (0.258± 0.022 µg PO₄-P mg^–1 C h^–1) of the daphnids fed with P-rich food than those fed with P-poor food (0.105 ± 0.047 µg PO₄-P mg^–1 C h^p–1), both the categories of the animals were apparently conserving P. A survey of the literature on zooplankton excretion shows that in Daphnia the excretion rates vary by a factor 30, irrespective of the species and size of animals and method of estimation and temperature used.About two-thirds of this variation can be explained by size and temperature. A major problem of comparability of studies on P-regeneration by zooplankton relates to the existing techniques of P determination, which necessitates concentrating the animals several times above the in situ concentration (crowding) and prolonged experimental duration (starving), both of which manifest in marked changes that probably lead to underestimation of the real rates.     

Relative importance of the trophic and direct pathways on PCB contamination in the rotifer species Brachionus calyciflorus (Pallas)

To determine the contribution of food ingestion (trophic pathway) to PCB contamination of zooplankton in the river Meuse (Belgium), we used ¹⁴C-labelled algae (Dictyosphaerium ehrenbergianum) to measure ingestion and assimilation rates in the rotifer species Brachionus calyciflorus. When the concentration of algae in the culture medium varied from 20 10³ to 200 10³ algal cells ml^–1 (0.12 to 1.18 mg Cl^–1), the Brachionus calyciflorus ingestion rate varied from 0.25 ± 0.12 to 1.52 ± 0.43 ng C ind^–1 h^–1 at 15 °C and from 0.74 ± 0.17 to 5.93 ± 0.61 ng C ind^–1 h^–1 at 20 °C. The assimilation efficiency (ratio of the assimilation rate to the ingestion rate) measured in a culture medium containing 200 10³ algal cells ml^–1 was 55.7 ± 5.8%. Since the PCB concentration measured in the phytoplankton of the river Meuse is about 3 µg PCBs g^–1 D.W., the estimated PCB contamination of zooplankton ascribable to the trophic pathway ranges from 0.22 ± 0.17 to 1.31 ± 0.77 µg PCBs g^–1 D.W. at 15 °C and from 0.64 ± 0.34 to 5.10 ± 2.10 µg PCBs g^–1 D.W. at 20°C. The lower figure based on measurements effected at 20 °C is comparable to the actual level measured in zooplankton samples collected in the river Meuse (0.69 ± 0.20 µg PCBs g^–1 D.W.). The applicability of the formula used in our estimate was checked in a 48-hour in vitro experiment in which the rotifers were fed contaminated algae. The PCB accumulation measured in the rotifers was found to coincide with the calculated PCB contamination. Additional experiments were carried out to determine the contribution of the direct pathway to PCB contamination of zooplankton living in the river Meuse (0.02 µg PCBs l^–1 of water; average dissolved organic matter: 3 mg C 1^–1). The PCB concentration in zooplankton resulting from direct uptake of PCBs from the water was estimated at 0.19 ± 0.05 µg PCBs g^–1 D.W. These results show that in zooplankton living in polluted ecosystems, PCBs are likely to accumulate via the trophic pathway to concentrations up to 30 times higher than by direct contamination. Furthermore, our estimates of PCB contamination via the trophic pathway coincide quite well with actual concentrations measured in situ.     

An approach for the assessment of risk from chronic radiation to populations of phytoplankton and zooplankton

A conceptual model of the effects of chronic radiation on a population of phytoplankton and zooplankton in an oceanic nutrient layer is presented. The model shows that there are distinct threshold dose rates at which the different plankton populations become unsustainable. These are 10,400 μGy h⁻¹ for phytoplankton and 125 μGy h⁻¹ for zooplankton. Both these values are considerably greater than the current screening values for protection of 10 μGy h⁻¹. The model highlights the effects of predator–prey dynamics in predicting that when the zooplankton is affected by the radiation dose, the phytoplankton population can increase. In addition, the model was altered to replicate the dose rates to the plankton of a previous ERICA Irish Sea assessment (24 μGy h⁻¹ for zooplankton and 430 μGy h⁻¹ to phytoplankton). The results showed only a 10% decrease in the zooplankton population and a 15% increase in the phytoplankton population. Therefore, at this level of dose, the model predicts that although the dose rate exceeds the guideline value, populations are not significantly affected. This result highlights the limitations of a single screening value for different groups of organisms.     

Characteristics of plankton communities in Chinese integrated fish ponds: effects of excessive grazing by planktivorous carps on plankton communities

Biomass and production of plankton communities were investigated in two Chinese integrated fish culture ponds in August, Dianshanhu Pond (with high density of planktivorous carp) and Pingwang Pond (with low density of planktivorous carp). The plankton communities were composed of rotifers, protozoans, phytoplankton (<40 µm) and bacteria. The large phytoplankton (>40 µm), cladocerans and copepods were rare because of grazing pressure by the carp. The density or biomass of bacteria (1.93 × 10⁷ and 2.20 × 10⁷ cells ml^–1 on average in Dianshanhu and Pingwang Ponds, respectively), picophytoplankton (24.6 and 18.5 mg m^–3 Chla on average) and rotifers (5372 and 20733 ind. 1^–1 on average) exceeded the maximum values reported for natural waters.The average [³H]thymidine uptake rates were 694 and 904 pmoles 1^–1 h^–1 (13.4 and 20.6 µgC 1^–1) and the bacterial production by the >2 µm fraction amounted 21–28% of total [³H] thymidine uptake rate in both ponds. The mean chlorophylla concentrations were 59.1 and 183 mg m^–3 in Dianshanhu and Pingwang Ponds, respectively. 82.4% and 65.3% of the total Chla was contributed by the <10 µm nano- and picophytoplankton in each pond, respectively. In particular, the picophytoplankton contribution amounted 41.2% of thtal Chla in Dianshanhu Pond. Primary production was 2.5 and 3.4 gC m^–2 d^–1 in each pond, respectively, and >50% of production was contributed by picophytoplankton. The mean biomasses of protozoa were 168 µg 1^–1 and 445 µg 1^–1 and those of rotifers were 763 µg 1^–1 and 1186 µg 1^–1 in Dianshanhu and Pingwang Ponds, respectively. The ecological efficiencies expressed in terms of the ratios of primary production to zooplankton production were 0.22 and 0.31, for the two ponds.     

Effect of the dilution rate on the biomass yield ofBacillus thuringiensis and determination of its rate coefficients under steady-state conditions

Depending on the biomass yield on glucose and the cell morphology ofBacillus thuringiensis, three different metabolic states were observed in continuous culture. At dilution rates between 0.18 h^–1 and 0.31 h^–1 vegetative cells, sporulating bacteria and spores coexisted, while glucose and amino acids were consumed. Only vegetative cells were observed at dilution rates between 0.42 h^–1 and 0.47 h^–1 and glucose was used as the main carbon and energy source. AtD = 0.50 h^–1 the biomass yield on glucose decreases sharply. To define better the specific growth rate range in which the microorganism uses mainly glucose, a dilution rate of 0.25–0.45 h^–1 was studied. The experimental data could be adjusted to a Monod model and the following rate coefficients and growth yields were determined: maximum specific growth rate 0.54 h^–1, saturation constant 0.56 mg glucose ml^–1, biomass growth yields 0.43 g cells (g glucose)^–1, and 0.76 g cells (g oxygen)^–1, and maintenance coefficients 0.065 g glucose (g cells)^–1 h^–1 and 0.039 g oxygen (g cells)^–1 h^–1.     

Influence of physiological conditions on EDTA degradation

Aerobic biodegradation of the chelating agent EDTA by a mixed bacterial culture was investigated. Bacterial growth and degradation of the substrate required the presence of sufficient metal ions in the culture fluid. Uncomplexed EDTA interacted negatively with the cell walls of the bacteria and completely inhibited bacterial growth, whereas Mg(II)/Ca(II)-EDTA was degraded up to an initial concentration of 4.7 g/l. Therefore, concentrations of metal ions must be stoichiometric to that of EDTA or higher. Specific degradation rates ranged between 120 mg EDTA g^–1 (cell dry weight) h^–1 and 285 mg EDTA g^–1 h^–1. In contrast, complexes with high thermodynamic stability constants such as Fe(III)-EDTA remained as inert compounds in the solution. Specific growth rates of the mixed culture were found to vary between 0.03 h^–1 and 0.07 h^–1, which could be explained by population dynamics within the synergistic mixed community. Growth was significantly accelerated by the addition of vitamins.     

Primary productivity and related parameters in three different types of inland waters in Sri Lanka

Gross and net primary production together with chlorophyll-a biomass were investigated with respect to depth and diurnal changes in three categories of inland waters (reservoirs, temporary ponds, brackish water lagoons) in Sri Lanka. Ten field sites, in both the dry and wet zones of the island, were investigated. Bimodal productivity profiles were recorded in two of the three reservoirs studied. The diel pattern of net photosynthetic rate varied between sites although peak photosynthetic efficiency occurred at solar noon. Surface photoinhibition was characteristic of the reservoirs and brackish water lagoons but not of the temporary ponds. Mean gross primary production was 3.02 g C m^–2 d^–1 but was higher in the temporary ponds than in the reservoirs. The gross primary production in the brackish water Koggala Lagoon at 0.08 g C m^–2 d^–1 is a record low for tropical lagoons and was 2.5 times less than the two other lagoons investigated. Variability in net primary production between sites was similar to the variation in gross production with a relatively low mean value for tropical inland waters of 0.495 C m^–2 d^–1. Mean maximum photosynthetic rate was 0.30 mg C m^–3 h^–1 but was lower in the reservoirs than in the temporary ponds and lagoons.     

The effect of fertilization regime on juvenile walleye growth and prey utilization in rearing ponds

Synopsis The effect of two contrasting fertilization regimes on juvenile walleye growth, survival and harvest was tested in six identical rearing ponds treated with fermented soybean meal at either a constant (36 g m^–3week^–1) or a progressively reduced (32 to 0 g m^–3week^–1) rate. Walleye length, percent survival and biomass harvest in constant fertilization ponds were 32, 83 and 294% greater, respectively, than those of reduced fertilization ponds. Chironomid larvae and pupae were the dominant prey (in terms of biomass) in juvenile walleye larger than 22 mm TL. Mean chironomid biomass was significantly higher in the constant fertilization ponds (5.1 vs. 1.7 g dry wt m^–2), particularly after peak emergence around week 4. Zooplankton were less important prey after week 2, and mean zooplankton density was not significantly different between treatments. From these data we conclude that better walleye performance in the constant fertilization ponds was due to higher chironomid density during the last half of the experiment. Our findings are reviewed in light of current knowledge of juvenile walleye feeding ecology and contemporary pond culture procedures.     

The aerobic mineralization of organic compounds in the saline Lake Grevelingen (The Netherlands) (VI)

Summary During a one year period the uptake of aspartic acid and of a mixture of amino acids was determined using¹⁴C-labeled substrates as described by WRIGHT and HOBBIE (1966). By this technique the activity is analyzed of that part of the bacterial population which is able to utilize the added substrate. For comparison purposes the activity of the total heterotrophic bacterial population was determined by measurement of the oxygen consumption rate. From the oxygen consumption rate (mg O₂.l^–1.h^–1) the carbon mineralization rate (mg C.l^–1.h^–1) was calculated by applying a conversion factor of 0.29.Aspartic acid was respired for 80% and the amino acid mixture for 43%. From the maximum uptake rates, the potential yearly uptake of the substrate in question can be calculated. These data indicate the relative importance of the several subpopulations in the carbon mineralization process as a whole. The highest value of the potential yearly uptake was obtained for the amino acid mixture; the comparable value for the uptake of aspartic acid was slightly lower.The carbon mineralization rate as calculated from the oxygen uptake experiments was about 150–200 g C.m^–2.year^–1. The potential yearly uptake as determined with the¹⁴C-labeled amino acid mixture was only 2.8% of the amount of mineralized carbon, as calculated from the oxygen uptake experiments. This percentage is very low in view of the fact that 35–55% of the organic carbon of living phytoplankton and zooplankton consists of protein (HAGMEIER, 1961) and that the aerobic mineralization of amino acids is a very common property among the heterotrophic bacterial population (SEPERS, 1979). The value of the applied activity measurements was investigated in order to obtain information about the relation between the uptake process as measured with¹⁴C-labeled substrates and the activity of the bacterial population in situ. The results of this study have been published bij SEPERS and VAN ES (1979).     

A laboratory study of phosphorus and nitrogen excretion of Euchlanis dilatata lucksiana

Phosphorus (PO₄-P) and nitrogen (NH₄-N) excretion rates of Euchlanis dilatata lucksiana, a rotifer, isolated from Lake Loosdrecht (The Netherlands) and cultured in the lake water at 18–19 °C, were measured in the laboratory.In a series of experiments, the effects of experiment duration on the P and N excretion rates were examined. The rates measured in the first half-hour were about 2 times higher for P and 2–4 times for N than the rates in the subsequent three successive hours which were quite comparable.Eight experiments were carried out in triplicate, 4 each for P and N excretion measurements, using animals of two size ranges: 60–125 µm and > 125 µm. The specific excretion rates varied from 0.06 to 0.18 µg P.mg^–1 DW.h^–1 and 0.21 to 0.76 µg N.mg^–1 DW.h^–1. Generally an inverse relationship was observed between the specific excretion rates and the mean individual weight. The excretion rates of Euchlanis measured by us are lower than those reported for several other rotifer species, most of which are much smaller than Euchlanis.Extrapolating the excretion rates of Euchlanis to the other rotifer species in Lake Loosdrecht, and accounting for their density, size and temperature, rotifer excretion appears to be a significant, potential nutrient (N,P) source for phytoplankton growth in the lake. The excretion rates for the rotifers appear to be about two thirds of the total zooplankton excretion, even though the computed rotifer mean biomass is about one-third of the total zooplankton biomass.     

Lactic acid productivity of a continuous culture of Lactobacillus delbreuckii

Summary Maximum volumetric productivities of biomass (1.40 gl^–1h^–1) and lactic acid (8.93 gl^–1h^–1) for a continuous culture ofLactobacillus delbreuckii occurred between dilution rates 0.35h^–1 and 0.40h^–1. All major nutrients were in excess in these cultures. Glucose utilisation was complete at dilution rates of 0.1h^–1 and lower. Product and biomass yields were constant in the dilution rate range studied (0.05h^–1 to 0.50h^–1).     

Overturn in a hypertrophic,warm, monomictic impoundment (Hartbeespoort Dam,South Africa)

A major decrease in air temperature with an increase in wind stress created thermal instability resulting in overturn. Surface oxygen values decreased from 12.6 (170% saturation) prior to overturn to 0.4 mg · l^–1 (5.7%) after overturn. Recovery of surface oxygen concentrations to supersaturation took approximately three months. Changes in the nitrogen balance after overturn indicated that extremely high rates of nitrification occurred. Little change in phosphorus concentration was observed. Surface chlorophyll concentrations decreased from 235 to approximately 2 mg · m^–3, resulting in very low primary production values (A_max = 12.4 mg C m^–3 h^–1). Reduced rates of primary production were maintained for several months after overturn, mainly owing to the low ratio of Z_eu/Z_m (0.17). No significant changes occurred in the total bacterial population other than a redistribution within the water column. Significant changes occurred in the distribution and density of the zooplankton population. No fish kills were observed although fish populations were oxygen-stressed.     

From triphenyltins to integrated management of the `pest' snail Cerithidea cingulata in mangrove-derived milkfish ponds in the Philippines

The potamidid snail Cerithidea cingulata is considered a pest in brackishwater milkfish ponds in the Philippines and has been controlled by the triphenyltin (TPT) compounds Aquatin and Brestan. But TPT is also toxic to other invertebrates, fishes, algae, bacteria and people, and high TPT residues occur in sea foods including milkfish. Thus, control of snails in milkfish ponds should be shifted from reliance on TPT to an integrated pest management (IPM) strategy. To formulate a responsible IPM, studies were conducted on C. cingulata in ponds and mangroves and the available data were synthesized with the relevant information from the literature. The deposit-feeding C. cingulata is a native resident of mangrove areas and becomes a problem in mangrove-derived ponds where the lack of competitors and predators results in 'ecological release' and population explosion. Snail densities ranged 1–470 m^–2 in the mangroves and 100–5000 m^–2 in ponds. In ponds, snails ranged 2–40 mm in shell length; those 25 mm long and 8 mm wide weighed 1 g on average, and had 150 mg flesh. Snails matured at 20 mm long and reproduced the whole year with a peak in Mar–Sep at water temperatures of 24–36 °C. Enriched sediments and stagnant water in ponds allowed fast growth and reproduction, low dispersal and high recruitment of snails. Snails were very tolerant to hypoxia and adverse conditions, but were killed within a week by sun-drying or by salinities of 48–70 and within 3 d by ammonium phosphate at 10 g l^–1 or urea at 5 g l^–1. IPM of snails requires changes in mind sets and perspectives of milkfish farmers and industry supporters and changes in farm practices and management. Snails must be viewed as a resource from which income can be made and employment can be generated. Harvest of snails for shellcraft and other enterprises also effectively removes the spawning population. Complete draining and sun-drying of ponds after harvest kills the adult snails and the egg strings on the bottom. Snails in puddles in the ponds may be killed by the usual nitrogen fertilizers and lime applied during pond preparation. Water input may be timed with periods of low veliger counts in the supply water. These IPM recommendations have yet to be verified.     

Degradation of sodium anthraquinone sulphonate by free and immobilized bacterial cultures

Aerobic biodegradation of a xenobiotic recalcitrant compound sodium anthraquinone-2-sulphonate (SAS), was investigated using as an inoculum a mixed microbial culture, which was activated sludge from industrial and domestic waste-water treatment plants. The difference in SAS degradation was examined using two main systems: (1) suspended cells and (2) immobilized cells, both in batch and in continuous culture. In the suspended cell system, under continuous culture conditions using SAS as a unique source of carbon and energy, it was possible to degrade about 95% of this substrate after 6 days. Maximal SAS removal rates in the suspended-cell system were 593 mg SAS l^–1 h^–1 and 88.7 mg SAS l^–1 h^–1 for dilution rates (D) of 0.05 h^–1 and 0.075 h^–1, respectively. In the immobilized-cell system, almost all SAS was degraded in 6 days and the maximal removal rate reached 88.7 mg SAS l^–1 h^–1 at D=0.05 h^–1. Application of a continuous-flow enrichment procedure resulted in selection of several kinds of micro-organisms and led to a progressive elimination of some species of Aeromonas. A stable microbial community of 11 strains has been established and characterized at D=0.075 h^–1. Most of them were Gram-negative and belonged to the genus Pseudomonas.     

Phosphorus and nitrogen excretion rates of zooplankton from the eutrophic Loosdrecht lakes,with notes on other P sources for phytoplankton requirements

Phosphorus and nitrogen excretion rates by zooplankton communities from two eutrophic and shallow Dutch lakes were measured in laboratory. The variations in excretion rates in the lakes (May–October) were caused mainly by fluctuation in zooplankton biomass. Mean summer excretion rates (June–September) were 2.4 and 0.9 µg PO₄P·1^–1·d^–1 in Lake Loosdercht and Lake Breukeleveen, respectively. This difference between the lakes was caused mainly by the lower zooplankton biomass in Lake Breukeleveen. The excretion of 2.4 µg PO₄P·1^–1·d^– compared with the calculated P-demand of phytoplankton of 8.0 µg PO₄P·1^–1·d^–1 is substantial in the summer (June–September) and far more important than the external P-supply of 0.4 µg P·1^–1·d^–1 and sediment release of 0.5 µg P·1^–1·d^–1. Both temperature and composition of zooplankton affected the weight specific excretion rates of the zooplankton community. The weight specific community excretion rates of P and N increased with temperature (exponential model); 1–8 g PO₄P·mg^–1 zooplankton-C·d^–1 and 5–42 µg NH₃N·mg^–1 zooplankton-C·d^–1 (10°C–20°C).     

Impact of cyprinids on zooplankton and algae in ten drainable ponds

To study the impact of cyprinids on algae, zooplankton and physical and chemical water quality, ten drainable ponds of 0.1 ha (depth 1.3 m) were each divided into two equal parts. One half of each pond was stocked with 0 + cyprinids (bream, carp and roach of 10–15 mm), the other was free of fish. The average biomass of the 0 + fish at draining of the ponds was 466 kg ha^–1, to which carp contributed about 80%.The fish and non-fish compartments showed significant differences. In the non-fish compartments the density of Daphnia hyalina was 10–30 ind. l^–1 and that of Daphnia magna 2–4 ind. l-^–1, whereas in the fish compartments densities were c. 1 ind. l^–1. Cyclopoid copepods and Bosmina longirostris, however, showed higher densities in the fish compartments. The composition of algae in the two compartments differed only slightly, but the densities were lower in the non-fish compartments. The significant difference in turbidity was probably caused by resuspension of sediment by carp. No significant difference in nutrient concentration between the compartments was found.