Zooplankton structure in the Loosdrecht lakes in relation to trophic status and recent restoration measures

A five-year zooplankton study (1982–86) on three shallow and highly eutrophic lakes in the Loosdrecht area (The Netherlands) did not reveal any significant changes following the considerable reduction in external P-loading (from about 1.0 g to 0.3 g P m^–2 year^–1) since mid-1984.The recent annual fluctuations in the rotifer and crustacean densities are within the range of those found before the restoration measure became operative. A decrease in the average size of the crustaceans and an absence of large-bodied forms reflects an increased fish predation rather than a change in the quality or quantity of their sestonic food ( < 150 µm) which continues to be dominated by filamentous cyanobacteria and Prochlorothrix hollandica, a prochlorophyte discovered in these lakes recently.     

Seasonal dynamics of rotifer and crustacean zooplankton populations in a eutrophic,monomictic lake with a note on rotifer sampling techniques

The abundances, biomass, and seasonal succession of rotifer and crustacean zooplankton were examined in a man-made, eutrophic lake, Lake Oglethorpe, over a 13 month period. There was an inverse correlation between the abundance of rotifers and crustaceans. Rotifers were most abundant and dominated (>69%) the rotifer-crustacean biomass during summer months (June–September) while crustacean zooplankton dominated during the remainder of the year (>89%). Peak biomasses of crustaceans were observed in the fall (151 µg dry wt l^–1 in October) and spring (89.66 µg dry wt l^–1 in May). Mean annual biomass levels were 46.99 µg dry wt l^–1 for crustaceans and 19.26 µg dry wt l^–1 for rotifers. Trichocerca rousseleti, Polyarthra sp., Keratella cochlearis and Kellicottia bostoniensis were the most abundant rotifers in the lake. Diaptomus siciloides and Daphnia parvula were the most abundant crustaceans. Lake Oglethorpe is distinct in having an unusually high abundance of rotifers (range 217–7980 l^–1). These high densities can be attributed not only to the eutrophic conditions of the lake but also to the detailed sampling methods employed in this study.The research was supported by National Science Foundation grants DEB 7725354 and DEB 8005582 to Dr. K. G. Porter. It is lake Oglethorpe Limnological Association Contribution No. 25 and Contribution No. 371 of the Harbor Branch Foundation, Inc.     

A mathematical model of the phosphorus cycle in Lake Loosdrecht and simulation of additional measures

The phosphorus cycle in the ecosystem of the shallow, hypertrophic Loosdrecht lakes (The Netherlands) was simulated by means of the dynamic eutrophication model PCLOOS. The model comprises three algal groups, zooplankton, fish, detritus, zoobenthos, sediment detritus and some inorganic phosphorus fractions. All organic compartments are modelled in two elements, carbon and phosphorus. Within the model system, the phosphorus cycle is considered as completely closed. Carbon and phosphorus are described independently, so that the dynamics of the P/C ratios can be modelled. The model has been partly calibrated by a method based on Bayesian statistics combined with a Range Check procedure.Simulations were carried out for Lake Loosdrecht for the periods before and after the restoration measures in 1984, which reduced the external phosphorus loading to the lake from ca. 2 mgP m^–2 d^–1 to 1 mgP m^–2 d^–1. The model outcome was largely comparable withthe measured data. Total phosphorus has slowly decreased from an average 130 µgP l^–1 to ca. 80 µgP l^–1, but chlorophyll-a (ca. 150 µg 1^–1, summer-averaged) and seston concentrations (8–15 mgC 1^–1) hardly changed since the restoration measures. About two-thirds of the seston consisted of detritus, while the phytoplankton remained dominated by filamentous cyanobacteria. The P/C ratio of the seston decreased from ca. 1.0% to 0.7%, while the P/C ratios of zooplankton, zoobenthos and fish have remained constant and are much higher. The system showed a delayed response to the decreased phosphorus loading until a new equilibrium was reached in ca. five years. Major reasons for the observed resilience of the lake in responding to the load reduction are the high phosphorus assimilation efficiency of the cyanobacteria and the high internal recycling of phosphorus. A further reduction of nutrient loading, perhaps in combination with additional measures like biomanipulation, will be the most fruitful additional restoration measure.     

Structure and feeding activities of zooplankton community in Lake Zwemlust,in the two years after biomanipulation

The biomanipulation study in Lake Zwemlust (area 1.5 ha; mean depth 1.5 m) is among the series of such investigations initiated recently in The Netherlands. The effects of the lake's reffilling (after it was first completely drained empty) almost entirely the nutrient-rich seepage water from the River Vecht flowing nearby and of removal of the planktivorous bream (Abramis brama), on zooplankton community structure and feeding activities of crustaceans were monitored for two successive years (1987, 1988). In these years a classical pattern of succession occurred, with the rotifer spring maximum preceding the crustacean maximum by about 3 weeks. Among the fiveDaphnia species, which appeared in quick succession during May–July, two were large-bodied forms (D. magna; D. pulex). OnlyD. pulex persisted and was the important grazer species in the second year, especially in spring. In the first year the crustacean grazing, with several values >100%.d^–1, contributed significantly to the lake's improved water clarity, with Secchi-disc transparencies of 1.5 m and more almost throughout the summer, compared with 0.3 m before the biomanipulation. Even though the water clarity climate in the second year was quite similar to that in the first, the causal factor was high macrophytic vegetation, rather than zooplankton grazing. The lake developed a rich littoral flora and fauna in the second year in response to the optimal light and nutrient conditions. Apparently, the predation by the introduced planktivorous rudd (Scardinius erythrophthalmus) on zooplankton was an important factor in the changes in zooplankton structure, and in the reduced role of zooplankton in seston elimination during the second year. There is some evidence from bioassay work that, simultaneously with the littoral development, nitrogen limitation of the phytoplankton also contributed to the improved light situation in the second year.     

Feeding in Euchlanis dilatata lucksiana Hauer on filamentous cyanobacteria and a prochlorophyte

Ingestion and assimilation rates of Euchlanis dilatata lucksiana Hauer, isolated from Lake Loosdrecht (The Netherlands) and cultured on lake water (seston < 33 µm), were measured in the laboratory using the ¹⁴C-tracer technique. The five taxa used as tracer foods, together with 6–7 mg C l^–1 of lake seston in each case, included four species of filamentous cyanobacteria (Oscillatoria redekei, O. limnetica, Aphanizomenon flos-aquae, Anabaena PCC 7120) and a prochlorophyte (Prochlorothrix hollandica). Except Anabaena, they are all commonly encountered in eutrophic Loosdrecht lakes, including Lake Loosdrecht, and their dimensions ranged between 150 and 250 µm in length and 2 and 3.5 µm in width. The small and large Euchlanis used as experimental animals had mean lengths of 217–223 µm and 276–305 µm, respectively. Euchlanis fed on all the taxa offered as food. Clearance rates ranged from 51 to 99 µl ind^–1 d^–1 for the large animals and from 22 to 41 µl ind^–1 d^–1 for the small animals. The highest ingestion rate observed, 1.7 µg ind^–1 d^–1, was for the large animals feeding on Aphanizomenon. The daily ration for both size classes far exceeded 100% of body weight, reaching up to 690% for the small animals feeding on Aphanizomenon. The small animals generally appeared to assimilate the ingested food more efficiently (assimilation efficiencies: 37–100%) than the large animals (34–77%). Compared with an earlier study in which only lake seston (<33 µm) was used as food, the specific clearance rates of Euchlanis on the cyanobacteria and Prochlorothrix were generally somewhat lower.     

Phytoplankton and zooplankton (Cladocera,Copepoda) relationship in the eutrophicated River Danube (Danubialia Hungarica,CXI)

The seasonal variation in primary production, individual numbers, and biomass of phyto- and zooplankton was studied in the River Danube in 1981. The secondary production of two dominant zooplankton species (Bosmina longirostris and Acanthocyclops robustus) was also estimated. In the growing season (April–Sept.) individual numbers dry weights and chlorophyll a contents of phytoplankton ranged between 30–90 × 10⁶ individuals, l^–1, 3–12 mg l^–1, and 50–170 µg l^–1, respectively. Species of Thalassiosiraceae (Bacillariophyta) dominated in the phytoplankton with a subdominance of Chlorococcales in summer. Individual numbers and dry weights of crustacean zooplankton ranged between 1400–6500 individuals m^–3, and 1.2–12 mg m^–3, respectively. The daily mean gross primary production was 970 mg C m^–3 d^–1, and the net production was 660 mg C m^–3 d^–1. Acanthocyclops robustus populations produced 0.2 mg C m^–3 d^–1 as an average, and Bosmina longirostris populations 0.07 mg C m^–3 d^–1. The ecological efficiency between phytoplankton and crustacean zooplankton was 0.03%.     

Structural and grazing responses of zooplankton community to biomanipulation of some Dutch water bodies

Structure and grazing activities of crustacean zooplankton were compared in five lakes undergoing manipulation with several unmanipulated eutrophic (shallow) and mesotrophic (deep) lakes in The Netherlands. The biomanipulated lakes had lesser number of species and their abundance, both of rotifers and crustaceans, and had much larger mean animal size (3–11 μg C ind.⁻¹) than in the unmanipulated eutrophic lakes (0.65 μG C ind.⁻¹). WhereasD. hyalina (=D. galeata) andD. cucullata generally co-occurred in the unmanipulated lakes, in the manipulated lakes bothD. hyalina and other large-bodied daphnids,D. magna,D. pulex (=D. pulicaria), were the important grazers. In the biomanipulated lakes an increase in the individual crustacean size and of zooplankton mass were reflected in a decrease in seston concentration, higher Secchi-disc depth and a marked decrease in the share in phytoplankton biovolume of cyanobacteria. Biomass relationship between seston (150 μm) and zooplankton indicated a Monod type relationship, with an initial part of the curve in which the zooplankton responds linearly to the seston increase up to aboutca. 2 mg C l⁻¹, followed by a saturation of zooplankton mass (0.39 mg C l⁻¹) at 3–4 mg C l⁻¹ seston, and an inhibitory effect on zooplankton mass at seston levels>4 mg C l⁻¹. This latter is related to predominance in the seston of cyanobacteria. In the biomanipulated lakes, the zooplankton grazing rates often exceeded 100% d⁻¹, during the spring, and food levels generally dropped to <0.5 mg C l⁻¹. The computed specific clearance rate (SCR) of zooplankton of 1.9 l mg⁻¹ Zoop C is well within the range of SCR values (1.7–2.2 l mg⁻¹ Zoop C) from deep and mesotrophic waters, but about an order of magnitude higher than in the eutrophic lakes, with the food levels 10-fold higher. For 25% d⁻¹ clearance of lake seston between 35 and 60 ind. l⁻¹ are needed in the biomanipulated lakes against 1200–1300 ind. l⁻¹ in eutrophic lakes. Similarly, about 10 to 15 times more crustacean grazers are required to eliminate the daily primary production in the eutrophic lakes than in the biomanipulated lakes. These numbers are inversely related to the differences in animal size. The corresponding biomass values of zooplankton needed to clear the daily primary production in the eutrophic waters were 0.1–0.2 mg C l⁻¹ in the biomanipulated lakes, but about 0.45 mg C l⁻¹ in the unmanipulated eutrophic waters. Only if the water was kept persistently clear by zooplankton was there a balanced seston budget between the inputvia primary production and elimination by zooplankton. Mostly, however, the input exceeded the assimilatory removal by zooplankton, such that the estimated seston loss could be attributed to sedimentation and mineralization.     

The longitudinal distribution and community dynamics of zooplankton in a regulated large river: a case study of the Nakdong River (Korea)

The longitudinal distribution and seasonal dynamics of zooplankton were examined along a 200-km section of the middle to lower Nakdong River, Korea. Zooplankton was sampled twice a month from January 1995 to December 1997 at five sites in the main river channel. There was considerable longitudinal variation in total zooplankton abundance (ANOVA, p < 0.001). All major zooplankton groups (rotifers, cladocerans, copepodids and nauplii) increased significantly with distance downstream along the river. There also were statistically significant seasonal differences in zooplankton abundance at the sampling sites (ANOVA, p < 0.01). Zooplankton abundance was high in spring and fall and low in summer and winter. The seasonal pattern of rotifers was similar to that of total zooplankton. This reflected the fact that rotifers (Brachionus calyciflorus, B. rubens, Keratella cochlearis and Polyarthra spp.) strongly dominated the zooplankton community at all locations. Among the macrozooplankton, small-bodied cladocerans (e.g. Bosmina spp.) dominated; the abundance of large-bodied cladocerans (e.g. Daphnia) was negligible (0–5 ind. l^–1). Among the environmental variables considered, partial residence time seemed to play the most important role in determining characteristics of the river zooplankton community.     

Predator-induced bottom-up effects in oligotrophic systems

Five treatments (replication n=2) were applied to mesocosms in an oligotrophic lake (TP=6–10 µg 1^-1) to assess the effects of fish on planktonic communities. The treatments were: (1) high fish (30 kg ha^–1 Lepomis auritus, Linnaeus), (2) low fish (10 kg ha^–1), (3) high removal of zooplankton, (4) low removal of zooplankton and (5) control. Total phosphorus, chlorophyll a, zooplankton biomass, and species richness decreased from high fish > low fish > control > low removal > high removal treatments. The fish treatments were dominated by crustacean zooplankton, while rotifers outnumbered the other zooplankters in the removal treatments. Calculations of zooplankton grazing rates suggested that clearance rates seldom exceeded 2% of the enclosure volume d^–1 and were unlikely to have had much influence on phytoplankton biomass. Calculations from a phosphorus bioenergetics model revealed that when fish were present, their excretion rates were higher than the rates ascribed to zooplankton. Diet analysis showed that the fish derived most of their energy from the benthos and periphyton, and that fish excretion and egestion made significant contributions to the very oligotrophic pelagic phosphorus pool. In the absence of fish, zooplankton excretion was highest in the control treatments and lowest in the zooplankton removal treatments. Our results suggest that in oligotrophic systems, planktivorous fish can be significant sources of phosphorus and that fish and zooplankton induced nutrient cycling have significant impacts on planktonic community structure.     

Dominant rotifers of Võrtsjärv (Estonia)

Rotifers form 71% of the zooplankton of the strongly eutrophic (total N 2 g m^–3, total P 53 mg m^–3) Võrtsjärv (Estonia). Altogether 150 taxa of rotifers occur. Species characteristic of oligo- and mesotrophic waters have totally disappeared during the last 30 years, or are disappearing now. Species whose numbers and biomass reached 20% or more of total zooplankton were considered dominants. These were Anuraeopsis fissa, Keratella cochlearis, K. quadrata frenzeli, Polyarthra dolichoptera, P. luminosa, Synchaeta verrucosa and Trichocerca rousseleti. The contribution of dominant rotifers to total zooplankton and its biomass is as follows: S. verrucosa 25% and 39%, P.dolichoptera 34% and 25%, K. cochlearis 25% and 7%, K. quadrata frenzeli 9% and 7%, A. fissa 28% and 0.4%, T. rousseleti 20% and 0.6%, respectively.