The possible origin ofDaphnia cucullata procurva Poppe 1887 in the lakes of the Pomeranian Lakeland by hybridization in the past

Besides the genusBosmina, the genusDaphnia offers considerable taxonomic difficulties due to the existence of a very great number of different morphotypes and their intergrades. Especially the morphotypes clustering aroundDaphnia galeata Sars, 1864,D. hyalina Leydig, 1860, andD. cucullata Sars, 1862 offer a bewildering picture. In the last years an attempt has been made to understand this phenomenon mainly as the result of hybridization and colonization of new biotopes by populations with hybridogenetic polymorphism and genotypic diversity (founder effect). Now it will be suggested on a morphological base thatD. cucullata procurva Poppe, 1887, a unique and aberrant formtype restricted to few lakes along the river Brda in Poland, is probably an old and stabilized hybrid originating from interspecific hybridization betweenD. galeata andD. cucullata.     

Observations on the zooplankton in the reedbelt area of the Neusiedlersee

Weekly sampling and simultaneous measurements of temperature, pH, and conductivity were made at three stations in the reedbelt, between 1980–82. 20 cladoceran and 14 copepod species were encountered, of whichCyclops vicinus vicinus was found in the lake for the first time. Of the measured environmental features, temperature showed the greatest seasonal changes; pH and conductivity were less variable. There were, both quantitatively and qualitatively, constant differences between the three stations, although they lay close to each other.     

Effects of recent acidification on Cladocera in small clear-water lakes studied by means of sedimentary remains

Cladoceran populations, as revealed by sedimentary remains, were studied in six lakes which by diatom evidence have become strongly acidified during the past 30 years. The accumulation rates of cladoceran remains and in two of the most strongly acidified lakes both relative and absolute numbers ofBosmina greatly increased during this period. Species numbers were not reduced, but there were shifts in relative abundances of cladoceran species.Bosmina longispina dominated all the lakes. Low pH (4.7) seemed not to be straight-forwardly harmful toDaphnia longispina, but the relative abundances of this taxon were reduced because ofBosmina.     

Functional bioassays utilizing zooplankton: a comparison

Functional zooplankton bioassays based on ingestion, reproduction and respiration are described, with methods for a new ingestion bioassay included. All bioassays are compared using three indices, including the variability of controls, the range of experimental responses, and a listing of contaminants causing inhibition/stimulation of response. The ingestion bioassay showed the greatest range of response, and was sensitive to pesticides, PCBs and heavy metals. It was also commonly characterized by a hormesis response. The reproduction bioassay showed the lowest variability, illustrated a reduced range of response, and was sensitive to nutrients and heavy metals. In one study, the respiration bioassay was sensitive only to PCBs.     

Relative importance of temporal and spatial heterogeneity in the zooplankton community of an artificial reservoir

Zooplankton samples were collected monthly at three stations in Ogochi Reservoir during 1980, and the temporal and horizontal variations in the density of dominant taxa were evaluated by a standard 2-way ANOVA with random effects. The analysis revealed that horizontal is greater than temporal variation in the warm water season (> 18 °C), whereas temporal variation is greater in the cold water season (< 18 °C). Horizontal heterogeneity of the zooplankton community found in the warm water season seems to be associated with differences in site-specific conditions.     

Observations on egg production rates and seasonal changes in the internal morphology of Mediterranean populations of Acartia clausi and Centropages typicus

Egg production rates in wild populations of Acartia clausi and Centropages typicus, sampled biweekly in the Gulf of Naples from October 1985 to July 1987, showed marked seasonal fluctuations with maximum values in early spring that proceeded the annual maxima for adult female densities in summer. A positive correlation between chlorophyll a concentrations and egg production was evident only during the early spring phytoplankton bloom. A strong diminution in egg deposition occurred later in spring and continued throughout the summer notwithstanding high chlorophyll concentrations. In winter, when population abundances for adult females were lowest, egg production rates were always higher than in summer. Differences in egg production rates coincided with pronounced morphological changes between summer and winter populations of both species. The most striking of these changes consisted, in winter, in the presence of a dark brown fluid-like mass of granular material that seemed to freely bathe the gonads. The presence of this substance only during periods of elevated egg production suggests that it may enhance egg production rates when the adult population reaches minimum annual levels. Such a mechanism of self-regulation may operate to dampen the effects of environmental variability thereby contributing to maintain a conservative structure in coastal copepod communities.     

Variations nycthémérales chez plusieurs taxons zooplanctoniques au cours de séries chronologiques de 24 heures dans le golfe de FOS-sur-Mer (Anse de Carteau et enceinte portuaire)

Preliminary review of diel variations in several zooplanktonic taxa during 24 h chronological series in the Gulf of Fos (Mediterranean sea).Time fluctuations in the zooplankton were studied in the Gulf of Fos during five chronological sampling series; samples were collected every 30 minutes during 24 h by means of a centrifugal pump in two areas of the Gulf of Fos (Carteau's bight and the inner basin number two of the harbour).The results show a great variability in the migratory behaviour of all species. According to this behaviour, it is possible to describe 3 groups of zooplanktonic species. The first group is strictly a nocturnal planktonic one: it includes some Meroplanktonic, holoplanktonic species and a lot of peracarida. The second group consist of species that perform more of less cyclic upward or downward migrations. The third group includes species with a distribution that is independent of diurnal variations.It seems that vertical migrations depend in part on irradiance but one can assume that high temperatures reduce migratory movements of a certain number of holoplanktonic species.

     

Morphology,physics, chemistry and biology of Lake Rara in West Nepal

A survey of oligotrophic Lake Rara, the biggest lake in Nepal, was carried out from 1982 till 1984. Mean depth is 100 m, and maximum depth is 167 m. The surface area covers 9.8 km², and the lake contains 0.98 km³ volume of water. Transparency was about 16 m, photoquantum yield decreased exponentially with depth below 5 m, and the extinction coefficient was 8.3 × 10⁻². The concentration of Chl.-a was in the range of 0.06–0.46 mg m⁻³, and total nitrogen was 18–30 μg 1⁻¹. The whole water column was well oxygenated. Primary productivity was extremely low. It has more than 30 inflowing brooks and one outlet. The water quality of the brooks changes drastically with their location. The pH, electrical conductivity, and EDTA hardness in the waters from a landslide area were high. In the waters from a rich pine forest they were extremely low. The zooplankton consisted of two species of protozoa, five species of rotifers, two species of Cladocera, and two species of Copepoda. The zooplankton density range was 6200–16200 individuals m⁻³. The minimum was on November 11th, 1983 and the maximum on August 19th, 1983.     

The impact of zooplankton size structure on phosphorus cycling in field enclosures

Plastic bag field enclosures (1570 1) were used to investigate the relationship between zooplankton size distributions and the turnover rate of pelagic phosphorus. Water within the enclosures was filtered to simulate either low (200 µm mesh net) or high (80 µm) planktivory. The bags were then spiked with radiolabelled phosphorus and sampled 7 times during the following 24 h period. Zooplankton communities dominated by smaller size classes cycled more phosphorus than those composed of larger species. These experiments reveal that higher trophic level interactions, such as size selective predation, may have a significant impact upon nutrient regeneration rates and, hence, primary production.     

Embryonic development times of entomostracan zooplankton from Lake le Roux (Orange River,South Africa), and their possible relationships to seasonal succession

Embryonic development times of the principal entomostracan zooplankton of Lake le Roux, Orange River, South Africa, are described. Durations decreased monotonically with temperature over the thermal tolerance range and are described using Blehrádek's equation. The present data are compared with other observations on rate-temperature responses of warm-water and temperate zooplankton. Small, but ecologically significant, differences are discerned. The possible significance of species-specific rate-temperature responses to the phenomenon of seasonal succession is explored.