Ecology of Lake District Ostracoda
Freshwater ostracods are potentially useful for environmental monitoring and, since their calcified valves may be preserved in lake sediments, are also valuable in palaeolimnological studies of environmental pollution such as eutrophication and surface water acidification. To establish a data-base of ostracod ecology, biannual survey work in 1989-1990 was performed in 75 lakes and tarns of a wide range of physico-chemical characteristics in the Lake District in Cumbria, ranging from large eutrophic lakes such as Windermere and Ullswater to tiny, upland acidic tarns. pH ranged from 4.3 (Black Pool) to 8.0 (Browns Tarn). Littoral margin samples were taken from all 75 sites and yielded 31 ostracod species, of which 8 were new to the Lake District fauna. The collection of multiple littoral samples from two sites demonstrated that a single sample produced an adequate faunal representation if it encompassed a variety of microhabitats. Statistical analysis, incorporating a multistage, multivariate technique, has shown that 18 species make up over 99% of the data set, and that 5 species, Cvpria ophthalmica, Cvclocypris ovum. Metacvpris cordata. Candona Candida and Cvpridopsis vidua. dominate the community in 71% of the sites containing ostracods, forming eight distinct assemblage groups. No ostracods were found in 13 of the 75 sites, 11 of which were acidic, having a pH of below 5.7. Equations were derived to predict both species distribution and diversity. Important predictors of community structures were shown to be pH, [Ca] 2+ , [Mg] 2+ , substrate, lake size and altitude. The equations were tested by further sampling of additional sites in the Lake District. Predictions of total species number and density generally provided an excellent fit to the observed data, although individual species predictions were poorer, especially in alkaline conditions. Substrate was not included in the analysis, due to quantitative difficulties, but this factor must be included in future predictive models as it was shown to be an important parameter in determining distribution. Deep-water sampling was carried out in 6 lakes. 10 species were collected, including Candona necrlecta, which was absent in the littoral samples. 9 species contributed to over 99% of the data set, and 2, Cypria ophthalmica and Candona Candida dominated the community in 75% of the sites containing ostracods. Community structure was predominantly determined by water depth (together with the associated temperature effect) and substrate. The sex ratio of Cyclocvpris ovum was shown be biased towards the female only at high alkalinities. Other species had sex ratios biased towards the male or female, the values independent of water quality. Large, swimming ostracod species were absent in sites containing fish. A series of laboratory experiments using three species of Ostracoda, (Cvpria ophthalmica. Cvpricercus fuscatus. and Eucvpris virens), and a predator (Gasterosteus aculeatus) correlated increasing ostracod size with an increased rate of predation, suggesting that predation could limit ostracod distribution. From the results of principal component analysis, it was concluded that the main characteristics that chemically differentiate the sites are calcium, magnesium, hydrogen and sodium ion concentrations. Toxicity tests were used to expose selected species to a wide range of calcium, magnesium, sodium and aluminium concentrations, at both neutral and acidic pH levels. Aluminium was selected as it has been highlighted as a major factor in the toxicity of acid waters. All species tolerated a wider spectrum of ionic concentrations than those in which they were recorded in the field, although the order of species survival in the experiments was similar to that found in the Lake District. It is suggested that whilst adult Ostracoda do not suffer from the acute toxicity of pH or aluminium, they may be unable to successfully reproduce in harsh environmental conditions. The waters of the English Lake District are not particularly species-rich due primarily to low alkalinity and low levels of dissolved cations, but also because they are cold. Only in small, ion-enriched pools is ostracod density sufficiently high to warrant their consideration as important detritivorous contributors in the cycling of nutrients. The Lake District fauna is compared with those recorded in other parts of Britain and Europe.