Master's Thesis from the year 2005 in the subject Biology - Zoology, grade: Degree Awarded, University College Cork, language: English, abstract: Ballincollig reservoir is a small eutrophic lake located west of Cork city and south of Ballincollig town in county Cork, Ireland. The reservoir was studied for six months, from 7th February through 27th July, for the factors that may influence or regulate population dynamics (i.e. abundance and size structure) of the crustacean zooplankton. Various physico-chemical and biological parameters were measured and related to abundance and size structure of the crustacean zooplankton species/groups using the non-parametric Spearman's correlation test. Friedman's test was employed for the purpose of data analysis. Three cladoceran species: B. coregoni Baird, B. longirostris O.F.Muller, D. hyalina Leydig, and a genus Ceriodaphnia Dana as well as cyclopoid copepods were investigated during the study period. Water variables such as subsurface water temperature, dissolved oxygen, conductivity, and pH as well as secchi depth and chlorophyll a concentration did not show statistically significant spatial variation within the reservoir where as they varied significantly over time. Factors such as fish predation (top-down control), type and availability of phytoplankton (bottom-up control), water temperature and competition for resources among the crustacean zooplankton were found to regulate or influence abundance and size structure of the crustacean zooplankton. Top-down control by fish was important particularly in structuring the spatial dynamics of the crustacean zooplankton population within the reservoir whereas bottom-up control, water temperature and competition regulated mainly the temporal trend of the population dynamics. The cyprinid fish fauna appeared to influence the temporal dynamics of the crustacean zooplankton rather indirectly through enhancement of phytoplankton availability by stirring up the nutrients from the s