Ecological determinants of rotifer vertical distribution in a coastal karst lake (Vrana Lake, Cres Island, Croatia)

Vrana Lake is a coastal karst lake on Cres Island within the eastern Adriatic coast. It appears to have formed as a cryptodepression on karstified carbonate rocks, and is filled with fresh water. It is classified as an oligotrophic hydrosystem and serves as a natural water supply reservoir (max. depth 74.5 m; surface area 5.5 km(2); water volume 220 x 10(6) m(3)). The aim of this study was to assess the influence of seasonally and spatially varying ecological conditions (i.e., thermal stratification, dissolved oxygen, orthophosphate and chlorophyll a concentrations) on the vertical distribution of the rotifer plankton community in this geomorphologically and hydrologically specific coastal lake. The rotifer community was surveyed monthly over a one year period. During the study period, a total of 21 rotifer taxa were identified. Among them Ascomorpha spp. (including A. ecaudis, A. ovalis and A. saltans), Filinia terminalis, Keratella cochlearis, Synchaeta gr. tremula-oblonga and Polyarthra vulgaris prevailed, representing 92% of the total rotifer abundance. Measured seasonal and spatial patterns of the rotifer assemblage along a vertical profile of Vrana Lake were considerably affected by temperature, food availability and biotic (competition, predation) interactions in the zooplankton community.