Temporal and spatial patterns of crustacean zooplankton dynamics in a transitional lagoon ecosystem

Patterns and mechanisms of plankton crustacean seasonal succession in the eutrophic freshwater Curonian lagoon (south-eastern Baltic Sea) were analysed on the basis of four-year (1995, 1996, 1998 and 1999) field sampling results. The seasonal crustacean zooplankton succession in the lagoon appears to be the consistent six-stage sequence of four distinct species complexes. Each stage is characterised by its individual species composition and quantitative characteristics. The uniform and periodic pattern of the limnetic zooplankton crustacean successional stages in the lagoon indicates that the seasonal succession of the limnetic zooplankton is not disturbed by unpredictable environmental fluctuations, such as brackish water inflows. Seasonal zooplankton succession is also comparatively uniform at a spatial scale. Not more than two adjacent successional stages were found across the northern part of the lagoon during each of 11 seasonal surveys. Comparison between monthly water residence time and dominant plankton crustacean species life cycle duration points to a more transitory plankton community in spring while in the summer it is not much influenced by lagoon hydrodynamics. Consequently, the Curonian lagoon crustacean community quite closely follows the Plankton Ecology Group (PEG)-described freshwater lake seasonal succession in summer and turns into a lentic-like system in spring and autumn.