DEVELOPMENT AND FATE OF EUTREPTIELLA-GYMNASTICA BLOOM IN NUTRIENT-ENRICHED ENCLOSURES IN THE COASTAL BALTIC-SEA

Response of the phytoplankton community to bottom-up (nutrients, organ ic carbon source) and top-down (fish) manipulations, both singly and t ogether, were studied daily during a 3 week period in July 1993 by usi ng eight 50 m(3) mesocosms in the coastal northern Baltic Sea. Nutrien t additions (once per week) invoked a series of blooms of Eutreptiella gymnastica Throndsen (Euglenophyceae) (up to 13 x 10(3) cells ml(-1)) which formed the major part (60-90%) of the total autotrophic biomass . After rapid depletion of nutrients (2-3 days) from the surface layer (0-6 m) downwards migration and a subsequent peak of E.gymnastica in the lower part of the water columns (6-12 m) followed. Settled materia l collected from the bottom of the enclosures contained a considerable amount of E.gymnastica cells and resting cysts. Nevertheless, sinking loss rates of E.gymnastica were estimated to be less than 1% day(-1) of the suspended cell numbers. The fate of E.gymnastica blooms was est imated to be grazing through mesozooplankton. However, provided the nu trients are plentiful in the water column, the growth potential of E.g ymnastica appears to exceed the ambient grazing pressure. If the nutri ents become depleted, it seems to be effectively controlled by mesozoo plankton grazing, which is probably limiting the likelihood of massive E.gymnastica blooms in the coastal Baltic Sea. Our study suggests tha t E.gymnastica appears to be a fast-growing fugitive (bloom) species w ith flexible behavioural(vertical migration) and life history (cyst fo rmation) adaptations which is able to exert dominant role and direct t rophic relations similar to other bloom species adapted for decaying t urbulence and high nutrient environments.