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, organic carbon source) and top-down (fish) manipulations, both singly and together, were studied daily during a 3 week period in July 1993 by using eight 50 m 3 mesocosms in the coastal northern Baltic Sea. Nutrient 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 Egymnastica in the lower part of the water columns (6-12 m) followed. Settled material 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 ELgymnastica blooms was estimated to be grazing through mesozooplankton. However, provided the nutrients are plentiful in the water column, the growth potential of E.gymnastica appears to exceed the ambient grazing pressure. If the nutrients become depleted, it seems to be effectively controlled by mesozooplankton grazing, which is probably limiting the likelihood of massive Egymnastica blooms in the coastal Baltic Sea. Our study suggests that E.gymnastica appears to be a fast-growing fugitive (bloom) species with flexible behavioural (vertical migration) and life history (cyst formation) adaptations which is able to exert dominant role and direct trophic relations similar to other bloom species adapted for decaying turbulence and high nutrient environments.