Dinoflagellate bloom formation in natural assemblages with diatoms: nutrient competition and growth strategies in Baltic spring phytoplankton

摘要

ABSTRACT: In temperate coastal waters, the spring bloom, which commonly dominates the seasonal productivity cycle, is typically composed of cold-water diatoms. In the Baltic Sea, dinoflagellates are a regular component of the spring phytoplankton assemblage, and recent evidence suggests that their proportion might be increasing relative to diatoms. A concomitant decrease in terrestrial Si discharges and an increase in other nutrients have been considered as a possible cause of such development, as they might lead to a deteriorating competitive position of co-occurring diatoms. Using mesocosms, we studied the effects of variable nutrient additions and nutrient supply ratios on species composition and dominance patterns of natural spring phytoplankton communities from the coastal northern Baltic Sea. The experiments were repeated in 3 consecutive years to investigate to what extent variability of initial community composition and relative abundance of co-occurring species affect bloom development and species dominance. The results showed that bloom formation and dominance of the dinoflagellate Woloszynskia halophila primarily depended on the size of the inoculum’s population and the relative abundance of co-occurring diatoms. Variations in dissolved inorganic silicon:(nitrate + phosphate) [DSi:(N+P)] ratios did not have major effects on phytoplankton development and composition. Additions of N and P, as well as increased irradiance, generally stimulated the growth of diatoms, but could not alter the outcome of competition between diatoms and dinoflagellates when the latter were initially dominant. Our results emphasize the importance of efficient recruitment strategies and initial conditions for dinoflagellate bloom formation. This implies dinoflagellate bloom sensitivity to hydrographic conditions and thus to projected climatic change, as well as different roles for diatoms and dinoflagellates along the coastal/offshore gradient.