Changes in Nutrient and Carbon Availability and Temperature as Factors Controlling Bacterial Growth in the Northern Baltic Sea

The effects of inorganic nutrient (N and P) and glucose C treatments on bacterial growth were followed for 3 days in natural surface and deep water bacterial samples during the main post-spring bloom stages of phytoplankton growth in the northern Baltic Sea. In addition, the importance of photochemical degradation of dissolved organic matter on bacterial growth was investigated vertically (0.1–2.0 m) and spatially, in a salinity gradient from river mouth to open sea. Bacterial production was consistently C limited in the surface layer, with N or both N and P as the secondary limiting nutrients from spring to early summer and in late summer, respectively. In deep water, bacterial growth showed combined temperature and C limitation, and in spring, this also appeared to be true with surface samples. The effect of 1-day sunlight pre-treatment varied from no effect up to a 44% production increase, with clear bacterial production responses only being seen at the surface (10 cm depth). The implications of bacterial C limitation for the structure and function of the surface plankton ecosystem, including its CO₂ exchange with the atmosphere are discussed.