Plankton distributions and processes across a front in the open Baltic Sea

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Abstract
Sections of the temperature-salinity structure in the southeastern Gotland Basin, central Baltic Sea, revealed the existence of a pronounced salinity front. A water mass with anomalously low salinity extended vertically across the horizontally uniform thermocline. Plankton distribution and primary productivity showed consistent features with the frontal structure: the near-surface primary productivity increased 7-fold adjacent to the front and leveled down farther away from it; the column chlorophyll had a broader and less distinct maximum on the high-salinity side of the front; the zooplankton community was similar in composition but its biomass more than doubled in the higher-salinity water. Upwelling of nutrient-rich water in the frontal zone had probably triggered the productivity peak; the subsequent advection and diffusion were instrumental for the broader chlorophyll maximum. Owing to the lower phytoplankton/zooplankton ratio, the nutrient cycling had to be faster in the higher-salinity water mass.