Coarse-Scale Spatial Structure of Phytoplankton Standing Crop in Relation to Hydrography in the Open Baltic Sea

Abstract

During a n interdisciplinary study of the BOSEX site in the Gotland Basin, central Baltic Sea, a number of mappings of the chlorophyll and hydrographic fields were made by vertical profilings on a regular grid of 20 X 25 nautical miles with a 5-mile station spacing. Spatial variability of the chlorophyll and density fields was partitioned into the fine-scale [-100 m) and coarse-scale [-10 km) components. The chlorophyll coarse-scale variance dominates over the fine-scale variance by an order of magnitude in the upper layer. The coarse-scale pattern of the phytoplankton standing crop, measured by the vertically integrated chlorophyll a concentration, shows 2.4-fold variations and coupling to the pattern of the intermediate layer thickness. The intermediate layer separates the upper layer, suitable for plant growth, and the deep layer, the source of nutrients. It is suggested that above a thin intermediate layer, by increased vertical current shear and breaking internal waves, the upward flux of nutrients is intensified causing a n enhanced phytoplankton growth and the increased standing crop.