Spatio-temporal dynamics of chlorophyll in the open Baltic Sea

Abstract

Five charts of the chlorophyll and hydrographic fields by vertical profiles of in situ fluorescence and CTD were made on a stationary grid of 20 x 25 nautical miles with a 5-mile spacing in the open Baltic Sea. Both chlorophyll levels and variability were maximal close to the spring bloom. High chlorophyll levels in summer are sustained by recurrent nutrient injections from the deep saline layer. Two of the surveys showed close coupling between the coarse-scale (- 10 km) chlorophyll distri- bution and the hydrographic structure determining the intensity of nutrient transfer. Vigorous advec- tion, stirring and current shear, associated with a strong mesoscale eddy, probably dominated the chlorophyll pattern on three surveys. The upward velocities in the cyclonic eddy resulted in accumula- tion of phytoplankton in the aphotic zone. Intensive heat input from the surface caused a sudden sink- ing of the phytoplankton and the formation of a pronounced sub-surface chlorophyll maximum.