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Seasonal distribution and production of phytoplankton in the southern Kattegat

The seasonal distributions of chlorophyll a and primary production throughout the water column were examined at a fixed station in the southern Kattegat. The most intensive data collection was in 1989 when the station was visited 55 times. Less intensive data collection was also carried out in 1988 and 1990. Surface chlorophyll distributions indicate the presence of a spring and autumn-winter 'bloom' in surface waters and relatively low chlorophyll a concentrations (< 2 pg I-') in the surface mixed layer during summer. Phytoplankton in surface waters during summer (mid-May to ca 1 October) appear to be nutrient limited. From mid July until ca 1 October, the entire euphotic zone appears to be nutrient depleted. The highest chlorophyll concentrations observed during the spring bloom are associated with frontal waters separating the Kattegat and waters of the Baltic outflow. From the period immediately following the spring bloom until ca 1 September, the highest chlorophyll concentrations were observed in association with the pycnocline (nutriciline). During late spring and early summer, the pycnocline usually occurs at a depth of between ca 10 and 20 m. The phytoplankton associated with the pycnocline during this period are usually photosynthetically active even at the photon flux densities encountered at the pycnocline. Thus, significant primary production occurs in this subsurface layer In 1989, total annual primary production at the study station was calculated to be ca 290 g C m-' Approximately 19 % of this production is estimated to occur during the spring bloom while on the order of 30 O/ O of the total is estimated to take place in subsurface phytoplankton populations during summer.