SUBSURFACE CHLOROPHYLL MAXIMA IN THE SKAGERRAK - PROCESSES AND PLANKTON COMMUNITY STRUCTURE

Subsurface chlorophyll maxima are common phenomena in both the coastal and open ocean. The main objective of this study was to clarify possi ble differences in the structure and function of the plankton communit y in subsurface chlorophyll maxima and at the surface. Sampling was pe rformed at seven stations in the Skagerrak, northeast Atlantic, during five cruises in May and August 1992 and April, May and August 1993. S ubsurface chlorophyll fluorescence maxima (FM) occurred on 25 out of 3 2 sampling occasions. The FMs were usually situated below the pycnocli ne and associated with the nutricline. The ratio of chlorophyll a to p articulate carbon and the light-saturated primary production were high er for plankton at the FM than at the surface, although assimilation n umbers (primary production rate:chlorophyll a) were not different from surface plankton. The light protective pigment diadinoxanthin occurre d in higher concentrations relative to chlorophyll a in surface plankt on than in FM plankton. Respiration was higher in the FM than at the s urface. This was not related to abundance of bacteria or bacterial pro duction since no differences between surface and FM values were detect ed for these parameters. FM plankton was characterized by high nutrien t uptake rates, but in this study there were no significant difference s compared to surface plankton. 'New' production was on average 25%, b ut up to about 50% in the western Skagerrak in spring. The average nit rogen uptake rates were dominated by the regenerated nutrients ammoniu m and urea, accounting for about 50 and 25%, respectively. The <3 mu m size fraction contributed significantly to concentrations of total ch lorophyll a, particulate carbon, and nitrogen as well as to nitrogen u ptake. Its contribution was highest when total values were low. Micros copical investigations and analysis of pigments specific to algal grou ps showed that diatoms dominated in the FMs in spring and that peridin in-containing dinoflagellates dominated in FMs in August. Autotrophic nanoplankton was dominated by the Prymnesiophyceae contributing about 50% of total cell numbers. Colonies of Phaeocystis sp. were abundant a long the NW Danish coast in April and May 1993, Autotrophic eukaryotic picoplankton occurred in cell numbers up to about 30 x 10(6) cells . dm(-3) along the Swedish coast. The highest cell numbers of cyanobacte ria of the Synechococcus-type, about 100 x 10(6) cells . dm(-3), were found in the central Skagerrak in August and the abundance of Synechoc occus was correlated with nitrogen uptake in the <3 mu m size fraction .