Dynamics of a subsurface phytoplankton maximum in the Skagerrak

Subsurface maxima in the distribution of autotrophic flagellates and diatoms were found in the nutricline on 3 successive transects across the Skagerrak between Denmark and Norway, during June 1990. The subsurface phytoplankton maximum increased during the 6 d investigation period both in intensity and horizontal extension and followed the nutricline, which descended from ca 15 to ca 25 m depth, while the pycnocline remained at 5 to 10 m depth.

The descent of the nutricline and the subsurface maximum is interpreted as the utilization of nutrients from the bottom water for new production. This interpretation is supported by a simple dynamic model simulating variations in biomass, irradiance and nutrient concentration with time and depth. About half of the primary production in the water column was associated with the subsurface maximum.

Experiments using 15N-nitrate and 32P-phosphate confirmed that the subsurface maximum was partly supported by new production, while surface phytoplankton relied mainly on regenerated nutrients. Measurements of heterotrophic activity and vertical carbon flux suggest that the net primary production in the subsurface maximum was roughly evenly allocated to biomass build-up, heterotrophic remineralisation and sedimentation.

The heterotrophic community responded to the subsurface phytoplankton maximum with corresponding maxima of bacterioplankton biomass and production and of biomasses of nano- and microzooplankton, whereas copepod biomass remained higher in the surface layer than in the subsurface phytoplankton maximum.