Multiple subsurface phytoplankton blooms occurring simultaneously inthe Skagerrak

Abstract

The distribution and characteristics of phytoplankton in the Skagerrak in August–September 2000 were analysed in order to evaluate the importance of subsurface phytoplankton peaks to water column ecology and primary production. In areas affected by outflow from the Baltic, enhanced chlorophyll concentrations were found in the warm surface waters (i.e. upper 10–20 m). However, for the central Skagerrak, the major part (50–80%) of the chlorophyll in the water column was found below the warm surface waters. The highest chlorophyll concentrations (up to >18 μg l⁻¹) in the study area were also found below the warm surface waters and up to 95% of total water column primary production was recorded below the warm surface water layer. Measurements of variable fluorescence (F_v/F_m) indicated the greatest potential capacity for electron flow in photosystem II in phytoplankton was located below the warm surface waters. Spectrophotometrically determined pigment ratios suggest that the enhanced capacity for photosynthesis in the deeper waters may be related to greater nutrient availability here than in surface waters. Subsurface chlorophyll distributions seen in relation to the different water masses identified in the area, as well as community analysis of the phytoplankton present in the subsurface peaks, indicate the presence of at least three distinct subsurface phytoplankton blooms in the Skagerrak during the study period. Local oxygen saturation maxima recorded immediately above the subsurface peaks provide in situ evidence that these peaks are photosynthetically active. This suggests that new production is taking place in these peaks, although quantification of this production is hampered due to a lack of information concerning the initial conditions in and lifetime of the subsurface peaks. The subsurface phytoplankton peaks were, generally, found immediately above an oxygen minimum that covered the entire study area. In the relatively cold deep Atlantic water found below the oxygen minimum layer, no or very little chlorophyll was recorded and oxygen concentrations increased. Thus, it is argued that the respiration of the organic material produced in the upper part of the water column during late summer may primarily occur in the intermediate layers of the water column.