Monitoring data (1999 to 2012) and data from a 2 wk field study at a seasonally stratified station in the Aarhus Bight near the Danish coast were used to demonstrate that the vertical distribution of photosynthesis influences both water column oxygen conditions and the fate of the organic material produced. The primary production (PP) occurring below the surface layer, i.e. in the pycnocline-bottom layer (PBL), is shown to contribute significantly to total PP. Oxygen concentrations in the PBL are shown to correlate significantly with the deep primary production (DPP) as well as with salinity, phosphate loading, wind and transparency in the surface layer. The phytoplankton communities detected in the surface layer and PBL during the field study were very different. Large cells, especially Ceratium spp., dominated in the PBL, while small diatoms, mainly Proboscis alata, dominated in surface waters. On the basis of chlorophyll-normalised photo synthetic parameters and variable fluorescence, it is shown that the 2 populations were physiologically distinct. The population in the PBL was photosynthetically active and adapted/acclimated to lower light than the population in the surface layer. Sinking rates (based on sediment trap collections) of carbon and nitrogen were highest in the PBL. Lyngsgaard et al. (2014; Limnol Oceanogr 59: 1679-1690) have demonstrated that the vertical distribution of PP in this region is influenced by anthropogenic nutrient loading. Thus, the present study indicates that eutrophication effects may include changes in the structure of planktonic food webs and element cycling in the water column, both brought about through an altered vertical distribution of PP.
Marine Ecology - Progress Series, 2014, Vol 517, p. 15-33