Description of the long-term (1991–2005) temporal and spatial distribution of phytoplankton carbon biomass in the Dutch North Sea

Since the beginning of the 1990s phytoplankton species composition and abundance have been monitored at a high frequency (bi-weekly in the growing season and monthly in winter) at a number of fixed stations on the Dutch Continental Shelf, of which 18 are used in this study. Phytoplankton carbon biomass has been calculated from species-specific biovolume/cell data and summed over all species per functional group enumerated in the samples. The species are divided into four functional groups i.e. diatoms, flagellates, autotrophic and mixotrophic dinoflagellates and Phaeocystis spp. The total number of phytoplankton samples analysed up to and including 2005 is almost 4000. The annual mean phytoplankton biomass over all stations remained stable at around 145 mg C m−3. However, the phytoplankton composition has changed significantly, with increases in diatoms and dinoflagellates and compensating decreases in flagellates and Phaeocystis. With increasing distance from the shore, coinciding with a decrease in nutrient availability and increasing water depth, total phytoplankton biomass as well as the biomass of diatoms, flagellates and Phaeocystis spp. decreased. This pattern was not true for the dinoflagellates, which occurred at more or less the same biomass throughout the region. Stations near river mouths and in the Wadden Sea outlets had much higher phytoplankton biomass than stations further from freshwater discharges. The data, split in two periods (1991–1998) and (1999–2005) and averaged over the whole Dutch Continental Shelf, had been aggregated into seasonal biomass distributions. The seasonal phytoplankton biomass distribution was unimodal in both periods, with similar spring maxima of around 300 mg C m− 3. The spring maximum occurred one month earlier, in April, in the second period. Phaeocystis over the whole study period remained the dominant near-shore species as it has been since the first phytoplankton observations at the end of the 19th century.