Frequency, composition, and causes of summer phytoplankton blooms in a shallow coastal ecosystem, the Kattegat

We propose a new operational definition for summer phytoplankton blooms based on station-specific chlorophyll a distributions. This definition has been applied to a large monitoring data set from the Kattegat, a shallow marginal sea affected by man-induced eutrophication, in order to describe spatial and temporal variations of summer blooms as well as their underlying cause. Blooms were associated with relatively higher salinity, nutrient concentrations, and wind speed prior to observation as well as relatively lower temperature, which suggests that entrainment and the upwelling of nutrient-rich bottom water are the main causes of summer blooms. The station-specific frequency of blooms was, on average, 8.7%, with higher frequencies in the frontal area toward the Skagerrak as well as in the shallow western area. Blooms generally had a limited extent in localized regions, and we suggest that blooms could be initiated in hydrodynamically active regions and subsequently transported to other parts of the Kattegat. There were no trends in the bloom frequency over the study period (1989-1999), but interannual variations were linked to external nitrogen input, through upwelling of regenerated nutrients, which suggests that blooms are increasing in frequency over longer timescales. Summer blooms were dominated by large species (Ceratium spp. and Rhizosolenia spp.), which indicates that size-dependent grazing affects the development and probably also the fate of summer phytoplankton blooms. This bloom definition can be generally applied to other data sets to investigate the properties of high phytoplankton biomass.