Phytoplankton blooms in response to nutrient enrichment

Delivery of continental nutrients to coastal waters generally results in phytoplankton blooms characterized by high biomass levels. Such is the case in the Belgian Coastal Zone (BCZ) where satellite images recurrently detect high Chl a concentrations (Fig. 3.1) particularly in the coastal waters influenced by the nutrient-enriched freshwater of the Scheldt and coastal tributaries (see Fig. 2.13 in Brion et al., 2008). These phytoplankton blooms are particularly significant in April and May, when Chl a concentrations higher than 10 and locally 25 mg m-3 are reached in Belgian and Dutch nearshore waters (Fig. 3.1). Phytoplankton blooms start in average in March, in the clearest Southwestern coastal waters (Fig. 3.1; Borges & Frankignoulle, 2002; Muylaert et al., 2006), and propagate in April-May in the almost entire BCZ and adjacent waters (Fig. 3.1). During summer and fall, moderate Chl a concentrations, i.e. 3-10 mg m-3, are still detected but are restricted to the very nearshore region (Fig. 3.1), presumably due to the local influence of nutrient enrichment at that time of the year. Satellite time series Chl a data retrieved for stations 230, 330 and 435 along an in-offshore transect (Fig. 3.1) reproduce well the phytoplankton bloom dynamics in the BCZ, highlighting a major spring bloom at all 3 stations but significant summer blooms at the coastal station 230 only (Fig. 3.2). The time series data show the transient nature of the phytoplankton blooms as well as a significant interannual variability in their magnitude and timing (Fig. 3.2).