Factors controlling the onset of spring blooms in the German Bight 2002–2005: Light, wind and stratification

Our quantitative understanding of the strong spatial and temporal variability in coastal algal spring blooms is yet much limited by the presence of changing physical forcing (e.g. tides, wind, precipitation and river runoff) on various spatio-temporal scales. In this study, we reconstructed the spring bloom dynamics of a shallow coastal marine ecosystem from 2002 to 2005 by combining three continuous time-series along a nearshore to offshore transect in the German Bight (GB). Nearshore, the timing and magnitude of the bloom remained nearly constant between years. At locations further offshore, bloom onset was related to water provenance, as expressed by salinity and the position of the tidal front. There, we could distinguish two regimes: (1) Under prevalence of turbid coastal waters, the bloom started early before stratification; Chl:C ratios increased with increasing mean water column irradiance, Im. (2) If the transitional water originated from the open North Sea, the bloom followed the stratification in late spring and depended less on Im; the Chl:C ratio was inversely related to Im and fluctuated widely. In both cases, we found a striking coincidence of wind-slack events (≤5ms−1) and bloom onset. Particularly, a bi-weekly period with low wind below 5 m s−1 on average favoured the formation of a pronounced bloom. Our study demonstrates the critical role of mesoscale spatial variations (i.e. lateral mixing and stratification) for coastal plankton dynamics in winter-spring.

► We combined physical and biological time-series along a nearshore–offshore transect. ► We assessed the role of atmospheric and hydrographic forcing on spring bloom dynamics. ► Chl:C variations represent variable biological response to mesoscale physical forcing. ► We found a predominant role of wind alterations on the bloom dynamics. ► This control acted on frontal dynamics or light penetration critical to algal growth.