Interannual variability of alongshore spring bloom dynamics in acoastal sea caused by the differential influence of hydrodynamicsand light climate

摘要

Timing and spatial distribution of phytoplanktonblooms in coastal oceans are highly variable. The interac-tions of various biological and physical factors leading to theobserved variability are complex and remain poorly under-stood. We present an example for distinct differences in thespatio-temporal chlorophyll a (CHL-a ) distribution on aninterannual scale, integrating high-frequency data from anautonomous measuring device (FerryBox), which operatedon an alongshore route in the coastal German Bight (NorthSea). While in one year the distribution of CHL-a was spa-tially homogeneous (2004), a bloom only developed in onepart of the transect in the following spring period (2005). Weuse a one-dimensional Lagrangian particle tracking model,which operates along the mean current direction, combinedwith a NPZ-model to identify the mechanisms controllingthe observed interannual bloom variability on the alongshoretransect. Our results clearly indicate that in 2004 the locallight climate determined the spatial and temporal dynam-ics of the spring bloom. In contrast, the import of a watermass with elevated CHL-a concentrations from the adjacentSouthern Bight triggered the spring bloom in 2005. The in-flow event did, however, not last long enough to spread thebloom into the eastern part of the study area, where highturbidity prevented local phytoplankton growth. The modelidentifies two interacting mechanisms, light climate and hy-drodynamics, that controlled the alongshore dynamics. Es-pecially the occurrence of a pronounced spring bloom despiteunfavourable light conditions in 2005 underlines the need tocarefully consider hydrodynamics to understand the dynam-ics of the plankton community in coastal environments.