BG Biogeosciences BG Biogeosciences 1726-4189 Copernicus Publications Göttingen, Germany 10.5194/bg-7-371-2010 Interannual variability of alongshore spring bloom dynamics in a coastal sea caused by the differential influence of hydrodynamics and light climate Brandt G. 1 2 Wirtz K. W. 1 GKSS-Research Centre, Institute for Coastal Research, Max-Planck-Str. 1, 21502 Geesthacht, Germany currently at: Bundesamt für Seeschifffahrt und Hydrographie, Bernhard-Nocht-Str. 78, 20359 Hamburg, Germany 29 01 2010 7 1 371 386 Copyright: © 2010 G. Brandt 2010 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://bg.copernicus.org/articles/7/371/2010/bg-7-371-2010.html The full text article is available as a PDF file from https://bg.copernicus.org/articles/7/371/2010/bg-7-371-2010.pdf

Timing and spatial distribution of phytoplankton blooms in coastal oceans are highly variable. The interactions of various biological and physical factors leading to the observed variability are complex and remain poorly understood. We present an example for distinct differences in the spatio-temporal chlorophyll <i>a</i> (CHL-<i>a</i>) distribution on an interannual scale, integrating high-frequency data from an autonomous measuring device (FerryBox), which operated on an alongshore route in the coastal German Bight (North Sea). While in one year the distribution of CHL-<i>a</i> was spatially homogeneous (2004), a bloom only developed in one part of the transect in the following spring period (2005). We use a one-dimensional Lagrangian particle tracking model, which operates along the mean current direction, combined with a NPZ-model to identify the mechanisms controlling the observed interannual bloom variability on the alongshore transect. Our results clearly indicate that in 2004 the local light climate determined the spatial and temporal dynamics of the spring bloom. In contrast, the import of a water mass with elevated CHL-<i>a</i> concentrations from the adjacent Southern Bight triggered the spring bloom in 2005. The inflow event did, however, not last long enough to spread the bloom into the eastern part of the study area, where high turbidity prevented local phytoplankton growth. The model identifies two interacting mechanisms, light climate and hydrodynamics, that controlled the alongshore dynamics. Especially the occurrence of a pronounced spring bloom despite unfavourable light conditions in 2005 underlines the need to carefully consider hydrodynamics to understand the dynamics of the plankton community in coastal environments.

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