Interannual variability of alongshore spring bloom dynamics in a coastal sea caused by the differential influence of hydrodynamics and light climate

G Brandt,K W Wirtz

doi:10.5194/bg-7-371-2010

Abstract

Abstract. 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 a (CHL-a) 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-a 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-a 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.

Highlights

The phytoplankton spring bloom drives food web dynamics and matter cycling in most temperate aquatic ecosystems (Sommer, 1998)
We present an example for distinct differences in the spatio-temporal chlorophyll a (CHL-a ) 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)
Though phosphate data are relatively sparse compared to Ferrybox measurements, it outline the high temporal dynamics of phosphate during spring (Fig. 2)

Summary

Introduction

The phytoplankton spring bloom drives food web dynamics and matter cycling in most temperate aquatic ecosystems (Sommer, 1998). While in deep waters the onset of a bloom typically follows stratification in spring, interannual variability and spatial heterogeneity are strong in shallow coastal seas (Thomas et al, 2003; Cloern, 1996). Strong turbulence, leading to vertical mixing, counteracts water column stratification and retards algal growth. It decreases light availability for phytoplankton by increasing turbidity, which is suggested to be pivotal for phytoplankton bloom control (Townsend et al, 1994). Vertical mixing may even raise phytoplankton mortality because of grazing by benthic filterfeeders (Cloern, 1996; Prins et al, 1996).

Methods

Results

Discussion

Conclusion