Characterizing the vertical distribution of chlorophyll a in the German Bight

Coastal and shelf seas display strong variability in the horizontal and vertical distributions of chlorophyll a (CHL). Detailed data are required to identify the processes that drive the observed spatio-temporal dynamics. A high-resolution, vertically resolved transect data set for biogeochemical and physical properties was collected in the inner German Bight (GB) from 2009 to 2011 on a seasonal basis. We used fluorescence as an indicator for phytoplankton biomass via the CHL concentrations. We classified profiles into different types by evaluating the heterogeneity of CHL vertical distribution and identifying vertical location (upper mixed layer, subsurface layer, bottom mixed layer of water column) of high CHL concentration in each profile. We analyzed the spatio-temporal occurrences of the different CHL vertical distribution types in the context of the hydrodynamic environment. More than half (68.7%) of all profiles showed vertically homogeneous CHL distributions. A smaller subset (3.2%) of all profiles showed subsurface CHL maximum layers (SCMLs) in the vicinity of the pycnocline, co-varying with strongly stratified conditions in deeper water. Profiles with highest concentration of CHL in the upper part of the water column (HCU) were observed in 11.5% of all profiles. Profiles with highest concentrations of CHL in the lower part of the water column (HCL) comprised 16.6% of all profiles. HCL profiles were extensively observed during the decay phase of the spring bloom and were associated with resuspension and erosion from pre-existing SCMLs, which could be driven by tide; photosynthetic activity below the pycnocline could also contribute. Under moderate weather conditions, tidal currents were the main driver of resuspension. This study highlighted the occurrence of SCMLs and HCL patterns in vertical CHL profiles in shallow shelf seas, such as the GB.