Effects of the flow field on small scale phytoplankton patchiness

We present an example of small scale (10–100 m) horizontal, subsurface patchiness of phytoplankton (Ceratium) during an intensive bloom in August 1993 and link it to the flow field. In the small Lake Belau (1.1 km²) in northern Germany large areas of the water surface are sheltered from wind, due to vegetation. Wind sheltering effects decrease with wind speed and below 2 m/s a spatially homogeneous wind field is observed. Under weak wind conditions near-surface Ceratium patches with local chlorophyll-concentrations up to 200 mg/m³ were observed in the south bay (0.3 m depth) as well as in parts of the central basin (1 m depth). Detailed flow simulations show very good agreement between location and size of current gyres and phytoplankton patches. Inside the gyres we find low flow velocities with low vertical turbulence. This allows Ceratium to form distinct vertical layers with high densities close to the water surface, according to the light gradient. Especially in the south bay flow eddies are determined by the course of the coastline and their location and intensity is, to a large degree, independent of prevailing wind directions.