Drifter and dye tracks reveal dispersal processes that can affect phytoplankton distributions in shallow estuarine environments

In shallow estuarine environments, the time scales of hydrodynamic processes that control particle distribution may outpace the time scales of phytoplankton patch formation through reproduction. Consequently, physical processes can dominate the distribution of the phytoplankton, but these processes and their dynamics are not well understood. Here we used flow measurements with a bottom mounted Acoustic Doppler Current Profiler (ADCP), shipboard hydrographic transects, drifter releases, and Rhodamine dye to characterize the small-scale flow environment and its effect on dispersion processes in a shallow estuarine environment, Apalachicola Bay, Florida. Spatial spectra of salinity and chlorophyll followed a power law behavior of −3 at length scales of 250 m–5 km. The ADCP data revealed the presence of a vertically sheared flow that was strongly modulated by tides and bottom topography. Tidal flows had a characteristic magnitude of 20–40 cm s−1, with durations of flow reversals between the near-surface and bottom flows. Drifter triplets indicated shear and strain rates on the order of 10−3 – 10−4 s−1, and single particle dispersion rates (diffusivity) of 0.1 m2 s−1. The area evolution of the dye patch observed by a drone corresponded to eddy diffusivity comparable to those estimated from drifters, or about 0.1 m2 s−1. The dye patch experiments demonstrate how physical processes at scales of 1–100 m can affect the shape and development of phytoplankton patches in the bay. Vertical shear, produced by wind directions deviating from flow direction, can broaden and divide a plankton patch by transporting different depths of a patch in different directions. When winds and currents are aligned, shear leads to elongation and narrowing of the patch. The results indicate that the small-scale flow environment in estuaries can be pivotal in controlling the distribution and dispersal of planktonic organisms and thereby becomes a decisive factor for the development and breakdown of phytoplankton communities.