Physical and biological controls of vertical gradients in phytoplankton

Abstract

Small-scale vertical heterogeneity in phytoplankton distributions is common in coastal waters and may be a critical feature influencing trophic coupling in planktonic systems. Here we develop a model to investigate the biological and physical dynamics that control vertical gradients in phytoplankton abundance. The model includes phytoplankton layer formation and layer destruction through mixing and predicts that the local maximum scaled phytoplankton gradient is controlled by the relative strengths of these dynamics. We compare the predictions of this model to highly resolved profiles of phytoplankton concentration and fluorescence collected using a free-falling planar laser imaging fluorometer (FIDO-Φ) and turbulence microstructure profiler data (TurboMAP-L). From these profiles, we estimate the model parameters: the maximum rate of layer formation and minimum possible layer thickness. The maximum rate of layer formation ranged from 0.46 to 0.94 d ^− 1, which is comparable to maximum reported growth rates of the most common phytoplankton taxa found in our samples. The minimum layer thickness estimated from our data suggests that persistent phytoplankton layers thinner than approximately 0.5 m may be rare in coastal waters. This study provides a mechanistic explanation for some of the underlying dynamics governing phytoplankton layer formation, maintenance, and destruction and will allow us to better predict the magnitude and occurrence of these ecologically important structures in the field.