Physical and biological controls of vertical gradients in phytoplankton

Lay Abstract As the base of the food web, phytoplankton perform vital functions in the ocean by fixing carbon through photosynthesis and by acting as prey for other organisms. Recently, scientists have observed layers of increased phytoplankton concentration in the ocean as thin as a meter or less. These findings have suggested that phytoplankton are rarely homogeneously distributed with depth, which has significant implications for predator foraging and the formation of marine snow—particles that are important to downward carbon flux. In this study, we have developed a mathematical model that describes the biological and physical mechanisms that allow these phytoplankton layers to form and persist. Our results suggest that the steepness of a phytoplankton layer is controlled by a balance of ocean mixing and the process that allowed the layer to form, such as phytoplankton growth or swimming. We then compare our model results to data of ocean mixing and phytoplankton distributions that were obtained from a free‐falling phytoplankton imaging system on a cruise in 2006.The comparison of the data to our model shows a strong fit and allows us to estimate the rate at which observed phytoplankton layers were formed. This project thus combined biological and physical measurements in the ocean with mathematical theory to provide an understanding of the mechanisms leading to phytoplankton layer formation and persistence in the ocean. This new understanding will allow us to better predict the occurrence of these features and quantify their importance to the functioning of the marine ecosystem.