A study of phytoplankton spatial distributions, flow structure and characteristics of mixing in a river reach with groynes

Prolonged retention of water in riverine recirculation zones and the consequences for phytoplankton growth are the subjects of ongoing debate and a challenge for intensive research. In the present study, conducted in a reach of the River Elbe with groynes, detailed field surveys of phytoplankton abundance and composition were accompanied by a comprehensive investigation of flow structure during low and high water stages. Settling of the suspended particulate matter, and the embedded phytoplankton, controls the phytoplankton dynamics. Processes of phytoplankton growth were shown to be insignificant because of insufficient residence times. The spatial patterns of the total suspended particulate matter (seston) concentration, the organic content, the chlorophyll a concentration, the mean velocity and turbulence fields exhibited a marked agreement, thus revealing the mutual relationship between phytoplankton abundance and local flow characteristics. The geometry of the groyne fields, expressed by the aspect ratio between length of groyne and the length of the side of the groyne field, was shown to determine these patterns. The study outlines the importance of numerical description of mixing processes in river flow and detailed field studies of turbulence for advanced research of phytoplankton dynamics in riverine ecosystems.