The influence of tidal straining and wind on suspended matter and phytoplankton distribution in the Rhine outflow region

To study the short-term effects of the physical environment on phytoplankton dynamics in the Rhine outflow area, changes in salinity, beam attenuation and phytoplankton distribution were observed in a series of high resolution measurements. During periods of weak wind and neap tide (low turbulent kinetic energy, TKE), the system was dominated by tidal straining that resulted in semi-diurnal oscillations in stratification. Phytoplankton was kept in resuspension at about 10–15 m depth. Subsequent strong wind and tidal mixing (high TKE) broke down stratification, enhanced horizontal gradients and suppressed estuarine transport. It is hypothesised that during low TKE, tidal straining and frontal circulation determined the phytoplankton distribution; and at high TKE, input by wind and tide enhanced conditions for growth, which resulted in a bloom in the surface layer of the Rhine Plume, where light and nutrients were available.