The Influence of tidal straining and wind on suspended matter and phytoplankton dynamics in the Rhine outflow region

To study the short-term effects of the physical environment on phytoplankton dynamics in the Rhine out¯ow 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±15m depth. Subsequent strong wind and tidal mixing (high TKE) broke down strati®cation, 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.