PLANKTON IN THE RIVER RHINE - STRUCTURAL AND FUNCTIONAL-CHANGES OBSERVED DURING DOWNSTREAM TRANSPORT

The growth dynamics of phytoplankton, zooplankton and bacterioplankton in the River Rhine were analysed simultaneously with a number of environmental factors in order to identify environmental steering factors and to describe some of their interrelations. Observations on the metabolic activity (for algae and bacteria) and density (for all organisms) were carried out three times in 1990 using successive sampling of the same water parcel during its transport in the lower 660 km reach of the river. High algal densities (up to 170.5 mug chlorophyll a l-1), rotifers (up to 1728 l-1), crustaceans (up to 65 l-1) and bacteria (up to 16 x 10(9) l-1) were found. Algae and rotifers showed a rapid successive development during transport, while crustaceans were only abundant in the tidal reach of the river. In May, a vigorous growth of phytoplankton, zooplankton and bacteria was found. The diatom-dominated phytoplankton depleted the dissolved silicate in the river water and this led to a collapse of the populations, indicated by a decreased specific rate of photosynthesis (measured by the C-14 method) and vigorous bacterial growth (measured by [H-3]thymidine incorporation). Subsequently, the remaining phytoplankton diminished to very low levels near the river mouth. In July and September, it seemed that biological interactions within the plankton populations or between plankton and benthos balance the population densities so that separate developmental stages, as in spring, were less prominent. Estimates of the growth rates and loss rates of the phytoplankton were made. Phytoplankton exerted a substantial influence on the partitioning of nutrients (nitrogen, phosphorus, silicate) over water and suspended matter (as analysed by element analysis). It seems likely that only the reduction of phosphate, as planned under the Rhine Action Programme, and not that of nitrogen, may restrict the peaks of plankton growth described here.