EFFECTS OF PHYTOPLANKTON ON METAL PARTITIONING IN THE LOWER RIVER RHINE

Since algal growth has been shown to play a key role in determining th e fate of metals in lakes and marine waters, we wondered if the Rhine phytoplankton, so much stimulated by nutrient input, would affect the partitioning of metals between the dissolved and particulate phases, t hereby altering the retention of metals in the Rhine delta. In a seaso nal study in which variations in the partitioning of Mn, Cu, Zn, Cd an d Pb (expressed as log K-p values) were correlated with phytoplankton parameters, it appeared that manganese occurred mainly In the particul ate form during algal blooms, whereas dissolved manganese predominated during periods low in phytoplankton. Photosynthetic activity (up to 7 00 mu g C 1(-1) h(-1)) correlated slightly better with the [log K-p]-M n than the chlorophyll a concentration (up to 140 mu g 1(-1)) and the pH (up to 8.35), suggesting that phytoplankton photosynthesis promotes Mn precipitation in the river. The variability in the partitioning of Cu, Zn, Cd and Pb in 1990 did not seem to be determined by the season al differences in phytoplankton and manganese partitioning, although i ncreased values of [log K-p]-Zn had been indicated for the summer of 1 983, when metal concentrations had generally been higher than in 1990. The lack of effect of riverine phytoplankton blooms on partitioning o f metals other than Mn contrasts with observations in stagnant waters. However, the low levels of cellular metal observed in cultures, along with the single growth pulse that phytoplankton shows during its shor t residence in the river, are consistent with the observations.