Effects of phytoplankton on metal partitioning in the lower River Rhine

Since algal growth has been shown to play a key role in determining the 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, thereby altering the retention of metals in the Rhine delta. In a seasonal study in which variations in the partitioning of Mn, Cu, Zn, Cd and Pb (expressed as log Kp values) were correlated with phytoplankton parameters, it appeared that manganese occurred mainly in the particulate form during algal blooms, whereas dissolved manganese predominated during periods low in phytoplankton. Photosynthetic activity (up to 700 μg C l−1 h−1) correlated slightly better with the [log Kp]-Mn than the chlorophyll a concentration (up to 140 μg l−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 seasonal differences in phytoplankton and manganese partitioning, although increased values of [log Kp]-Zn had been indicated for the summer of 1983, when metal concentrations had generally been higher than in 1990. The lack of effect of riverine phytoplankton blooms on partitioning of 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 short residence in the river, are consistent with the observations.