MONITORING OF ALGAE IN DUTCH RIVERS - DOES IT MEET ITS GOALS

Observations on phytoplankton in the lower reaches of the rivers Rhine and Meuse were carried out in the framework of the national monitorin g programme of the main water systems in The Netherlands. Seasonal cha nges in density and species composition were analysed to detect the ma jor variables and to discuss whether this monitoring meets its goals. Phytoplankton reached peak densities of 140 and 65 mu g L-1 Chi alpha in the Rhine and Meuse, respectively. Overall density was correlated w ith seasonal variation in water discharge rather than with nutrient co ncentrations, that are high in both rivers. The position of sampling s ites in relation to the downstream development of the plankton was ver y important, especially in the regulated Meuse. Despite hydrographic a nd chemical differences between the two rivers, many species, predomin antly diatoms and green algae, were shared. The occurrence of the diat om Skeletonema subsalsum in the Rhine and the green algae Neodesmus da nubialis, Micractinium pusillum and Pseudotetrastrum punctatum in the Meuse (1992, but not 1996) was interpreted as a feature related, respe ctively, to the high salinity of the Rhine and specific riverine condi tions of the Meuse. In general the potamoplankton was characterised as an opportunistic assemblage exploiting the high nutrient contents and disturbed hydrography of both rivers. Nevertheless, the phytoplankton contributed to the characterisation of the present river communities, ones that differ rather strongly in historic records. Although year t o year variation in phytoplankton density is mainly related to variati on in water discharge, phytoplankton biomass did increase as a result of eutrophication over a period of decades. The widespread nature of m any of today's potamoplankton species in the two rivers render these o rganisms less useful for indicating short-term changes in water qualit y. However the importance of phytoplankton in trophic relationships me rits its inclusion in the monitoring of riverine ecosystems.