Comparision of historical and recent data on hydrochemistry and phytoplankton in the Rijnland area (The Netherlands)

In two canals and two lakes in the western part of the Netherlands a comparision is made between data on water chemistry and phytoplankton from 1941/1942 with recent data. Orthophosphate in particular, but also inorganic nitrogen, has increased tremendously, especially in the Gouwe canal, where Rhinewater enters the area. The inorganic N/P mass ratio decreased in the last 45 years, indicating that the limiting nutrient has changed from phosphate in 1941 to nitrogen in 1987.The average seston volume, measured by filtering 1001 water through a plankton net (50 µm), has doubled. In the early 1940s the blue-green alga Microcystis aeruginosa regularly formed waterblooms, as it does now. The plankton composition seems to have become impoverished in the last decennia, since several taxa have disappeared. Others are strongly reduced in number. The saprobic index has not changed.Based on the relations between chlorophyll-a and biological oxygen demand (BOD) and between transparency and seston volume, BOD, dry weight and ash free dry weight in 1987 chlorophyll-a and transparency in 1941 are estimated. The average chlorophyll-a concentration in the lakes has doubled or tripled in the last 45 years and the mean transparency in the Gouwe canal declined from 75 to 50 cm. Submerged higher plants at some sites have disappeared in the last decennia. The results can be used to develop ecological objectives for combatting eutrophication in canals and lakes and possibly in the river Rhine.