Changes in plankton communities in regulated reaches of the lower river rhine

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Abstract
The Dutch section of the Lower Rhine is highly modified by regulating measures, such as the construction of weirs and the closure of estuarine river mouths, which seriously affect sedimentation of suspended material. During 1987 the fate of plankton communities in this part of the Rhine was investigated. Water samples were taken fortnightly at the German‐Dutch border and in the main branch of the Rhine, just before it debouches into the North Sea. Dry weights of suspended matter, bacterial numbers, phytoplankton biomass (as chlorophyll a) and zooplankton composition and biovolumes were determined, and, at two occasions, phytoplankton composition and biovolumes as well. Supended matter and bacteria disappeared during transport in the Rhine Delta. phytoplankton biomass (primarily diatoms) showed typical spring and summer peaks and also decreased in the investigated river reach. In contrast, zooplankton developed a higher biomass due to an increase in the numbers of rotifers, crustaceans, and molluscan larvae, whereas arcellas and ciliates decreased in numbers.It is concluded that the Rhine, after flowing more than 800 km, meets in its delta the proper conditions for a true plankton community to develop: settling and trophic interactions become of increasing importance in determining the structure of plankton communities.