Ruurd Noordhuis
watervogels, voedselrijkdom, slib, mosselen, spiering
Summary
Water quality and ecological changes in Lake Markermeer-IJmeer
Waterbirds, nutrients, silt, mussels, smelt
During the 1990s, water quality and ecology of Lake Markermeer-IJmeer changed. Water transparency diminished and numbers of several species of waterbirds decreased, following the decline of their main prey, zebra mussels (Dreissena polymorpha) and smelt (Osmerus eperlanus). Lake Markermeer-IJmeer has high concentrations of suspended sediment. However, not an increase of suspended silt, but rather a decrease of nutrient concentrations caused the changes. Phytoplankton became phosphorous limited instead of light limited and species composition changed in favour of species with low phosphorous demand. Hence, food quality of phytoplankton decreased. In addition, f locculation of algae with silt particles removed part of the algae from the food web. Growth rate and caloric value of zebra mussels decreased. Numbers of several species of benthivorous birds declined. However, remaining birds now have a more diverse diet, and some species benefit from increase of macrophytes, foraging on plants, gastropods and gammarids. The explanation of the decrease of the smelt population is more complex, entailing interaction of decreasing food availability, climate change and commercial fishery. Some of the piscivorous bird species may benefit from increased habitat diversity as well, if this leads to a more diverse fish stock.
