The impact of suspended sediments and phosphorous scarcity on zebra mussel and quagga mussel growth
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Dreissenids are a key stone species for the IJsselmeer area in the Netherlands. However, the zebra mussel (Dreissena polymorpha) populations in the IJsselmeer and Markermeer are in poor conditions relative to 20 years ago. Nowadays, quagga mussels (Dreissena rostriformis bugensis) are dominant in both lakes after their recent invasion. It was tested whether the high suspended sediment concentrations in the Markermeer and/or the relatively low phosphorous content of the phytoplankton in both lakes might underlie the observed population developments. Two full factorial microcosm experiments were conducted with zebra and quagga mussels. In the first experiment, the mussels were reared under four different suspended sediment concentrations (0, 32, 80 and 200 mg l-1). In the second experiment, zebra and quagga mussels were fed with Scenedesmus obliquus with C:P ratios of 520 and 287. Growth rates were very constant over the suspended sediment levels and did not differ between both species but at the highest sediment level, where zebra mussel growth rate peaked. Mussel tissue dry weight was significantly lower in the high C:P level than in the low C:P level. However, the mussel tissue C:P ratio was not significantly different between the two C:P ratio treatment levels. These results suggest an upper tissue C:P limit of 110 for zebra mussels and 150 for quagga mussels. Furthermore, quagga mussels appeared to have lower phosphorous requirements than zebra mussels. The current suspended sediment levels in the Markermeer are unlikely to hinder zebra mussel growth or to benefit quagga mussel growth over zebra mussel growth. In contrast, the relative low phosphorous content of the phytoplankton in both lakes might be detrimental to zebra mussel growth, while quagga mussel growth will be less affected.