Silica uptake of the marine sponge Halichondria panicea in Kiel Bight

The silica uptake of Halichondria panicea Pallas, 1766 was measured in laboratory experiments from February until August 1995. Uptake rates were determined by measuring the decrease of dissolved silica in the surrounding seawater. All specimens were collected in Kiel Bight and maintained in aquaria up to several weeks prior to the experiments. Up to 5.27 mu mol Si g(-1) AFDW h(-1) were incorporated by the sponges, with a strong, size-independent individual variation. A positive correlation between the content of dissolved silica in the seawater and the silica uptake was found. Temperature had no decisive effect on uptake rates. The nutritional condition of the sponges turned out to be of great importance. After 1 wk of starvation, Halicondria panicea uptake rates were only 15% of the previous amount, which indicates that silica transport is an energy-consuming process. During the most intense phase of reproduction activity in spring, female specimens showed a significant drop in their silica uptake. Obviously they did not produce spicules during this time. An attempt to estimate the influence of H. panicea on its habitat as a consumer of dissolved silica led to the conclusion that in summer the sponges might affect the phytoplankton species composition by competing with diatoms for dissolved silica.