In the present study we estimated differences in shell growth rates ofChamelea gallina (Linnaeus), from relatively food poor areas with growth rates from specimens collected from a frontal area, which has been regarded as highly productive. The results have been compared with growth rates obtained for two other benthic interface feedersArctica islandica (Linnaeus) andEchinocardium cordatum (Pennant) from the same geographical area.
The results suggest that skeletal growth of above three macro benthic invertebrates in organically poor sediments is faster when compared to growth rates of the same species from a food rich frontal region with fine grained sediment. This led to the idea that growth rates and food quantity are at least partly uncoupled. As an alternative hypothesis we think that food quality is one of the key factors to explain the observed differences.
To test this hypothesis we tried to estimate the differences in the quantity and quality of near bottom suspended matter at two opposing but hydrodynamically representative regimes in the southern North Sea. At these sites we deployed a new sediment water interface sampler, which caught particles from approximately 10 cm above the bottom. These samples were analysed for their ratio of chlorophyll to total dry matter to give an impression of the food quality.
In the shallow sandy sediments with strong tidal currents the highest ratio of chlorophyll: dry matter was found. In the silty frontal area with weaker tidal currents, which was supposed to be food rich, the food quality was poor because it was mixed with large quantities of resuspended silt. This dilution effect appeared to be related to the tidal cycle in combination with the difference in settling velocities of the mineral particles at slack tide. In the southern area the coarse sand grains quickly settle at slack tide. In the frontal area fine silt particles remain long in suspension thereby depressing the quality of suspended matter. This difference is believed to be an important mechanism behind the unexpected low growth rates in the frontal area when compared to the supposedly food deprived Southern Bight.