In Dutch marine circumstances, sand extraction releases silt into the water column. The extra silt can reduce light penetration into the water and consequently algal growth. To predict potential effects of an expansion of sand extraction activities it is necessary to know possible impacts on the environment. Ensis directus, a dominant species web of the North Sea coastal zone, has a key position in the food web. Therefore, it was selected as model species in this study to predict the effects of the reduced food conditions due to sand extraction on the growth of E. directus. A DEB (Dynamic Energy Budget) model is in development. This study describes the basic experiments that have been done to determine empirical relations between clam size or food concentration and filtration, respiration and growth rates necessary for the DEB modelling. Also, the basic values on physiology itself have their value because little is known on this species. Filtration and respiration rates were measured at four food levels (2, 5, 20 and 40 μg chlorophyll a/l). Clam shell length varied from 42 to 135 mm. Filtration rate decreased with an increase in clam size from maximally 3.3 lh-1 g-1 ash-free dry weight (ADW) to 0. lh-1 g-1 ADW. There was no relation between food concentration on filtration rate. Respiration rates showed a similar decrease with clam size from maximally 5000 mg O2 lh-1 g-1 ADW to 1500 mg O2 lh-1 g-1 ADW. In addition, an increase in respiration rate was found with an increase in food concentration. In the growth experiment five food levels were tested (0, 2, 5, 20 and 40 μg chlorophyll a/l).Clams smaller than 75 mm shell length showed more growth (up to 1% increase in wet weight (WW) per day or 0.3% shell length per day) than larger clams (maximally 0.16% increase in WW per day or 0.01% shell length per day). Growth rates showed an increase with increased food concentration.
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