Arenicola marina is the largest and quantitatively most important deposit feeder in the Wadden Sea. Sediment reworking by Arenicola was measured as the amount of faeces produced. At two stations on tidal flats near the southern part of the island Texel faeces was collected regularly over periods of one year in 1910 and in 1975. A large day to day variation was observed. Values for 1970 compare well with those for 1975. Annual sediment reworking at the two locations was estimated to correspond with a sediment layer of 14 and 33 cm, respectively. The difference between the 2 locations is largely due to a difference in density of (adult) Arenicola, being on average 42.5 and 85 per m², respectively. A density of 85 per m² is near the maximum density in the Dutch Wadden Sea. The average density of Arenicola on tidal flats in the Dutch Wadden Sea was estimated by BEUKEMA (1976) to be 17 per m². This tigure corresponds with a sediment layer of 6 to 7 cm reworked annually for the whole Dutch Wadden Sea. Sediment reworking showed a marked seasonal cycle, summer values being about one order of magnitude higher than winter values. Temperature alone cannot explain this as maximum summer temperatures do not coincide with maximum faeces production, and spring values for faeces production are higher than autumn values at the same temperature. Food availability also seems to be an important factor. As already observed by others, feeding activity of Arenicola results in a biogenic graded bedding in the sediment. Sediment particles smaller than 300 to 400 µm for preference. As a result coarser particles are concentrated at feeding depth. Organic carbon and organic matter reach highest values at this depth due to a concentration of undigestible peat detritus. Water content of the sediment was not affected by the activity of Arenicola
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