The Netherlands’ coast being sandy and muddy is not suitable for most algal growth. Dikes, piers and harbour-works replace the rocks elsewhere. The pier at Hoek van Holland is one of these artificial rocky coasts. It has been constructed with basalt blocks and other hard stones, viz. the so called “Nilvoordste steen” and “Doornikse steen”, both from Belgium. The pier projects into the sea about 1350 m. On the southern side it is washed by the mouth of the Nieuwe Waterweg, on the northern side by the North Sea. A little more (fig. 1) up the Nieuwe Waterweg there are several basins enclosed by a dam parallel to the pier and a number of dams at right angles to it, thus replacing the rockpools of natural rocky shores.