Two-third of the area of the Netherlands is flood prone. Storm surges at the North Sea, floods in the rivers, failure of secondary dikes, as well as heavy rainfall may cause flooding. Most of the flood prone areas rely for their existence on drainage by pumping, because their surface level may be permanently or during floods up to several metres below the surrounding water levels. During the past decades climate change is generally felt as a reason for major concern. However, the impacts of climate change on increase in extreme conditions may be up to 45% per century. If we look at the man-induced changes in land use, due to increase in population and rapid increase in the value of public and private property, then one may observe an increase of 100–1,000% per century. One should therefore wonder what would have to be our major concern.

In the paper the relevant processes are described, based on some characteristic data on water management and flood protection in the Netherlands. It is shown that impacts of climate change on water management and flood protection may be expected, but that such impacts can easily be accommodated during improvement works on water management systems and flood protection provisions that from time to time will be required. It will be much more important to take carefully into account the risk of flooding in the planning of land use development, especially for valuable types of land use like urban and industrial areas, green houses and recreation areas.

The occurrence of 18 species of algae was stated in the investigated region. Among them the following were predominant: Himantothallus grandifolius, Desmarestia menziesii, Cystosphaera jacqninotii, Ascoseira mirabilis. Leptosomia simplex. Adenocystis utricularis. Monostroma hariotii, Iridaei: obovata, Hildenbrandia lecunnellieri, Plocamium coccineum and Phycodrys antarctica. Vertical stratification of the distribution of three singled out communities of algae was observed downwards to the depth of 90 m, which is the limit of the occurrence of the algae in the Bay. The process of decomposition depends on the place where it occurs, the kind of the thalluses and the season of the year. The quickest decomposition of algae was observed on the shore, in the summer and spring. The total quantity of algal matter washed ashore along 15.8 km of the coast line of Admiralty Bay, during the period between February and October 1979, was estimated at 279 metric tons of dry weight matter. From this quantity, in result of decomposition of the algae on the shore, 75 tons of the matter were released during an average time of 12 days. The remaining 204 tons of partially decomposed algal matter are driven by winds farther inshore or into the waters of the Bay or remain ashore among the stony rubble.