Eutrophication and algal growth in the North Sea. In: Symposium Mediterranean Seas 2000

Abstract

The North Sea is a shallow shelf sea with an area of 0.6 million km^2, surrounded by densely populated land areas. The general water circulation is counter-clockwise, and an annua1 freshwater input of 400 km^3 results in characteristic coastal water masses and currents. The North Sea receives an annual input of about l million tons of nitrogen (N) per year by 3rivers and run-off. Most of this nitrogen input is of anthropogenic origin and comes to the shallow southern part of the North Sea. The nitrate concentration of major rivers is 300-500 uM in winter and the annual mean N concentration of the freshwater discharge to the southern North Sea is about 300 uM. When diluted 23-fold, corresponding to a salinity of 33.5 if diluted with Atlantic water, the freshwater N is approximately equal to the natural N content of the Atlantic seawater. This level of anthropogenic N loading affects about 30 % of the volume flow through the central and southern North Sea. The average NIP ratio for the annua1 inputs of N and phosphorus (P) in 1990 was 30, which is almost twice as high as the typical ratio of 16 for marine waters. This leaves a large surplus of about 400 thousand tons of N remaining when P is depleted by algal growth in the coastal water. The excess N is exported with the coastal currents and is utilized under P limitation over an extended area. The increase in nitrate-based new production due to anthropogenic eutrophication is about 30 % for the whole North Sea. The change to P limitation of large coastal water masses is perhaps the most significant ecological change due to eutrophication. This could have particular importance for the strongly stratified waters of Kattegat and inner Skagerrak. Here nutrients advected from the southern North Sea by the Jutland Current are entrained from below the pycnocline into the euphotic zone, whereas local input of nutrients comes primarily to the upper layer. Eutrophication of stratified waters may be particulary inducive of promoting growth of flagellates, including harmful and toxic species. P limitation may aggrevate this situation. Dinoflagellates are as a group characterized by slow growth, and their ecological success is assumed to reflect a compensatory high ability to survive. A relationship between P requirement and growth rate is postulated, reflecting the general proportionality between content of nucleic acids and growth rate. Based on this, a hypothesis is put forward that suggests a shift towards greater dominance of slow-growing algae, including red tide dinoflagellates, with increasing NIP ratio of marine systems. Recent blooms of toxic Prymnesiophytes, such as the 1988 bloom of Chrysochromulina polylepis in Kattegat and Skagerrak, suggest amother link between P limitation and harmful algae. Prymnesiophytes seem to have a good ability to grow on organic P and to be good competitors under P limitation. It is hypothesized that Chrysochromulina type algae are usually living in association with organic marine snow aggregates which presumably are P limited microenvironments. Massive blooms of these organisms may be exceptional events made more likely by changes due to eutrophication. Increased nutrient loading and associated P limitation may cause changes in nutrient conditions of macroscale coastal environments which resemble the conditions of their microscale habitats.