PELAGIC ECOSYSTEM RESPONSES TO NUTRIENT ADDITIONS IN ACIDIFIED AND LIMED LAKES IN SWEDEN

This article summarizes a series of experiments involving low-dose add itions of phosphorus, nitrogen, and organic carbon, performed to test hypotheses concerning the oligotrophication of acidified and limed lak es. Gradual fertilization of a whole lake stimulated productivity, via chrysophytes and cladoceran zooplankton, through the entire pelagic f ood chain without causing structural damage to the system. Although P is considered the primary limiting nutrient for phytoplankton growth i n acidified and limed lakes, in two out of four lakes we found a close balance between P and N limitation. In acidified Lake Njupfatet, phyt oplankton community structure changed depending on the form in which n itrogen, added together with phosphorus, was supplied. A cyanophyte wa s favored with ammonium, whereas a dinoflagellate dominated when nitra te was the nitrogen source. Chrysophytes, the desired ''edible'' phyto plankton, were only stimulated when organic carbon was added together with N and P. Many flagellates are mixotrophic and they may have respo nded to an increase in heterotrophic bacterial growth. Nutrient additi ons during late summer, when cyanophytes dominated, did not improve th e growth of any component of the plankton community. Liming of Lake Nj upfatet resulted in decreases both in phosphorus concentration and in standing stock of phytoplankton. The phytoplankton community after lim ing changed markedly, the cyanophytes disappeared, while the zooplankt on community composition remained typical of acidified lakes. Nutrient additions during late summer resulted in increased growth of both phy to- and zooplankton. We argue that controlled nutrient additions may b e a necessary complement to liming in order to restore acidified lakes .