Pelagic ecosystem responses to nutrient additions in acidified and limed lakes in Sweden

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