Algal co-limitation by N and P persists after 30 years in Mirror Lake (New Hampshire, USA)

摘要

The fundamental question of limits to ecosystem productivity is a continually important topic in the field of limnology. Historically, phosphorus (P) has usually been considered the most important limiting nutrient for primary production in freshwater lakes (SCHINDLER 1977). However, the recognition of limitation or co-limitation by other nutrients, in particular nitrogen (N), has generated further interest and understanding about processes that control lake productivity (ELSER et al. 1990). In light of persistent nitrogen pollution in many regions, including the northeastern United States (GALLOWAY & COWLING 2002, ABER et al. 2003), there is even more impetus to examine the potential for nitrogen limitation. Nonpoint sources of nitrogen pollution are considered to be a key factor in eutrophication of inland waters (CARPENTER et al. 1998). Indeed many examples show lakes becoming less limited by nitrogen, or increased productivity due to nitrogen stimulation (JASSBY et al. 1995, BERGSTROM & JANSSON 2006). Several methods were used to asses the nutrient limitation status of Mirror Lake in the early 1970s (GERHART 1975, GERHART & LIKENS 1975). In those studies, N and P co-limited phytoplankton primary production. Mirror Lake is located in New Hampshire, United States (LIKENS 1985) in an area known for moderately high deposition of atmospheric nitrogen (OLLINGER et al. 1993). Long-term studies of chemical mass balances conducted from 1980 to 2000 suggested the possibility that the nutrient limitation status of Mirror Lake may have changed; therefore, we repeated one of Gerhart's experiments with the intent of discovering whether there had been any change in nutrient limitation in Mirror Lake. Specifically we addressed the question of whether N and P were still co-limiting in Mirror Lake. Surprisingly, we again found co-limitation by N and P after 30 years of continued atmospheric nitrogen pollution.