Nitrogen Fixation Does Not Axiomatically Lead To Phosphorus Limitation In Aquatic Ecosystems

A long-standing debate in ecology deals with the role of nitrogen and phosphorus in management and restoration of aquatic ecosystems. It has been argued that nutrient reduction strategies to combat blooms of phytoplankton or floating plants should solely focus on phosphorus (P). The underlying argument is that reducing nitrogen (N) inputs is ineffective because N-2-fixing species will compensate for N deficits, thus perpetuating P limitation of primary production. A mechanistic understanding of this principle is, however, incomplete. Here, we use resource competition theory, a complex dynamic ecosystem model and a 32-year field data set on eutrophic, floating-plant dominated ecosystems to show that the growth of non-N-2-fixing species can become N limited under high P and low N inputs, even in the presence of N-2 fixing species. N-2-fixers typically require higher P concentrations than non-N-2-fixers to persist. Hence, the N-2 fixers cannot deplete the P concentration enough for the non-N-2-fixing community to become P limited because they would be outcompeted. These findings provide a testable mechanistic basis for the need to consider the reduction of both N and P inputs to most effectively restore nutrient over-enriched aquatic ecosystems.