Nitrogen, macrophytes, shallow lakes and nutrient limitation: resolution of a current controversy?

Phosphorus (P) is conventionally thought to limit production in freshwaters and nitrogen (N) that in the sea. Before much human activity, however, co-limitation by N and P was probably normal, with systems developing ratios of N to P tending to the Redfield ratio. Single-factor limitation may be a symptom of human activity in many cases. It is widely believed that N fixation should compensate for N shortage because N fixers are ubiquitous and versatile, but this is not always the case and the issue has hitherto been considered largely with respect to plankton communities. Effects of N on macrophyte communities provide justification for control of both nutrients, at least in shallow lakes and estuaries. Increased N loading reduces plant biodiversity, changes the structure, and is associated with eventual loss, of macrophyte communities. P control alone may suffice in many deep lakes where denitrification is low and stratified conditions favour cyanobacterial development. Therein may lie a resolution to current controversies.