Studies on the ability of a Potamogeton pectinatus community to remove dissolved nitrogen and phosphorus compounds from lake water.
Enclosures (4.5 m3) in a dense Potamogeton pectinatus community were enriched by weekly additions of NaNO3 and KH2PO4 at four rates (max. 1000 mu g N l-1 week-1 and 100 mu g P l-1 week-1) over a period of 15-23 weeks. The most marked effect of enrichment was the development of a dense filamentous algal population. There was no parallel development of planktonic algae, and chlorophyll a levels remained low in all treatments. Enrichment did not increase Potamogeton biomass. After 9 weeks at all levels of enrichment the community removed all the added N and P within 1 day. 32P tracer experiments showed that most of the added P was absorbed, in the short term (2 h), by the filamentous algae associated with the macrophytes. Concentrations of N and P per unit dry mass in the algae, macrophytes and sediments were increased significantly only by the highest enrichment treatment. Decomposing filamentous algae apparently provided the major input of N and P to the sediments in this treatment. The data indicate that dense submerged macrophyte beds with their associated epiphytic algae may in some areas be useful nutrient filters.