Exchange of Nutrients Between Sediments and Water After 15 Years of Experimental Eutrophication

Increases in the concentrations of nitrogen and phosphorus occurred in the water column of Lake 227 during 15 years of fertilization with phosphoric acid and sodium nitrate. The sources of the increases were deduced by comparing the budgets of N and P to the budget for sodium, using a simple model, and from the chemistry of sediment pore water.Total inventories for sodium since fertilization began confirmed that the element was almost perfectly conservative, as expected. When model results for phosphorus and sodium were calculated, the apparent increase in phosphorus was found to be due entirely to lower water renewal rates during recent years. This agrees with the observation that phosphorus concentrations in sediment pore water did not change between 1975 and 1982. Both phosphorus and sodium in the lake have been in equilibrium with external loading for the past several years.In contrast, the observed increase in nitrogen could not be accounted for by the decreased water renewal rate, implying that the element was either becoming more efficiently recycled or that there was an increasing, unmeasured external source of the element. An increase in pore water ammonium between 1975 and 1982 was balanced by an increase in the concentration in overlying waters, so that diffusive fluxes did not change. Increasing blooms of nitrogen-fixing bluegreen algae after 1975 indicate that N2 fixation is the increasing source of nitrogen to the lake. No tendency for the nitrogen:sodium ratio to stabilize has been observed, indicating that the nitrogen budget has still not reached a new steady state.