Forest detritus and cycling of nitrogen in a mountain lake

Small lakes of forested watersheds can receive large subsidies of forest matter, but little is known about the material's role in the cycling of nutrients within these ecosystems. This paper examines the influence of detritus and dissolved nitrogen from a forest on the nitrogen cycle of a small subalpine lake in the Cascade Mountains of Washington during the ice-free period (98 days). Relationships between changing detrital microbial biomass, oxygen uptake rates, and water conditions indicate that dissolved inorganic nitrogen concentrations and water temperatures control the decomposition of the nitrogen-depleted detritus. The microbial respiration rates suggest the probable co-occurrence of several microbial oxidation and reduction reactions that could be cycling nitrogen in oxic–anoxic interfaces of detrital deposits, sediments, and riparian areas. Estimates of nitrogen gains and losses (3 and 7%, respectively) by forest detritus are low in comparison with total nitrogen uptake and releases within the lake during the study period (378 and 347 mg•m−2, respectively) and point to the need to examine other methods for measuring detrital nitrogen fluxes. The total nitrogen input to the lake (2600 mg•m−2 for the study period) from the watershed exceeds the lake output (2120 mg•m−2 for the study period). The low output of total nitrogen appears to be due to retention of dissolved inorganic nitrogen and particulate organic nitrogen within the lake. Most dissolved inorganic nitrogen retained is nitrate suggesting possible losses through denitrification. Dissolved organic nitrogen is the major proportion of the total nitrogen fluxes, but related mass balance errors indicate the need for further definition of both the sources and fates of dissolved organic nitrogen for the ecosystem.