Sources and Influences of Allochthonous Inputs on the Productivity of a Subalpine Lake

Allochthonous and autochthonous inputs of soluble reactive phosphate, dissolved organic P, particulate P, dissolved inorganic N, total N, dissolved organic C, and particulate organic C were measured seasonally over a 2—yr period in a subalpine lake of a coniferous forest. Such data have not previously been collected for this lake type. Using a new error analysis technique, nutrient budgets were constructed and analyzed for patterns and relative importance of material flow pathways. Of total annual particulate organic C inputs to the water column of 15.9 g/m2, °83% was allochthonous, mainly from fluvial and snowpack inputs in the spring and litter inputs in the fall. Annual allochthonous inputs of soluble reactive P and dissolved inorganic N averaged 0.12 g/m2 and 2.24 g/m2, respectively; spring fluvial sources provided 33%—50% of these annual incomes. Zooplankton excretion of soluble reactive phosphate was comparable to allochthonous inputs of soluble reactive phosphate, and °18% of total inputs of dissolved inorganic N was autochthonous. Particulate losses were assignable to both fluvial and sedimentation processes, whereas °70% of soluble reactive phosphate and dissolved inorganic N incomes were lost through the outflow and the rest to uptake. Allochthonous dissolved organic carbon inputs (68 g/m2) and allochthonous dissolved organic P inputs (0.21 g/m2) were mostly fluvial and °100% were lost fluvially. These data are related to those of other lakes, and patterns and apparent anomalies are discussed. These results suggest that terrestrial inputs to the sediments, and the subsequent cycling of nutrients in the sediments, have a major influence on biological activity in both benthic and water—column environments during most of the growing season.