Sources of Phosphorus and Nitrogen for Lakes on Afognak Island1

The contribution of inlet streams to the phosphorus and nitrogen budgets of two lakes on Afognak Island (Kodiak Region), Alaska has been studied with special reference to the morphology of the associated watersheds. One lake drains a steep, vast watershed; the other, a relatively flat, restricted one. Streams coming from the steeper slopes show a lower PO 4 ‐P concentration than those coming from shallow slopes as might be expected from runoff— residence considerations. Exactly the opposite is found when the nitrate concentration of these streams is examined, i.e. , only the streams arising on steep slopes show appreciable amounts of nitrate. Phosphorus appearing in the inlets probably originates in the Katmai ash which lies just under the humus layer of soil. Three distinct layers of ash occur normally. A complex pattern of phosphorus distribution can probably be attributed to differences in the permeability of these layers to water. A model has been constructed as an aid to interpretation of the data. Thickets of nitrogen fixing alders, found only along steep stream banks, may be the source of the nitrate content of these streams.