High-elevation lakes in the Sierra Nevada are experiencing changing nutrient loading with concomitant shifts between P to N limitation of phytoplankton growth. Because N inputs have remained relatively constant (20+ years), increases in P supply are presumably influencing changes. We hypothesized that internal P loading from lake sediments and altered biogeochemical cycling in watershed soils, due to climate variations, are increasing P supply to high-elevation lakes. We evaluated P pools in soils and sediments using a sequential fractionation procedure and measured rates of P release during in-situ sediment core incubations.
Results/Conclusions
We found that high-elevation soils are not P-deficient and on average contain 867 µg P g-1 in A horizons and 597 µg P g-1 in B horizons. Of the total P found in soils, organic P (Po) accounts for ca. 60%, with labile pools of Po approximately doubling in content from winter through summer. In sediments, P concentrations are highest at the surface (1,445 µg P g-1; 0-2 cm) and decrease gradually with depth (793 µg P g-1; 20-30cm), presumably suggesting a shift to a more eutrophic state. However, of the total P, only 18 % is freely exchangeable or associated with reducible metal hydroxides. In both soils and sediments, Al controls the biogeochemistry of P, such that P retention in soil is high, and P is not significantly released from sediments even during hypolimnetic anoxia (0.05-0.1 µg P L-1 h-1). In lakes, 57 % of the variation in total dissolved P is best explained by Si, particulate C, and dissolved organic N concentrations and not by sediment P content. We conclude that internal P loading is not driving the eutrophication of Sierran lakes, and propose that a synergistic effect between a warming climate and increased rates of atmospheric N and P deposition, may be enhancing C storage in soils and thereby the soil Po pool, of which labile forms of P may be transferred to Sierran lakes.