Sustained stoichiometric imbalance and its ecological consequences in a large oligotrophic lake.

ABSTRACT

Considerable attention is given to absolute nutrient levels in lakes, rivers, and oceans, but less is paid to their relative concentrations, their nitrogenphosphorus (NP) stoichiometry, and the consequences of imbalanced stoichiometry. Here, we report 38 y of nutrient dynamics in Flathead Lake, a large oligotrophic lake in Montana, and its inflows. While nutrient levels were low, the lake had sustained high total N total P ratios (TNTP 60 to 901 molar) throughout the observation period. N and P loading to the lake as well as loading NP ratios varied considerably among years but showed no systematic long-term trend. Surprisingly, TNTP ratios in river inflows were consistently lower than in the lake, suggesting that forms of P in riverine loading are removed preferentially to N. In-lake processes, such as differential sedimentation of P relative to N or accumulation of fixed N in excess of denitrification, likely also operate to maintain the lake's high TNTP ratios. Regardless of causes, the lake's stoichiometric imbalance is manifested in P limitation of phytoplankton growth during early and midsummer, resulting in high CP and NP ratios in suspended particulate matter that propagate P limitation to zooplankton. Finally, the lake's imbalanced NP stoichiometry appears to raise the potential for aerobic methane production via metabolism of phosphonate compounds by P-limited microbes. These data highlight the importance of not only absolute N and P levels in aquatic ecosystems, but also their stoichiometric balance, and they call attention to potential management implications of high NP ratios.