Nutrient concentrations, loading, and N:P stoichiometry (1983 - 2020) and impacts in Flathead Lake (Montana, USA)

Considerable attention is given to absolute nutrient levels in lakes, rivers, and oceans but less is paid to their relative concentrations, their N:P stoichiometry, and to the consequences of imbalanced stoichiometry. Here we report 38 years of nutrient dynamics in Flathead Lake, a large oligotrophic lake in Montana (USA), and its inflows. While nutrient levels were low, the lake had sustained high total N : total P ratios (TN:TP: 60-90:1 molar) throughout the observation period. N and P loading to the lake as well as loading N:P ratio varied considerably among years but showed no systematic long-term trend. Surprisingly, TN:TP 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 TN:TP ratios. Regardless of causes, the l...