期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2022
卷号：119
期号：30
DOI：10.1073/pnas.2202268119
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：Significance
Both nitrogen (N) and phosphorus (P) drive water quality and are heavily altered by human activities that amplify their supplies to lakes, rivers, and oceans. Considerable attention is given to management of absolute nutrient levels but less to their relative abundances, their N:P stoichiometry. This study documents high N:P ratios in low-nutrient Flathead Lake that persist despite considerably lower N:P ratios in river nutrient inputs. The lake’s elevated N:P ratios are associated with phytoplankton P limitation, impaired food quality for zooplankton, and potential production of the greenhouse gas methane by P-limited microbes. These findings highlight the need to consider not only absolute levels of N and P in aquatic ecosystems, but also their stoichiometric balance.
Considerable attention is given to absolute nutrient levels in lakes, rivers, and oceans, but less is paid to their relative concentrations, their nitrogen:phosphorus (N:P) 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 (TN:TP: 60 to 90:1 molar) throughout the observation period. N and P loading to the lake as well as loading N:P ratios 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 lake’s stoichiometric imbalance is manifested in P limitation of phytoplankton growth during early and midsummer, resulting in high C:P and N:P ratios in suspended particulate matter that propagate P limitation to zooplankton. Finally, the lake’s imbalanced N:P 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 N:P ratios.
