Abstract
Concentrations of nitrogen (N) and phosphorus (P) in lakes may be differentially impacted by climate-driven changes in nutrient loading and by direct impacts of temperature and wind speed on internal nutrient cycling. Such changes may result in systematic shifts in lake N:P under future climate warming. We used 21 years of monitoring data to compare long-term and intra-annual trends in total N (TN), total P (TP) and TN:TP at 15 sites in Lake Champlain to concurrent measurements of watershed nutrient inputs and meteorological drivers. TN:TP declined sharply lake-wide, particularly in the past decade, yet the drivers of this trend varied based on site depth. In deep sites, declines were driven by changes in watershed loading of dissolved P and N and (in some cases) by decreases in hypolimnetic dissolved oxygen. In shallow sites, declines in TN:TP were primarily driven by long-term increases in temperature and decreases in wind speed, and exhibited systematic seasonal variability in TN:TP due to the timing of sediment P loading, N removal processes, and external nutrient inputs. We developed a conceptual model to explain the observed trends, and suggest that while climate drivers have affected nutrient dynamics in shallow and deep sites differently, both deep and shallow sites are likely to experience further declines in N:P and increases in cyanobacteria dominance if recent climate trends continue.
Highlights
Nitrogen (N) and phosphorus (P) are the primary nutrients limiting phytoplankton production in lakes (Guildford and Hecky 2000)
Lake Champlain contains five major basins separated by human-made and natural barriers, and different regions of the lake experience a wide range of trophic conditions (Xu et al 2015b)
Annual total N (TN):TPlake of the deep, central Main Lake station was significantly correlated with aggregate annual dissolved nutrients (DN):DPriver (p = 0.00733, R2 = 0.322), but was not correlated with aggregate annual TN:TPriver. These results suggest that dissolved riverine nutrient inputs had more influence on TN:TPlake than total riverine nutrient inputs in the main lake basin and that changes to their ratio in river water were driving the observed trends over the monitoring period in deeper areas of Lake Champlain
Summary
Nitrogen (N) and phosphorus (P) are the primary nutrients limiting phytoplankton production in lakes (Guildford and Hecky 2000). Climate change may affect lake N:P indirectly through changes in mass fluxes of water and nutrients to lakes mediated by lake catchments, or directly through changes in fluxes of energy to lakes (air temperature, wind, or solar radiation) (Leavitt et al 2009; Blenckner 2005). Recent studies suggest that external loads of P have already increased in some lakes as a result of changing climate patterns, despite reductions in anthropogenic inputs (Jeppesen et al 2009). Higher energy storms are expected to lead to increased transport of particulate nutrients, while the effects on dissolved nutrient transport are less clear (Sharpley et al 2008). Because P transport is frequently dominated by the particulate fraction while most N is transported in dissolved form, Biogeochemistry (2017) 133:201–217 stronger storms may result in changing N:P of river loads (Rosenberg and Schroth 2017). Climate-driven changes in N and P loading are likely to interact with changes in land use and land management practices, potentially resulting in further changes to lake N:P (Michalak et al 2013)
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