Long-term trends in particulate nutrients in offshore waters of the Laurentian Great Lakes

The Laurentian Great Lakes have undergone significant ecological changes in recent decades. We characterized patterns and long-term trends in particulate nutrient concentrations and seston stoichiometry from 1997 to 2019 across all five Great Lakes using spring and summer particulate nutrient data routinely collected by the US EPA GLNPO Water Quality Monitoring Program. We observed broad differences between the upper and lower Great Lakes, likely driven by varying surrounding land use, nutrient dynamics, light and hydrological regimes, and internal processes. Lakes Huron and Michigan exhibited the most significant changes in particulate nutrient concentrations throughout the study period. Despite changes in particulate nutrient concentrations, no significant trends in seston stoichiometric ratios were observed in any of the lakes. We identified two periods of abrupt change (mid-2000s and 2012) in the particulate nutrient time series. The first period of change was characterized by significant declines in total suspended solids, particulate phosphorus, and particulate organic carbon concentrations, concurrent with the timing of documented lower food web changes in Lakes Huron and Michigan but preceding the period of rapid quagga mussel expansion into the Great Lakes. Since 2012, particulate nitrogen has abruptly increased in all lakes, resulting in increasingly imbalanced seston N:P and C:N. These shifts may have broader ecological implications, potentially affecting organisms higher up the food web through altered nutrient availability and balance. Tracking particulate nutrients will continue to be critical for lake management.