Relative contributions of terrestrial and oceanic nutrient loading on phytoplankton concentration in an estuary

Watershed-sourced nutrient loading to estuaries depends on both the freshwater flow into the estuary and the concentration of nutrients in that flow. Similarly, oceanic-sourced nutrient loading depends on estuarine exchange flow rates and the concentration of nutrients in that flow. Regardless of the source, the loading responds to climate change, which results in changes to precipitation, freshwater flows, ocean currents, upwelling, and land-use changes, which alter both nutrient concentrations and the amount and timing of freshwater flows. The interaction of physical and biological processes in the estuary determine the location and magnitude of phytoplankton blooms that may be triggered or fueled by these terrestrially or oceanic-sourced nutrients. These blooms can result in oxygen depletion and large-scale ecosystem impacts. This study utilizes a novel, idealized physical-biological box model of a hypothetical estuary to separate the physical and biological processes driving the development of the bloom from both terrestrial and oceanic nutrient sources. We find that the magnitude of blooms is largely governed by the nutrient concentrations in the river or oceanic flows, but the location and extent of the blooms are dominated by physical processes driven by the river and oceanic flows. River-sourced nutrients tend to have a stronger influence on blooms than ocean-sourced nutrients at the same concentration since they enter the estuary in the photic zone.