Freshwater inflow, riverine nutrient load and salinity: Unraveling dominant drivers of dissolved inorganic nutrients...

Nutrient dynamics in bay-estuary systems are shaped by the complex interplay between hydrology, biogeochemistry and hydrodynamics. Riverine nutrient loads are widely recognized as primary drivers of nutrients, yet how these loads translate into internal nutrient dynamics across varying hydrologic and salinity regimes remains poorly resolved. We addressed this gap by investigating long-term (19 years) dynamics of dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) loads and their role in governing nutrient variability under varying freshwater inflow and salinity regimes in Apalachicola Bay, Florida. We applied a series of generalized additive models (GAMs) that incrementally accounted for freshwater inflow, nutrient load and estuarine salinity alongside Neural Additive Models (NAMs) with concurvity regularization to account for the correlation among these drivers. Our results showed that riverine nutrient load is the dominant driver of estuarine nutrient dynamics. Although freshwater inflow and riverine nutrient loads are often used interchangeably in past studies, we show that "inflow as a proxy for nutrient load" is an oversimplification, particularly under low-flow conditions. We further showed that estuarine DIN exhibits more predictable, seasonally coupled variability with freshwater inflow and salinity. In contrast, episodic and localized biogeochemical processes make DIP dynamics less predictable and obscure the influence of salinity on DIP at monthly timescales. However, we showed that the dependence of estuarine DIN concentrations on the riverine load is strongly structured by salinity regimes, with fresher conditions favoring this dependence. Increasing variability in sea level and freshwater supply under climate change can make these dynamics less predictable.