Systematic Quantification of Nearshore and Offshore Submarine Groundwater Discharge Along Florida Coasts

Abstract

Submarine groundwater discharge (SGD), comprising both nearshore and offshore components, plays a vital role in water cycling and solute transport in coastal areas, and affects coastal marine ecosystems. Previous estimations of SGD based on seepage meters, geochemical tracers, water balances, analytical, and numerical approaches frequently overlooked offshore contributions driven by oceanic currents, waves, and tides, resulting in an incomplete understanding of SGD dynamics and its ecological consequences. Therefore, this study quantified the total SGD by integrating offshore (current-, wave-, and tide-driven SGD) and nearshore (fresh SGD and tide-driven SGD) components in Florida coasts. The calculated total SGD was approximately 15.08% of annual precipitation volume in Florida, with 14.09% offshore SGD (0.7%, 8.2%, and 5.2% from currents, waves, and tides, respectively) and ∼0.986% nearshore SGD (0.44% and 0.55% from fresh and recirculated SGD), underscoring offshore SGD as a major driver of groundwater discharge extending across the continental shelf. Moreover, nearshore SGD-derived dissolved inorganic nutrient fluxes were estimated as $\left(4.2\pm 0.2\right)\times {10}^5$ kg/yr for nitrogen and $\left(6\pm 1\right)\times {10}^4$ kg/yr for phosphorus, whereas offshore SGD-derived nutrients were $\left(2.7\pm 0.5\right)\times {10}^{10}$ kg/yr for nitrogen and $\left(6.2\pm 1.5\right)\times {10}^9$ kg/yr for phosphorus. On average, these nutrient inputs were approximately 6 and 4 times greater than those from surface water nutrient fluxes from coastal river discharge for dissolved inorganic nitrogen and dissolved inorganic phosphorus, respectively, highlighting the significant role of SGD in nutrient cycling in Florida. Additionally, we identified 54 SGD hotspots, which are generally aligned spatially with the distribution of coastal springs. Therefore, future research should evaluate the impact on nutrient loads to enhance coastal water management and sustainability.