Abstract
Submarine groundwater discharge (SGD) to coastal zones contributes terrestrial freshwater and nutrients that may support harmful algal blooms (HABs). The magnitude of nutrient exports via SGD depends on volumes of fresh groundwater discharge, its chemical composition, and modifications by biogeochemical processing within subterranean estuaries. Thus, the ability to upscale SGD exports requires knowing the range of chemical composition of inland groundwater and how those compositions may be transformed as fresh and saltwater mix within subterranean estuaries. These processes may create heterogeneous magnitudes of solute exports, even at small spatial scales, and such heterogeneities have rarely been assessed for regional or global SGD nutrient export estimates. To evaluate heterogeneity in subterranean estuary processes and nutrient export, we collected seasonal pore water samples in 2015–2016 at three proximal (<20 km) subterranean estuary sites in Indian River Lagoon, FL. Sites have homogenous hydrogeological settings, but differ in land use and coastal features, and include a mangrove site, an urban site, and a site offshore of a natural wetland. All sites exhibit little variation through time in nutrient concentrations and modeled SGD rates. In contrast, each site exhibits significantly different nutrient concentrations of potential fresh groundwater sources, fresh groundwater discharge volumes, and nutrient transformations within subterranean estuaries. Groundwater specific discharge correlates with nutrient concentrations, suggesting that higher residence times in the subterranean estuary increase biogeochemical transformations that reduce anthropogenic nutrient loads but increase in situ nutrient sources derived from organic matter remineralization. The differences in transformations lead to SGD nutrient contributions that differ by orders of magnitude between sites and have N:P ratios that are greater than the Redfield ratio (15) for the mangrove (29) and urban sites (28), but less than the Redfield ratio for the wetland site (8). These results indicate that heterogeneity of both absolute and relative nutrient export via SGD complicates integration of nutrient fluxes across regional coastal zones and evaluations of its impacts to coastal ecosystems. A better understanding of the drivers of heterogeneity, including subterranean estuary processes, land use, coastal topography, and vegetation dynamics could improve assessments of regional nutrient loading and upscaling for estimates of global solute cycles.
Highlights
Groundwater is an important component of coastal water budgets and contributes terrestrial solutes to coastal zones through submarine groundwater discharge (SGD)
SGD includes both fresh groundwater (; fresh SGD), whose discharge is driven by inland groundwater hydraulic head, and recirculated seawater (: saline SGD), whose discharge is driven by processes including tidal pumping and bioirrigation (Martin et al, 2006; Taniguchi et al, 2019)
We evaluate SGD dynamics and potential nutrient loading from three seepage faces offshore of regions of distinct inland land use: Eau Gallie North (EGN; Figure 1B) is offshore of a highly developed commercial area, Riverwalk Park (RWP; Figure 1C) is offshore of the last remaining natural wetland bordering Indian River Lagoon, and Banana River Lagoon (BRL; Figure 1D) is offshore of moderately developed residential properties
Summary
Groundwater is an important component of coastal water budgets and contributes terrestrial solutes to coastal zones through submarine groundwater discharge (SGD). Terrestrial solutes transported by fresh SGD are increasingly recognized to affect coastal chemical budgets (Luijendijk et al, 2020) and SGD may be their principal or sole source where surface water runoff is limited (Burnett et al, 2006; Pain et al, 2020) Both terrestrial and recycled marine nutrients transported by SGD have been implicated as drivers of benthic primary productivity (Carruthers et al, 2005) and harmful algal blooms (Phlips et al, 2002; Hu et al, 2006). Methods to estimate water fluxes range from water mass balance approaches to the measurement of conservative geochemical tracers (typically Ra and Rn isotopes; methods extensively reviewed in Taniguchi and others, 2019) While these methods provide a means to estimate total contributions of freshwater to the coast, they can only be used to estimate total nutrient inputs when coupled with an appropriate estimate of average discharging groundwater composition. The assumptions made in estimating groundwater composition are challenged by heterogeneity in inland groundwater composition in space and time, which can be related to hydrogeological factors (Zamrsky et al, 2020), vegetation type and coverage (McGowan and Martin, 2007; Humphries et al, 2011; Alaghmand et al, 2014), and anthropogenic nutrient sources including fertilizers and wastewater
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