NOAA RESTORE Science Program

Abstract Anthropogenic production of greenhouse gases (GHGs) has intensified the need to constrain estimates of natural atmospheric sources from both terrestrial and marine systems. Estuaries are known sources of carbon dioxide (CO2) and methane (CH4); however, less is known about GHG dynamics in subterranean estuaries (STEs). We evaluate CO2 and CH4 dynamics in three proximal STEs bordering Indian River Lagoon, Florida, where groundwater flows through siliciclastic sediments with minor carbonate mineral contents. Although the three STEs have similar mineralogical and flow characteristics, CO2 and CH4 concentrations vary by orders of magnitude. Nonconservative mixing of both gases is observed, and CH4 is generally produced while CO2 is sequestered. The extent of methanogenesis is linked to the redox potential of inflowing groundwaters, as well as degree of CH4 oxidation, which results mostly from anaerobic oxidation of methane. Methane concentrations vary by orders of magnitude, and stable isotopic signatures suggest differences in the microbial production pathway between sites. CO2 is sequestered due to the production of alkalinity relative to dissolved inorganic carbon, which occurs both through rapid CaCO3 dissolution at the shoreline as low‐pH groundwater from the siliciclastic aquifer interacts with carbonate minerals in lagoon sediments, as well as redox reactions, particularly sulfate reduction and denitrification. These results demonstrate a high variability in CO2 and CH4 concentrations, and thus fluxes, even among geographically constrained and hydrogeologically similar STEs. Although STEs are sources of both CO2 and CH4 to surface waters, the variability of production and consumption complicates global estimates of GHG fluxes from STEs.

Abstract Submarine groundwater discharge (SGD) is a source of water and bioreactive solutes to coastal zones but may be modified by organic matter (OM) remineralization dynamics within subterranean estuaries (STEs). We hypothesize that bioreactive solute fluxes should depend on water residence time in STEs, but links between OM transformations and residence time in STEs are poorly characterized. To test this hypothesis, we compare dissolved OM (DOM) quantity and quality in two hydrologically distinct STE systems: a reef lagoon on the east coast of the Yucatan Peninsula, Mexico, where semidiurnal mixing in submarine springs of a carbonate karst aquifer results in short residence times, and a barrier lagoon on the east coast of Florida, where slow flow through siliciclastic sediments results in long residence times. We measured dissolved organic carbon concentrations and characterized colored DOM (CDOM) with ultraviolet spectroscopy and fluorescence combined with Parallel Factor Analysis. Both sites exhibit similar shifts in OM quality with salinity and reflect a marine source of labile OM to the STEs. Nonconservative mixing and CDOM production occurs at all sites but the long water residence times in the siliciclastic STEs cause orders of magnitude greater production than the carbonate STE. Consistent CDOM production across sites with disparate characteristics indicates that STEs are common sources of CDOM to surface water. However, observed variation in the magnitudes of CDOM production indicates that estimating global, and even regional, solute fluxes associated with SGD will be complicated by hydrologic control on extents of OM remineralization.

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.

Depth is a fundamental and universal driver of ocean biogeography but it is unclear how the biodiversity patterns of larger, more mobile organisms change as a function of depth. Here, we developed a predictive biogeography model to explore how information of mobile species' depth preferences influence biodiversity patterns. We employed a literature review to collate shark biotelemetry studies and used open‐access tools to extract 283 total records from 119 studies of 1133 sharks from 35 species. We then matched field guide reported depth ranges and IUCN habitat associations for each shark species to use as covariates in a hurdle variant of ensemble random forests. We successfully fit this model (R 2 = 0.63) to the noisy time‐at‐depth observations and used it to predict the time budgets of the northeast Pacific shark regional pool (n = 52). We then assessed how occurrence diversity patterns, informed by minimum and maximum depth of occurrence, compared to time‐at‐depth weighted diversity patterns. Time‐at‐depth weighted richness was highest between 0 and 25 m and at the upper part of the mesopelagic zone, 250–300 m; resulting in little similarity to common depth or elevational biodiversity patterns while the occurrence‐weighted richness pattern was similar to the ‘low‐plateau' pattern. In the phylogenetic and functional dimensions of biodiversity and over three different distance metrics, we found strong but haphazard differences between the occurrence‐ and time‐at‐depth weighted biodiversity patterns. The strong influence of time budgets on biodiversity led us to conclude that occurrence data alone are likely insufficient or even misleading in terms of the depth‐driven biogeographic patterns in the open ocean. Utilizing the increasing amount of time‐at‐depth information from biotelemetry studies in predictive biogeographic models may be critical for capturing the preferences of pelagic, mobile species occupying the largest biome on the planet.