Submarine Groundwater Discharge Exceeds River Inputs as a Source of Nutrients to the Great Barrier Reef

Rivers are often assumed to be the main source of nutrients triggering eutrophication in the Great Barrier Reef (GBR). However, existing nutrient budgets suggest a major missing source of nitrogen and phosphorus sustaining primary production. Here, we used radium isotopes to resolve submarine groundwater discharge (SGD)-derived, shelf-scale nutrient inputs to the GBR. The total SGD was ∼10-15 times greater than average river inputs, with nearshore groundwater discharge accounting for ∼30% of this. Total SGD accounted for >30% of all known dissolved inorganic N and >60% of inorganic P inputs and exceeded regional river inputs. However, SGD was only a small proportion of the nutrients necessary to sustain primary productivity, suggesting that internal recycling processes still dominate the nutrient budget. With millions of dollars spent managing surface water nutrient inputs to reef systems globally, we argue for a shift in the focus of management to safeguard reefs from the impacts of excess nutrients.

Submarine groundwater discharge and associated metal elements into an urbanized bay

In this study, geochemical tracers (radium isotopes) and heavy metals (Pb, Zn, Cd, Cr and As) were analyzed to derive the submarine groundwater discharge (SGD) and associated metal fluxes during four seasons in an urbanized bay (Daya Bay, China). Results showed that Pb and Zn were the main pollutants in bay water. SGD was found to exhibit an obvious seasonal trend (autumn > summer > spring > winter). Such seasonal patterns may be related to the hydraulic gradient between groundwater level and sea level, storm surges and tidal range. SGD was a dominant source of marine metal elements, contributing 19 %-51 % of the total inputs of metals into Daya Bay. The bay water was classified as slight pollution to heavy pollution, which could be linked to SGD-derived metal fluxes. This study provides a better understanding of the important role that SGD plays in metal budgets and ecological environments of coastal waters.

Autonomous Large-Scale Radon Mapping and Buoyant Plume Modeling Quantify Deep Submarine Groundwater Discharge: A Novel Approach Based on a Self-Sufficient Open Ocean Vehicle

Groundwater discharge into the sea occurs along many coastlines around the world in different geological settings and constitutes an important component of global water and matter budget. Estimates of how much water flows into the sea worldwide vary widely and are largely based on onshore studies and hydrological or hydrogeological modeling. In this study, we propose an approach to quantify a deep submarine groundwater outflow from the seafloor by using autonomously measured ocean surface data, i.e., ²²²Rn as groundwater tracer, in combination with numerical modeling of plume transport. The model and field data suggest that groundwater outflows from a water depth of ∼100 m can reach the sea surface implying that several cubic meters per second of freshwater are discharged into the sea. We postulate an extreme rainfall event 6 months earlier as the likely trigger for the groundwater discharge. This study shows that measurements at the sea surface, which are much easier to conduct than discharge measurements at the seafloor, can be used not only to localize submarine groundwater discharges but, in combination with plume modeling, also to estimate the magnitude of the release flow rate.

Physicochemical coastal groundwater dynamics between Kauhakō Crater lake and Kalaupapa settlement, Moloka'i, Hawai'i

Land-based sources of groundwater pollution can be a critical threat to coral reefs, and a better understanding of "ridge-to-reef" water movement is required to advance management and coral survival in the Anthropocene. In this study a more complete understanding of the geological, atmospheric, and oceanic drivers behind coastal groundwater exchange on the Kalaupapa peninsula, on Moloka'i, Hawai'i, is obtained by analyzing high resolution geochemical and geophysical time-series data. In concert with multiyear water level analyses, a tidally and precipitation-driven groundwater connection between Kauhakō Crater lake and submarine groundwater discharge (SGD) fluxes are demonstrated. Results include an average discharge rate of 190 cm d^-1 and the detection of water-flow pathways past cesspools that likely contribute to higher nutrient loading near the SGD sites. This underlines the importance of managing anthropogenic nutrients that enter the shallow freshwater lens such as through cesspools and are consequently discharged via SGD onto coral reef habitats.

Radon and Salinity Mass Balance Constraints on Groundwater Recharge on a Semi-Arid Island (Catalina, California)

Quantifying the freshwater component of submarine groundwater discharge (SGD) is critical in the analysis of terrestrial influences on marine ecosystems and in assessing the water budget and groundwater recharge of coastal aquifers. In semi-arid to arid settings, this quantification is difficult because low SGD rates translate into low concentrations of groundwater solutes in coastal waters. In this study, fresh SGD (FSGD) was quantified for Toyon Bay on Catalina Island, California, for wet and dry seasons using a combination of radon and salinity mass balance models, and the results were compared to watershed-specific groundwater recharge rates obtained from soil water balance (SWB) modeling. Calculated FSGD rates vary only slightly with season and are remarkably similar to the recharge estimates from the SWB model. While sensitivity analyses revealed FSGD estimates to be significantly influenced by uncertainties in geochemical variability of the groundwater end-member and fluctuations of water depth, the results of this study support the SWB-model-based recharge rates. The findings of this study highlight the utility of the radon-and-salinity-mass-balance-based FSGD estimates as groundwater recharge calibration targets, which may aid in establishing more refined sustainable groundwater yields.

Measurement of submarine groundwater discharge (SGD) into Tiruchendur coast at southeast India using 222Rn as a naturally occurring tracer

Application of natural tracers such as radon isotope mass balance has been useful in estimating the submarine groundwater discharge (SGD). This study used ²²²Rn and evaluated the magnitude of SGD at Tiruchendur coast of southeast India in the Gulf of Mannar (Indian Ocean). Higher magnitudes of ²²²Rn in the porewater and seawater in comparison with the groundwater suggest simultaneous SGD with fluxes of 0.1-0.25 m³ m^-2 d^-1 at offshore and 0.4-0.20 m³ m^-2 d^-1 at the near shore. These baseline data would contribute to the management and protection of the Gulf of Mannar region in near future.

Bacterial-derived nutrient and carbon source-sink behaviors in a sandy beach subterranean estuary

Microbial communities in subterranean estuaries play important roles in the biogeochemical cycle. However, the microorganisms associated with biogeochemical behaviors in subterranean estuaries have received little attention. Here, the bacterial communities were compared between the fresh and saline groundwater in a subterranean estuary. Correlation analysis between bacterial groups and salinity indicated that different species represented different groundwater types. The key bacterial groups found along the subterranean estuaries have been shown to influence organic pollutant degradation and nitrate utilization. These species may be potential candidates for the in situ bioremediation of subterranean estuaries that are contaminated with pollutants. The utilization of nitrate and organic pollutants by bacteria in subterranean estuaries serves as a nitrate sink and inorganic carbon source. Our results show the role of bacteria in remediating pollutants through submarine groundwater discharge (SGD) to the coastal ocean, and specific species may be helpful in selecting reasonable groundwater end-members and reducing SGD uncertainties.

Microalgae, a Boring Bivalve and a Coral-A Newly Described Association Between Two Coral Reef Bioeroders Within Their Coral Host

Bioeroding organisms play an important part in shaping structural complexity and carbonate budgets on coral reefs. Species interactions between various bioeroders are an important area of study, as these interactions can affect net rates of bioerosion within a community and mediate how bioeroders respond to environmental change. Here we test the hypothesis that the biomass of endolithic bioeroding microalgae is positively associated with the presence of a macroboring bivalve. We compared the biomass and chlorophyll concentrations of microendolithic biofilms in branches of the coral Isopora palifera (Lamarck, 1816) that were or were not inhabited by a macroboring bivalve. Those branches with a macroborer present hosted ∼80% higher microbial biomass compared to adjacent branches from the same coral with no macroborer. Increased concentrations of chlorophyll b indicated that this was partly due to a greater abundance of green microalgae. This newly described association has important implications for the coral host as both the bivalve and the microalgae have been hypothesized as symbiotic.

Seabird nutrients are assimilated by corals and enhance coral growth rates

Nutrient subsidies across ecotone boundaries can enhance productivity in the recipient ecosystem, especially if the nutrients are transferred from a nutrient rich to an oligotrophic ecosystem. This study demonstrates that seabird nutrients from islands are assimilated by endosymbionts in corals on fringing reefs and enhance growth of a dominant reef-building species, Acropora formosa. Nitrogen stable isotope ratios (δ¹⁵N) of zooxanthellae were enriched in corals near seabird colonies and decreased linearly with distance from land, suggesting that ornithogenic nutrients were assimilated in corals. In a one-year reciprocal transplant experiment, A. formosa fragments grew up to four times faster near the seabird site than conspecifics grown without the influence of seabird nutrients. The corals influenced by elevated ornithogenic nutrients were located within a marine protected area with abundant herbivorous fish populations, which kept nuisance macroalgae to negligible levels despite high nutrient concentrations. In this pristine setting, seabird nutrients provide a beneficial nutrient subsidy that increases growth of the ecologically important branching corals. The findings highlight the importance of catchment–to–reef management, not only for ameliorating negative impacts from land but also to maintain beneficial nutrient subsidies, in this case seabird guano.

Fresh and Recirculated Submarine Groundwater Discharge Evaluated by Geochemical Tracers and a Seepage Meter at Two Sites in the Seto Inland Sea, Japan

Submarine groundwater discharge (SGD) consists of fresh submarine groundwater discharge (FSGD) and recirculated submarine groundwater discharge (RSGD). In this study, we conducted simultaneous 25-hour time-series measurements of short-lived 222Rn and 224Ra activities at two sites with differing SGD rates in the central Seto Inland Sea of Japan to evaluate SGD rates and their constituents. At both sites, we also quantified the total SGD, FSGD, and RSGD using a seepage meter to verify the water fluxes estimated with 222Rn and 224Ra. SGD rates estimated using 222Rn and 224Ra at the site with significant SGD approximated the total SGD and RSGD measured by the seepage meter. However, SGD rates derived using 222Rn at the site with minor SGD were overestimated, since 222Rn activity at the nearshore mooring site was lower than that in the offshore area. These results suggest that the coupling of short-lived 222Rn and 224Ra is a powerful tool for quantification of FSGD and RSGD, although it is important to confirm that tracer activities in coastal areas are higher than those in offshore.

A multi-omics based ecological analysis of coastal marine sediments from Gladstone, in Australia's Central Queensland, and Heron Island, a nearby fringing platform reef

The impact of anthropogenic factors arising from point and non-point pollution sources at a multi commodity marine port and its surrounding ecosystems were studied using sediment samples collected from a number of onshore (Gladstone Harbour and Facing Island) and offshore (Heron Island and Fitzroy Reefs) sites in Australia's Central Queensland. Sediment samples were analyzed for trace metals, organic carbon, polycyclic aromatic hydrocarbons (PAH), emerging chemicals of concern (ECC) and sterols. Similarly, the biological and biochemical interaction between the reef and its environment was analyzed by the multi-omic tools of next-generation sequencing characterization of the bacterial community and microbial community metabolic profiling. Overall, the trace elements were observed at the lower end of the Australian environmental guideline values at the offshore sites, while higher values were observed for the onshore locations Nickel and copper were observed above the high trigger value threshold at the onshore sites. The levels of PAH were below limits of detection across all sites. However, some of the ECC and sterols were observed at higher concentrations at both onshore and offshore locations, notably, the cholesterol family sterols and 17α-ethynylestradiol. Multi-omic analyses also indicated possible thermal and photo irradiation stressors on the bacterial communities at all the tested sites. The observed populations of γ-proteobacteria were found in combination with an increased pool of fatty acids that indicate fatty acid synthesis and utilisation of the intermediates of the shikimate pathways. This study demonstrates the value of applying a multi-omics approach for ecological assessments, in which a more detailed assessment of physical and chemical contaminants and their impact on the community bacterial biome is obtained.

Microphytoplankton variations during coral spawning at Los Roques, Southern Caribbean

Phytoplankton drives primary productivity in marine pelagic systems. This is also true for the oligotrophic waters in coral reefs, where natural and anthropogenic sources of nutrients can alter pelagic trophic webs. In this study, microphytoplankton assemblages were characterized for the first time in relation to expected coral spawning dates in the Caribbean. A hierarchical experimental design was used to examine these assemblages in Los Roques archipelago, Venezuela, at various temporal and spatial scales for spawning events in both 2007 and 2008. At four reefs, superficial water samples were taken daily for 9 days after the full moon of August, including days before, during and after the expected days of coral spawning. Microphytoplankton assemblages comprised 100 microalgae taxa at up to 50 cells per mL (mean ± 8 SD) and showed temporal and spatial variations related to the coral spawning only in 2007. However, chlorophyll a concentrations increased during and after the spawning events in both years, and this was better matched with analyses of higher taxonomical groups (diatoms, cyanophytes and dinoflagellates), that also varied in relation to spawning times in 2007 and 2008, but asynchronously among reefs. Heterotrophic and mixotrophic dinoflagellates increased in abundance, correlating with a decrease of the diatom Cerataulina pelagica and an increase of the diatom Rhizosolenia imbricata. These variations occurred during and after the coral spawning event for some reefs in 2007. For the first time, a fresh-water cyanobacteria species of Anabaena was ephemerally found (only 3 days) in the archipelago, at reefs closest to human settlements. Variability among reefs in relation to spawning times indicated that reef-specific processes such as water residence time, re-mineralization rates, and benthic-pelagic coupling can be relevant to the observed patterns. These results suggest an important role of microheterotrophic grazers in re-mineralization of organic matter in coral reef waters and highlight the importance of assessing compositional changes of larger size fractions of the phytoplankton when evaluating primary productivity and nutrient fluxes.

Coastal acidification induced by tidal-driven submarine groundwater discharge in a coastal coral reef system

We identified a barely noticed contributor, submarine groundwater discharge (SGD), to acidification of a coastal fringing reef system in Sanya Bay in the South China Sea based on time-series observations of Ra isotopes and carbonate system parameters. This coastal system was characterized by strong diel changes throughout the spring to neap tidal cycle of dissolved inorganic carbon (DIC), total alkalinity, partial pressure of CO2 (pCO2) and pH, in the ranges of 1851-2131 μmol kg(-1), 2182-2271 μmol kg(-1), 290-888 μatm and 7.72-8.15, respectively. Interestingly, the diurnal amplitudes of these parameters decreased from spring to neap tides, governed by both tidal pumping and biological activities. In ebb stages during the spring tide, we observed the lowest salinities along with the highest DIC, pCO2 and Ra isotopes, and the lowest pH and aragonite saturation state. These observations were consistent with a concurrent SGD rate up to 25 and 44 cm d(-1), quantified using Darcy's law and (226)Ra, during the spring tide ebb, but negligible at flood tides. Such tidal-driven SGD of low pH waters is another significant contributor to coastal acidification, posing additional stress on coastal coral systems, which would be even more susceptible in future scenarios under higher atmospheric CO2.

Oceanic forcing of coral reefs

Although the oceans play a fundamental role in shaping the distribution and function of coral reefs worldwide, a modern understanding of the complex interactions between ocean and reef processes is still only emerging. These dynamics are especially challenging owing to both the broad range of spatial scales (less than a meter to hundreds of kilometers) and the complex physical and biological feedbacks involved. Here, we review recent advances in our understanding of these processes, ranging from the small-scale mechanics of flow around coral communities and their influence on nutrient exchange to larger, reef-scale patterns of wave- and tide-driven circulation and their effects on reef water quality and perceived rates of metabolism. We also examine regional-scale drivers of reefs such as coastal upwelling, internal waves, and extreme disturbances such as cyclones. Our goal is to show how a wide range of ocean-driven processes ultimately shape the growth and metabolism of coral reefs.

In situ coral reef oxygen metabolism: an eddy correlation study

Quantitative studies of coral reefs are challenged by the three-dimensional hard structure of reefs and the high spatial variability and temporal dynamics of their metabolism. We used the non-invasive eddy correlation technique to examine respiration and photosynthesis rates, through O₂ fluxes, from reef crests and reef slopes in the Florida Keys, USA. We assessed how the photosynthesis and respiration of different reef habitats is controlled by light and hydrodynamics. Numerous fluxes (over a 0.25 h period) were as high as 4500 mmol O₂ m⁻² d⁻¹, which can only be explained by efficient light utilization by the phototrophic community and the complex canopy structure of the reef, having a many-fold larger surface area than its horizontal projection. Over diel cycles, the reef crest was net autotrophic, whereas on the reef slope oxygen production and respiration were balanced. The autotrophic nature of the shallow reef crests implies that the export of organics is an important source of primary production for the larger area. Net oxygen production on the reef crest was proportional to the light intensity, up to 1750 µmol photons m⁻² s⁻¹ and decreased thereafter as respiration was stimulated by high current velocities coincident with peak light levels. Nighttime respiration rates were also stimulated by the current velocity, through enhanced ventilation of the porous framework of the reef. Respiration rates were the highest directly after sunset, and then decreased during the night suggesting that highly labile photosynthates produced during the day fueled early-night respiration. The reef framework was also important to the acquisition of nutrients as the ambient nitrogen stock in the water had sufficient capacity to support these high production rates across the entire reef width. These direct measurements of complex reefs systems yielded high metabolic rates and dynamics that can only be determined through in situ, high temporal resolution measurements.