Submarine Groundwater Discharge Dynamics in Coastal Ecosystems

Submarine groundwater discharge (SGD) constitutes a critical but often invisible component of the coastal hydrological cycle, in which fresh or saline groundwater seeps from coastal aquifers into the nearshore ocean. Operating over a range of spatial and temporal scales, SGD delivers heat, dissolved nutrients, trace elements and contaminants to shallow marine environments, with profound implications for biogeochemical cycling, ecosystem health and coastal resource management. The magnitude and composition of SGD fluxes are controlled by coastal hydrogeology, terrestrial recharge rates, tidal pumping and density-driven circulation. In permeable sediments, porewater advection can further modulate the delivery of alkalinity and dissolved inorganic carbon, thereby influencing carbonate chemistry along reef and estuarine gradients. Globally, SGD supplies a non-negligible fraction of nutrient and solute inputs to coastal waters, with spatial hotspots where groundwater-borne fluxes rival or exceed riverine discharge. These fluxes can either sustain productive marine communities or drive eutrophication and harmful algal blooms, depending on nutrient composition, residence time and mixing dynamics. Emerging evidence also highlights the role of SGD in thermal refugia creation, pollutant transport and utilisation as a freshwater resource by local populations. Integrating multidisciplinary observations with tracer-based quantification and numerical modelling has advanced our understanding of SGD pathways, yet challenges remain in resolving small-scale heterogeneity, seasonal variability and the influence of human activities on coastal aquifer systems.

Research from Nature Portfolio

Recent studies in cold marine settings have revealed the importance of hydrothermal SGD in sustaining atypical biological communities. Investigations in a Siberian Arctic bay demonstrate that warm, nutrient-rich groundwater discharge creates a subsurface water mass, underpinned by cyclonic mixing, which delivers heat and elevated chlorophyll concentrations to an otherwise frigid environment. This process has enabled boreal zooplankton and epibenthic assemblages to flourish, underscoring the role of SGD in expanding ecological niches at high latitudes.

On a global scale, spatially resolved modelling has provided the first systematic quantification of freshwater SGD contributions to oceans. While globally SGD accounts for less than one per cent of total freshwater inputs, its contribution to solute delivery—particularly carbon, nitrogen and silica—can exceed two per cent of riverine fluxes. Moreover, for a significant fraction of estuaries, marshes and coral reefs, SGD-derived groundwater supplies over a quarter of the total freshwater and nutrient budget, highlighting critical hotspots for monitoring and management.

Submarine Groundwater Discharge Dynamics in Coastal Ecosystems publication trend

The graph below shows the total number of articles in submarine groundwater discharge dynamics in coastal ecosystems across all publications each year (not limited to Nature Index journals).

Technical terms

Submarine groundwater discharge (SGD): The process by which groundwater flows from coastal aquifers into the ocean, carrying dissolved constituents and heat.

Nutrient flux: The rate of transport of dissolved nutrients (such as nitrogen, phosphorus, silica) across a boundary per unit area, influencing primary productivity and eutrophication.

Radium isotope tracer: Naturally occurring radium isotopes used to identify and quantify the rate of groundwater discharge and associated solute inputs in marine environments.

References

1. A unique warm–water oasis in the Siberian Arctic’s Chaun Bay sustained by hydrothermal groundwater discharge. Communications Earth & Environment (2024).
2. The connection between Submarine Groundwater Discharge and seawater quality: The threat of treated wastewater injected into coastal aquifers. The Science of The Total Environment (2024).
3. Fresh groundwater discharge insignificant for the world’s oceans but important for coastal ecosystems. Nature Communications (2020).
4. Societal use of fresh submarine groundwater discharge: An overlooked water resource. Earth-Science Reviews (2017).
5. Radium isotopes as submarine groundwater discharge (SGD) tracers: Review and recommendations. Earth-Science Reviews (2021).
6. Impact of Submarine Groundwater Discharge on Marine Water Quality and Reef Biota of Maui. PLOS ONE (2016).
7. How significant is submarine groundwater discharge and its associated dissolved inorganic carbon in a river-dominated shelf system?. Biogeosciences (2012).
8. Groundwater and porewater as major sources of alkalinity to a fringing coral reef lagoon (Muri Lagoon, Cook Islands). Biogeosciences (2013).