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  • Review Article
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Submarine groundwater discharge impacts on coastal nutrient biogeochemistry

Nature Reviews Earth & Environment volume 2, pages 307–323 (2021)Cite this article

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Abstract

Submarine groundwater discharge (SGD) links terrestrial and marine systems, but has often been overlooked in coastal nutrient budgets because it is difficult to quantify. In this Review, we examine SGD nutrient fluxes in over 200 locations globally, explain their impact on biogeochemistry and discuss broader management implications. SGD nutrient fluxes exceed river inputs in ~60% of study sites, with median total SGD fluxes of 6.0 mmol m−2 per day for dissolved inorganic nitrogen, 0.1 mmol m−2 per day for dissolved inorganic phosphorus and 6.5 mmol m−2 per day for dissolved silicate. SGD nitrogen input (mostly in the form of ammonium and dissolved organic nitrogen) often mitigates nitrogen limitation in coastal waters, since SGD tends to have high nitrogen concentrations relative to phosphorus (76% of studies showed N:P values above the Redfield ratio). It is notable that most investigations do not distinguish saline and fresh SGD, although they have different properties. Saline SGD is a ubiquitous, diffuse pathway releasing mostly recycled nutrients to global coastal waters, whereas fresh SGD is occasionally a local, point source of new nutrients. SGD-derived nutrient fluxes must be considered in water quality management plans, as these inputs can promote eutrophication if not properly managed.

Key points

  • Submarine groundwater discharge (SGD) is an essential component of biogeochemical budgets. Fresh SGD is a source of new nutrients, whereas saline SGD often releases recycled nutrients from sediments.

  • SGD-derived nitrogen fluxes exceeded river inputs in ~60% of the reviewed cases and usually counteracted nitrogen limitation in coastal waters due to high N:P exceeding the Redfield ratio.

  • Positive impacts of SGD on coastal ecosystems include enhanced coral calcification, primary productivity, fisheries, denitrification and pollutant attenuation.

  • Negative impacts of SGD include eutrophication, algal blooms, deoxygenation and localized ocean acidification, depending on site-specific conditions.

  • Considering SGD is crucial to reach the United Nations Sustainable Development Goals pollution targets. The US Supreme Court decision to consider SGD under the Clean Water Act represents a positive policy change, signalling broader appreciation of SGD impacts.

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Fig. 1: The nitrogen cycle in sandy, muddy and rocky coastal aquifers.
Fig. 2: SGD rates from study cases reviewed here.
Fig. 3: SGD-derived DIN, DIP and DSi fluxes based on different spatial scales and ecosystem types.
Fig. 4: Nutrient limitation and speciation in SGD versus rivers.
Fig. 5: River and SGD-derived nutrient inputs to the ocean.
Fig. 6: The biological impacts of SGD.

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Acknowledgements

This Review was initiated with support from the Australian Research Council (FT170100327) and concluded with support from the Swedish Research Council to I.R.S. H.-M.C. was supported by the National Research Foundation of Korea (2020R1F1A1071423) and Inha University Research Grant (2020). X.C. acknowledges the National Natural Science Foundation of China (42006152). N.D. was supported by a University of Alabama sabbatical fellowship. V.R. acknowledges the Beatriu de Pinós postdoctoral programme of the Catalan Government (2017-BP-00334). H.L. acknowledges the National Natural Science Foundation of China (41972260, 41430641). R.S. acknowledges the Japan Society for the Promotion of Science (18KK0428). S.B. was supported by the Swedish Research Council Formas (2017-01513). A.H.S. acknowledges the National Science Foundation (NSF EAR-1752995).

Author information

Authors and Affiliations

  1. Department of Marine Sciences, University of Gothenburg, Gothenburg, Sweden

    Isaac R. Santos & Stefano Bonaglia

  2. National Marine Science Centre, Southern Cross University, Coffs Harbour, Australia

    Isaac R. Santos

  3. Key Laboratory of Coastal Environment and Resources of Zhejiang Province, School of Engineering, Westlake University, Hangzhou, China

    Xiaogang Chen & Ling Li

  4. Department of Natural and Applied Sciences, Lynn University, Boca Raton, FL, USA

    Alanna L. Lecher

  5. School of Earth Sciences, The Ohio State University, Columbus, OH, USA

    Audrey H. Sawyer

  6. Leibniz Centre for Tropical Marine Research (ZMT), Bremen, Germany

    Nils Moosdorf & Mithra-Christin Hajati

  7. Institute of Geosciences, University of Kiel, Kiel, Germany

    Nils Moosdorf

  8. Institut de Ciència i Tecnologia Ambientals, Universitat Autònoma de Barcelona, Bellaterra, Spain

    Valentí Rodellas

  9. Department of Ocean & Earth Sciences, Old Dominion University, Norfolk, VA, USA

    Joseph Tamborski

  10. Department of Ocean Sciences, Inha University, Incheon, Korea

    Hyung-Mi Cho

  11. Department of Geological Sciences, University of Alabama, Tuscaloosa, AL, USA

    Natasha Dimova

  12. Research Center for Marine Bioresources, Fukui Prefectural University, Fukui, Japan

    Ryo Sugimoto

  13. State Environmental Protection Key Laboratory of Integrated Surface Water-Groundwater Pollution Control, School of Environmental Science and Engineering, Southern University of Science and Technology, Shenzhen, China

    Hailong Li

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  1. Isaac R. Santos
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Contributions

I.R.S. conceived the paper with input from all authors and wrote several passages. X.C. did most of the data compilation with support from all authors. A.L.L. and R.S. wrote most of the biological implications section. A.H.S. wrote about scales of SGD and made global maps. N.D. wrote about methods of SGD. N.M. and H.L. wrote most of the societal implications section. V.R. and J.T. wrote some of the river versus SGD and global distribution sections. H.-M.C. wrote some of the nitrogen speciation section. M.-C.H. and L.L. performed some of the data analysis. S.B. wrote about nitrogen cycling. All authors edited the manuscript and contributed to general discussions and literature reviews.

Corresponding author

Correspondence to Isaac R. Santos.

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Supplementary information

Glossary

Submarine groundwater discharge

The flow of water through continental margins from the seabed to the coastal ocean, with length scales of metres to kilometres, regardless of fluid composition or driving force.

Permeability

A measure of the ability of unconsolidated rocks and sediments to allow groundwater flow.

Hydraulic heads

Vertical and horizontal pressure gradients driving groundwater flow.

Biogenic silica

Mineral containing silicon often produced by plankton (such as diatoms and radiolarians) and often well preserved during sedimentation and burial.

Denitrification

Microbial process in the nitrogen cycle that converts nitrate to nitrogen gas that flows to the atmosphere.

Karst

Landscape formed by carbonate rocks often weathered by dissolution and with abundant conduits for fast groundwater flow.

Unconfined aquifers

Surficial aquifers situated above a low-permeability layer of sediment or rock, and with the upper water layer at atmospheric pressure.

Oxidation-reduction potential

Measure of the tendency of a chemical species to acquire electrons, to be reduced or to lose electrons, or to be oxidized.

Subterranean estuaries

The locations in coastal aquifers where there is mixing between fresh groundwater and seawater, and chemical reactions modify the composition of submarine groundwater discharge.

Nitrogen fixation

Microbial process that leads to the conversion of nitrogen gas into ammonia/ammonium.

Dissimilatory nitrate reduction to ammonium

Microbial process in the nitrogen cycle that converts fixed nitrogen from nitrate to ammonium.

Diatom

Microscopic algae (unicellular and non-flagellate) with a characteristic wall made up of silica and are one of the most important groups of planktonic marine microalgae.

Dinoflagellates

Group of microscopic algae (mostly unicellular and flagellate) representing one of the most important groups of both marine and freshwater phytoplankton.

Evapotranspiration

The quantity of water that moves to the atmosphere from the plants and soil; describes the joint effect of transpiration, through the plants, and evaporation, directly from the soil.

Microphytobenthos

Living organisms, such as unicellular eukaryotic algae (mainly diatoms) and cyanobacteria, growing in the upper layers of illuminated aquatic sediments.

Cyanobacteria

Ubiquitous phylum of single-celled bacteria that carry out photosynthesis.

Macrophytes

Large aquatic plants and multicellular algae widespread in marine, brackish and freshwater environments, which are referred to as macrophytes to distinguish from unicellular algae (phytoplankton).

Acid sulfate soils

Naturally occurring soils usually found in coastal wetlands with a high content of iron sulfide minerals, such as pyrite; when disturbed by dredging or drainage, the soils come into contact with oxygen, oxidizing pyrite and releasing sulfuric acid (H2SO4).

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Santos, I.R., Chen, X., Lecher, A.L. et al. Submarine groundwater discharge impacts on coastal nutrient biogeochemistry. Nat Rev Earth Environ 2, 307–323 (2021). https://doi.org/10.1038/s43017-021-00152-0

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