Award: OCE-1923877

Submarine groundwater discharge (SGD), the flow of water from the land through the marginal seabed, is a land-sea connection that affects coastal ecosystems worldwide. The biogeochemical composition of SGD is unique from seawater, often carrying high concentrations of nutrients, carbon dioxide, and other dissolved organic and inorganic solutes. Many coastal ecosystems, like coral reefs, may depend on SGD. However, the disruption of the groundwater composition and flux to the coastline from anthropogenic processes including pollution, sea-level rise, and climate change could alter the relationship between SGD and coastal ecosystem processes. The goals of our project were to 1) characterize the biogeochemistry of SGD in relation to geology, environmental settings, and human influence and 2) understand the impact of SGD on coral reefs, from organismal to ecosystem processes.

Key findings:

The biogeochemical composition of coastal reef waters is fundamentally linked to terrigenous inputs.  We demonstrated how the biogeochemical composition (i.e., what solutes are present) and the concentration (i.e., how much of each solute is in the water) of SGD differ among representative sites in Moorea and in Hawaii. We linked nutrient composition to land use: locations with higher population density also had higher nutrient concentrations. Additionally, the geology of the aquifer drove changes in the SGD carbonate chemistry, which could influence the buffering capacity of seawater.

Land use has fundamental controls over the biogeochemistry of coral reef waters. In addition to exploring SGD, we published 3 peer-reviewed manuscripts on the impacts of sewage pollution on coral reefs in Hawaiʻi. We also explored larger-scale island mass effect impacts on kilometer-scale nutrient biogeochemistry around islands and circum-island scale nutrient connections to land-based source pollutions.

SGD alters the physiology and population biology of corals, algae and microbes. Through a combination of both lab and field experiments, we showed that multiple coral species and algal and microbial communities are highly sensitive to SGD. Specifically, we found that SGD significantly alters coral growth rates, rates of photosynthesis, respiration, and calcification, endosymbiont densities, and chlorophyll-a concentration. We also showed that moderate levels of SGD have the capacity to increase coral recruitment rates, alter algal community structure, alter algal carbon uptake rates, and change the composition of both planktonic and biofilm microbial communities.

Coral declines are fundamentally linked to impacts from warming, nutrients and sedimentation. This work supported our collaborations with several projects in three major peer-reviewed journal articles exploring how bleaching associated with heat stress causes differential impacts to corals depending on resource availability. We also wrote two high impact peer-reviewed reviews of the literature synthesizing how 1) nutrients and 2) sediments impact the physiology and growth of corals.

SGD influences benthic and pelagic biodiversity. In the benthic community, our results showed that biodiversity is highest at moderate levels of SGD. We also showed that SGD alters early successional benthic communities, where SGD promoted a shift from calcifying to fleshy macroalgae on new substrates. In the water column, we showed that SGD creates distinct gradients in microbial communities, where microbial taxa commonly found within freshwater and wastewater are present in areas of the reef with high concentrations of SGD. We also showed that SGD alters the composition of fish communities, with high SGD favoring more herbivorous and omnivorous species.

SGD influences community and ecosystem metabolism. Our results highlight that SGD leads to both direct and indirect effects on community metabolic rates, including net community production and net community calcification. Communities composed of organisms commonly found in high SGD locations had lower net calcification and net production rates than communities commonly found in low SGD locations. Lastly, we showed that SGD augments ecosystem metabolism through a “biogeochemical cascade”, which we define as a process indirectly affecting biogeochemistry by changing the patterns of uptake and release of carbon by benthic organisms.

Overall, our research provides valuable insights into the interplay between terrigenous inputs and coral reef ecosystems, advancing our understanding of coastal carbon cycling and the broader implications of land-based inputs on ecosystem functioning.

Broader impacts: Our project supported the training of dozens of undergraduate and graduate students, research technicians, and postdoctoral researchers across two institutions.  We developed programming for local middle school students on freshwater impacts to coral reefs, which have already been taught in dozens of schools.  We worked with artists and filmmakers to share the results of our work with the broader community through creative media. We also created a set of short films on SGD that have since been featured in international film festivals. Our research has been published in 18 peer-reviewed journal articles and presented to scientists, managers, and the broader public through presentations at local, national, and international conferences.

 

 

Last Modified: 12/29/2025
Modified by: Craig E Nelson