Award: OCE-1924281

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 due to anthropogenic disturbances 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 at two unique watersheds with different geology, environmental settings, and human influence and 2) understand the impact of SGD on coral reef organismal to ecosystems processes. Key findings: The biogeochemical composition of SGD varies substantially by watershed. Both the biogeochemical composition (i.e., what solutes are present) and the concentration (i.e., how much of each solute is in the water) were different between our two sites. The location 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. SGD alters the physiology and population biology of corals. Through a combination of both lab and field experiments, we showed that multiple coral species 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. 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 modifies species interactions. SGD altered the competitive dynamics between adult corals, where corals interacting with other corals of the same species led to metabolic depression in high SGD environments. In the fish community, we showed that herbivory rates increased under high SGD conditions. 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 SGD 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. Specifically, we collaborated on a coral reef art exhibition in Miami, Florida that was visited by hundreds of people. We also collaborated on a set of short films on SGD that have since been featured in international film festivals. Our research has been published in peer-reviewed literature and presented to scientists, managers, and the broader public through presentations at local, national, and international conferences. Last Modified: 01/30/2025 Submitted by: NyssaSilbiger