Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • Data Processing Description
  • BCO-DMO Curation Notes
  • Problem Description
• Related Publications
• Related Datasets
• Parameters
• Instruments
• Project Information
• Funding

Water sample collection

Field sampling was conducted at Cabral and Varari, two fringing coral reef sites on the western coast of Mo'orea, French Polynesia, during the dry season (August 2021) and wet season (March 2022). Discrete seawater samples were collected approximately 20 cm above the benthos using acid-washed high-density polyethylene (HDPE) bottles or a Subsurface Automated Sampler (SAS) following Enochs et al. (2020). Sampling locations included reef sites, submarine groundwater discharge (SGD) seepage points, and inland groundwater sources. Geographic coordinates were recorded for each sampling location, and samples were collected across seasonal, tidal, and spatial gradients to characterize the influence of submarine groundwater discharge on reef biogeochemistry.

Samples collected for nutrient and fluorescent dissolved organic matter (fDOM) analyses were filtered through 0.22 µm Sterivex filters either in situ prior to entering acid-washed Mylar bags within the SAS or immediately after collection using sterile syringes. Nutrient samples were stored frozen (-20°C) in 50 mL Falcon tubes until analysis. fDOM samples were stored refrigerated (4°C) in amber glass bottles and analyzed within two months of collection.

Samples for total alkalinity (TA) were collected into Tedlar bags pre-fixed with 50% saturated HgCl₂ in deionized water or fixed immediately after collection. Samples were transferred to amber HDPE bottles and stored in a cool, dark environment until analysis. Groundwater and low-salinity samples (salinity <20) were collected in duplicate following Mos et al. (2021), with one sample preserved using HgCl₂ and one filtered and refrigerated. Differences between preservation methods were consistently less than 5 µmol kg⁻¹, and preserved samples were used in all analyses.

Salinity and pH were measured immediately after collection. pH was measured on the total hydrogen ion concentration scale (pHT) using a Tris-calibrated glass electrode following Dickson Standard Operating Procedure (SOP) 6a. In situ temperature was measured using conductivity-temperature loggers deployed at each study site, and in situ pH was subsequently back-calculated using the seacarb package in R. Salinity was measured immediately after collection using a conductivity electrode. All sample containers and analytical vials were acid washed and rinsed three times with sample water prior to use.

Laboratory analyses

Dissolved inorganic nutrients (NOx, NH₄⁺, SiO₃²⁻, and PO₄³⁻) were analyzed using a Seal Analytical AA3 nutrient autoanalyzer at the University of Hawaiʻi School of Ocean and Earth Science and Technology (SOEST) Laboratory for Analytical Biogeochemistry. Analytical precision (coefficient of variation) was approximately 0.3% for NOx, 1.3% for NH₄⁺, 0.5% for SiO₃²⁻, and 0.2% for PO₄³⁻.

Fluorescent dissolved organic matter (fDOM) was analyzed on 0.22 µm filtered seawater using a HORIBA Aqualog three-dimensional scanning fluorometer following Nelson et al. (2015). Fluorescence excitation-emission matrices (EEMs) were processed to quantify individual fluorescence components. For statistical analyses, fluorescence peaks were grouped into Humics (Marine Humic-Like and Visible Humic-Like fluorescence) and Proteinaceous (Tryptophan-Like and Tyrosine-Like fluorescence) following Coble et al. (2014).

Total alkalinity was measured using a Mettler-Toledo T5 automatic titrator following Dickson SOP 3b. Instrument accuracy was verified daily using Certified Reference Material supplied by the A. G. Dickson laboratory at Scripps Institution of Oceanography, with analytical accuracy better than 5 µmol kg⁻¹.

This data and code package available at Zenodo supports the results publication (Silbiger et al., 2025, Ecological Monographs). The repository contains R scripts for data processing, Bayesian structural equation modeling, and figure generation, along with associated output files. The dry and wet season discrete water-sample datasets in the repository are derived from the biogeochemical observations in this BCO-DMO dataset. See the references of this dataset to find a link to this Zenodo software holding.

Curation Actions Performed on Data File

- "NA" treated as missing data values and converted to blank values in the data file.
- Converted Date from %m/%d/%Y to %Y-%m-%d.
- Created ISO_DateTime_UTC by combining Date and Time, interpreting the values as Pacific/Tahiti local time, converting them to UTC, and formatting the result as %Y-%m-%dT%H:%M:%SZ.
- Renamed Date to Date_Local and Time to Time_Local.
- Set column types: string for Location, LocationID, Plate_Seep, Tide, and Season; number for Lat, Long, Salinity, Temperature, TA_umol_kg, pH_t, Phosphate_umolL, Silicate_umolL, NN_umolL, Ammonia_umolL, MarineHumic_Like, VisibleHumidic_Like, Tryptophan_Like, and Tyrosine_Like; date for Date_Local; time for Time_Local; and datetime for ISO_DateTime_UTC.
- Updated metadata for all columns, including descriptions, standard name IDs, and supplied units.
- Marked ISO_DateTime_UTC, Lat, and Long as primary parameters.
- Saved the final table as 1010349_v1_submarine_groundwater_discharge_biogeochemistry_data.csv.

NSF Award Abstract:
Submarine groundwater discharge (SGD) is the flow of water from land through the coastal seafloor into the nearby ocean. Approximately 13,000 cubic kilometers of groundwater is discharged into coastal environments every year, yet the effects of this fresh and often nutrient rich SGD are still poorly understood for coral reefs. This SGD input is driven by changes in precipitation, human land use, sea-level rise, tidal amplitude, and groundwater usage, many of which are rapidly changing with climate and human impacts. This project improves our understanding of SGD effects on coral reefs to better predict how both natural and human-induced changes will affect coastal ecosystem functioning in the future. Working in one of the most comprehensively studied coral reef ecosystems in the Pacific (Mo'orea, French Polynesia, home of the Mo'orea Coral Reef Ecosystem LTER); this project tests the influence of SGD on individual, community, and ecosystem-scale coral reef processes. Using mensurative studies, caging experiments, and a synthetic model, the investigators: 1) characterize SGD gradients and relate it to high resolution coral reef cover data, 2) determine how individual to ecosystem processes are influenced by SGD, and 3) develop a synthetic model to show how changes in SGD fluxes will alter reef ecosystem functioning. As SGD is a common feature on nearshore coral reefs worldwide, the results of this study have global implications for understanding the performance of coral reefs, which are essential economic, cultural, and scientific resources. This project is structured to provide training across multiple career levels, linking 13 undergraduate students, 2 graduate students, 2 senior personnel, 1 postdoctoral researcher, 1 female beginning lead investigator, and 2 senior co-investigators, with a focus on encouraging participation from underrepresented groups (e.g., through the Alaska Native and Native Hawaiian, Asian American and Native American Pacific Islander, and Hispanic-Serving Institutions of California State University Northridge, the University of Hawaiʻi at Mānoa, and California State University Long Beach). The investigators work with local K-12 students and teachers in Mo'orea and collaborate with an artist-in-residence to communicate science to the broader public through interactive and immersive art experiences in Mo'orea, Miami, and Los Angeles.

SGD is a natural and understudied feature of many nearshore coral reef ecosystems, which can contribute substantial changes to marine biogeochemistry, with impacts for coastal organisms such as reef-building corals, macroalgae, and bioeroders. SGD may play a key role in coral reef ecosystem functioning because it alters key physicochemical parameters (e.g., temperature, salinity, and nutrient and carbonate chemistry) that substantially affect both biotic and abiotic processes on coral reefs. This project (i) characterizes the spatial extent and biogeochemical signal of SGD in Mo'orea, French Polynesia, (ii) identifies how SGD influences microbial processes, benthic organism growth rates and physiology, species interactions between corals, macroalgae, and herbivores, and net ecosystem calcification and production rates, and (iii) quantitatively assesses how changes in SGD fluxes will alter reef biogeochemistry and ecosystem functioning through an integrative modelling effort. Specifically, the hydrogeological, biogeochemical, and ecological data collected in this study are synthesized in a Bayesian structural equation model. This project characterizes and quantifies how SGD directly and indirectly affects ecosystem functioning via changes in biogeochemistry and altered individual to ecosystem responses, thereby providing a better capacity to track and predict alterations in reef ecosystem function.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.