Dataset: Biogeochemistry from submarine groundwater discharge in Mo'orea, French Polynesia from August 2021 to March 2022

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.