Diel (36-hour) Variability in Seawater Carbonate Chemistry from a Fringing, Groundwater-Influenced Jamaican Reef (Turtle Crawle, Portland, Jamaica)

Description

The primary objective of this study was to characterize the high-frequency diel variability of the marine carbonate system and stable carbon isotopes (δ13C-DIC) in a high-advection coral reef environment. We hypothesized that despite significant physical forcing and events driven by advection, the underlying metabolic "breath" of the reef - specifically the organic carbon sources fueling nighttime respiration - could be isolated and identified using isotopic mass balance (Keeling plots). We specifically sought to test the hypothesis that certain reef systems are fueled primarily by the consumption of externally acquired organic matter (zooplankton) rather than locally produced autotrophic carbon. This dataset contains a 34-hour time-series of surface seawater carbonate chemistry collected at Turtle Crawle (TC), Jamaica, in June 2023. The data captures a full diel cycle (Day-Night-Day) featuring clear patterns of: Daytime DIC drawdown (photosynthesis) and nighttime DIC accumulation (respiration); A significant advective "reset" event occurring between 23:00 and 03:00 on Night 1, characterized by a sharp increase in salinity (+0.58) and a simultaneous drop in DIC concentration; High-precision δ13C-DIC measurements that track the integrated history of benthic metabolism, reaching an absolute minimum at dawn despite the physical reset of carbon concentrations. The data demonstrates that physical advection (likely brine-enriched groundwater or offshore water) can mask biological signals in raw concentration data (DIC and TA), leading to poor stoichiometric correlations. Despite these sources of physical "noise," the raw 1/DIC vs. δ13C-DIC relationship remained robust, yielding a stable metabolic intercept of -19.4 ± 5.4‰. The intercept indicates that the reef's nighttime respiration is fueled by a heterotrophic source (zooplankton/POM) rather than local coral-associated carbon. A key finding of this research is that traditional salinity-normalization (Friis et al., 2003) can introduce mathematical artifacts in high-advection settings where physical and chemical signals are decoupled, producing physically nonsensical results. Methods: Seawater was collected in 300 mL borosilicate BOD bottles and poisoned with saturated mercuric chloride (HgCl2) to halt biological activity. Analyses were performed using a coupled UIC MODICA and Picarro G-2131i cavity ring-down spectroscopy system. DIC (DIC; µmol/kg) and pH (total scale) were measured via phosphoric acid titration and spectrophotometric determination. δ13C-DIC was calibrated using a regional field anchor from the 2021 A22 Repeat Hydrography Cruise to reconcile the instrumental scale with the VPDB scale (constant offset of –11.1‰). Total Alkalinity (TA; µmol/kg) and in situ pH (total scale) were calculated using CO2SYS v3.0 (equilibrium constants from Lueker et al., 2000).

Institutions

• Georgia Institute of Technology

  Georgia, Atlanta
• The University of Texas System

  Texas, Austin
• University of the West Indies

  Kingston, Kingston

Categories

Funders

• U.S. National Science Foundation

  Government of the United States of America

  Alexandria

  Grant ID: 2325619

Licence