Implications of salinity normalization of seawater total alkalinity in coral reef metabolism studies

Authors

Travis A. Courtney, University of California San Diego; University of Puerto Rico Mayagüez
Tyler Cyronak, Nova Southeastern UniversityFollow
Alyssa J. Griffin, University of California Davis
Andreas J. Andersson, University of California San Diego

ORCID

0000-0003-3556-7616

Document Type

Article

Publication Title

PLOS One

ISSN

1932-6203

Publication Date

12-29-2021

Abstract

Salinity normalization of total alkalinity (TA) and dissolved inorganic carbon (DIC) data is commonly used to account for conservative mixing processes when inferring net metabolic modification of seawater by coral reefs. Salinity (S), TA, and DIC can be accurately and precisely measured, but salinity normalization of TA (nTA) and DIC (nDIC) can generate considerable and unrecognized uncertainties in coral reef metabolic rate estimates. While salinity normalization errors apply to nTA, nDIC, and other ions of interest in coral reefs, here, we focus on nTA due to its application as a proxy for net coral reef calcification and the importance for reefs to maintain calcium carbonate production under environmental change. We used global datasets of coral reef TA, S, and modeled groundwater discharge to assess the effect of different volumetric ratios of multiple freshwater TA inputs (i.e., groundwater, river, surface runoff, and precipitation) on nTA. Coral reef freshwater endmember TA ranged from -2 up to 3032 μmol/kg in hypothetical reef locations with freshwater inputs dominated by riverine, surface runoff, or precipitation mixing with groundwater. The upper bound of freshwater TA in these scenarios can result in an uncertainty in reef TA of up to 90 μmol/kg per unit S normalization if the freshwater endmember is erroneously assumed to have 0 μmol/kg alkalinity. The uncertainty associated with S normalization can, under some circumstances, even shift the interpretation of whether reefs are net calcifying to net dissolving, or vice versa. Moreover, the choice of reference salinity for normalization implicitly makes assumptions about whether biogeochemical processes occur before or after mixing between different water masses, which can add uncertainties of ±1.4% nTA per unit S normalization. Additional considerations in identifying potential freshwater sources of TA and their relative volumetric impact on seawater are required to reduce uncertainties associated with S normalization of coral reef carbonate chemistry data in some environments. However, at a minimum, researchers should minimize the range of salinities over which the normalization is applied, precisely measure salinity, and normalize TA values to a carefully selected reference salinity that takes local factors into account.

DOI

10.1371/journal.pone.0261210.g001

Volume

16

Issue

12

First Page

e0261210

Comments

This work was supported by funding from the National Oceanographic and Atmospheric Administration Ocean Acidification Program Award# NA15OAR4320071 (AJA, TAC). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Creative Commons License

This work is licensed under a Creative Commons Attribution 4.0 International License.

NSUWorks Citation

Travis A. Courtney, Tyler Cyronak, Alyssa J. Griffin, and Andreas J. Andersson. 2021. Implications of salinity normalization of seawater total alkalinity in coral reef metabolism studies .PLOS One , (12) : e0261210 . https://nsuworks.nova.edu/occ_facarticles/1236.