A Unique Diel Pattern in Carbonate Chemistry in the Seagrass Meadows of Dongsha Island: The Enhancement of Metabolic Carbonate Dissolution in a Semienclosed Lagoon

Wen-Chen Chou,Hsiao-Chun Tseng,Chin-Chang Hung,Chang-Chang Yang,Ying-Hsuan Chen,Cheng-Kuan Su,Wei-Jen Huang,Hung-Yu Chen,Keryea Soong,Lan-Feng Fan,Yung-Yen Shih,Gwo-Ching Gong

doi:10.3389/fmars.2021.717685

Abstract

In contrast to other seagrass meadows where seawater carbonate chemistry generally shows strong diel variations with higher pH but lower partial pressure of CO2 (pCO2) during the daytime and lower pH but higher pCO2 during nighttime due to the alternation in photosynthesis and respiration, the seagrass meadows of the inner lagoon (IL) on Dongsha Island had a unique diel pattern with extremely high pH and low pCO2 across a diel cycle. We suggest that this distinct diel pattern in pH and pCO2 could be associated with the enhancement of total alkalinity (TA) production coupled to carbonate sediment dissolution in a semienclosed lagoon. The confinement of the IL may hamper water exchange and seagrass detritus export to the adjacent open ocean, which may result in higher organic matter loading to the sediments, and longer residence time of the water in the IL, accompanied by microbial respiration (both aerobic and anaerobic) that may reduce carbonate saturation level to drive carbonate dissolution and thus TA elevation, thereby forming such a unique diel pattern in carbonate chemistry. This finding further highlights the importance of considering TA production through metabolic carbonate dissolution when evaluating the potential of coastal blue carbon ecosystems to buffer ocean acidification and to absorb atmospheric CO2, in particular in a semienclosed setting.

Highlights

As one of the most productive ecosystems on Earth, seagrass meadows have been recognized for their important role in “blue carbon” storage (Duarte et al, 2010; Fourqurean et al, 2012)
To the best of our knowledge, the seasonal recurrence of high pH and low pressure of CO2 (pCO2) values across a diel cycle within seagrass meadows, such as those at the inner lagoon (IL) site on Dongsha Island, has never been reported in other marine ecosystems. We suggest that this unique diel pattern in carbonate chemistry could be associated with the semienclosed nature of the IL site, including, higher organic matter (OM) loading, finer-grained sediment, smaller sediment permeability and longer residence time of the porewater, that may stimulate both aerobic and anaerobic respiration leading to CaCO3 dissolution and total alkalinity (TA) production
Our results demonstrate that CaCO3 dissolution in vegetated reef sediments can be enhanced in a semienclosed environmental setting

Summary

Introduction

As one of the most productive ecosystems on Earth, seagrass meadows have been recognized for their important role in “blue carbon” storage (Duarte et al, 2010; Fourqurean et al, 2012). Hendriks et al (2014) reported diel pH changes in Mediterranean seagrass meadows, where the pH during 47% of the observation time was lower than the source seawater pH, suggesting a certain period of time when the seagrass meadows could not mitigate OA but exacerbated OA. These surveys clearly demonstrate that the OA buffering potential of seagrass meadows may have considerable temporal variability

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