Alkalinity cycling and carbonate chemistry decoupling in seagrass mystify processes of acidification mitigation

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Amanda L. Kelley, Marine Biology, University of Alaska Fairbanks, Fairbanks, AK and Cale A. Miller, Laboratoire d’Océanographie de Villefranche, Sorbonne Universitè, Villefranche-sur-Mer, France

Background/Question/Methods
The adverse conditions of ocean and coastal acidification on sensitive marine organisms has led to the investigation of bioremediation methods as a way to abate localize acidification. This phytoremediation, by macrophytes, is expected to reduce the severity of acidification in nearshore habitats on short timescales. Characterizing the efficacy of phytoremediation can be challenging as residence time, tidal mixing, freshwater input, and a limited capacity to fully constrain the carbonate system can lead to erroneous conclusions. Here, we present in situ observations of carbonate chemistry relationships to seagrass habitats in coastal Alaska by comparing dense grass (DG: 62 % of pool area), patchy grass (PG: 26 % of pool area), and no grass (NG: 0 % of pool area) Zostera marina pools in the high intertidal experiencing intermittent flooding and prolonged periods of emersion.
Results/Conclusions
High-frequency measurements of three carbonate system parameters--pH, alkalinity (TA), and total-CO2 elucidate extreme diel cyclicity in all variables. The DG pool displayed frequent decoupling between pH and aragonite saturation state (Ωarg) suggesting pH-based inferences of acidification remediation by seagrass can be misinterpreted as pH and Ωarg can be independent stressors for some bivalves. Estimates show the DG pool had an integrated ΔTA of 550 µmol kg-1 over a 12 h period, which is ~60 % > the PG and NG pools. The extreme diel cyclicity of TA observed in this study is unprecedented for nearshore macrophyte habitats in temperate locations and refutes assumptions of its invariability and strong correlation with salinity, a relationship often used to constrain the carbonate system. The modulation of pHT, TCO2, and TA in each of the pools exceeds those that would be derived based on any two of the carbonate chemistry parameters and exemplifies an uncoupled system in seagrass habitats likely overlooked by previous studies. Our results suggest that phytoremediation is best achieved in habitats with mixed photosynthesizers (i.e., PG pool with seagrass and filamentous algae) which result in less decoupling between pH and Ωarg.