Alkalinity Cycling And Carbonate Chemistry Decoupling in Seagrass Mystify Processes of Acidification Mitigation

Abstract

The adverse conditions of acidification on sensitive marine organisms has led to the investigation of bioremediation methods as a way to abate local 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 by comparing dense (DG), patchy (PG), and no grass (NG) Zostera marina pools in the high intertidal experiencing intermittent flooding. High-frequency measurements of pH, alkalinity (TA), and total-CO2 elucidate extreme diel cyclicity in all parameters. 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. We conclude habitats with mixed photosynthesizes (i.e., PG pool) result in less decoupling between pH and Ωarg.

Advancing real-time pH sensing capabilities to monitor coastal acidification as measured in a productive and dynamic estuary (Ría de Arousa, NW Spain)

“…The incorporation of other sensors, such as dissolved oxygen concentration into upcoming versions of the SURCOM instrument, or as part of an acidification sensor suite, would represent a significant improvement, as it would allow a better assessment of biological processes and its impacts as drivers for pH oscillations. Recent studies have shown that ecosystem metabolism can cause significant short-term pH changes in addition to being a strong seasonal driver (Feely et al, 2018;Pacella et al, 2018;Lowe et al, 2019;Miller and Kelley, 2021). The information obtained by pH observation networks coupled to other biogeochemical sensors is necessary for understanding the impact of acidification on coastal ecosystems, for the development of predictive models (Mongin et al, 2016;Siedlecki et al, 2016;Turi et al, 2016) and for the design of response strategies to ocean acidification by policy makers.…”

“…A better analysis of the sources of uncertainty could be approached by deploying an observational mesh of SURCOM sensors over a more long period, that would allow to improve the correction of the impact of the large spatial and temporal variability of these environments, and also to apply different quality control techniques (McLaughlin et al, 2017). In any case, the precision of the SURCOM instrument was comparable to the performance of c o m m e r c i a l pH s e n s o r s f o u n d in s i m i l a r c o a s t a l implementations (Hoppe et al, 2012;Wootton and Pfister, 2012;McLaughlin et al, 2017;Gonski et al, 2018;Miller and Kelley, 2021). Moreover, this uncertainty of ±0.029 is in the range of magnitude to 0.02, an acidification measurement criterion that distinguishes "weather" quality datasets (Newton et al, 2015) proving the potential suitability of this sensor for studies at this level of quality (Gonski et al, 2018) once we gain increased expertise in the equipment and protocols (McLaughlin et al, 2017).…”