Primary producers may ameliorate impacts of daytime CO2 addition in a coastal marine ecosystem.

Matthew E.S Bracken,Nyssa J Silbiger,Genevieve Bernatchez,Cascade J.B Sorte

doi:10.7717/peerj.4739

Matthew E.S Bracken, Nyssa J Silbiger + Show 2 more

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https://doi.org/10.7717/peerj.4739

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Abstract

Predicting the impacts of ocean acidification in coastal habitats is complicated by bio-physical feedbacks between organisms and carbonate chemistry. Daily changes in pH and other carbonate parameters in coastal ecosystems, associated with processes such as photosynthesis and respiration, often greatly exceed global mean predicted changes over the next century. We assessed the strength of these feedbacks under projected elevated CO2 levels by conducting a field experiment in 10 macrophyte-dominated tide pools on the coast of California, USA. We evaluated changes in carbonate parameters over time and found that under ambient conditions, daytime changes in pH, pCO2, net ecosystem calcification (NEC), and O2 concentrations were strongly related to rates of net community production (NCP). CO2 was added to pools during daytime low tides, which should have reduced pH and enhanced pCO2. However, photosynthesis rapidly reduced pCO2 and increased pH, so effects of CO2 addition were not apparent unless we accounted for seaweed and surfgrass abundances. In the absence of macrophytes, CO2 addition caused pH to decline by ∼0.6 units and pCO2 to increase by ∼487 µatm over 6 hr during the daytime low tide. As macrophyte abundances increased, the impacts of CO2 addition declined because more CO2 was absorbed due to photosynthesis. Effects of CO2addition were, therefore, modified by feedbacks between NCP, pH, pCO2, and NEC. Our results underscore the potential importance of coastal macrophytes in ameliorating impacts of ocean acidification.

Highlights

Increased concentrations of CO2 in the atmosphere have already altered oceanic carbonate chemistry, resulting in a decline in overall pH by ∼0.1 units since the year 1800 (Sabine et al, 2004; Orr et al, 2005)
Biological processes resulted in substantial changes in both pH and pCO2 in tide pools over the course of a single low tide: pH increased by ∼0.4 units and pCO2 decreased by ∼233 μatm in both control and +CO2 pools (Fig. 1; Table S4)
The effects of our experimental CO2 additions on pH and pCO2 were masked by the dominant effects of primary producers on carbonate chemistry; there was no apparent difference in either the pH (t = 0.6, df = 9, P = 0.591; Fig. 1A) or the pCO2 (t = 0.3, df = 9, P = 0.777; Fig. 1B) of control versus +CO2 pools

Summary

Introduction

Increased concentrations of CO2 in the atmosphere have already altered oceanic carbonate chemistry, resulting in a decline in overall pH by ∼0.1 units since the year 1800 (Sabine et al, 2004; Orr et al, 2005). Predicting the impacts of ocean acidification on marine communities and ecosystems is critical These predictions are difficult in coastal ecosystems, where carbonate chemistry is already extremely dynamic (e.g., Wootton, Pfister & Forester, 2008; Thomsen et al, 2010; Hofmann et al, 2011; Guadayol et al, 2014; Chan et al, 2017; Koweek et al, 2017; Silbiger & Sorte, 2018)

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