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Seagrass habitat metabolism increases short-term extremes and long-term offset of CO2 under future ocean acidification
Citation:
Pacella, S., C. Brown, G. Waldbusser, R. Labiosa, AND B. Hales. Seagrass habitat metabolism increases short-term extremes and long-term offset of CO2 under future ocean acidification. PNAS (PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES). National Academy of Sciences, WASHINGTON, DC, 115(15):3870-3875, (2018). https://doi.org/10.1073/pnas.1703445115
Impact/Purpose:
This study provides the first estimates of how high-frequency pH, aragonite saturation state (Ωaragonite), and pCO2 dynamics are altered by ocean acidification (OA) in an estuarine habitat. The impact of OA has been well described and accepted for open ocean environments, but generally overlooked or dismissed as unimportant in estuarine environments. As a result, the impact of OA in productive, nearshore estuarine environments remains poorly characterized, despite these areas being some of the most economically, ecologically, and culturally important habitats in the marine waters. Our study utilized a combination of high-resolution observations, mechanistic modeling, multiple techniques for estimating anthropogenic carbon loading into the system, and hindcasting/forecasting models to determine the impacts of OA in a seagrass habitat in Puget Sound, WA, USA. We find that as anthropogenic carbon continues to increase in the atmosphere and enters coastal waters, the carbonate system of an estuarine seagrass bed becomes less able to buffer natural sources of CO2 variance, causing non-linear amplification of naturally extreme events. This altered buffering capacity causes the rates of change for the most biologically relevant carbonate system indices (e.g. minimum pH is often used as a biological threshold for many estuarine organisms) to outpace rates of OA published for the open-ocean, as well as observed and predicted rates of atmospheric CO2 change. Net community metabolism of the seagrass bed is found to currently increase the incidence of both harmful and favorable bio-calcifying conditions, while ultimately reducing organismal exposure to harmful conditions in future high-CO2 scenarios. The significance of these findings are several fold. There has been virtually no appreciation for the role of interacting natural CO2 variations with a baseline CO2 shift (i.e. OA) in accelerating biogeochemical change in near-shore systems. Our results also directly contribute to ongoing discussions in the research and management community regarding the nature and timing of coastal water quality standard non-attainment attributable to OA, and the potential role of seagrass habitats as OA refugia. We highlight how the non-linear amplification of carbonate parameters relevant for water quality standards with increasing OA has been previously unrecognized, causing departures of greater than 0.2 pH units from pre-industrial conditions earlier than expected otherwise. We discuss how the future success of estuarine OA-sensitive organisms will likely depend on their resilience to increasingly extreme carbonate chemistry, as well as the ability to capitalize on short windows of favorable conditions.
Description:
The role of rising atmospheric CO2 in modulating estuarine carbonate system dynamics remains poorly characterized, likely due to myriad processes driving the complex chemistry in these habitats. We reconstructed the full carbonate system of an estuarine seagrass habitat for a summer period of 2.5 months utilizing a combination of time-series observations and mechanistic modeling, and quantified the roles of aerobic metabolism, mixing, and gas exchange in the observed dynamics. The anthropogenic CO2 burden in the habitat was estimated for the years 1765-2100 to quantify changes in observed high-frequency carbonate chemistry dynamics. The addition of anthropogenic CO2 alters the thermodynamic buffer factors (e.g., the Revelle factor) of the carbonate system, decreasing the seagrass habitat's ability to buffer natural carbonate system fluctuations. As a result, the most harmful carbonate system indices for many estuarine organisms [minimum pHT, minimum Ωarag, and maximum pCO2(s.w.)] change up to 1.8×, 2.3×, and 1.5× more rapidly than the medians for each parameter, respectively. In this system, the relative benefits of the seagrass habitat in locally mitigating ocean acidification increase with the higher atmospheric CO2 levels predicted toward 2100. Presently, however, these mitigating effects are mixed due to intense diel cycling of CO2 driven by aerobic metabolism. This study provides estimates of how high-frequency pHT, Ωarag, and pCO2(s.w.) dynamics are altered by rising atmospheric CO2 in an estuarine habitat, and highlights nonlinear responses of coastal carbonate parameters to ocean acidification relevant for water quality management.
URLs/Downloads:
DOI: Seagrass habitat metabolism increases short-term extremes and long-term offset of CO2 under future ocean acidification