Seagrass habitat metabolism increases short-term extremes and long-term offset of CO ₂ under future ocean acidification

Abstract

The role of rising atmospheric CO 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 CO 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 CO 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 pH, minimum Ω, and maximum pCO] 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 CO levels predicted toward 2100. Presently, however, these mitigating effects are mixed due to intense diel cycling of CO driven by aerobic metabolism. This study provides estimates of how high-frequency pH, Ω, and pCO dynamics are altered by rising atmospheric CO in an estuarine habitat, and highlights nonlinear responses of coastal carbonate parameters to ocean acidification relevant for water quality management.

Seasonal Asymmetry in the Evolution of Surface Ocean pCO2 and pH Thermodynamic Drivers and the Influence on Sea‐Air CO2 Flux

“…The findings at Irminger and DP S reveal that a larger amplitude seasonal cycle in pCO 2 BP versus pCO 2 T (or vice versa) does not necessarily equate to biophysical (thermal) dominance during both summer and winter, and thus, R T BP À1 does not provide information about the time of year that biophysical or thermal processes dominate pCO 2 variations. Importantly, although our focus is on pCO 2 , the linear relationship between pCO 2 and [H + ] indicates that these asymmetries also hold true for ocean acidification, as recently discussed for estuarine habitats by Pacella et al (2018). Further investigation of seasonally asymmetric [H + ] (and pH) changes in fully coupled model runs and observations is needed to assess the implications for ocean ecosystems, building on prior work by Kwiatkowski & Orr, 2018;McNeil & Sasse, 2016;Sasse et al, 2015;Shaw et al, 2013, and others.…”

“…However, in the past decade, several independent modeling studies have determined that the seasonal cycle of carbon variables in the surface ocean is not expected to be stationary, but rather to exhibit an increasing (and for some parameters decreasing) seasonal amplitude associated with the invasion of anthropogenic CO 2 (Gorgues et al, ; Hauck et al, ; Hauck & Völker, ; Kwiatkowski & Orr, ; McNeil & Sasse, ; Pacella et al, ; Riebesell et al, ; Rodgers et al, ; Sasse et al, ). Very recently, observation‐based evidence of p CO 2 seasonal cycle amplification across broad ocean realms was presented by Landschützer et al (), suggesting that expectations from modeling studies are presently manifesting in the environment.…”

Elevated CO2 impairs olfactory‐mediated neural and behavioral responses and gene expression in ocean‐phase coho salmon (Oncorhynchus kisutch)

“…Furthermore, while our study did not investigate recovery of normal behavioral function following cessation of the exposures, there is evidence that such recovery does happen in fish (Chivers et al, ; Jarrold, Humphrey, McCormick, & Munday, ). The environment that salmon reside in (i.e., open ocean vs. nearshore environment, time of year they reside in each environment, and the water depth they reside at) is important to consider going forward as the degree of neural impairment driven by elevated CO 2 could vary (Jarrold et al, ; Pacella, Brown, Waldbusser, Labiosa, & Hales, ).…”

Controls on Carbonate System Dynamics in a Coastal Plain Estuary: A Modeling Study

“…CD was highest within mid bay bottom waters that also had low pH, and CD was an important buffering mechanism for pH changes in late summer, actually leading to stable or slightly higher pH values in late summer despite persistent respiration and associated hypoxic conditions ( Figure 10). While few, if any calcification and dissolution measurements are available for the water-column in this system to identify the specific source of CaCO 3 , a growing body of literature links seagrass bed carbonate system dynamics to buffering changes (Hendriks et al, 2014;Pacella, et al, 2018). Recently expanded SAV communities now cover up to 70% of the broad, tidal freshwater region at the mouth of Susquehanna River (Susquehanna Flats; Gurbisz & Kemp, 2014) and occupy several other low-salinity regions of Chesapeake Bay (Orth et al, 2017).…”

“…Process-based models of coupled hydrodynamics and biogeochemistry to investigate carbon cycling and ocean acidification in coastal estuaries are very limited but growing in number (Fennel et al, 2008;Hauri et al, 2013;Laurent et al, 2017;Pacella et al, 2018). Numerical models can quantify competing physical and biological processes on estuarine acidification and can play a central role in analyzing system responses to altered nutrient loading, altered carbonate chemistry under elevated atmospheric CO 2 , and variability in climatic forcing.…”