Evaluating the drivers of air-sea CO2 exchange and ocean acidification in coastal waters around New Zealand

Ocean absorption of anthropogenic carbon dioxide (CO2) emissions mitigates the impacts of climate change but also causes ocean acidification, putting marine organisms and ecosystems at risk. Carbon dynamics in coastal environments, driven by interactions with terrestrial processes, benthic ecosystems and unique physical oceanographic processes make these ecosystems more vulnerable to ocean acidification and compounding stressors such as eutrophication, hypoxia, and warming due to climate change. Coastal margins, including shelf seas, represent areas of high biological productivity that provide important economic and cultural marine ecosystem services and play a significant role in the global carbon cycle. However, coastal processes are typically not well represented or constrained in Earth System Models despite their significance to the global carbon budget and sensitivity to human impacts. Understanding the drivers of regional carbon cycles is necessary for predicting how a system will respond to anthropogenic perturbation. The research presented herein focuses on investigating the drivers of seasonal carbon cycle, air-sea CO2 exchange and implications for ocean acidification in the coastal margins around New Zealand. Observational data collected regularly since 1998 at stations spanning subantarctic and subtropical waters, along with 4 years (2015 – 2019) of observations from a coastal ocean observing network were used to evaluate regional carbon cycles. Methods to integrate modelled and reanalysis data with observational data were developed to leverage sparsely sampled datasets to better understand the processes that control the seasonal carbon cycles and drivers of long-term variability. Nearshore coastal environments exhibited the largest seasonal to interannual variability in pH compared to shelf seas, consistent with the influence of terrestrial processes, freshwater fluxes, and dominance of benthic ecosystems on carbonate chemistry in these systems. Overall, subtropical shelf waters in the northern North Island are a stronger sink for atmospheric CO2 (4.66 mol C m-2 y-1) than subantarctic waters off the South Island (0.84 mol C m-2 y-1). CO2 fluxes are driven by air-sea gradients that are controlled by seasonal thermodynamics, biological production, and physical transport. Subtropical sites exhibit a dominance of seasonal temperature variability compared to subantarctic sites. Circulation was found to play a large role the seasonal carbon cycles around New Zealand. Advective fluxes export dissolved inorganic carbon (DIC) from the northeastern shelf of the North Island while they add DIC along the southeastern shelf break of the South Island. Decadal variability in advection along the southeastern shelf is correlated with the El Nino Southern Oscillation and Southern Annual Mode, which has reduced advection of DIC into this region, so maintaining the regional sink strength for atmospheric CO2. These changes in ocean circulation and warming due to climate change have also reduced solubility of CO2 during 2009-2018 by 2%. Simulations using the Regional Ocean Modelling System (ROMS) were used to improve understanding of how terrestrial interactions affect seasonal mixed layer dynamics and carbonate chemistry in coastal and shelf waters off the southeast the South Island. Terrestrial freshwater fluxes were shown to have a large impact on the seasonal salinity and heat budgets which are dominated by advection and turbulent mixing along the Subtropical Front. A coupled biogeochemical ROMS was developed for a domain along the northeastern shelf of the North Island which included the Firth of Thames and Hauraki Gulf. A hydrological model was used to estimate terrestrial fluxes of freshwater, nutrients, organic matter, dissolved oxygen, and heat, which enabled sensitivity analysis of coastal carbonate chemistry and air-sea gas exchange to terrestrial inputs and deconvolution of the seasonal carbon budget. Inner Firth primary production was driven nearly entirely by terrestrial nitrate loading but sensitivity to loading diminished along the land-shelf spatial gradient. Terrestrial organic matter had limited impact on seasonal air-sea exchange and carbon export. The total carbon exported from the Hauraki Gulf was estimated to be ~8 Tg C y-1 with the model, but this may represent an overestimate due to the simplicity of the biological model used. Although this model showed high skill in reproducing seasonal phytoplankton biomass, it did not reproduce hypoxic conditions observed seasonally due to inadequately represented benthic processes. This modelling framework was successful in informing drivers of the seasonal air-sea CO2 exchange across this land-ocean gradient. These studies indicate vulnerability of New Zealand’s coastal ecosystems to climate change and anthropogenic stressors. The results show the relative importance of ocean circulation, biological processes, changes in ocean heat and salinity, and land-ocean interactions in modulating carbon cycling over seasonal to decadal time scales. Methods developed within this research show how model and observational data can be combined to investigate climate change questions important for sustainable resource management.

Vance J. M., 2023. Evaluating the drivers of air-sea CO2 exchange and ocean acidification in coastal waters around New Zealand. PhD thesis, University of Otago, 391 p. Thesis.

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