Abstract

Deep coastal inlets are sites of high sedimentation and organic carbon deposition that account for 11 % of the world's organic carbon burial. Australasia's mid to high latitude regions have many such systems. It is important to understand the role of climate forcings in influencing hypoxia and organic matter cycling in these systems, but many such systems, especially in Australasia, remain poorly described. We analysed a decade of in-situ water quality data from Macquarie Harbour, Tasmania, a deep coastal inlet with more than 180,000 tons of organic carbon loading per annum. Monthly dissolved oxygen, total Kjeldhal nitrogen, dissolved organic carbon, and dissolved inorganic nitrogen concentrations were significantly affected by rainfall patterns. Increased rainfall was correlated to higher organic carbon and nitrogen loading, lower oxygen concentrations in deep basins, and greater oxygen concentrations in surface waters. Most notably, the Southern Annular Mode (SAM) significantly influenced oxygen distribution in the system. High river flow (associated with low SAM index values) impedes deep water renewal as the primary mechanism driving basin water hypoxia. Climate forecasting predicted increased winter rainfall and decreased summer rainfall, which may further exacerbate hypoxia in this system. Currently, the Harbour basins experience frequent (up to 36 % of the time) and prolonged (up to 2 years) oxygen-poor conditions with the potential to promote greenhouse gas (CH4, N2O) production. Increased greenhouse gas production will alter the processing of organic matter entering the system. The increased winter rainfall predicted for the area will potentially increase greenhouse gas emissions due to increased spread and duration of hypoxia in the basins. Further understanding of these systems and how they respond to climate change will improve our estimates of future organic matter cycling (burial vs export) and greenhouse gas production.

Highlights

  • IntroductionFjords and fjord-like estuaries ( called Deep Coastal Inlets – DCI; Keith et al 2020) are sites of high sedimentation and organic carbon (OC) burial

  • Fjords and fjord-like estuaries are sites of high sedimentation and organic carbon (OC) burial

  • It is important to understand the role of climate forcings in influencing hypoxia and organic matter cycling in these systems, but many such systems, especially in Australasia, remain poorly described

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Summary

Introduction

Fjords and fjord-like estuaries ( called Deep Coastal Inlets – DCI; Keith et al 2020) are sites of high sedimentation and organic carbon (OC) burial These systems account for approximately 11% of the world's annual OC burial (Smith et al 2015). Compared to other marine benthic environments (e.g. sediments along the continental shelf, deeper pelagic sediments, 35 shallow-water carbonate sediments), they bury the most OC per unit area (Smith et al 2015; Bianchi et al 2018, 2020). Their location within mid to high latitude coastal margins and disproportionate role in geochemical cycling make these systems especially vulnerable to anthropogenic pressure (Walinsky et al 2009; Gilbert et al 2010; Bianchi et al 2018, 2020). One of the critical issues facing coastal environments is the expansion of poor oxygen conditions due to increased anthropogenic organic matter loadings (Diaz and Rosenberg 2008; Oschlies et al 2018; Breitburg et al 2018; Pitcher et al 2021)

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