The influence of mesoscale climate drivers on hypoxia in a fjord-like deep coastal inlet and its potential implications regarding climate change: examining a decade of water quality data

Abstract

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 t of organic carbon loading per annum. Monthly dissolved oxygen, total Kjeldahl 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 predicts increased winter rainfall and decreased summer rainfall, which may further exacerbate hypoxia in this system. Currently, Macquarie Harbour's basins experience frequent (up to 36 % of the time) and prolonged (up to 2 years) oxygen-poor conditions that may promote greenhouse gas (CH4, N2O) production altering the processing of organic matter entering the system. The increased winter rainfall predicted for the area will likely promote the 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).

Nitrous oxide (N2O) in Macquarie Harbour, Tasmania

“…Hells Gates muffles tidal forcing resulting in harbour water levels primarily determined by river flow and wind set up (Hartstein et al, 2019). The morphology of this system results in sharp gradients of DO,salinity,and 80 temperature which are seasonally dependant (Creswell et al, 1989;Hartstein et al, 2019;Maxey et al, 2022).…”

“…In surface waters DO concentrations are nearly always in equilibrium with the air but decrease sharply through the halocline (~8m to 15m). Subhalocline layers (~15m to a few meters from the bottom) are observed to be below 62.5 µM more than 50% of the time (see Maxey et al, 2022). Near the seabed, episodic marine intrusions (deep water renewal) refresh the supply of DO.…”

“…Near the seabed, episodic marine intrusions (deep water renewal) refresh the supply of DO. In the upper reaches of the harbour marine intrusions are much 85 less common (see Hartstein et al, 2019;Maxey et al, 2022). In these areas the DO concentration falls below 31 µM nearly a third of the time (Maxey et al, 2022).…”

“…There is almost no DO produced below the halocline (8m to 12m deep) as the overlying freshwater lens is high in chromophoric dissolved organic matter (CDOM) limiting light available to primary producers in the surface water layers (within 3m) (Maxey et al, 2017(Maxey et al, , 2020. Basin water oxygen concentrations and salinity are largely influenced by advection of marine water over Hells Gates (Andrewartha and Wild-Allen 2017;Hartstein et al, 2019;Maxey et al, 2022). This 95 process is driven by low atmospheric pressure, sustained NW winds, and low catchment rainfall which itself is influenced by Southern Annular Mode (SAM) (Hartstein et al, 2019;Maxey et al, 2022) The main source of freshwater to the harbour is located on its southeast end (the Gordon River) and drains a 105 nearly pristine catchment (including the Franklin River) of approximately 5,682 km 2 (Macquarie Harbour Dissolved Oxygen Working Group, 2014; Fig 1).…”

“…Basin water oxygen concentrations and salinity are largely influenced by advection of marine water over Hells Gates (Andrewartha and Wild-Allen 2017;Hartstein et al, 2019;Maxey et al, 2022). This 95 process is driven by low atmospheric pressure, sustained NW winds, and low catchment rainfall which itself is influenced by Southern Annular Mode (SAM) (Hartstein et al, 2019;Maxey et al, 2022) The main source of freshwater to the harbour is located on its southeast end (the Gordon River) and drains a 105 nearly pristine catchment (including the Franklin River) of approximately 5,682 km 2 (Macquarie Harbour Dissolved Oxygen Working Group, 2014; Fig 1). The Gordon River discharges an estimated 180,000 tons organic carbon (OC) per year (Maxey et al, 2020(Maxey et al, , 2022 into the estuary.…”