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).