The Current Status of Dissolved Oxygen in Macquarie Harbour
North West Macquarie Harbour – Segment 121
Within Segment 121 (Figure 1), dissolved oxygen reported as the median concentration (mg/l) averaged across five monitoring sites (KR1, CHN, CC, SB, HG1) during the 12-month period between December 2023 and November 2024 (Figure 2), are within the interim Default Guideline Value range, with the exception of the 5 metre depth.
For more information on how values are calculated see
Where does this information come from? below and for more information on interim Default Guideline Values please visit
Interim Default Guideline Values Macquarie Harbour - Segment 121.
Figure 1:
Segment 121: NW Macquarie Harbour. Salmon aquaculture leases depicted by rectangles, long term EPA water quality monitoring depicted by black circles, 14 water quality monitoring sites routinely sampled by the salmon industry depicted by grey triangles.
Figure 2:
Water depth profile illustrating the median dissolved oxygen concentration (mg/l) for Segment 121 – North West Macquarie Harbour between December 2023 and November 2024.
Central Macquarie Harbour – Segment 120
Within Segment 120 (Figure 3) dissolved oxygen reported as the median concentrations (mg/l) averaged across five monitoring sites (C10, CHE, C8, CH5 and PET3) during the 12- month period between December 2023 and November 2024 (Figure 4), are within the interim Default Guideline Value range at all sampling depths below 10m depth.
Dissolved oxygen concentrations are reported below the interim Default Guideline Values at 5m, 8m, 9m, 10m and 15m depths. This indicates that dissolved oxygen concentrations dipped below the interim Default Guideline Value for periods of time during the 12-month period between December 2023 and November 2024 at these depths.
For more information on how values are calculated see Where does this information come from? below and for more information on interim Default Guideline Values please visit Interim Default Guideline Values for Aquatic Ecosystems Macquarie Harbour - Segment 120.pdf
Figure 3:
Segment 120: Central Macquarie Harbour. Salmon aquaculture leases depicted by rectangles, long term EPA water quality monitoring depicted by black circles, 14 water quality monitoring sites routinely sampled by salmon industry depicted by grey triangles.
Figure 4:
Water depth profile illustrating the median dissolved oxygen concentration (mg/l) for Segment 120 – Central Macquarie Harbour between December 2023 and November 2024. Values are derived from 5 monitoring sites within Segment 120.
South East Macquarie Harbour – Segment 119
Within Segment 119 (Figure 5) dissolved oxygen reported as the median concentrations (mg/l) averaged across four monitoring sites (WH1, WH2, WHN, GR2) during the 12-month period between December 2023 and November 2024 (Figure 6), are within the interim Default Guideline Value range at most sampling depths.
Eight sampling depths (5m, 9m, 10m, 18 - 20m, 23m and 24m) are reported below the interim Default Guideline Values. This indicates that dissolved oxygen concentrations dipped below the interim Default Guideline Values for periods of time during the 12-month period between December 2023 and November 2024 at these depths.
For more information on how values are calculated see
Where does this information come from? below and for more information on interim Default Guideline Values please visit Interim Default Guideline Values for Aquatic Ecosystems Macquarie Harbour - Segment 119.pdf
Figure 5:
Segment 119: SE Macquarie Harbour. Salmon aquaculture leases depicted by rectangles, long term EPA water quality monitoring depicted by black circles, 14 water quality monitoring sites routinely sampled by the salmon industry depicted by grey triangles.
Figure 6:
Water depth profile illustrating the median dissolved oxygen concentration (mg/l) for Segment 119 – SE Macquarie Harbour between December 2023 and November 2024. Values are derived from 4 monitoring sites within Segment 120.
Where does this information come from?
Macquarie Harbour has been split into three defined areas (segments) for the purpose of monitoring, analysing and assessing water quality including dissolved oxygen concentration and comparing this to interim Default Guideline Values (DGVs).
Interim DGVs have been published for:
The interim DGVs have been derived from data collected by the EPA between May 1993 and October 2009 across 40 sites within Macquarie Harbour. As such, the DVGs represent historical water quality prior to the expansion of salmon farming.
Interim DGVs presented for the less than 6 metres and
greater than 25 metre depths have not yet been published on the EPA website. These have been included
on figures 2, 4 and 6 to assist with interpreting the dissolved oxygen measurements.
Figure 7:
Macquarie Hbr Segments 121, 120, 119. Salmon aquaculture leases depicted by rectangles, long term EPA water quality monitoring depicted by black circles, 14 water quality monitoring sites routinely sampled by salmon industry depicted by grey triangles.
Water quality monitoring (including measurement of dissolved oxygen concentration) is undertaken once a month at the 14 water quality monitoring sites routinely sampled by the salmon industry (Figure 7, grey triangles). Samples are taken at specified sampling depths – this sampling is undertaken as part of the requirements for environmental monitoring by the salmon industry.
The information from this monitoring is routinely analysed and used to understand the status of dissolved oxygen concentration in the harbour.
There are many ways of analysing and interrogating water column dissolved oxygen data which can include looking at change over time at individual monitoring sites and depths or exploring spatial variability across the harbour.
Each of these approaches can provide different information, often very valuable to better understanding the dynamics of dissolved oxygen concentration and how it is influenced by local events (e.g. recharge events, water temperature, organic matter).
One approach that has been developed to incorporate elements of time and space (because dissolved oxygen concentrations can vary significantly over time and space) has been to create depth profiles for each segment of the harbour that represent a composite indication of dissolved oxygen concentration at each depth within that segment over the preceding 12-month period.
In other words, each data point within these depth profiles integrates 12 dissolved oxygen concentrations measured at monthly intervals at each monitoring site by calculating a 12-month median for each sampling depth (incorporating the time element) and using these to calculate an average dissolved oxygen concentration for each sampling depth within each segment (incorporating the space element).
Water Stratification and Dissolved Oxygen in the Harbour
In Macquarie Harbour, dissolved oxygen naturally varies with depth (Figure 8). Surface waters generally show higher concentrations of dissolved oxygen (pale grey line in Figure 8) while deeper waters often show significantly lower concentrations (red line in Figure 8).
Figure 8:
Graph illustrating variability in dissolved oxygen saturation (%) at 0-5m depth, 5-15m depth, 15-25m depth, 25-35m depth and 35-45m depth over the period July 1993 to December 2024 at the EPA long term monitoring site within Central Macquarie Harbour.
Macquarie Harbour was created by a glacier and is similar to many fjords in the Northern Hemisphere, which have a shallow sill at their mouth (the area connecting the fjord with the open ocean).
Because of the shape of its basin and the supply of fresh oxygenated water from the Gordon River (Figure 9), Macquarie Harbour is a highly stratified water body – meaning that there are layers of water (water masses) within the harbour that have very different characteristics to each other (Figure 9). The thickness of these layers and the amount of dissolved oxygen within them varies over time.
The sill at the harbour mouth restricts the amount of water exchange between the open ocean and the harbour basin. Ocean water does periodically move across the sill and enters the harbour, bringing with it ‘new’ dense salty ocean water with relatively high dissolved oxygen concentrations that sinks as it enters the harbour and gradually moves south east, mixing with and displacing existing deep waters which move up to create an intermediate water layer (older basin water) – we call this a recharge event as it is the main mechanism for recharging the Harbour with dissolved oxygen.
Meanwhile, the Gordon River also brings a supply of fresh, oxygenated river water into the harbour that pushes north west towards the sill. Because this fresh water is more buoyant than the denser salt water below it, it forms a surface water layer on the top of the harbour.
Figure 9:
Conceptual model of Macquarie Harbour, illustrating its shape (including the shallow sill), the water layers and highlighting the drivers of deep-water recharge (re-printed from Hartstein et al. 2019).
The density differences between the fresh river water, the older basin water and the 'new' dense salty ocean water that enters the harbour during recharge events cause the water stratification that we see (Figure 8 and Figure 9). While slow and limited mixing does occur between these three distinct water layers, each has distinct physical and chemical characteristics which can be identified by water
monitoring.
It should be noted that other factors including water temperate can influence a water body's ability to absorb dissolved oxygen.
When do recharge events occur?
Recharge events appear to occur episodically and vary in their magnitude over time. While large recharge events may only occur every few years, evidence suggests that many small recharge events can occur within a year.
Our current understanding is that the frequency and magnitude of recharge events is influenced by:
- Wind strength
- Wind direction
- Air pressure
- River flow (and therefore rainfall and catchment management)
- Tidal forcings
Research has indicated that high magnitude recharge events are more likely to occur under conditions of high north or northwesterly winds at the same time as low air pressure and low river flow.
Why do recharge events matter?
Because recharge events are the main way of increasing the dissolved oxygen concentration of middle and deeper waters within the harbour, dissolved oxygen concentrations slowly decline between recharge events as oxygen within the water column is used up by the breakdown of organic matter and dissolved nitrogen. This process is referred to as ‘biological oxygen demand’.
The biological oxygen demand within the harbour at any one time will depend on the amount of organic matter and dissolved nitogen present within the water column and on the sea floor. Increasing the organic matter and dissolved nitrogen present in the environment will result in an increase in the biological oxygen demand, leading to a more rapid decline in dissolved oxygen concentration over time (in the absence of any recharge events).
While dissolved oxygen concentrations do naturally vary within the harbour, the deposition of organic matter and the release of dissolved nitrogen from human activities (including salmon farming and sewage outfalls) and the inflow of riverine inputs can also influence the system. To see what the salmon industry is doing to offset their oxygen consumption in the harbour, their dissolved oxygen and mitigation plan can be viewed here.
It is important that dissolved oxygen concentrations are within a range that promotes the natural ecosystem functioning expected within the harbour. Interim Default Guideline Values provide a range of ‘acceptable’ dissolved oxygen levels for Macquarie Harbour – see Where does this information come from? for more information on Interim Default Guideline Values .
Therefore, in a system that becomes depleted in dissolved oxygen over time, recharge events are the primary source of ‘additional’ oxygen, allowing dissolved oxygen concentrations to increase without reducing the biological oxygen demand.