The replenishment of the deeper bottom waters reported in late 2016 and early 2017 has continued through spring and winter 2017, albeit intermittently. These oxygen recharge events were most evident at the sensor string deployed closest to the harbour entrance and in the deeper bottom waters at the Strahan monitoring site. There was also evidence of an increase in DO in the bottom waters at the Franklin site in the south of the harbour, but the influence of the DO recharge was clearly reduced with distance from the harbour entrance. DO levels were typically < 20% saturation at depths below 20m at the new string deployed in the World Heritage Area. Although there would appear to have been some improvement in the DO levels in the middle of the water column (i.e. between 20 - 30m depth), oxygen levels in this part of the water column still remain low. This is most pronounced at the monitoring sites furthest from the harbour entrance, with DO levels approaching 10% saturation in the mid-waters at the Franklin and World Heritage Area monitoring sites. It is also clear from the latest DO observations that bottom water concentrations are once again decreasing with the onset of spring and as a consequence there is still potential for DO concentrations to return to critically low levels.

The bottom water recharge that commenced in early November again appeared strongly tied to salinity at the surface, and hence river flow. Through winter and into early spring river flow was high and surface salinities low and this coincided with the decline in DO levels at depth. From October river flow decreased, surface salinity increased and in early November DO levels at depth increased markedly. These are the same combination of conditions that have coincided with previous oxygen recharge events. An important feature from a farming and management perspective are the concomitant changes observed in DO levels towards the surface. In the weeks prior to the onset of the oxygen recharge (from late October through November), DO levels at shallower depths (5-15m) declined significantly, most notably at the 5 and 7.5m sensor depths, before increasing again in early December. This would appear to be strongly related to the reduction in river flow, as evidenced by increased surface salinities, and likely reflects the lower DO, more saline water that had previously been pushed deeper in the system now moving closer to the surface. During a recharge event this situation may be further exacerbated, as the denser more saline oceanic water entering the bottom waters will effectively enhance the displacement of this water upwards from below.

In the October 2017 survey we reported that the early signs of recovery in benthic fauna across lease sites observed in May 2017 had declined, concomitant with the return of very low DO concentrations. Faunal abundances had returned to levels similar to those observed in the October 2016 and January 2017 surveys. The number of species recorded also decreased but not to the same extent. In the January 2018 survey there was little change from October 2017 in faunal abundance and number of species recorded at most of the leases, except at the more southern leases where there had been a further decline. At the majority of external sites (>1km from the leases), the improvements seen in May 2017 remained in October 2017; there has been little further change in January 2018. At the deeper external sites to the south, faunal abundances continue to remain low relative to observations prior to the noted decline in spring 2016 – early 2017, although there has been some improvement at the shallowest of these sites.

The May and October 2017 video surveys indicated a clear reduction in the presence of Beggiatoa at both lease and external sites compared to the October 2016 and January 2017 surveys. In the January 2018 video survey there was an increase in the number of dives that Beggiatoa was observed on, but this reflected more observations of patches of Beggiatoa at lease sites and fewer observations of Beggiatoa forming extensive mats relative to the previous two surveys. At the external sites, Beggiatoa was observed at the same number (2) of the 28 sites in the January 2018 survey as it was in the September 2017 survey, which is significantly fewer than reported in January 2017 and October 2016. The January 2018 ROV has also seen a reduction in the number of sites where Dorvilleid polychaetes were observed; there also appears to have been a reduction in the 4

number of sites with very high Dorvilleid scores (>300 abundance categories).

1. the maintenance of the CSIRO profiler and ongoing delivery of near real time and short term forecast model results
2. updates to the visualisation dashboard
3. a description of the simulated harbour water quality, evaluation of oxygen and nitrogen budgets, scenario results and analysis
4. analysis of observational process studies for phosphate addition and oxygen drawdown

The improved oxygen concentrations observed in the middle and bottom waters during summer 2017/18 extended through to the middle of 2018.  This was reflected in the June 2018 survey of benthic fauna; the abundance and number of species increasing at all five leases from that observed in the October 2017 and January 2018 surveys.  Faunal abundance and species numbers at the majority of the external sites in the June 2018 survey remain typical of those recorded historically and consistent with observations in recent surveys. At the deeper external sites to the south where abundances have remained low relative to observations prior to the noted decline in spring 2016 – early 2017, there has been some improvement at two of these sites.

Following the June 2018 faunal survey, oxygen concentrations in the middle and bottom waters again declined into early spring. However,  this decline wasn’t as low as observed in the previous two springs and didn’t continue for as long.   Improved oxygen concentrations in the middle and bottom waters have extended from the middle of spring 2018 through to late autumn 2019 due to recharge events that commenced in early October 2018.   The faunal survey in January 2019 demonstrates that benthic conditions have improved relative to that observed at the same time of year in 2018 and 2017.  Notably, at the deeper external sites to the south where abundances have remained low relative to observations prior to the decline in spring 2016 – early 2017, faunal numbers had improved at 3 of the 4 sites in the latest survey; abundance and the number of species have returned to or are closely approaching the range observed prior to the decline.

In Spring 2018thedecline in bottom water oxygen concentrations wasn’t as low as observed in the previous two springs and didn’t continue for as long due to a sustained period of recharge and replenishment of bottom waters that began in middle of spring 2018 and extended through to lateautumn2019.The faunal survey in January 2019 demonstrated that benthic conditions had improved relative to that observed at the same time of year in 2018 and 2017.  This pattern has continued in the latest faunal survey in June 2019. Notably, at the majority of both lease and external sites faunal abundance and the number of species have returned to or are closely approaching the range observed prior to the decline in spring 2016-early 2017.

In spring 2019 oxygen concentrations in the middle and bottom waters again declined to low levels and these conditions persisted for longer than in spring 2018. Oceanic recharge of bottom water was not observed until late December 2019 when the river flows decreased, and the halocline became shallower. Despite the extended period of low bottom water dissolved oxygen concentrations, the macrofaunal abundances and diversity in January 2020 was similar that observed in the January 2019 survey. Faunal abundance and the number of species at the external sites throughout the harbour have returned to and remain well within the range reported before the decline in spring 2016 – early 2017.

This study, undertaken by the Institute for Marine and Antarctic Studies (IMAS) and the Commonwealth Scientific and Industrial Research Organisation (CSIRO) was commenced to investigate deteriorating benthic conditions in Macquarie Harbour; in particular the very low dissolved oxygen (DO) levels observed in the middle and bottom waters in spring 2016 (Ross & Macleod 2016a). This project was designed to provide a clearer understanding of both the effectiveness of fallowing and passive remediation for benthic recovery, and the drivers and importance of oxygen dynamics for recovery of benthic conditions in Macquarie Harbour.