Macquarie Harbour (TAS, 2013-2020)

Fast Facts

  • Completed 2020.
  • Co-invested funding from CSIRO and FRDC project 2016 – 067 ‘Understanding oxygen dynamics and the importance for benthic recovery in Macquarie Harbour’ via sub-contract to IMAS (FRDC project lead).
  • Further Information: Dr Mike Herzfeld (hydrodynamics), Karen Wild-Allen (biogeochemistry).

Background

Macquarie Harbour is located on Tasmania’s west coast and is one of the largest salt-wedge estuaries in Australia. It is home to a growing salmon farming industry, and also adjoins the South-West World Heritage Area containing the Gordon & Franklin rivers.

The harbour is approximately 35 by 9 km in size, and is fed by fresh water from a catchment of 6900 sq. km, the majority of which belongs to the Gordon and King Rivers. Both rivers are regulated to some extent by hydro-electric power schemes and have average flows of approximately 265 & 55 cumecs respectively. But these numbers can vary markedly with rainfall and power generation The power schemes control about 25% of the flow in the Gordon (capable or outputting 280 cumec), and 60% of the flow in the King. The water from the Gordon River is of high quality, while that of the King is very poor owing to the effects of the mining industry

The harbour is connected to the Indian Ocean through a relatively narrow entrance, 400m wide but containing a much narrower navigable channel. Currents of up to 4 knots prevail during strong tides.

Depths in the harbour range up to 50m, but much of it is very shallow, particularly the perimeter and the sill separating the harbour from the ocean.

Field studies by Cresswell et al (1989) have suggested that the harbour consists of a 3-layer system ie a top layer controlled by river run-off, seasonal heating and cooling, a mid layer at about 20m which has a long residence time and is low in oxygen, and a lower layer within the deep basins that contains modified marine water brought in by tides over the sill.

Project Achievements

A numerical model of Macquarie Harbour was originally developed as part of the Aquafin CRC (Volkman et al) to demonstrate the transferability of modelling approaches from the then existing aquaculture projects (D’Entrecasteaux Channel and Port Lincoln) to other sites. The model was updated to take advantage of new forcing data sets that enable its running in near real-time with and extended forecast out to +10 days, using BOM ACCESS-G, although the accuracy of the harbour forecast is limited by the accuracy of the meteorological forecast and estimated river flows, both of which can deteriorate significantly over this extended forecast period.

A CSIRO profiling mooring delivered an impressive dataset of hourly profiles of temperature, salinity, dissolved oxygen, chlorophyll, BGA and CDOM fluorescence in near real time from 2018 – 2020 with very little down time.  These data (together with available IMAS mooring string data and fish farm data) were routinely used to assess the performance of an operational hydrodynamic and oxygen tracer model that ran in near real time with a short-term forecast.

Model results, profiling mooring observations and analysis were delivered on a live web based Macquarie Harbour Dashboard, to inform stakeholders of evolving and predicted harbour conditions.  An example demonstrating stakeholder use of the dashboard occurred in February 2020, when farmers were informed (with 3 days notice), of a predicted intrusion event and risk of reduced near surface dissolved oxygen in areas of the harbour shown on the dashboard.

A biogeochemical and optical water quality model was implemented for Macquarie Harbour and generally reproduced the hydrodynamics, biogeochemical cycling and dissolved oxygen conditions observed in 2017-18 very well.  Surface water in the harbour is seasonally warmer in summer and cooler in winter resulting in a persistent temperature inversion in winter and spring, maintained by the strong salinity driven density gradient.  Should low river flow occur in winter or spring this could potentially allow overturning of the water column to re-establish a stable thermal structure.

Surface waters had very low phosphorous content with model and observations suggesting a deficit in phosphorous supply for phytoplankton growth (c.f. Redfield 16 mol N : 1 mol P).  Simulated nutrient concentrations at depth were generally higher in 2017 and lower in 2018, particularly for nitrogen, due to a corresponding reduction in fish farm nutrient load in 2018.  The largest input of nitrogen to the harbour in 2017-18 was from rivers (45%), fish farms and sewerage (25%), and marine intrusions (23%) [note that for the Gordon river only 30% of the nitrogen load was labile (reactive) material c.f. 100% of fish farm and sewerage treatment plant waste]; nitrogen loss from the harbour was by export to the ocean (80%), and by sediment and water column denitrification (20%).  Phytoplankton growth throughout the harbour was limited by light (CDOM and detrital matter dominate total absorption) and low nutrient concentrations in surface waters.  Peak concentrations of chlorophyll were simulated at around 15m depth towards the northern end of the harbour which was subject to greater marine phosphorous supply and more transparent water in summer.

Dissolved oxygen concentrations in harbour surface waters were generally high, whilst concentrations at depth were depleted, primarily due to stratification and slow flushing of deep water in the harbour.  No anoxic (<1% oxygen saturation) water or sediment areas were simulated by the biogeochemical model.  Mean conditions simulated in 2017-18 showed that 14% of the whole harbour water volume and 33% of the sediment surface area was hypoxic (1-30% oxygen saturation) with the World Heritage Area (WHA) more impacted than the other basins.  Oxygen budget analysis found the largest influx of oxygen to the harbour was from rivers (66%), marine input (10%) and air-sea flux (6%); the greatest loss terms were from export to the ocean (87%), biogeochemical remineralisation processes in the sediment (8%) and estimated farmed fish respiration (3%).  Seasonal analysis for 2017-18 showed a net increase in harbour oxygen in winter and less consistently in autumn, and a net loss of oxygen in spring.

Scenario simulations were achieved for drier (lower river flow) and wetter (higher river flow) conditions.  Under reduced river flow, there was ~20% greater influx of marine water into the deeper parts of the harbour and reduced hypoxia; with increased river flow, marine influx was suppressed (by ~20%), deep water residence time increased and hypoxia increased.  These patterns were persistent when scenarios were extended for a further 2 years and strong linear relationships were found between river flow, marine influx of oxygen and hypoxia.  Further scenario simulations were achieved to explore reduced anthropogenic load on harbour water quality (by omission of fish farm respiration and nutrient loads).  This reduction in anthropogenic load resulted in a 50% reduction in hypoxic water and a 40% reduction in hypoxic sediment compared to the 2017-18 model run and was persistent in the extended model run.  The reduced anthropogenic load scenarios showed a larger reduction in hypoxia under comparable ocean oxygen influx c.f. all other scenarios.

This study has demonstrated that river flows control the flushing time and influx of marine water and together with anthropogenic load, this ultimately defines the hypoxic condition of the harbour.  Our observations (profiling mooring and process studies) confirmed the skill of the model and identified the key microbial communities driving the biogeochemical cycling of nutrients, the remineralisation of organic matter and the ecological drawdown of oxygen.  The findings documented Wild-Allen et al., 2020 are available to inform ongoing sustainable management of the harbour to minimise the deleterious impacts of anthropogenic activities on water quality and local

Acknowledgments

The global ocean operational model OceanMAPS was used to provide offshore ocean conditions on the open boundaries. OceanMAPS is operated by BOM (Bureau of Meteorology) and is a data assimilating, eddy resolving model with 10 km resolution in the region surrounding Australia.  Surface fluxes were derived from the atmospheric model ACCESS-A & ACCESS-G, operated by the Bureau of Meteorology.  River flow and temperature data were provided by Hydro Tasmania.  AVHRR SST images processed by the CSIRO Remote Sensing Project were included for comparison with model output.  We thank the EPA, DPIPWE Marine Farming Branch, Tassal, HAC and Petuna for making data available to the project.  We thank HAC, Petuna and Tassal for operational support in maintaining the CSIRO profiling mooring and IMAS staff for assistance with fieldwork.  We thank Lesley Clementson, Monika Wozniak, Bozena Wojtasiewicz and Dion Frampton for assistance with optical and picoplankton sample analysis and Tim Malthus for constructive review.

References

Cresswell G.R., Edwards R.J. and Barker B.A. (1989) Macquarie Harbour Tasmania, Seasonal Oceanographic Surveys in 1985, Papers and Proceedings of the Royal Society of Tasmania, Vol. 123.

Koehnken L. (2005) Overview of Water Quality in Macquarie Harbour and Assessment of Risks due to Copper Levels. Prepared for DPIWE March 2005.

Palmer L., McConachy F. and Peterson J. (2001) Basslink Integrated Impact Assessment Statement, Potential Effects of Changes to Hydro Power Generation, Appendix 2, Gordon River Hydrology Assessment. Prepared for Hydro Tasmania June 2001.

Tyler P.A., Terry C. and Howland M.B. (2001) Basslink Integrated Impact Assessment Statement, Potential Effects of Changes to Hydro Power Generation, Appendix 11, Gordon River Meromictic Lakes Assessment. Prepared for Hydro Tasmania June 2001.

Volkman J.K., Thompson P., Herzfeld M., Wild-Allen K., et al (2009) A Whole-of-Ecosystem Assessment of Environmental Issues for Salmonid Aqualculture. Aquafin CRC Project 4.2(2).

Wild-Allen, K., Andrewartha, J., Baird, M., Bodrossy, L., Brewer, E., Eriksen, R., Skerratt, J., Revill, A., Sherrin, K., Wild, D. (2020) Macquarie Harbour Oxygen Process model (FRDC 2016-067): CSIRO Final Report.  CSIRO, Australia