Confidential Report
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CONFIDENTIAL: VERSION 3 (FINAL) CONFIDENTIAL REPORT MACQUARIE HARBOUR DISSOLVED OXYGEN WORKING GROUP 6 OCTOBER 2014 CONFIDENTIAL: VERSION 3 (FINAL) SUMMARY AND RECOMMENDATIONS 4 1. Changes in dissolved oxygen levels 4 2. Organic carbon loads and biological oxygen demand 4 3. Replenishment of dissolved oxygen by physical processes 4 4. Attribution 5 5. Recommendations 5 1. BACKGROUND 7 Biochemical processes 8 Physical processes 9 2. DATA DESCRIPTION AND PROCESSING 12 3. ORGANIC CARBON LOADS AND BIOLOGICAL OXYGEN DEMAND 14 Organic carbon loads 14 Biological oxygen demand 17 4. REPLENISHMENT OF DISSOLVED OXYGEN BY PHYSICAL PROCESSES 27 Long-term trends 27 High river discharge mixing events 27 Deep water recharge events – low frequency variability 35 Deep-water recharge events – higher frequency characteristics 36 4. DISCUSSION 40 REFERENCES 42 APPENDIX A: TERMS OF REFERENCE 44 Macquarie Harbour Dissolved Oxygen (MH DO) Working Group 44 APPENDIX B: CATCHMENTS AND RIVER DISCHARGES 47 Catchment data 47 Gordon River catchment 49 King River catchment 51 2 CONFIDENTIAL: VERSION 3 (FINAL) Other catchments 53 APPENDIX C: SEALEVEL AND TIDES 54 APPENDIX D: WIND CHARACTERISTICS 55 APPENDIX E: WATER TEMPERATURE 58 In-situ temperature data 61 APPENDIX F: DISSOLVED OXYGEN QA/QC 63 3 CONFIDENTIAL: VERSION 3 (FINAL) Summary and Recommendations 1. Changes in dissolved oxygen levels 1.1. There is a clear downward trend in the dissolved oxygen (DO) levels of the deep-waters (> 15m) of Macquarie Harbour over the period 2009- present. 1.2. DO levels less than 2 mg/l are now very common below 20 m and occasionally come to within 12 m of the surface. 1.3. There have been a number of significant changes over the period from 2009-present. River flow was historically low between 2009-12 and historically high in 2013. This period also coincides with a major expansion of salmon aquaculture. 2. Organic carbon loads and biological oxygen demand 2.1. Biological oxygen demand (BOD) in the harbour is controlled by the supply of labile (biologically available) organic carbon and potentially the conversion of refractory organic carbon to labile carbon. 2.2. It has been estimated that the total organic carbon load associated with river discharge into Macquarie Harbour is around 100 times that of salmon production in the harbour (ocean inputs are also expected to be small). However, the fraction of organic carbon in river discharges that is labile has not been measured in Macquarie Harbour and in other well forested catchments has been found to be small (Sun et al. 1997, Moran et al. 1999). The fractions of labile and refractory organic carbon exported to the ocean from the harbour are also unknown. 2.3. Based on the data available from recent in situ measurements aquaculture is estimated to be responsible for between 3 and 12% of the benthic BOD in Macquarie Harbour (for sediments deeper than 15 m). The remaining benthic BOD is presumably associated with particulates in river discharge and detritus derived from biological production within the harbour. 2.4. Previous studies have found that pelagic BOD is significantly higher than benthic BOD in many estuaries. However, pelagic BOD has not yet been measured in Macquarie Harbour. 3. Replenishment of dissolved oxygen by physical processes 3.1. Dissolved oxygen in the deep waters of Macquarie Harbour is mainly replenished through (i) mixing with higher DO surface waters; and (ii) higher DO ocean waters overflowing the sill at the mouth of the harbour and descending as a dense plume that recharges DO near the bottom. 3.2. Mixing with higher DO surface waters is most effective following flood events, where a large fraction of the upper water column has been displaced by relatively fresh high DO water. Weak stratification within this upper layer then allows energy from winds and currents to penetrate and continue to mix DO into the deep waters. This process has been observed to influence DO through the entire water column during 4 CONFIDENTIAL: VERSION 3 (FINAL) flood events such as the series of events that occurred in July and August 2013. However, these conditions were rare through the low river discharge period of 2009-2012. 3.3. Recharge of bottom water is most common under low river discharge conditions when ocean inflow is not obstructed by the outflow of freshwater over the harbour sill. Wind, tide and pressure also play an important part in determining if salt water from outside of Hells Gate can enter the harbor. A significant number of recharge events were observed over the period 2009-present. However, with the exception of summer 2011-2012, the total input of oxygen during each event was quite small and enhanced DO near the bottom of the harbour tended to be rapidly diffused and/or consumed through respiration. 4. Attribution 4.1. There are a number of limitations in our knowledge about the biogeochemical processes in Macquarie Harbour that preclude any definitive attribution of the recent decline in deep-water DO. Primary among these are the absence of reliable estimates of labile organic carbon fluxes associated with river discharge and export to the ocean, and levels of pelagic BOD. 4.2. Relatively stable levels of average DO between 1993 and 2009 suggest there is tight coupling between demand and replenishment of DO in Macquarie Harbour, which could be influenced by additional labile organic carbon loads. However, limitations with the data available preclude an unambiguous assessment of the drivers of the deep water decline in DO since 2009. 4.3. Historically low and high river discharge and associated organic carbon loads are likely to have influenced both the physical resupply of oxygen and the BOD of bottom waters. 4.4. Historically, DO levels appear to have been maintained by a combination of regular (bottom-up) recharge and (surface-down) mixing events driven by highly variable river discharge. However, for 2009-present rainfall and river discharge conditions may have altered the frequency of these events. Detailed hydrodynamic modelling and continuous monitoring are required to quantify this relationship. 4.5. The rapid transition to high river discharge (2013) after a prolonged period of low discharge (2009-2012) is likely to have delivered an accompanying influx of accumulated organic carbon from the catchment. However, the influence of this influx on BOD in bottom waters is unclear. Measurements of labile and refractory loads and modelling to estimate retention verses export to the ocean are required to assess the role of river organic carbon loads on bottom water BOD. 5. Recommendations 5.1. Existing monitoring, including recently incorporated TOC, DOC and TN sampling, should be continued and supplemented by a strategic field campaign aimed at measuring the influence of organic carbon loads in river discharge. The latter should measure: (i) labile and refractory 5 CONFIDENTIAL: VERSION 3 (FINAL) loads; (ii) rates of deposition and retention in the harbor; (iii) conversion of refractory to labile carbon; and (iv) the relative importance of pelagic and benthic BOD. 5.2. Similarly, the role of fish farm carbon and nitrogen inputs on BOD should be extended to provide greater resolution of the spatial extent of deposition and the influence on benthic and pelagic BOD. 5.3. Measurements in 5.1 and 5.2 should be used to support development of a more detailed quantitative carbon/nitrogen/oxygen budget for Macquarie Harbour. 5.4. Hydrodynamic and biogeochemical models should be used to help constrain the carbon/nitrogen/oxygen budgets and to explore past conditions and future scenarios relating to changes in river flows and/or salmon production. 5.5. Options for adaptive farm management approaches that allow timely responses to DO fluctuations should be explored. Such approaches could include implementing a Decision Support System (DSS) with a particular focus on oxygen. 6 CONFIDENTIAL: VERSION 3 (FINAL) 1. Background In February 2014 the Tasmanian Salmon Growers Association (TSGA) established the Macquarie Harbour Dissolved Oxygen (MHDO) Working Group with the purpose of verifying the scope of dissolved oxygen (DO) reductions in the bottom waters of Macquarie Harbour and, to the extent allowed by available data, determine attribution. The Terms of Reference for this group are provided in Appendix A. This report documents the finding of the MHDO Working Group. Macquarie Harbour is a large estuary supplied with freshwater from the Gordon and King Rivers (Figure 1.1). The harbour is approximately 33 km long and 9 km wide with a total surface area of 276 km2. While its maximum depth is around 50 m, a shallow sill at its mouth (< 5 m) restricts exchanges with the ocean. This isolation of deep water in the harbor has resulted in a naturally low DO environment (Cresswell et al. 1989) that may be vulnerable to further increases in oxygen demand. DO levels have been declining since 2009 (Figure 1.2) and further decreases may have a direct impact on both the ecology of the harbour and aquaculture production. Figure 1.1: Bathymetry in Macquarie Harbour (Lucieer et al. 2009). 7 a) EPA Site 12 b) EPA Site 12 ) 120 35 n o i t a 30 r 100 u t a 25 s ) 80 t p % p ( ( 20 n y t e 60 i g n i y l 15 x a O S 40 d 10 e v l o 20 5 s s i D 0 0 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 9 9 9 9 9 9 9 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 9 9 9 9 9 9 9 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 9 9 9 9 9 9 9 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 9 9 9 9 9 9 9 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1