Unravelling the importance of benthic mineralisation and nutrient cycling in Macquarie Harbour, Tasmania

Sediment biogeochemistry was studied in Macquarie Harbour, which is located on the west coast of Tasmania, Australia. Rates of respiration and fluxes of nutrients, which are either released or taken up during mineralisation of organic matter, were investigated at a series of harbour wide sites and at sites under and adjacent to fish farms. Environmental characteristics of the sediments and overlying water column were also measured to examine their role in determining sediment biogeochemistry of the harbour. The purpose of this study was to understand benthic mineralisation and nutrient cycling, particularly nitrogen cycling, in Macquarie Harbour and the environmental controls of these processes. This study also assessed the influence of organic loading from fish farming activities on sediment biogeochemistry. Sediments cores were collected from nine sites across the harbour in November 2012; these sites were located well outside the influence of benthic enrichment from fish farms. Sediments were also collected from nine other sites to examine the influence of organic enrichment from fish farming; six sites were located directly under the outer edge of fish cages and three sites 50 m from the edge of the cages. To investigate the influence of farm management practices, the cages, 50 m from cage and a subset of the control sites were sampled again in January, May and September 2013. The harbour wide study showed that the sediments in Macquarie Harbour contained mainly terrestrial sourced organic matter based on its depleted carbon signatures. Characterisation of the organic matter also showed gradients commonly observed in estuaries where the isotopic signatures of carbon and nitrogen increased and the percentage of total organic carbon and total nitrogen contents generally decreased from the upper towards the lower estuary. Oxygen consumption and dissolved inorganic carbon (DIC) production rates suggest that aerobic respiration were the dominant respiration pathway except at a few sites where anaerobic respiration were prevalent. The uptake of nitrate from the water column for denitrification at the majority of sites suggest that uncoupled nitrification-denitrification was an important anaerobic pathway in the harbour. The release of ammonium at most of the sites further suggest that nitrification in the sediments were limited, most likely due to low oxygen penetration of sediments. Multiple regression of individual fluxes shows that oxygen was a clear controlling factor for benthic mineralisation and nutrient cycling in Macquarie Harbour. The influence of fish farming on the benthic processes was clearly evident. Oxygen consumption was six times higher at cage sites compared to the control sites. While oxygen consumptions at 50 m from the cage sites were often elevated compared to the control sites, the rates were not significantly higher. The high DIC/O2 consumption ratio, especially at the cage sites, indicates that the main degradation pathway was through anaerobic respiration. The presence of a white filamentous mat on the sediments taken from the fish cages observed during the experiment highlight the likely role of anaerobic sulfate reduction. Ammonium fluxes from sediments into the water column were also significantly higher at the fish cages and conversely, nitrate uptake from the water column were higher at cage sites. This is consistent with a reduction in sediment nitrification due to increased bacterial respiration and low oxygen concentrations and a greater dependence on nitrate from the water column for denitrification (i.e. uncoupled nitrification-denitrification). There was also evidence that some of the nitrate is reduced to ammonium via dissimilatory nitrate reduction to ammonium (DNRA). The release of phosphate from the sediments occurred at the cage and 50 m sites, in contrast to phosphate uptake at control sites. Despite the observed higher ammonium release from farm enriched sediments, there has been no significant change in bottom water ammonium concentrations during the period of significant industry expansion based on data from the monthly water quality monitoring program. This suggests that more broadly across the harbour, nitrification in the water column and sediment denitrification may be buffering the system against the increased loads. This remains to be tested. Based on the flux rates, there was no clear indication that there was any temporal changes in Macquarie Harbour. This may reflect relatively stable bottom water conditions throughout the year more broadly in the harbour. Sediment conditions showed improvement at some of the cage sites during fallowing. However, the sediment response to fallowing at some areas was not as expected. In one of the leases, benthic flux rates did not show signs of recovery during fallowing (i.e. lower benthic respiration rates, lower nutrient flux rates), and conversely, during stocking the flux rates declined instead. This suggest that other factors are likely to be responsible for organic loading and mineralisation of organic matter. This study is the first assessment of benthic nutrient cycling in Macquarie Harbour sediments. Importantly it describes sediment function more broadly in the harbour and in response to organic enrichment due to fish farming. The more recalcitrant organic matter brought in by the rivers has kept the background oxygen consumption and nutrient levels low. Despite low oxygen consumption, bottom water oxygen concentrations are naturally low due to the long residence times and reduced mixing with surface waters. Sediment nitrification appears to be reduced as a result, with sediment denitrification relying on nitrate sourced from the water column. Lastly, considering that oxygen concentration is an important driver of sediment function in Macquarie Harbour, the recently observed decline in bottom water dissolved oxygen concentration and its implication for nutrient cycling warrants attention.