Seasonal variation in the emission of greenhouse gases and biogeochemical cycling of nutrients in a farm dam of south-eastern Australia

Farm dams are integral part of Australian agriculture and they contain a substantial fraction of inland fresh water on the Australian continent. Storage of water can alter the physical, chemical and biological processes occurring within the dam and subsequently, any downstream receiving water body. However, limited research has been directed into understanding the factors controlling biogeochemical processes in the dam and the effect of the dam on downstream water quality. In this thesis, a dam (11.8 ML) with a catchment area of 1.3 km2 on a dairy farm at Poowong East (ca. 130 km south-east of Melbourne, VIC) was studied to investigate: (1) the seasonal variations in nutrient concentrations and greenhouse gas (N2O and CH4) emissions, (2) the role of aquatic sediments in the emission of the greenhouse gases, and (3) the biogeochemical cycling of nitrogen and phosphorus in the dam. Fluxes of N2O, CH4 and bioavailable inorganic nutrients (NH4+, NOx and FRP) between the sediment and the water column at 3 different sites within the dam were determined over a period of 18 months (from July 2010 to December 2011). Factors controlling the gas and nutrient fluxes (i.e. temperature, dissolved oxygen, nutrient availability and faunal abundance) were analysed. The physicochemical characteristics of the creek upstream and downstream of the dam were also measured throughout the sampling period. This study also involved sediment core incubations, deployment of equilibrium dialysis samplers (pore water peepers) and planar optode experiments. The dam was found to be a major source of N2O during periods when both NOx and O2 concentrations were high in the water column. Sedimentary CH4 flux increased gradually during summer when the overlying water was very close to, or completely, anoxic. The production rate of CH4 was almost 3 times greater in the deeper site (3.5 metres) than in the shallow site (1 metre) and the highest fluxes were associated with the bottom water temperature maxima. The dam was a net source of NH4+ but a net sink for NOx and FRP. The internal load of NH4+ was almost 3 times higher than the external load on an annual basis. Conversely, the dam removed about 14% and 5% of the annual external NOx and FRP loads, respectively. There was a significant seasonal variation in inorganic nutrient fluxes in the dam. The diffusive flux of nutrients across the sediment-water interface, particularly during summer months, emphasized the role of the bottom sediment as an alternative source of bioavailable nutrients into the water column. This study found that benthic macrofauna (here both Chironomidae and Oligochaeta) did not impose any significant effect on in situ N2O production but the results suggested a perhaps important involvement in the cycling of biogenic CH4. Chironomidae larvae significantly increased sediment oxygen consumption rates and decreased the NH4+ efflux. Chironomidae larvae also appreciably increased the water column NOx-derived denitrification (Dw) as well as the coupled nitrification-denitrification (Dn) rates in the sediment. Overall, this study produced three key findings: (1) Farm dams can be a major source of N2O and CH4 depending on the season and nutrient availability. (2) Seasonal variation of nutrient processing within the dam resulted in significant effects on the downstream water quality. In most sampling months, the outflowing water failed to meet the state and regional water quality standards particularly in terms of turbidity, dissolved oxygen and different forms of nutrients. (3) Long term monitoring of the seasonal variation of the nutrient processing in farm dams is essential to predict the influence of such dams on the downstream water bodies and to assess the effectiveness of any management actions.