Concentrations of macronutrients (e.g. nitrate and phosphate) in the surface ocean play an important role in controlling the ocean’s primary productivity (the rate at which new organic matter is developed at the base of the food web). Surface ocean waters around Australia typically have low macronutrient concentrations. The supply of nutrients into the upper ocean is facilitated primarily by seasonal movement of the mixed layer and eddy-driven mixing (Falkowski et al. 1998, Doney 2006). Wind-induced upwelling is confined to a few localised regions (e.g. along the Bonney Coast in the South-east Marine Region). Although land-based sources of nutrients can be significant, they are largely seasonal as a result of climatic variability in rainfall and confined to localised, nearshore regions (e.g. the inner lagoon of the Great Barrier Reef; Revelante et al. 1982).
As the ocean warms around Australia, it is expected that the upper ocean will become more stratified, which could result in a decline in the vertical supply of nutrients to the surface, reducing primary productivity (Bopp et al. 2013, Lenton et al. 2015; see also Box MAR1). This will have flow-on impacts on marine productivity and fisheries, and, in turn, on higher-order marine animals such as turtles, sharks and seabirds (Brown et al. 2010). In inshore areas, changes in precipitation associated with climate change will influence the frequency and intensity of flooding events, which will have flow-on impacts on sediment and nutrient flows into estuarine and coastal regions (see the Coasts report for further details). Increased eddy activity because of the strengthening of EAC eddy transport (see Ocean currents and eddies) may compensate for a decline in the vertical nutrient supply in the Tasman Sea (Matear et al. 2013, 2015).
At present, observations of nutrients in the shelf and oceanic waters around Australia are only sufficient to document the mean state of the ocean, and insufficient data are available to quantify recent trends. It is therefore unclear whether nutrient supply is changing in Australian waters.
Oxygen is consumed in aerobic respiration, and most marine ecosystems comprise aerobic organisms that need oxygen to survive. The oxygen content of the ocean varies spatially and temporally, reflecting areas of varying oxygen production and consumption.
Because of the distribution of the highest abundances of aerobic organisms, and therefore the highest rate of oxygen consumption, the dissolved oxygen concentrations in the ocean are lowest in the intermediate water (300–1000 metres; Riser & Johnson 2008). In some inshore regions with limited circulation, and in several subsurface oceanic zones, biological consumption of oxygen can lower oxygen concentrations considerably further; they can reach ultralow values that can be up to 50 times lower (e.g. less than 20 micromoles per litre [μmol/L] and reaching 1 μmol/L at their core) than the oxygen minimum found in intermediate water (Paulmier & Ruiz-Pino 2009). These low oxygen concentrations can lead to ecosystem-wide changes, including loss of biomass of species and food-web complexity, and potentially diminished ecosystem services (Chu & Tunnicliffe 2015).
At the scale of ocean basins, deoxygenation has been observed during the past 50 years and is projected to continue to occur because of warming waters from climate change (Joos et al. 2003, Helm et al. 2011, Andrews et al. 2013). This will result in an overall reduction in dissolved oxygen and an expansion of areas with low oxygen, known as oxygen minimum zones.
Observations of dissolved oxygen in shelf and offshore regions identify Australian waters as being generally well oxygenated with little spatial and temporal variability (Figure MAR8). However, current observations are not sufficient to determine decadal trends on regional scales (CSIRO 2014). Comprehensive measurements from inshore regions outside estuarine and embayment habitats are largely lacking. The application of new oxygen sensor technology through observing platforms, such as the Integrated Marine Observing System (IMOS) National Reference Stations (Lynch et al. 2014) and autonomous profiling floats deployed as part of IMOS, has the potential to enable monitoring of trends in dissolved oxygen.