Physical, biogeochemical, biological and ecological processes are an important component of marine ecosystem function. Together with the status and trends of marine habitats, communities and species groups, they provide an indication of the health of the marine ecosystem (e.g. Rombouts et al. 2013). Ecosystem health affects the services provided by the environment, and the industries and societies that use the marine environment, either directly (e.g. fishing) or indirectly (e.g. carbon sequestration and climate).
Several initiatives at local, regional and international levels recognise that monitoring key processes as indicators of marine ecosystem health is required to assess, adapt and revise management actions. As a result, there has been considerable discussion in the scientific and management community about the appropriate variables to measure and monitor. They include identifying the processes and components of the marine environment that managers and society value, such as those that support ecosystem services. Examples include ‘key biological areas’ (Eken et al. 2004), ‘key ecological features’ (Dambacher et al. 2012), ‘ecologically or biologically significant marine areas’ (Dunn et al. 2014, Bax et al. 2015) and the ‘key environmental variables of the Global Ocean Observing System’ (see Sustained ocean monitoring). Processes for identifying measurable variables also vary; they include simple selection criteria–based and more complex model-based frameworks (e.g. Hayes et al. 2015).
Within the Australian marine environment, the identification of key ecological features has been central to the marine bioregional planning process (see Box MAR10). Considerable effort has gone into identifying important ecosystem components and processes associated with each key ecological feature, and the biological variables that have high commonality across the features, which could therefore comprise essential variables for measurement and monitoring (Hayes et al. 2015). To date, this process has been completed for 32 of the 53 key ecological features. For pelagic key ecological features, identified indicators include biogeochemical (nutrients) and biological (phytoplankton) indicators at the bottom of the food web, and predators (large pelagic predatory fish and seabirds) at the top of the food web. In shelf systems, identified indicators include those that are habitat forming (macroalgae and coral; see Hayes et al. 2015). Further detail on Australian and global efforts to identify key indicators for measurement and monitoring is provided in Sustained ocean monitoring.
The biophysical and ecological indicators of marine health discussed here were identified in SoE 2011, and, for comparative purposes, we update them here. These include indicators of physically driven processes (water column turbidity and connectivity), productivity (microbes, phytoplankton and zooplankton), food webs (trophic processes), disease and outbreaks, and invasive species.
Overall, biophysical and ecological indicators of marine health within the Australian marine environment are in good condition, although several indicators are highly spatially and temporally variable. The methods used to measure each indicator are also variable. Current monitoring of many indicators is not spatially and temporally comprehensive enough to capture such dynamics in a robust manner. Therefore, assessment at a national scale and determination of trends for these indicators are difficult. Where indicators are highly dynamic (i.e. there is high variability), it is often difficult to distinguish trends from variability (i.e. the signal from the noise). Care must be taken in deriving trends across short timeseries, because these may capture only a portion of a highly variable signal and may not be indicative of longer-term trends (Hobday & Evans 2013, Harrison & Chiodi 2015). In addition, interpreting any observed trends requires identifying and understanding the relevant components of ecosystem structure, which can vary depending on interpretation, and between different areas or systems. Given the limited spatial and temporal extent of most information and data available, state and trends of these processes will be provided more generally for the Australian region rather than for each marine region.
Physical, biogeochemical and biological processes
Water column turbidity and transparency
Australian marine waters are generally low in turbidity and colour, and high in transparency (Shi & Wang 2010). In oceanic and outer continental-shelf waters, the major determinant of turbidity, transparency and colour is the biomass of phytoplankton, whereas, in inshore regions, sediment flows from river systems or land run-off and high tidal flows have the most influence. Observations from the network of IMOS National Reference Stations show low suspended solids across all stations except Darwin (Figure MAR31).
Although spatially and temporally variable overall, the transparency of the water column in open-water environments has significantly increased since 1997, largely associated with improved wastewater treatment, reduced nutrient inputs, and improved management of agricultural practices and associated run-off (see the Coasts report for further detail on inshore, embayment and estuarine regions, and see GBRMPA [2014a] for an assessment of waters associated with the Great Barrier Reef). In regions not greatly affected by these processes, transparency has remained stable (Figure MAR31), with this stability reflected in the generally comparable grade and trend of SoE 2016 to SoE 2011.