Level of potential risk to Great Barrier Reef ecosystems 2014-15

As described in Section 4.7, the river plume maps and plume water type maps can be overlaid with information on the presence or distribution of GBR ecosystems. This method can help identify ecosystems which may experience acute or chronic high exposure to contaminants in river plumes (exposure assessment) and thus, help to evaluate the susceptibility of GBR ecosystems to land-sourced contaminants. The framework to produce river plume risk maps for seagrass and coral ecosystems is based on a simplified risk matrix assuming that ecological responses will increase linearly with the pollutant concentrations and frequency of river plume exposure and was shown in Figure 4-3 and Figure 4-4.

Measuring the magnitude of the river plume risk to coral reefs and seagrass beds can be challenging because of the combination of different stressors in river plume waters, the difficulty in sampling a plume sufficiently to characterise it fully to assess its effects, and the inherent complexity of hydrodynamics in the region. Devlin et al. (2012b) underscored the need to develop risk models that incorporate the cumulative effects of pollutants. Detailed methods of how these figures are derived are included in Appendix A1.9. The actual risk is not validated against ecological health data and is at this stage theoretical. The lowest risk categories (I and II) are characterised by low frequency of the Primary and Secondary plume types, and the highest risk categories (III and IV) are characterised by high frequency of Primary and Secondary plume types.

It is important to note that: (i) Any results obtained in the Cape York NRM should be considered with care. Cape York is a shallow and optically complex environment where the true colour method hasn’t been fully validated; and (ii) Only surface areas inside the GBR

marine boundaries are reported. It is also acknowledged that this assessment does not take into account current condition of GBR ecosystems, and long term impacts on these communities. For example, it is recognised that inshore communities may be adapted to plume type and exposure history, so the highest risk of an ecological response could be during large events when Primary/Secondary waters extend into otherwise low risk (Tertiary) areas. In the future, these maps could be presented in the context of a longer term mean or median result, and present the current year in context of likely impacts, i.e., the predicted impacts are proportional to the deviation from the mean when past some biological threshold.

Coastal areas have the highest frequency of occurrence of Primary plume waters (see section 5.3.1) and thus coastal ecosystems are most potentially exposed to the highest risk categories (category III and IV). Inversely, offshore areas are less frequently exposed to plume waters and, when exposed, are more likely reached by the Tertiary plume water type.

Thus, offshore ecosystems are most potentially exposed to lower river plume risk categories.

Inshore ecosystems are located in transitional zones seeing an alternation of plume water types and frequencies depending on the wet season characteristics.

Figure 5-9 presents the potential river plume risk map of the 2014-15 wet season, showing that the GBR lagoon was most exposed to the lowest categories of potential river plume risk (category I and II). Approximately 53% of the total area of the GBR was exposed to surface river plume waters, with only 7% exposed to the higher risk categories (category III and IV) (Table 5-2). The proportion of each NRM region exposed to river plumes ranged from 37%

in the Burnett Mary region to 58% in Cape York (note low confidence) and 50% in the Fitzroy region. However, the proportion of the regions in the highest potential risk categories (III and IV) were much lower with 2% of the Burnett Mary region, 5% of Cape York and 8% of the Cape York region.

Figure 5-9: Map of potential risk from GBR river plumes in 2014-15, where class I is the lowest potential risk (blue) and class IV is the highest potential risk (dark red). Risk categories are defined as a combination of the plume frequency (5 categories: 0-0.2, 0.2-0.4, 0.4-0.6, 0.6-0.8, 0.8-1.0) and plume water type (3 categories:

Primary, Secondary and Tertiary) as presented in Figure 5-7 and Figure 5-8.

Table 5-2: Areas (km²) and percentage (%) of the GBR lagoon exposed to different categories of surface river plume frequency and river plume-related risk within the GBR and each NRM. Surface areas south of the GBR Marine Park boundary (Hervey Bay) are not included.

NRM Total Risk category Total categories of potential river plume risk within each NRM, exhibiting a wide range of exposure (areas, % and categories of risk). In 2014-15, it was estimated that 62% of GBR coral reefs were exposed to the lowest categories of potential river plume risk (I and II) and only a very small area of reefs (<1% of the area) were in the highest risk category (class IV). The assessment indicates that only coral reefs in the Mackay Whitsunday and Fitzroy regions were potentially exposed to category III risk from river plume exposure, however these areas were also small (3%).

Table 5-3: Areas (km²) and percentage (%) of the coral reefs exposed to different categories of surface river plume frequency and river plume-related risk within the GBR and each NRM region. Surface areas south of the GBR marine park boundary (Hervey Bay) are not included.

Coral reefs Total Risk category Total

exposed

Table 5-4 presents the areas (km²) and percentage (%) of seagrass beds (surveyed, deep-water modelled and total (surveyed + deep-deep-water modelled) exposed to different categories of potential river plume risk within each NRM, exhibiting a wide range of exposure (areas, % and categories of risk). In 2014-15 GBR surveyed seagrass beds were mostly exposed to the medium categories of potential river plume risk (categories II and III). Exposure categories for deep-water modelled seagrass were more variable but were predominantly exposed to the lowest categories of potential river plume risk (I and II).

It is estimated that 97% of the GBR surveyed seagrasses were exposed to surface river plumes, with most of them in the potential risk categories II and III from the river plume exposure. The largest areas of coastal seagrasses exposed to river plumes were located in the Cape York (97% but note low confidence), Burdekin (97%) and Fitzroy (95%) regions.

Excluding the Cape York region, surveyed seagrasses of the Burdekin and Fitzroy regions had the highest potential risk from river plume exposure 83% and 95 % respectively).

For deep-water modelled seagrass, 94% were exposed to surface river plumes, with most of them (92%) in the lowest potential risk categories (I and II). In all NRM regions, 90 to 100% of deep-water modelled seagrass areas were exposed to river plumes, except in the Burdekin where the proportion of meadows exposed was about 74%. Note that seagrass meadows in Hervey Bay (outside of the GBR southern boundary) were not included in the risk analysis. There were no deep-water modelled seagrass in the highest risk category, and a relatively small proportion was in the potential risk category III in the Fitzroy and Mackay Whitsunday regions (5% and 13% respectively).

The assessment of the total seagrass area (see Table 5-4) indicates that while 94% of the seagrasses were potentially exposed to surface river plumes, a majority of these were in the lowest risk categories (I and II).

Table 5-4: Areas (km²) and percentage (%) of surveyed, deepwater modelled and total (surveyed plus deepwater modelled) seagrass exposed to different categories of surface river plume frequency and river plume-related risk within the GBR and each NRM region. Surface areas south of the GBR Marine Park boundary (Hervey Bay) are not included.

Total seagrass Total Risk category Total

exposed

Offshore seagrass Total Risk category Total

Total seagrass Total Risk category Total

exposed

5.3.4 Loading maps for dissolved inorganic nitrogen, particulate nitrogen and