Cumulative effects of multiple stressors impact an endangered seagrass population and fish communities.

Coastal ecosystems are becoming increasingly threatened by human activities and there is growing appreciation that management must consider the impacts of multiple stressors. Cumulative effects assessments (CEAs) have become a popular tool for identifying the distribution and intensity of multiple human stressors in coastal ecosystems. Few studies, however, have demonstrated strong correlations between CEAs and change in ecosystem condition, questioning its management use. Here, we apply a CEA to the endangered seagrass Posidonia australis in Pittwater, NSW, Australia, using spatial data on known stressors to seagrass related to foreshore development, water quality, vessel traffic and fishing. We tested how well cumulative effects scores explained changes in P. australis extent measured between 2005 and 2019 using high-resolution aerial imagery. A negative correlation between P. australis and estimated cumulative effects scores was observed (R2 = 22 %), and we identified a threshold of cumulative effects above which losses of P. australis became more likely. Using baited remote underwater video, we surveyed fishes over P. australis and non-vegetated sediments to infer and quantify how impacts of cumulative effects to P. australis extent would flow on to fish assemblages. P. australis contained a distinct assemblage of fish, and on non-vegetated sediments the abundance of sparids, which are of importance to fisheries, increased with closer proximity to P. australis. Our results demonstrate the negative impact of multiple stressors on P. australis and the consequences for fish biodiversity and fisheries production across much of the estuary. Management actions aimed at reducing or limiting cumulative effects to low and moderate levels will help conserve P. australis and its associated fish biodiversity and productivity.

Decadal dynamics of subtidal barrens habitat.

Rocky reef barrens that are devoid of macroalgae can be created by various mechanisms, and are often maintained in the long-term by grazing urchins. The persistence of barrens varies greatly among locations, although few studies have investigated the stability of this habitat over multiple decades, particularly at large spatial scales. We used aerial images to test for differences in temporal trajectories of shallow (2-10 m) barrens at 21 sites (average size 12 ha) over 30 years across 500 km of coastline in New South Wales, Australia. Longer-term (40-68 yr) trajectories of barrens cover were documented for five of the sites and these generally reflected the 30-year patterns. Averaged across all sites, barrens area increased at a rate of 19.9 ± 8.4 m2 per year per hectare of reef from 1980s-2010s. Importantly, however, 55% of sites had stable or fluctuating (±10% cover) barrens over this period, rather than displaying continual increases. Although the extent of shallow barrens increases with latitude, the temporal dynamics of barrens did not differ among three latitudinal regions where barrens are the most extensive. Associations between variability in barrens cover and environmental variables indicated that reef topography might pay a role in influencing barrens. Examples of such long-term persistence of extensive barrens are relatively rare and potential reasons for this and possible future changes are discussed.

Differences in rocky reef habitats related to human disturbances across a latitudinal gradient.

This study tested for differences in the composition of intertidal and shallow subtidal rocky reef habitats subjected to a range of human pressures across ∼1000 km of coastline in New South Wales, Australia over 5 years. Percentage covers of habitats were sampled using aerial photography and a large grain size (20 m2 intertidal; 800 m2 subtidal) in a nested hierarchical design. Results were consistent with anthropogenic impacts on habitat structure only around estuaries with the most heavily urbanised or agriculturally-intense catchments. The most convincing relationships documented here related to environmental variables such as SST, latitude, reef width and proximity to large estuaries irrespective of human disturbance levels. Moreover, there were suggestions that any influences of estuarine waters (be they anthropogenic or natural) on reef assemblages could potentially extend 10s of kilometres from major estuaries. In general, our results supported those of studies that utilised smaller grain sizes (greatest variability often at smallest spatial scales), but we found that variability over scales of 100s of km can be similar to or greater than variability over scales of 10s of metres.