Interactive effects of local and global stressors on soft sediment communities of southeast Australian estuaries

Estuaries are highly productive and provide valuable ecosystem services and fisheries resources. They are, however, also the sites of human population centres, and are consequently under growing pressure from multiple stressors introduced by coastal development. These stressors may act interactively to affect estuarine biodiversity and functioning. They may also modify the response of coastal systems or their resilience to climate change. In this thesis I considered two of the major threats to estuarine ecosystems: coastal urbanization and nutrient pollution and how they may exacerbate and interact with the effects of global stressors associated with climate change. -- Artificial structures, such as sea-walls, are a common feature of urbanized coastal areas. Their presence can impede the landward migration of the coastline caused by sea level rise, resulting in a reduction of the intertidal area. I assessed what the ramifications of a reduction in intertidal area might be for 1) the quality and decomposition of seagrass detritus, an important resource for macroinvertebrates, and 2) the macroinvertebrate communities supported by live seagrass. Results indicated that tidal elevation influenced seagrass decomposition directly, by changing the decomposition environment, rather than inducing changes in detrital quality. Tidal elevation also influenced seagrass-dwelling macroinvertebrates both as a direct effect of changes in depth and an indirect effect of changes in seagrass morphology. Hence, a reduction in intertidal area is likely to have sizeable ecological ramifications for tidal flat communities dependent on seagrass resources. -- Warming is anticipated to enhance the susceptibility of estuaries to eutrophication by increasing metabolic rates of oxygen consumption, and by increasing the stratification of waterways. I assessed how climatic setting modifies the relationship between diffuse nutrient pollution and invertebrate assemblage composition and their carbon and nitrogen sources, sampling 16 estuaries, receiving varying levels of nutrient pollution, across a 7° gradient in latitude. Overall, macrofaunal communities were only weakly correlated with nutrient enrichment and this relationship was dependent on sediment grain-size, rather than climatic setting. The nitrogen content of mangrove and seagrass increased with latitude and this pattern was weakly, but significantly, correlated with a shift in the proportion of food sources assimilated by a detritivorous gastropod, but not by a predatory polychaete. Consequently, it appears that present rates of diffuse nutrient loading to oligotrophic southeast Australian estuaries may be insufficient to cause persistent degradation of benthic sediments, although the possibility for localized eutrophication caused by point nutrient sources remains. Future studies should determine threshold nutrient loadings at which environmental deterioration starts to be seen. -- In summary, this thesis has enhanced our understanding of when and where two consequences of coastal development, coastal armoring and nutrient enrichment, will have greatest impacts on estuarine tidal flat communities. In doing so, it will assist managers in ascertaining when, where and how to expend limited resources to best sustain estuarine biodiversity and its important functions.