Unpacking effects of multiple stressors on estuarine meiobenthos

Estuaries are increasingly exposed to a multitude of stressors, introduced by anthropogenic activities. Although these stressors often overlap in time and space, ecological studies predominantly consider their impacts in isolation of one another, and often through highly contrived experiments in the laboratory. Of the relatively few multiple stressor studies that have been done, the great majority provide stressors simultaneously, failing to acknowledge that stressors may occur asynchronously, and that the timing and order of stressors may influence their cumulative impact. This thesis considered how two important estuarine stressors - nutrient enrichment and physical disturbance of sediments - independently and interactively influence estuarine meiobenthic communities. The meiobenthos, though important links in food webs and critical to nutrient cycling, are a rarely studied group, especially along the east coast of Australia. First, to assess the spatial scales across which estuarine meiobenthos naturally vary, and how this relates to estuarine nutrient loading, an observational field study was conducted. Meiobenthos were sampled from seagrass beds at 16 sites, of 8 estuaries, spanning ~1000 km of the coast of New South Wales, Australia. Four of the estuaries had elevated nutrient loadings, relative to pre-European settlement, while 4 were relatively unmodified. I found that across the sites sampled, sediment grain size was a more important determinant of the meiobenthic community structure than nutrient load or other predictor variables such as latitude. Second, using small-scale field experiments I investigated how nutrient enrichment and physical disturbance interact to influence meiobenthic communities at each of two field sites. Whereas previous mesocosm experiments found that two stressors have interactive effects, I found predominantly additive effects. Effects varied between the two study sites, demonstrating the importance of environmental factors in mediating stressor impacts. Finally, I assessed how the synchrony and order of the application of the two stressors (nutrient enrichment and physical disturbance) influences their cumulative impact. The study found that although total abundance and Shannon diversity were insensitive to the timing of stressor application, taxonomic richness was more negatively impacted by simultaneous than sequential stressor application. Individual taxa varied in their response to the timing and order of stressor application, with responses often site-dependent. This was expected given that taxa vary markedly in their tolerance to particular stressor. My research has made an important contribution to stressor ecology, providing one of the first empirical tests of ecological theory regarding the effect of the synchrony of multiple stressors on their cumulative impact. It has also addressed a major knowledge gap regarding the sensitivity of meiobenthic communities of south-east Australian estuaries to two major stressors. The knowledge generated by this thesis will assist estuarine managers and policy makers to develop strategies for managing and mitigating a variety of multiple stressor scenarios.