Linking the trophic function of estuaries to characteristics of their catchments

Our understanding of the interactions between terrestrial river catchments and estuarine ecology needs to be strengthened. The conversion of natural catchments to human land uses generally increases nutrient inputs to estuaries. Nutrient enrichment can disrupt ecological processes, including primary and secondary production and consumer trophic and population dynamics. These changes can alter the composition and trophic characteristics of consumer assemblages. This thesis details four complementary studies investigating relationships among catchment land use, nutrient loading, the instream consequences of nutrient loading, and the composition, trophic organization and nutritional support of estuarine fish assemblages. The estuary catchments investigated spanned a gradient of land use from highly altered to some largely intact catchments. Catchment land use was associated with the composition of estuarine fish assemblages (Chapter 2). The proportion of demersal species (i.e. species that inhabit the water column near to, and influenced by, the benthos) in fish assemblages was associated negatively with catchment land uses that are expected to increase nutrient loads (e.g. animal production, horticulture, industry and urbanization). My subsequent investigations suggested that this relationship reflected changes in habitat, especially the dominance of macroalgae over seagrass, which is induced by nutrient enrichment (Chapter 5). The proportion of urbanization in catchments was positively related to the nitrogen stable isotope (δ¹⁵N) values of large-bodied fish (Chapter 3). There was no relationship between δ¹⁵N and agricultural land uses, which substantially contribute to nitrogen loads of many Victorian estuaries. The lack of relationships between δ¹⁵N values in estuaries and the extent of agriculture in catchments suggests that bulk δ¹⁵N values are affected by mixing of isotopic sources or by the non-uniformity in nitrogen transformations among catchments distributed over hundreds of kilometres. Nutrient loading to estuaries influenced the trophic organization of fish assemblages (Chapter 4). Greater trophic diversity and less redundancy was seen at moderate to high inorganic nitrogen loading. This relationship may reflect the greater productivity and availability of vegetated demersal habitat. My subsequent investigations showed the contribution of demersal primary producers to fish nutrition did not reflect the relative availabilities of those resources in estuaries, which may be due to hypoxic conditions and habitat structure restricting the trophic transfer of some demersal resources to fish (Chapter 5). Findings from this thesis emphasise the need to integrate more fully the ecology of estuaries with that of their catchments, and then apply this to the management of estuarine ecosystems, particularly in the current era of rapid coastal development.