Trophic dynamics in Australian seagrass beds : unravelling linkages from nutrients to fish
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The study of food webs and trophic dynamics has been a major endeavour for biologists for over half a century. The major aim of trophic ecology has been to explain the complex structures and interaction of food webs using simple models and generalities. A key debate in this research field has been the extent to which trophic structures of communities are driven by resource or consumer control. This issue has important implications for the management of marine ecosystems with anthropogenic disturbance rapidly altering bottom-up (e.g. nutrient enrichment) and top-down (e.g. fisheries exploitation) processes. To enhance understanding of bottom up and top-down influences in productive estuaries, this study investigated a prominent four level food chain (epiphytic algae, mesograzers, juvenile fish, and piscivorous fish) in seagrass habitats. Large-scale mensurative field experiments were combined with manipulative experiments to gain an improved understanding of changes in biomass and community structure of trophic levels under different nutrient loading and to determine the type of functional response driving these changes. In general, epiphyte and grazer biomass were greater in developed catchments with higher nutrient loads during long-term field sampling of systems approaching equilibrium. However, a decoupling from the epiphyte trophic pathway was evident in higher trophic levels. The biomass of carnivorous juvenile fish and large piscivores displayed no significant change across the nutrient gradient of this study. This pattern indicates that a ratio-dependent functional response would be most appropriate for modelling the lower trophic levels and their responses to nutrient enrichment. However, more complex models taking into account other factors such as trophic transfer and subsidy are required for models across all four trophic levels. Short-term experiments involving epiphytes, grazers and juvenile fish that manipulated nutrient levels and predation rates often behaved in ways that contradicted the patterns described above for long-term studies. In particular, manipulative studies showed strong evidence for top-down control sometimes resulting in lower or similar levels of epiphytes in enriched plots compared to ambient controls. The structure of fish assemblages was influenced by the nutrient status of the waters they inhabited. Assemblages from developed and undeveloped catchments separated into distinct communities with higher abundances of fish in low nutrient waters. The species that contributed most to these differing assemblages were small pelagic carnivores (Ambassis jacksoniensis and Redigobius macrostoma). These species appeared to be sensitive to poor water quality making them candidates for bio-indicators of anthropogenic disturbance in catchments. Enclosure experiments and mensurative field sampling found that growth, survival and condition of juvenile trumpeter (Pelates sexlineatus) were greater under ambient nutrient levels despite increased prey items in nutrient enriched sites. This suggests that the positive indirect effects of increased trophic support from nutrient enrichment may have been countered by direct negative effects of toxicity either from the nutrients in their dissolved forms of ammonia; nitrate; or from other anthropogenic pollutants such as metals or hydrocarbons. Overall, nutrient enrichment of coastal waters in south-eastern Australia promoted epiphyte growth that may compete with seagrasses for light, causing stress and possible declines in seagrass health and distribution. My study, however found greater seagrass biomass at some sites with nutrient enrichment suggesting moderate increases to nitrogen-limited systems may be beneficial to seagrasses. It also appears that the increase in primary productivity associated with nutrient loading did not translate into an increase in biomass of juvenile fish or piscivores in seagrass habitats, providing no benefit to fisheries production. Given the potential for nutrient enrichment to alter trophic structure in coastal waters and to have negative impacts on seagrass health it is recommended that an integrated system of management be implemented. This should include further targeted research for a better understanding of seagrass trophic systems, monitoring of environmental indicators that alert catchment managers to early signs of eutrophication, remediation of existing eutrophic systems through nutrient reduction mechanisms and planning policies that identify and afford greater protection to ecologically-important habitats such as seagrass; and estuaries with high residence times that are more vulnerable to eutrophication.
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