The nature and significance of 'macroalgae-epifauna-invertivorous fish' trophic links within a macroalgal-dominated reef ecosystem

Abstract

Canopy-forming macroalgae are important primary producers and habitat constructors within marine environments worldwide. In tropical regions, macroalgae have been much maligned in terms of their negative competitive interactions with corals. However, macroalgal habitats are themselves extremely valuable. They provide food, shelter and nurseries for associated invertebrates and fishes, including economically-important species, thereby supporting high biodiversity and with the potential to drive production of biomass. Compared to other marine habitats such as coral reefs and mangroves, our understanding of the productivity and ecosystem functioning of macroalgal meadows is much more limited. Using field data collected from meadows of the brown macroalga Sargassum in Ningaloo Marine Park, Western Australia, my PhD examines key aspects of the way in which these habitats function including: (1) the role of epifaunal invertebrates as links in the food chain between primary producers (macroalgae) and secondary consumers (invertivorous fishes); (2) the response of epifaunal community structure to primary producers, predators and habitat settings across seascapes; (3) the effect of seasonal fluctuations in macroalgal canopy structure on epifaunal production and the implications of changes in meadow coverage that could occur under conditions of ocean warming; (4) the foraging preferences of key secondary consumers (invertivorous fishes) of epifauna with the potential and the importance of niche partitioning within shifting environments; and (5) the impacts of predation by invertivorous fishes on epifaunal biomass associated with these macroalgal habitats and experimental estimates of the transfer of epifaunal production to the next trophic level. As the rate of global changes increases, the structure and functioning of marine ecosystem will alter radically. As a pivotal component in marine food webs, epifauna are rarely studied and are ambiguously defined. In chapter 2 I build a systematic framework for the study of epifaunal communities within marine habitats, which offers a consensus-based definition of epifauna as an aid for unifying different research areas. In chapters 3 and 4 I examine the community structure and quantify the secondary productivity of epifauna within a macroalgal-dominated reef, and assess how these variables relate to biotic and abiotic environmental factors. Macroalgal canopy size, canopy cover, predator biomass and seascape setting were key drivers for the spatial and temporal fluctuations in epifaunal community structure and productivity. Sensitivity analysis also indicated that even small reductions in canopy size and/or canopy cover can lead to dramatic drop in epifaunal production. Chapters 5 and 6 move on to examining elements of the trophic linkages between macroalgal-associated epifauna and invertivorous fish community. Invertivorous fishes in Ningaloo were chiefly represented by species belonging to the Labridae, Lethrinidae and Mullidae. Individual species demonstrated strong foraging microhabitat preferences and can be grouped into three foraging categories: 'canopy forager', 'generalist' and 'abiotic forager'. The use of classic predator-exclusion caging experiments showed that predation by canopy foragers has a significant influence on epifaunal density within macroalgal canopy and enabled quantification of productivity transfer from epifauna to invertivorous fishes. My research highlights the importance of trophic links between canopy-forming macroalgae, epifaunal invertebrates and invertivorous fishes within tropical seascapes, as well as the vulnerability of these links to changes in habitat structure that are forecast to occur under global change. The results of my research will also enhance our understanding of the functional impacts of macroalgal-associated species, as well as our understanding of trophic flows in marine food webs for appropriate management and conservation.