WAMSI Node 3.2.2 - Ecosystem impacts of human usage and the effectiveness of zoning for biodiversity conservation. Trophic effects through herbivory at Ningaloo

We used a range of approaches to gain an understanding of spatial and species-related patterns in herbivory in five distinct studies. Using underwater video cameras and Sargassum myriocystum assays, 23 different fish species were observed consuming macroalgae, but seven species (Naso unicornis, Kyphosus sp., K. vaigiensis, Siganus doliatus, Scarus ghobban, S. schlegeli and initial-phase Scarus sp.) together accounted for 95% of the observed bites across five regions. Of these species, three were identified as the most important in consuming macroalgae: N. unicornis, Kyphosus sp. and K. vaigiensis. These results were supported by stable isotope analyses that incorporate nutrients from food sources over far longer periods than those examined using the assay approach. We quantified spatial patterns of macroalgal consumption and food sources across a range of scales. Firstly, across reef habitats separated by hundreds of meters, herbivory was always greatest in the structurally complex coral-dominated outer reef and reef flat habitats, which are also characterised by the highest biomass of herbivorous fish. Secondly, we showed a high degree of variability in grazing rates among regions separated by 100s km in the marine park, with different species responsible for macroalgal removal among those regions. Either N. unicornis or Kyphosus spp. were responsible for the majority of the grazing. Thirdly, we showed variability in the importance of different food sources across both habitats and regions for some consumers (e.g. Siganus spp.) but consistency for other species (e.g. Naso unicornis, Kyphosus spp.), which is likely to reflect shifts in food source availability or feeding preferences. Lastly, we found strong transcontinental differences between Keppel Islands in the Great Barrier Reef (GBR) on the east coast of Australia and Ningaloo Reef in both the diversity of the species observed feeding and on the species composition of the roving herbivorous fish community. In Ningaloo Reef, 23 species were observed biting on Sargassum, compared with just 8 in the Keppel Islands. Sargassum consumption in the Keppel Islands was dominated by a small number of species and supports the identification of Naso unicornis as a key browser species. The larger number of species feeding on macroalgae in Ningaloo Reef suggests that there may be higher functional redundancy among macroalgal consumers in this system. We also characterised the benthic community dynamics of the reef-flat and lagoon habitats to identify seasonal patterns and we experimentally determined the importance of herbivory on algae recruitment in these two habitats. Differences among habitats in algal biomass were strongly influenced by season. Lagoon habitats only had higher biomass than reef-flat habitats during part of the year (late summer/ early autumn). Herbivory had an equally strong effect on the community composition of algal recruits in the lagoon and reef flat habitats, despite the reef flat hosting a herbivorous fish community that was an order of magnitude greater in terms of biomass than the lagoon, which is characterised by younger and smaller fish (e.g. Scarus initial phase).

Organization: CSIRO Marlin Data Catalogue