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Analysis of Sources of Primary Production Supporting the Apex Predator-dominated Ecosystem in the Northwest Hawaiian Island Coral Reef Ecosystem Reserve(2007)

Project Description:
Information on the food sources and trophic position of the fish occupying the Northwest Hawaiian Islands (NWHI) will provide resource managers with insight on the degree and mechanisms by which alterations in the oceanographic regime vs. changes in the reef benthic environment will affect fishery productivity at the NWHI Coral Reef Ecosystem Reserve. The Northwest Hawaiian Islands are a relatively pristine coral reef ecosystem and exhibit a trophic structure dominated by apex predators. Preliminary estimates are that sharks and jacks constitute over 50% of the fish biomass associated with the reefs. This suggests an abundant source of primary production available to herbivores, which must be heavily preyed upon. It is unknown whether the bulk of this primary production is autochthonous, that is, associated with the reef environment and surrounding sediments, or allochthonous, that is, associated with the North Pacific Gyre and brought in to the reef ecosystem on ocean currents. Autochthonous production would be primarily associated with benthic algae and coral organisms, while allochthonous production would be associated with oceanic phytoplankton production. These two sources of primary production are distinguishable by their stable C isotope signature, or 13C value. The C isotope signatures of fish and shark tissue will provide an index of the amount of benthic (autochthonous) and planktonic (allochthonous) production supporting fishery production. N isotopes do not differ much between benthic and pelagic primary producers in oceanic environments, but they are excellent indicators of the trophic position of consumers. Compared to their food, consumer organisms are enriched in 15N in a significant and predictable pattern, and so can be used to determine whether a fish is feeding 2, 3, 4 or more steps above the base of the food chain. Objectives are: a. Determine the role of benthic and planktonic primary production in supporting secondary production of apex predators in the NWHI reserve. Assess vertical trophic linkages between primary producers and consumers, and horizontal trophic linkages between the reef environment, the adjacent shallow seafloor, and deep-water fishes. b. Obtain estimates of benthic microalgal and phytoplankton biomass from the reef, surrounding sediments, and water column of the Northwest Hawaiian Islands. c. Obtain taxonomic description of dominant microphytobenthos in NWHI shallow sediments, to contribute to biodiversity assessment. d. Provide data on distribution and abundance of primary producers and food web structure in a GIS-compatible format for integration into biogeographic and ecosystem function models.

Expected Outcome:
This project will provide important information on the trophic linkages between primary producers and fishery organisms in the NHWI. The analysis of food webs supporting fish and particularly apex predators will tell us the degree to which shallow benthic habitats which are dominated by algae contribute to overall fishery productivity in the NWHI. It is anticipated that we will find that these relatively nondescript habitats are vitally important to sustaining ecosystem production in the NWHI. Information on food webs, algal biodiverisy, biomass and production can be added to GIS databases to characterize NHWI habitats, and also will contribute to predictive models used to forecast the ecosystem response to environmental change and stressors.