Luke McPhan

Project Carbon flux in Aquatic food webs: Investigating the influence of Basal carbon on the daily growth of larval fish and their dietary nutrition in the southern Murray-Darling basin
Candidate Luke McPhan
Contactluke.mcphan@unsw.edu.au
Supervisor/sProfessor Andy Baker (UNSW)
Dr Kim Jenkins (UNSW/CSU)
Dr Paul Humphries (CSU)
Dr Lee Baumgartner (MDFRC)
Duration July 2013 – July 2016

Summary

As the dominant building block of life, carbon is incorporated by organisms across trophic levels of food webs. From origins at primary production through to predatory vertebrate biomass, carbon is recycled by microbial food webs and accumulates at different trophic levels in the form of organism biomass. Moving through multiple pathways, the stable isotopes of carbon present an appropriate measure for determining origins of basal carbon and which processes (e.g. terrestrial/aquatic primary production or microbial heterotrophy) are important for specific taxa and their growth (biomass accumulation).  Fish larvae are some of the fastest growing organisms within the freshwater systems of the Murray-Darling basin, though, larvae are also characterised by high mortality (>99%, Chambers and Trippel 1997). To know what organisms are being consumed by fish larvae, gut content analysis and carbon signatures of larval fish tissue and prey items, will be used to determine the importance of different primary production sources for riverine food webs and ultimately larval fish growth.

Though autotrophs; plants, macrophytes and algae, in both riparian and aquatic habitats are thought to dominate carbon supply to food webs, labile dissolved carbon (<0.45µm), in the form of dissolved organic matter (DOM) is also incorporated to the food web through heterotrophic bacteria. With aged terrestrial carbon being shown recently to be extremely labile for microbial communities (Fellman et.al. 2014) this flux may have been under estimated in past years of food web modelling in freshwater systems.  My research aims to determine the importance of carbon uptake by heterotrophic bacteria through fluorescence of DOM (fDOM) and stable isotope analysis of both dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC). This will be compared with stable isotopic signatures of riparian and aquatic vegetation and incorporated in isotopic bi-plots of the entire food web for all trophic levels below fish larvae. Fish larvae will be measured for total length, dissected for sagittal otoliths to determine daily growth and also dried for  isotope analysis.

Understanding the flux of carbon in freshwater systems is of upmost importance when considering the persistence of native fish in Australian rivers. As anthropogenic flow regulation by dams and weirs continues to disrupt carbon flow along rivers, impacts of carbon supply to riverine food webs remain unknown with potential impacts on the growth, conservation and management of riverine taxa from riparian vegetation to aquatic vertebrates.