Arid zone estuaries: nekton and trophic connectivity over heterogeneous landscapes.

Helen Penrose (2011). Arid zone estuaries: nekton and trophic connectivity over heterogeneous landscapes. PhD Thesis, School of Biological Sciences, The University of Queensland.

       
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Author Helen Penrose
Thesis Title Arid zone estuaries: nekton and trophic connectivity over heterogeneous landscapes.
School, Centre or Institute School of Biological Sciences
Institution The University of Queensland
Publication date 2011-05
Thesis type PhD Thesis
Supervisor Professor Catherine Lovelock
Associate Professor Greg Skilleter
Professor Alistar Robertson
Total pages 149
Total colour pages 11
Total black and white pages 138
Subjects 06 Biological Sciences
Abstract/Summary Abstract Estuaries comprise key habitats that support primary and secondary production in the coastal zone. The degree of aridity plays a major role in shaping the physical conditions found within an estuary and, ultimately, the composition and distribution of intertidal habitats and associated fauna. Our current knowledge of estuarine carbon movement and fish trophic dynamics is derived from studies in temperate and wet tropical climates. Many regions of the world have arid tropical climates where extensive intertidal salt flats are encrusted with cyanobacterial mats. Whilst the role of cyanobacterial mats in delivering nutrients to coastal waters is well established, their use and trophic reliance by fish and prawns (nekton) is largely unknown and unquantified. Fundamental to understanding large-scale ecological processes and managing the integrity of coastal landscapes is knowledge of the connectivity between habitats. In this thesis I examined connectivity in relation to three major processes: i) physical habitat connectivity via tidal migrations of nekton (Chapter 2) ii) trophic connectivity via the assimilation and movement of carbon by nekton (Chapter 3), and iii) ontogenetic connectivity via the tidal movements of an elasmobranch (Chapter 4). The study location was the Exmouth Gulf of north-western Australia; one of the largest, unmodified arid tropical estuaries in the world, where high intertidal flats are the largest habitat in areal extent and are intermittently available to estuarine consumers on high spring tides. To define the assemblage of nekton relevant to the functioning of arid tropical estuaries, nekton was captured with fyke nets that were positioned within two contrasting landscapes (habitat mosaics): (1) cyanobacterial mats with spatially adjacent, extensive fringing mangroves (CM+M mosaic), and (2) cyanobacterial mats “without” extensive fringing mangroves (CM-M mosaic). A total of 32, 330 fish from 61 species within 32 families, and a total of 82, 996 crustaceans representing nine species within three families, were caught. Twenty five percent of the total fish catch were caught on the high intertidal cyanobacterial mat, representing 33 species. The abundance and composition of the nekton assemblage on the high intertidal cyanobacterial mat was thought to be influenced by the structural habitat complexity and/or geomorphology of the adjacent intertidal habitat(s). The landscape configuration is thought to influence habitat use by specific species and potentially the trophic pathways within and between the intertidal and subtidal habitats. Stable isotope (13C and 15N) and gut content analyses were used to test the hypothesis that fish use of cyanobacterial mat-associated resources was influenced by the landscape configuration. Cyanobacteria were the dominant carbon source for juvenile whiting (Sillago burrus: Sillaginidae), silver biddy (Gerres oyena: Gerreidae), north-west hardyhead (Craterocephalus capreoli: Atherinidae), and a new genus/species of family Gobiidae. The composition of prey items observed in the stomachs of fish was influenced by the landscape configuration for G. oyena, C. capreoli and estuarine glassfish (Ambassis vachelli: Ambassidae), as was the source of assimilated dietary carbon for G. oyena. Cyanobacteria were the dominant carbon source assimilated by these fish species in both mosaic types. The western school prawn, Metapenaeus dalli (Penaeidae), relied on cyanobacteria in the mosaic with adjacent mangroves, suggesting that the habitat value of the mangroves may facilitate habitat and trophic connectivity via the tidal movements of this species. Terrestrial sources were a major dietary component of the fish using the high intertidal habitat. My research establishes the important functional role of high intertidal cyanobacterial mats in supporting fisheries production. The role of arid tropical estuarine habitats in providing trophic support for the juvenile life stage of the giant shovelnose ray (Glaucostegus typus: Rhinobatidae) were examined with stomach content and dual-tissue (liver and white muscle) stable isotope analysis. Juvenile G. typus forage on benthic macroinvertebrate communities that rely on cyanobacteria and seagrass as a food source. This energy is transferred to the offshore pelagic food web during ontogenetic dietary shifts, where the rays are a food source for sharks. Although stomach content data indicated feeding in nearshore benthic habitats, the reliance on benthic carbon sources is not reflected in liver or muscle tissue until sufficient growth has occurred, diluting the maternal feeding signature that was derived from adults foraging in the pelagic food web. Isotopic equilibrium with the post-natal diet in G. typus is thought to occur when the animals reach between 60 and 90 cm total length. This thesis provides an assessment of the functional role of an important, but largely unstudied component of arid tropical estuaries; high intertidal cyanobacterial mats. I found that the functional role of arid tropical cyanobacterial mats lies in the trophic support of fisheries production and the landscape configuration influences the composition of the nekton assemblage and their diet. This research increases our understanding of tropical marine systems through the addition of knowledge of the ecosystem functioning of arid tropical coasts, where previously the focus has primarily been on the wet tropical systems.
Keyword cyanobacterial mats
Mangroves
Arid
estuary
Nekton
Trophic
connectivity
Diet
Stable Isotopes
Exmouth Gulf
Additional Notes Colour pages (pdf pages): 1, 21, 31, 32, 38, 60, 61, 85, 111, 114, 115

 
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