Abstract
Both brown (detrital-based) and green (algal-based) food pathways support freshwater food webs, although the importance of either source may vary within species, regions and different phases of the flow regime. The bony bream (Nematalosa erebi Clupeidae: Dorosomatinae) is one of Australia’s most widely distributed freshwater fish species and is a key component of freshwater food webs, especially in northern Australia. We sought to better define the feeding habits of this species, previously classified as a detritivore, algivore or zooplanktivore (or combinations thereof), by undertaking meta-analyses of published accounts based on stomach content analysis and 13C and 15N stable isotope analysis. Stomach content analysis clearly indicated that detritus was the dominant food item, although benthic algae could be an important dietary component in some habitats (inland river flood plains) and during the wet season. Zooplankton were important for small fish (i.e. juveniles <100mm in length). When data were pooled across a large number of locations, stable isotope analysis indicated that detritus derived from terrestrial vegetation was better aligned isotopically with values for both adult and juvenile bony bream, whereas algae were comparatively 13C enriched, indicating the latter source was not the dominant contributor to the biomass of this species. However, using site-specific data and a regression approach, a significant relationship was revealed between algal carbon and that of large fish, suggesting that carbon derived from benthic algae contributed ~20% of the carbon of adult bony bream. Zooplankton contributed a similar amount. Zooplankton provided the majority of carbon for small fish. We contend that detritus derived from terrestrial vegetation is the likely remaining carbon source for large bony bream, and this interpretation was supported by the outcomes of multiple regression analyses. Although previous studies of aquatic food webs in northern Australia have emphasised the importance of high-quality algal basal resources, this study indicates that terrestrial sources may be important for some species and demonstrates the need to better consider the circumstances that cause biota to switch between different food sources.
Highlights
Most plant matter ends up as detritus and most community food webs contain both detrital and living primary producer energy channels
The data used in this study were drawn from a larger dataset being used to examine the trophic ecology of Australian freshwater fishes in which diet was apportioned to proportional contributions within 15 categories
Consumption of zooplankton was greatest in arid zone or southern regions; high consumption of zooplankton was recorded in northern regions
Summary
Most plant matter ends up as detritus and most community food webs contain both detrital and living primary producer energy channels (brown and green food chains respectively; Moore et al 2004; Rooney et al 2006). Models of aquatic ecosystem function emphasised the importance of terrestrial or aquatic vascular plant material in supporting the biomass of aquatic consumers via a detrital breakdown pathway (Vannote et al 1980; Junk et al 1989). Qualification of this viewpoint has included the inclusion of microbiota as both conditioners of detritus that make nutrients and energy more available and as constituents, which are themselves consumed Algal carbon is easier to digest and assimilate than that of vascular plant material (Brett et al 2017). Algae contain higher quantities of polyunsaturated fatty acids (PUFA), which are essential for metazoan growth (Guo et al 2016a)
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