Big floods, big knowledge gap: Food web dynamics in a variable river system

Abstract

While flow regime is widely recognised as an over‐arching feature structuring aquatic ecosystems, the influence of flood events on feeding habits in fish assemblages inhabiting variable environments remains largely unknown. In this study, we investigated how diets of a fish assemblage changed in response to fluctuations in hydrology in a highly variable, wet‐dry tropical Australian river. We compared dietary habits, trophic guilds and intra‐ and interspecific dietary overlap and diet breadth across different seasons. Wet‐season floods acted as major geomorphic and ecological perturbations that exerted substantial effects on the trophic dynamics of the terapontid assemblage, particularly through the removal of in‐stream plant production and detritus. Forced by major shifts in food source availability due to seasonal flooding, fishes responded to these perturbations by marked changes in diet, with significant guild switching following wet‐season floods. Rather than the predictable, gradual, changes in consumer–resource interactions associated with the gentler seasonal flood pulse evident in other tropical river systems, study results instead emphasised rapid and profound changes to species' diets and trophic interactions following abrupt flood pulses. These sudden shifts in food sources may also explain some of the lack of clarity in discrimination of specific carbon sources or species' trophic levels evident in isotopic food web studies from the region. Results indicate that the effects of flood pulses on assemblage trophic interactions may differ among ecosystem types and across the broad potential scale of hydrological disturbance mediated by flood events.

The behavioural ecology toolkit for fish management and conservation

“…Mark‐recapture (PIT tags, subcutaneous dyes), passive or active telemetry using radio, acoustic or satellite technologies, or downstream trapping of migratory stream fishes (reviewed in Crossin et al, 2017) can indicate habitat affinity and survivorship if the study period is sufficiently long (Stewart et al, 2017). Large‐scale life history movements, habitat residence time (via types of resources consumed; Brush et al, 2012; Davis et al, 2018), or migrations can also be tracked using stable isotope analyses from soft tissue or otoliths (Furey et al, 2018). Finally, the energetic costs of movement are quantified by bioenergetic modelling (individual‐based models [IBMs], dynamic energy budgets) and may be used to evaluate the physiological implications of movements related to protected or restored habitats (Budy et al, 2011; Hansen et al, 1993; Snyder et al, 2019).…”

The role of floods and droughts on riverine ecosystems under a changing climate

“…The additional influx of nutrients supports rapidly growing populations of macroinvertebrates (Hickey & Salas, 1995). Allochthonous inputs and high autochthonous floodplain production dominate ecological processes (Davis et al, 2018;Humphries, Keckeis, & Finlayson, 2014). This creates an abundance of prey for fish (Allen, 1993;Junk, 2005).…”

“…In lowland rivers with extensive floodplains, flood energy is more easily dissipated and water velocity and shear stress may not increase significantly. Nutrients previously deposited on the floodplain are also mobilised, affecting water quality and potentially greatly increasing primary production rates (Davis, Pusey, & Pearson, ; Edwards, Baker, Dunbar, & Laize, ). Floods fill wetlands, anabranches and flood runners with a slow‐moving flow that recedes slowly, and deposits sediments and organic particles upon the floodplain.…”

Sea catfishes (Ariidae) feeding on freshwater floodplains of northern Australia

“…Ecological studies during this phase of the hydrological cycle are logistically difficult due to the remoteness of the area (accessibility is limited and expensive to overcome) and infrequently undertaken. Consequently, our understanding of ecological dynamics during this phase of the hydrological cycle remains an important knowledge gap (Davis et al 2018), highlighting the value of information gathered incidentally during the conduct of any research activities. We opportunistically gathered information on the diet and condition of catfishes collected from the lower-most reaches of main river channels and the adjacent inundated floodplain habitats during short fieldwork campaigns in the Mitchell and Flinders rivers, northern Australia, described elsewhere (e.g.…”