Dynamic stability in dry season food webs within tropical floodplain rivers

Debate exists about the effects of hydrological variation on food web dynamics and the relative importance of different sources of organic carbon fuelling food webs in floodplain rivers. Stable carbon isotope analyses and ecological stoichiometry were used to determine the basal sources in dry season macroinvertebrate food webs in two floodplain river systems of Australia’s wet-dry tropics that have contrasting flow regimes. Algae, associated with phytoplankton and biofilm, were the primary food source, potentially contributing > 55% organic carbon to the biomass of a wide range of primary and secondary consumers. However, many consumers assimilated other sources in addition to algae, e.g. detritus from local C3 riparian vegetation. Food webs were characterised by substantial flexibility in the number and types of sources identified as important, which was indicative of generalist feeding strategies. These findings suggest ‘dynamic stability’ in the food webs, which imparts resilience against natural disturbances like flow regime seasonality and variation in hydrological connectivity. This adaptation may be characteristic of macroinvertebrate assemblages in highly seasonal river systems or in those with high levels of flow variability. Additional key words: Algae, terrestrial detritus, carbon: nitrogen, macroinvertebrates, hydrological connectivity, Gulf of Carpentaria Introduction Food web analysis helps improve our understanding of ecosystem function by revealing the key sources of organic matter that fuel metazoan consumers (Thorp et al. 1998). In river systems, a number of conceptual models emphasise the relative importance of different forms and origins of organic matter in driving aquatic food webs, depending upon on the type of system for which each model was developed (e.g. Vannote et al. 1980; Junk et al. 1989; Thorp and Delong 1994). Despite the disparate predictions from