Effects of freshwater flow extremes on intertidal biota of a wet-dry tropical estuary

Abstract

Freshwater flow into estuaries during periods of high rainfall is considered to en - hance the biomass of primary producers, with positive effects propagating to higher trophic groups. We examined the effect of flow on chlorophyll a (chl a) concentrations and meiofaunal abundance on the intertidal mudflats in a wet-dry tropical estuary in the Norman River, northern Australia. Meiofaunal density and sediment chl a concentrations (microphytobenthos) were signifi cantly reduced during 2 consecutive wet season floods (the first, a major flood; the second, minor). Additionally, a short-term study of primary productivity on the mudflats during the minor flood measured rates below detection limits. The effects of salinity and burial by sediment on chl a concentrations and meiofaunal abundance were examined experimentally. Both the sudden and prolonged changes in salinity and sediment movement, and subsequent sedimentation as a result of wet season floods were the likely causes of the reduced meiofaunal density, chl a concentrations and benthic primary production. In the short term, large floods in the wet-dry tropics appear to act more as a disturbance event than a subsidy for benthic estuarine biota, and any positive effects of the flood may not occur until flooding eases. Additionally, we propose that there is a transitional period between the typical wet and dry seasons in the wet-dry tropics, during which flood waters are dissipating and the initial impact of the flood has abated, and there are positive effects on meiofauna and microphytobenthos biomass. © Inter-Research 2014.