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 enhance 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.

Physiological mechanisms used by fish to cope with salinity stress

“…Such events can cause large and sudden increases or decreases in habitat salinity, e.g. during flooding associated with tsunamis or rainstorms in intertidal, coastal or desert habitats (Illangasekare et al, 2006;Drake et al, 2013;Duggan et al, 2014). Climate change-induced floods have already caused salinity stress resulting in significant mortality in coral reefs and other coastal marine ecosystems (Huang et al, 2014).…”

Inundation of saline supratidal mudflats provides an important source of carbon and nutrients in an aquatic system

“…During our study, the wet season in Year 1 (2008−2009) had a major flood, while the second wet season (2009−2010) had a moderate flood ( Fig. 1b; Duggan et al 2014). The estuary has a relatively simple morphology, with a main river channel (36 km 2 ), fringing tidal mudflats, a narrow strip of mangrove forest and beyond this, extensive supratidal mudflats (356 km 2 ) (National Land & Water Resources Audit 2001; http:// trove.nla.gov.au/people/1306148?c=people).…”

“…22.5−44.5 mg m −2 , followed by the supratidal mudflats (when inundated), i.e. 19.8 mg m −2 , then phytoplankton 9.2−19.2 mg m −2 (Burford et al 2012, Duggan et al 2014. PP per unit area was similar between the 3 habitats when supratidal mudflats in the wet season were compared with intertidal mudflats and phytoplankton in the dry season.…”

Restored freshwater flow and estuarine benthic communities in the northern Gulf of Mexico: research trends and future needs

“…Estuaries generally support high biomass of benthic invertebrates, with varying salinity tolerances, but many are not tolerant to fresh conditions (Fredette & Diaz, 1990). Benthic infaunal macroinvertebrates (macrobenthos >one mm body length) may decline under oligohaline conditions due to high metabolic cost and mortality (Duggan et al, 2014). However, smaller opportunistic meiobenthos (benthic invertebrates between 0.063 mm and 0.5 mm body length) may increase under these conditions (Alongi & Robertson, 1995;Giere, 2009).…”