Deoxygenation potential of the Richmond River Estuary floodplain, northern NSW, Australia

Periodic deoxygenation events (DO < I mg/L) occur in the Richmond River Estuary on the east coast of Australia following flooding and these events may be accompanied by total fish mortality. This study describes the deoxygenation potential of different types of floodplain vegetation in the lower Richmond River catchment and provides a catchment scale estimate of the relative contribution of floodplain vegetation decomposition to deoxygenation of floodwaters. Of the major vegetation types on the floodplain slashed pasture was initially (first 5 to 7 h) the most oxygen demanding vegetation type after inundation (268 mg 02 m(-2) h(-1)), followed by dropped tea tree cuttings (195 +/- 18mg O-2 m(-2) h(-1)) and harvested cane trash (110 +/- 8mg O-2 m(-2) h(-1)). However, 10 h after inundation the oxygen consumption rates of slashed pasture (105 +/- 5mg O-2 m(-2) h(-1)) and tea tree cuttings (59 +/- 7 mg O-2 m(-2) h(-1)) had decreased to a rate less than the harvested cane trash (110 +/- 8mg O-2 m(-2) h(-1)). The oxygen demands of the different floodplain vegetation types when inundated were highly correlated with their nitrogen content (r(2) = 0.77) and molar C:N ratio (r(2) = 0.82) reflecting the dependence of oxygen demand of vegetation types on their labile carbon content. The floodplain of the lower Richmond River (as flooded in February 2001) has the potential to deoxygenate about 12.5 x 10(3) mL of saturated freshwater at 25 degrees C per day which is sufficient to completely deoxygenate floodwater stored on the floodplain with 3 to 4 days. In addition, oxidation of Fe2+ mobilized during the decomposition of floodplain vegetation via iron reduction and discharged from groundwater and surface runoff in acid sulfate soil environments could account for about 10% of the deoxygenation of floodwater stored on the floodplain. Management options to reduce floodplain deoxygenation include removing cuttings from slashed pasture and transporting off-site, reducing slashed pasture windrow loads by using comb-type mowers, returning areas of the floodplain to wetlands to allow the establishment of inundation tolerant vegetation and retaining deoxygenated floodwaters in low lying areas of the floodplain to allow oxygen consumption process to be completed before releasing this water back to the estuary. Copyright (c) 2006 John Wiley & Sons, Ltd.