Remnant riparian vegetation, sediment and nutrient loads, and river rehabilitation in subtropical Australia

Abstract

A decline in the ecosystem health of Australia's Moreton Bay, a Ramsar wetland of international significance, has been attributed to sediments and nutrients derived from catchment sources. To address this decline the regional management plan has set the target of reducing the loads by 50%. Reforestation of the channel network has been proposed as the means to achieve this reduction, but the extent of revegetation required is uncertain. Here we test the hypothesis that sediment and nutrient loads from catchments decrease proportionally with the increasing proportion of the stream length draining remnant vegetation. As part of a routine regional water quality monitoring program sediment and nutrient loads were measured in 186 flow events across 22 sub-catchments with different proportions of remnant woodland. Using multiple linear regression analysis we develop a predictive model for pollutant loads. Of the attributes examined a combination of runoff and the proportion of the stream length draining remnant vegetation was the best predictor. The sediment yield per unit area from a catchment containing no remnant vegetation is predicted to be between 50 and 200 times that of a fully vegetated channel network; total phosphorus between 25 and 60 times; total nitrogen between 1.6 and 4.1 times. There are~48 000 km of streams in the region of which 32% drain areas of remnant vegetation. Of these 17 095 km are above the region's water storage dams. We estimate that decreasing the sediment and phosphorus loads to Moreton Bay by 50% would involve rehabilitating~6350 km of the channel network below the dams; halving the total nitrogen load would require almost complete restoration of the channel network.

Science to Support Management of Receiving Waters in an Event-Driven Ecosystem: From Land to River to Sea

“…This role may be even more important in event-driven systems where high rainfall and runoff generate high stream power and strong erosive forces on the channel network. Intact riparian vegetation can significantly reduce the export of this material from the terrestrial to aquatic environment [15,16]. Retaining these sediment and nutrient resources also enhances the ability of catchments to deliver valuable ecosystem services by, for example, supporting agricultural production with reduced requirement for artificial fertilizer inputs [17].…”

“…This has led to the determination of the effect of remnant riparian vegetation on sediment and nutrient loads delivered to waterways in the region [16]. Using event-based water quality data, Olley et al found that the sediment yield per unit area from cleared catchments is between 50 and 200 times that of fully vegetated catchments, with a best estimate of 100 times [16]. For total phosphorus it is between 25 and 60 times, with a best estimate of 40 times, and for total nitrogen between 1.6 and 4.1 times.…”

“…For total phosphorus it is between 25 and 60 times, with a best estimate of 40 times, and for total nitrogen between 1.6 and 4.1 times. Using these results, it has been estimated that reducing sediment and phosphorus loads to Moreton Bay by 50% would involve rehabilitating 6000 km of the 24,000 km of degraded channel network to the same condition as the remnant woodland areas (assuming that this is possible) [16]. This is probably an over estimate of the proportion of the channel that will require rehabilitation, however, because it is based on the assumption that the poorly vegetated sections of the channel network contribute uniformly to the sediment load.…”

“…For SEQ, researchers have suggested that conservation efforts should first focus on protecting areas where the riparian cover is in relatively good condition [66] and then on revegetating the channel network in priority areas [16]. This will significantly alter the channel roughness, slowing the flows and increasing the extent of floodplain inundation [64], and such an approach is likely to result in the improved water quality and overall ecosystem health of the region's rivers and coastal waters, including the ecologically significant Moreton Bay.…”

Tracing the influence of land-use change on water quality and coral reefs using a Bayesian model

“…Deforestation and farming increase run-off of nutrients and sediment that can flow out to the ocean 4 . In the ocean, sediment and nutrient pollutants can decrease water clarity and shade or smother habitats, reducing diversity of benthic organisms 5 , habitat complexity and fish diversity 6, 7 .…”