Regional scale nutrient modelling: exports to the Great Barrier Reef World Heritage Area

Clearing of native vegetation and replacement with cropping and grazing systems has increased nutrient exports to the Great Barrier Reef (GBR) to a level many times the natural rate. We present a technique for modelling nutrient transport, based on material budgets of river systems, and use it to identify the patterns and sources of nutrients exported. The outputs of the model can then be used to help prioritise catchment areas and land uses for management and assess various management options. Hillslope erosion is the largest source of particulate nutrients because of its dominance as a sediment source and the higher nutrient concentrations on surface soils. Dissolved nutrient fractions contribute 30% of total nitrogen and 15% of total phosphorus inputs. Spatial patterns show the elevated dissolved inorganic nitrogen export in the wetter catchments, and the dominance of particulate N and P from soil erosion in coastal areas. This study has identified catchments with high levels of contribution to exports and targeting these should be a priority.     

Estimates of sediment and nutrient loads in 10 major catchments draining to the Great Barrier Reef during 2006-2009

The Great Barrier Reef (GBR) catchment area has been monitored simultaneously for sediment and nutrient exports from 10 priority catchments discharging into the GBR lagoon between 2006 and 2009. This allows GBR catchment-wide exports to be estimated and spatially compared within a discrete time-frame. Elevated levels of sediment and nutrient exports were recorded in all monitored catchments as compared to pre-European estimates, but vary around previous estimates of mean annual loads. During the period of monitoring, the Burdekin and Fitzroy catchments contributed the highest sediment and nutrient exports, however when loads were normalised for area, these catchments produced the lowest unit yields. In contrast, the highest yields were produced in the wetter and proportionately more intensively cultivated Johnstone, O'Connell, and Pioneer catchments particularly for dissolved nitrogens. This assessment offers the necessary scientific foundation for future monitoring, assessment, and management of sediment and nutrient loads entering the GBR.     

Alkalinity and dissolved inorganic carbon exports from tropical and subtropical river catchments discharging to the Great Barrier Reef,Australia

Dissolved inorganic carbon (DIC) transport by rivers is an important control on the pH and carbonate chemistry of the coastal ocean. Here, we combine DIC and total alkalinity (TAlk) concentrations from four tropical rivers of the Great Barrier Reef region in Australia with daily river discharge to quantify annual river loads and export rates. DIC in the four rivers ranged from 284 to 2,639 μmol kg⁻¹ and TAlk ranged from 220 to 2,612 μmol kg⁻¹. DIC:TAlk ratios were mostly greater than one suggesting elevated exports of free [CO₂*]. This was pronounced in the Johnstone and Herbert rivers of the tropical wet north. The largest annual loads were transported in the two large river catchments of the southern Great Barrier Reef region, the Fitzroy and Burdekin rivers. The carbon stable isotopic composition of DIC suggests that carbonate weathering was the dominant source of DIC in the southern rivers, and silicate weathering was likely a source of DIC in the northern Wet Tropics rivers. Annual loads and export rates were strongly driven by precipitation and discharge patterns, the occurrence of tropical cyclones, and associated flooding events, as well as distinct seasonal dry and wet periods. As such, short-lived hydrological events and long-term (seasonal and inter-annual) variation of DIC and TAlk that are pronounced in rivers of the tropical and subtropical wet and dry climate zone should be accounted for when assessing inorganic carbon loads to the coastal ocean and the potential to buffer against or accelerate ocean acidification.     

Assessing the potential underestimation of sediment and nutrient loads to the Great Barrier Reef lagoon during floods

Much of the sediment and nutrient load to the Great Barrier Reef (GBR) lagoon happens during over bank floods, when discharge can be significantly underestimated by standard river gauges. This paper assesses the potential need for a flood load correction for 28 coastal rivers that discharge into the GBR lagoon. For each river, daily discharge was divided into flows above and below a 'flood' threshold to calculate the mean annual percentage flow above this threshold. Most GBR rivers potentially need a flood load correction as over 15% of their mean annual flow occurs above the minor flood level; only seven rivers need little/no correction as their flood flows were less than 5% of the mean annual flow. Improved assessment of the true load of materials to the GBR lagoon would be an important contribution to the monitoring and reporting of progress towards Reef Plan and associated marine load targets.     

Sources of sediment to the Great Barrier Reef World Heritage Area

To reduce sediment exports discharging to the Great Barrier Reef (GBR), it is essential to identify the sources of exported sediment. We used modelling of spatial sediment budgets (the SedNet model) to identify sources and deposition of sediment as it is transported through river networks. Catchments with high levels of land clearing, cattle grazing and cropping show the largest increases in sediment export compared with natural conditions. Hillslope erosion supplies 63% of sediment to the rivers. Gully erosion and riverbank erosion are lower sources of sediment at the GBR catchment scale, but they are important in some catchments. Overall, 70% of sediment exported from rivers comes from just 20% of the total catchment area, showing that much of the problem can be addressed in a relatively small area. This is a much more manageable problem than trying to reduce erosion across the entire GBR catchment. Areas of high contribution are all relatively close to the coast because of the high erosion and high sediment delivery potential.     

Quantifying Inputs of Pesticides to the Great Barrier Reef Marine Park – A Case Study in the Herbert River Catchment of North-East Queensland

The Great Barrier Reef Marine Park (GBRMP) is an ecosystem whose ecological features are recognized internationally. There is a need to evaluate the impacts of historical and contemporary changes in land use on the GBRMP. This paper quantifies pesticide inputs in the Herbert River catchment of North Queensland in the context of changes in land use over the last 100 years. We show that three major phases of rural land use and land cover change have occurred, with large areas of native vegetation converted to agricultural production. The increase in agricultural land has seen a corresponding increase in the area receiving pesticides. We present data showing application histories for organochlorine, organophosphate, phenoxy, triazine, urea, mercurial and azole group pesticides. While the fate of these pesticides in aquatic and marine systems is largely unknown, these trends pose a significant challenge for agricultural industries in complying with the principles of ecologically sustainable development (ESD).     

Quantifying the sources of pollutants in the Great Barrier Reef catchments and the relative risk to reef ecosystems

Development of the Great Barrier Reef (GBR) catchments in the last 150 years has increased the loads of suspended sediment, nutrients and pesticides ('pollutants') delivered to the GBR. The scale and type of development, the pollutants generated and the ecosystems offshore vary regionally. We analysed the relative risk of pollutants from agricultural land uses and identified the sources of these pollutants from different land uses for each region to develop priorities for management. The assessment showed the Wet Tropics and Mackay Whitsunday regions to be of relatively high risk dominated by sugarcane cultivation, contributing pesticide and dissolved inorganic nitrogen (DIN). The Burdekin and Fitzroy ranked medium-high risk dominated by grazing suspended sediment inputs for both, and additionally sugarcane DIN and pesticide inputs for the Burdekin. The Burnett Mary ranked medium risk, dominated by grazing and sugarcane. Cape York was not formally ranked but is considered to be low risk.     

Statistical power of detecting trends in total suspended sediment loads to the Great Barrier Reef

The export of pollutant loads from coastal catchments is of primary interest to natural resource management. For example, Reef Plan, a joint initiative by the Australian Government and the Queensland Government, has indicated that a 20% reduction in sediment is required by 2020. There is an obvious need to consider our ability to detect any trend if we are to set realistic targets or to reliably identify changes to catchment loads. We investigate the number of years of monitoring aquatic pollutant loads necessary to detect trends. Instead of modelling the trend in the annual loads directly, given their strong relationship to flow, we consider trends through the reduction in concentration for a given flow. Our simulations show very low power (<40%) of detecting changes of 20% over time periods of several decades, indicating that the chances of detecting trends of reasonable magnitudes over these time frames are very small.     

Trace metals in algae from the Great Barrier Reef

Zinc, copper, cadmium, nickel, lead and mercury concentrations were examined in 48 species of algae from the Great Barrier Reef. In general the data showed considerable inter- and intra-specific variation with mean levels for each metal ranging from 0.27 to 38.6; 0.74 to 11.2; 0.08 to 2.2; 0.41 to 46.1; <0.45 to 6.7, and 0.005 to 0.320 μg g⁻¹ dry weight, respectively. Not-withstanding this, levels of all metals were substantially lower than reported values for algae from metal contaminated environments. The green alga, Chlorodesmis fastigiata, was the only species examined which was abundant at all study sites and, as a consequence, was considered to be the most suitable candidate for continued surveillance studies in the area.     

Trace metals in corals from the Great Barrier Reef

Various octocorallian and scleractinian corals from within the Great Barrier Reef Province were analysed for zinc, copper, cadmium, nickel and lead by atomic absorption spectroscopy. Of the two coral groups, the octocorals accumulated significantly greater amounts of all detectable metals. The soft coral, Sarcophyton sp., revealed distinct inter-locational differences for zinc, copper and cadmium. The potential of this genus for monitoring trace metal pollution in tropical reef waters is briefly discussed.     

Terrestrial discharge into the Great Barrier Reef Lagoon: nutrient behavior in coastal waters

Pollution of coastal regions of the Great Barrier Reef World Heritage Area (GBRWHA) is dominated by river discharge associated with agricultural development of the adjacent catchments. Runoff of sediment, nutrients and pesticides has sharply increased since European settlement. Since 1991 plumes from river discharge entering the GBRWHA have been mapped by aerial mapping of plume edges and concentrations of contaminants in plumes measured. Plume dispersion is governed primarily by wind speed and direction. Most plumes spread in a band up to 50 km from the coast. Particulate material discharged in the plumes is trapped within 10 km of the coast. Dissolved nutrients disperse much further and elevated nutrient concentrations are measurable at distances of hundreds of kilometres from river mouths. This differential transport of particulate versus dissolved nutrients is important for the potential effects of these materials and management of their generation on the Great Barrier Reef catchment.     

Trace metals in surface water from the Great Barrier Reef

Filtered (0.45 μm), unacidified, surface seawater samples, collected at 3-monthly intervals for 9 months from three locations within the Great Barrier Reef Province, were analysed for zinc, copper, nickel, cadmium and lead by atomic absorption spectroscopy (air/acetylene flame) following Chelex-100 (ammonium form) extraction. Mean levels of each metal ranged from 0.03–0.35; 0.11–0.24; 0.06–0.16; <0.01–0.06; all less than 0.06 μg l⁻¹ respectively. Significant interlocational differences were observed for copper and nickel and clear temporal trends were identified for copper (location-dependent) and cadmium.     

In the other 90%: phytoplankton responses to enhanced nutrient availability in the Great Barrier Reef Lagoon

Our view of how water quality effects ecosystems of the Great Barrier Reef (GBR) is largely framed by observed or expected responses of large benthic organisms (corals, algae, seagrasses) to enhanced levels of dissolved nutrients, sediments and other pollutants in reef waters. In the case of nutrients, however, benthic organisms and communities are largely responding to materials which have cycled through and been transformed by pelagic communities dominated by micro-algae (phytoplankton), protozoa, flagellates and bacteria. Because GBR waters are characterised by high ambient light intensities and water temperatures, inputs of nutrients from both internal and external sources are rapidly taken up and converted to organic matter in inter-reefal waters. Phytoplankton growth, pelagic grazing and remineralisation rates are very rapid. Dominant phytoplankton species in GBR waters have in situ growth rates which range from approximately 1 to several doublings per day. To a first approximation, phytoplankton communities and their constituent nutrient content turn over on a daily basis. Relative abundances of dissolved nutrient species strongly indicate N limitation of new biomass formation. Direct ((15)N) and indirect ((14)C) estimates of N demand by phytoplankton indicate dissolved inorganic N pools have turnover times on the order of hours to days. Turnover times for inorganic phosphorus in the water column range from hours to weeks. Because of the rapid assimilation of nutrients by plankton communities, biological responses in benthic communities to changed water quality are more likely driven (at several ecological levels) by organic matter derived from pelagic primary production than by dissolved nutrient stocks alone.     

River loads of suspended solids, nitrogen, phosphorus and herbicides delivered to the Great Barrier Reef lagoon

Degradation of coastal ecosystems in the Great Barrier Reef (GBR) lagoon, Australia, has been linked with increased land-based runoff of suspended solids, nutrients and pesticides since European settlement. This study estimated the increase in river loads for all 35 GBR basins, using the best available estimates of pre-European and current loads derived from catchment modelling and monitoring. The mean-annual load to the GBR lagoon for (i) total suspended solids has increased by 5.5 times to 17,000ktonnes/year, (ii) total nitrogen by 5.7 times to 80,000tonnes/year, (iii) total phosphorus by 8.9 times to 16,000tonnes/year, and (iv) PSII herbicides is 30,000kg/year. The increases in river loads differ across the 10 pollutants and 35 basins examined, reflecting differences in surface runoff, urbanisation, deforestation, agricultural practices, mining and retention by reservoirs. These estimates will facilitate target setting for water quality and desired ecosystem states, and enable prioritisation of critical sources for management.     

Assessing the additive risks of PSII herbicide exposure to the Great Barrier Reef

Herbicide residues have been measured in the Great Barrier Reef lagoon at concentrations which have the potential to harm marine plant communities. Monitoring on the Great Barrier Reef lagoon following wet season discharge show that 80% of the time when herbicides are detected, more than one are present. These herbicides have been shown to act in an additive manner with regards to photosystem-II inhibition. In this study, the area of the Great Barrier Reef considered to be at risk from herbicides is compared when exposures are considered for each herbicide individually and also for herbicide mixtures. Two normalisation indices for herbicide mixtures were calculated based on current guidelines and PSII inhibition thresholds. The results show that the area of risk for most regions is greatly increased under the proposed additive PSII inhibition threshold and that the resilience of this important ecosystem could be reduced by exposure to these herbicides.