What river impacts this reef? A simple reef exposure model.

M. Maughan1, J. Brodie1 and J. Waterhouse2 1 Australian Centre for Tropical Freshwater Research

James Cook University Townsville QLD 4811

AUSTRALIA E-mail: Mirjam.Maughan@jcu.edu.au 2 CSIRO Sustainable Ecosystems

Davies Laboratory Townsville QLD 4811

AUSTRALIA

Abstract

Since land-based pollution is a major cause of decline of reefs, a simple reef exposure model was developed to model and visualise current contaminant exposure to reefs and future management options. Total suspended sediments, dissolved inorganic nitrogen, total phosphorus and herbicides were modelled for the whole Great Barrier Reef coast, using data inputs such as monitored contaminant load, river volume and variability, distance from river to reef, and bearing. Realistic contaminant reductions were used to calculate and visualise management options for the Tully/Murray, Burdekin, Mackay Whitsunday, Fitzroy and Burnett regions, and the results will assist the creation of Water Quality Improvement Plans for those regions.

1. INTRODUCTION

The Great Barrier Reef (GBR) is a reef system extending more than 2300km along the NE Australian coast (EPA 2003). Over fifty rivers flow into the GBR lagoon from the adjacent Great Barrier Reef Catchment Area (Geoscience Australia 2006) (Figure 1). Since land-based pollution is an important reason for worldwide decline of reefs (Pandolfi et al. 2003), it is important to investigate the extent and influence of individual rivers and their contaminants, as well as the impact of the combination of those rivers. Agricultural development of the GBR coast in the last 150 years has increased the sediments, nutrients (nitrogen and phosphorus) and herbicides running off to the reef. Major industries in the region such as beef grazing have caused an increase in soil erosion and 4-5 times the amount of sediments is now delivered to the reef compared to pre-European settlement. The use of fertilizer associated with horticulture (including sugar cane and cotton) has increased the nutrient level in waters of the coastal area, and the herbicides and pesticides used in this industry were non-existent before European settlement. Urban centres along the GBR coast also contribute to increased nutrient levels (Brodie et al. 2003). This material is impacting on mangroves (Duke et al. 2005) and coral reefs (Fabricius et al. 2005) causing reduced health, dieback, and reduced richness and abundance of those species. This simple model attempted to visualise the exposure of reefs to certain river contaminants: total suspended sediments (TSS), dissolved inorganic nitrogen (DIN), total phosphorus (TP) and “Diuron” type herbicides, taking into account the river volume, variability of river floods flushing out to the ocean, distance from the river mouth, wind and current directions, and contaminant dispersal. It built on the exposure model developed by Devlin et al. (2003), but took into account more reef data points (4200 points versus 30 points), and modeled the exposure by each contaminant individually, making it easier to visualize the influence of one single contaminant, and creating more flexibility for ‘weighting’ when adding the four contaminants together. The results can be analysed from the reef perspective, as in ‘what rivers impact this reef’, as well as the river perspective: “how far does the influence of this river reach”. It modeled scenarios for pre-

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European arrival (natural exposure), current exposure (using monitored and modeled input data), and management scenarios using achievable management strategies with the predicted reduction of contaminant load. Certain scenarios incorporated the influence from all GBR rivers, other scenarios focused on the influence of the rivers of 1 region only, while a third group of scenarios looked at the result of management in one region while the other rivers maintained the same contaminant load. The results of this modeling exercise have assisted the development of regional Water Quality Improvement Plans (WQIPs) in the GBR region.

Figure 1 Location of modelled catchments

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2. METHODOLOGY

2.1. DATA INPUTS

From the rivers that flow into the GBR lagoon, 51 rivers (Geoscience Australia 2006) were chosen because of their size, significance or location. For the distance and direction analysis between each river and reef, point data was needed, so a GIS layer was created with exit points for each of the rivers. Centre points were created for each of the approximately 3000 reefs (GBRMPA 2003), forcing the centre point to be located within the reef-polygon. Additional points were added to this layer where points were scarce, so enough points were available for a reasonable performance of the interpolation. Current contaminant load data for TSS, TP and DIN was derived from Brodie et al. (2003). For each of the rivers, the river and basin catchment areas had to be determined, because the monitored and modeled contaminant load data was presented per basin, not per river. The percentage of surface area was used to derive the river (vs. the basin) contaminant load and concentration (for more details see Maughan et al. 2007). Little data was available for herbicide load (White et al. 2002, Mitchell et al. 2005, Lewis et al. 2007), so the existing data was used to predict herbicide load in other catchments, using the percentage of sugar and cotton production (QDNRW 2002) in each catchment as an indicator. River volume was derived from Furnas (2003) and river flow variability is calculated using data from Watershed (QDNRW 2006). This data was used to calculate an ‘index’ for each of the contaminants at each river exit: multiplying variability with discharge and the individual contaminant concentration (see Maughan et al. 2007). Contaminant load data for the pre-European scenario was derived from Brodie et al. (2003), which estimated the pre-1850 sediment exports by using loads from current near-pristine catchments such as the Normanby in the dry tropics, and Dalrymple Creek (Herbert catchment) in the wet tropics. Management scenarios were developed specifically with the region’s WQIPs in mind: “each WQIP identifying key threats to water quality entering the reef, developing end-of-catchment targets, prioritizing actions and activities to meet these targets and formulate long term monitoring, modeling and adaptive management strategies to evaluate progress of WQIP Implementation” (Queensland Government 2008). Modeled scenarios include TSS management by fencing and rotational spelling for land used for beef grazing (Coughlin et al. 2007); banded application whereby herbicide is spread over the crop row only, reducing the cost by 60% and reducing soil residue problems (DPI 2007); and “6 easy steps” management for DIN, a precision farming technique matching the amount of fertilizer to the crop’s needs (sugarcane and horticulture), using soil analysis and knowledge of crop nitrogen and phosphorus uptake (Canegrowers 2007).

2.2. ANALYSIS

The spatial analysis, using ArcGIS9.2 software, started with calculating the distance between the river exits and reef centre points, limited to a 500km radius. The resulted table, with close to 100 000 records, was joined with the original tables, for the purpose to add columns with reef and river names, and reef and river point coordinates. Since TSS and TP have different dispersal behaviours from DIN and herbicides, another two columns were created: one calculating 1/d2 to be used with TSS and TP calculations, and one with the values of 1/d for DIN and herbicides. A script (Snyder 2004) was used to calculate the direction between river and reef point, and a direction value (Figure 2) was assigned, giving a base table to be used for multiple scenarios. The preferential direction of river influence used in Figure 2 was derived from modeled and monitored flood plume studies in the GBR (King et al. 2001, Devlin & Brodie 2005): the northwestern movement caused by the Coriolis effect (Brodie et al. 2006), prevailing south easterly winds in the summer months (Devlin et al. 2001), the northward flowing Hiri current, and the southward flowing East Australian Current (Brinkman et al. 2002). The contaminant indexes for each river were then joined to this distance/bearing table. Exposure was estimated by multiplying the individual river-contaminant index by bearing value and distance value. The result gives individual values for each river-reef combination, so a pivot table was used to total the

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exposure for each of the reef points. The result was then joined with the reef centre points, and inverse distance weighted interpolation was used to estimate and visualise the area between those points. A logarithmic scale was used when displaying the results in five classes (see Maughan et al. 2007). The process was then repeated for a new contaminant, scenario or region, using the base (distance/bearing) table to calculate new exposure values.

Figure 2 Bearing values used in exposure calculation

3. RESULTS & DISCUSSION

Numerous maps were created (Maughan et al. 2007) showing either the whole GBR region (Figure 3) or zooming into one of the 5 regions that prepared a WQIP report (Tully/Murray region, Burdekin region, Mackay Whitsunday region, Fitzroy region and Burnett region) (Figure 4). Maps were created showing the influence of all rivers (Figure 5) or the influence of only one region (Figure 6). Combinations with different management in different regions were also modeled in the report (Maughan et al. 2007). Tables were created for chosen reefs, showing the percentage influence from individual rivers (per contaminant) and showing the % reduction of exposure under certain management scenarios. Taking one contaminant (DIN) in one region (Tully region) as an example, figure 3 shows current DIN exposure for the whole coastal region, showing a high to very high DIN exposure along the Wet Tropics coast, and patches of high to very high exposure close to the Normanby, Burdekin and Fitzroy rivers. When zooming in to the Tully region (Figures 4-10) more detail can be observed. The pre-European scenario (Figure 4) shows that the region naturally had a high to very high DIN exposure close to the Tully river mouth, with patches of medium exposure along the coast, and most mid/outer-shelf reefs off the Tully coast (such as Beaver Reef) having very low exposure to DIN. The DIN levels have increased in the last 150 years to very high exposure patches at most river mouths (Figure 5), with a now continuous area of high exposure level close to shore. Most mid/outer shelf reefs see a change from pre-European very low exposure to current low or medium exposure: the exposure of Beaver reef increased threefold (2.99) from very low to medium exposure. The current exposure of coastal King reef is 3.32 times higher than pre-European settlement, and changed from low to high exposure. The increase of DIN levels in flood plumes can cause algal blooms in the naturally nutrient-poor waters of the GBR, negatively impacting seagrass and coral reefs (Brodie et al. 2007). Figure 6 shows the movement of a flood plume after a large rainfall event in 2007, with the green colour on the imagery associated with algal growth in nutrient rich waters. During the first days of the flood plume the green colour is close to shore, equivalent to our prediction of high exposure to DIN (Figure 5). It slowly moves further east, covering the mid- to outer-shelf reefs on February 13, which is similar to our prediction of medium exposure in Figure 5. The Tully River is the main source of DIN for the King, Beaver and Stingaree Reefs with 33-62% of the exposure originating from the Tully River, while Brook Island Reef receives 30% of its DIN from the Herbert River and 25% from the Tully River (Table 1). To be able to show the influence of management in the Tully region, the influence of only the Tully region’s rivers is visualised in Figure 6, which is smaller than the ‘all river influence’ because of DIN sourced from other rivers (Table 1).

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Figure 3 Dissolved Inorganic Nitrogen, all river influence

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Figure 4 Dissolved Inorganic Nitrogen, pre European scenario, all river influence

Figure 5 Dissolved Inorganic Nitrogen, current scenario, all river influence

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Figure 6 Imagery of Wet Tropics flood plume, 2007 (Brodie et al. 2007)

When predicting the influence of DIN management regimes within the Tully region only (Figure 8), a small change can be detected when the “6 easy steps” are applied, with the exposure of all reefs decreasing by 23%, which changes Stingaree Reefs from high to medium level, and King Reefs from medium to low level. But other rivers also influence the reefs, so when “6 easy steps” management is applied in the Tully region, and using current levels for the remaining rivers (Figure 9), levels at Brook Island and King Reef are only reduced by 6.8% (Brook Island Reef) to 15.3% (Stingaree Reef), since they also receive DIN from the Herbert River and Maria Creek. This indicates that management of the reefs should be a combined effort. When multiple regions, such as the Tully/Murray, Burdekin, Mackay-Whitsunday, Fitzroy and Burnett regions, apply the “6 easy septs” management, the exposure is reduced between 7.5% (Brook Islands) and 15.5% (Stingaree Reefs), compared to current levels of DIN, slightly more than the scenario where only the Tully region manages the DIN levels in run-off. The results of modeling contaminant exposure of the whole GBR region (Maughan et al. 2007) shows that the catchments causing most concern for DIN are the catchments of the Wet Tropics coast including the Herbert, Tully and Johnstone rivers, caused by fertilizer use in sugar and horticulture. Herbicide exposure is largest in areas associated with sugar cane, off the Wet Tropics and Mackay Whitsunday coast. Increased levels of TSS are most of concern in the Burdekin and Fitzroy regions, where beef grazing is the major land use.

Table 1 River source of DIN (in %) for Tully focus reefs, rivers arranged from north to south. Only the top 10 results are displayed, which together contribute to more than 90% of the exposure for each of the

focus reefs.

Catchment River King Reefs

Beaver Reef

Stingaree Reefs

Brook Island Reefs

Normanby Normanby R 0.5 1.1 0.4 0.9 Mulgrave Russell Mulgrave Russell R 2.7 4.3 1.8 3.0

Johnstone R 9.4 10.6 5.0 7.3 Liverpool Ck 8.4 3.0 1.9 2.1

Johnstone

Maria Ck 18.1 1.9 1.7 1.6 Tully R 37.2 33.0 62.1 25.4 Tully-Murray Murray R 2.4 2.4 3.4 2.0

Herbert Herbert R 9.9 18.2 12.0 30.7 Burdekin Burdekin R 7.2 14.5 7.0 16.9 Pioneer Pioneer R 0.8 1.9 0.8 1.7

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Figure 7 Dissolved Inorganic Nitrogen, current scenario, Mackay Whitsunday influence only

Figure 8 Dissolved Inorganic Nitrogen, “6 easy steps” scenario, Mackay Whitsunday influence only

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Figure 9 DIN "6 easy steps" management in Tully region, all other regions as current

Figure 10 DIN "6 easy steps" in the Tully, Burdekin, Mackay Whitsunday, Fitzroy and Burnett regions

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4. CONCLUSION

The presented model showed how the complex influence of rivers (each having a different contaminant concentration and volume), to reefs (each in a different distance and bearing from those rivers) can be calculated and visualised in a simplified spatial model. The modeled current influence of rivers seems to correspond with satellite imagery taken after large rainfall events. This model was then used to predict the reef exposure to contaminants for realistic management scenarios, assisting natural resource managers in decision making (EPA in prep., Rudd et al. 2007), envisaging how management reduces the extent of exposure. Future development might include modeling additional regions such as the Townsville catchments or regions north of Tully; modeling more management scenarios; or using additional data from monitoring to improve the accuracy of the inputs.

5. REFERENCES

Brinkman R., Wolanski E., Deleersnijder E., McAllister F. and Skirving W. (2002), Oceanic inflow from the Coral Sea into the Great Barrier Reef. Estuarine, Coastal and Shelf Science 54 (4), 655-668. Brodie, J., McKergow, L.A., Prosser, I.P., Furnas, M., Hughes, A.O. & Hunter, H. (2003), Sources of sediment and nutrient exports to the Great Barrier Reef World Heritage Area. ACTFR Report No. 03/11, Australian Centre for Tropical Freshwater Research, James Cook University, Townsville. Available from: http://www.actfr.jcu.edu.au/idc/groups/public/documents/technical_report/jcudev_015447.pdf Brodie J., Dekker A.G., Brando V.E., Masters B., Faithful J., Noble R. & Rohde K. (2006). Extent and duration of the algal bloom in the Great Barrier Reef Lagoon following river discharge events in the Mackay Whitsunday Region, Australia. In: Proceedings of the 13th Australasian Remote Sensing and Photogrammetry Conference: Earth Observation - from Science to Solutions, Canberra, November, 2006. Available from: http://www.actfr.jcu.edu.au/idc/groups/public/documents/journal_article/jcudev_015597.pdf

Brodie J., Mitchell A., Lewis S., BainbridgeZ., Faithful J., Hateley L., Armour J., Maughan M. and Reghenzani J. (2007). Water Quality Issues in the Tully Region. ACTFR report no. 07/04, Australian Centre for Tropical Freshwater Research, James Cook University, Townsville. Available from: http://www.actfr.jcu.edu.au/idc/groups/public/documents/technical_report/jcudev_015402.pdf Canegrowers (2007), BSES Six Easy Steps – a joint initiative aimed at improved nutrient management. Online resource {Accessed 25 Jan 2008.} Available from: http://www.canegrowers.com.au/publications-centre/canegrower-magazine-articles-2007/magazine-2007-08-13/2007-08-13-nutrientmangement.aspx Coughlin T., Nelson B. and O’Reagin P. (2007), Grazing Land Best Management Practices (BMPs) Draft Guidelines. Burdekin Dry Tropics NRML: Townsville. Available from: http://www.burdekindrytropics.org.au/cci/grazinglands/downloads/Draft-final-for-Web.pdf Devlin M., Waterhouse J., Taylor J. and Brodie J. (2001), Flood Plumes in the Great Barrier Reef: Spatial and Temporal Patterns in Composition and Distribution. Great Barrier Reef Marine Park Authority, Townsville. Available from: http://www.gbrmpa.gov.au/corp_site/info_services/publications/research_publications/rp068/index.html Devlin M., Brodie J., Waterhouse J., Mitchell A., Audas D., and Haynes D. (2003), Exposure of Great Barrier Reef Inner-Shelf Reefs to River-borne Contaminants. In: Proceedings of the 2PndP National Conference on Aquatic Environments: Sustaining our aquatic environments – Implementing solutions. Queensland Department of Natural Resources and Mines, Brisbane, Australia.

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Devlin M. and Brodie J. (2005), Terrestrial discharge into the Great Barrier Reef Lagoon: nutrient behavior in coastal waters. Marine Pollution Bulletin 51, 9-22. DPI (2007), Weed management principles. Online resource. {Accessed 25 January 2008} Available from: http://www.dpi.qld.gov.au/cps/rde/xchg/dpi/hs.xsl/26_4255_ENA_Print.htm. Duke N.C., Bell A.M., Pederson D.K., Roelfsema C.M. and Bengtson Nash S. (2005), Herbicides implicated as the cause of severe mangrove dieback in the Mackay region, NE Australia: consequences for marine plant habitats of the GBR World Heritage Area. Marine Pollution Bulletin 51 (1-4), 308-324. EPA (2003), State of the Environment Queensland 2003. Environmental Protection Agency, Brisbane, Australia. EPA (in prep), Draft 2007 water quality report: interim to a water quality report card for the Great Barrier Reef catchments and inshore ecosystems. Queensland Environmental Protection Agency, Brisbane. Fabricius K., De’Ath G., McCook L., Turak E. and Williams D. (2005), Changes in algal, coral and fish assemblages along water quality gradients on the inshore Great Barrier Reef. Marine Pollution Bulletin 51 (1-4), 384-398. Furnas, M. (2003), Catchment and corals: terrestrial runoff to the Great Barrier Reef. AIMS: Townsville. GBRMPA (2003), Major Coral Reefs within the Great Barrier Reef World Heritage Area. Great Barrier Reef Marine Park Authority, Townsville, Australia. Geoscience Australia, (2006), Geodata topo 250k Series 3 Shape files format. Geoscience Australia, Canberra. King B., McAllister F., Wolanski E., Done T. & Spagnol S. (2001), River plume dynamics in the central Great Barrier Reef. In: Wolanski E. (ed), Oceanographic Processes of Coral Reefs: Physical and Biological Links in the Great Barrier Reef. CRC Press, Boca Raton, 145-160. Lewis S., Davis A., Brodie J., Bainbridge Z., McConnell V. and Maughan M. (2007), Pesticides in the lower Burdekin and Don River catchments. Report no. 07/05, Australian Centre for Tropical Freshwater Research, James Cook University, Townsville. Available from: http://www.actfr.jcu.edu.au/idc/groups/public/documents/technical_report/jcudev_015403.pdf Maughan M., Brodie J. and Waterhouse J. (2007), Reef exposure model for the Great Barrier Reef Lagoon. ACTFR Report no. 07/19, Australian Centre for Tropical Freshwater Research, James Cook University, Townsville. Available from: http://www.actfr.jcu.edu.au/idc/groups/public/documents/technical_report/jcudev_015634.pdf Mitchell C., Brodie J., and White I. (2005), Sediments, nutrients and pesticide residues in event flow conditions in streams of the Mackay Whitsunday Region, Australia. Marine Pollution Bulletin, 51, 23-36. Pandolfi J.M., Bradbury R.H., Sala E., Hughes T.P., Bjorndal K.A., Cooke R.G., McArdle D., McClenachan L., Newman M.J.H., Paedes G., Warner R.R., and Jackson J.B.C. (2003), Global Trajectories of the Long Term Decline of Coral Reef Ecosystems. Science 301, 955-958 Queensland Department of Natural Resources and Water (2002), Queensland Land Use Mapping Program. Queensland Department of Natural Resources and Water, Indooroopilly

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Queensland Department of Natural Resources and Water (2006), Watershed database. Online resource. Available from: http://www.nrw.qld.gov.au/watershed/ {Accessed July 2007} Queensland Government (2008), Reef Water Quality Protection Plan, Action D4: Promote development of water quality improvement plans. Online resource, available from: www.bdtnrm.org.au/projects/rmworkshop_april07.html {Accessed 23 January 2008} Rudd K., Garrett P, and O’Brien K. (2007), Labor’s Reef Rescue Plan. Election 2007 Policy Document. Australian Labor, Canberra. Snyder C. (2004). Calculate the north azimuth bearing between two sets of x, y coordinates in an attribute table - and the distance tool. Online resource {Accessed July 2007}. Available from: http://arcscripts.esri.com/details.asp?dbid=13813 White, I., Brodie, J. & Mitchell, C. (2002), Pioneer River catchment event based water quality sampling. Healthy Waterways Program, Mackay Whitsundays Regional Strategy Group, Mackay. Available from: http://www.actfr.jcu.edu.au/idc/groups/public/documents/journal_article/jcudev_015631.pdf

6. ACKNOWLEDGEMENTS

Thanks to Terrain NRM, Black Ross Basin WQIP, Burdekin Dry Tropics NRM, Mackay Whitsunday NRM Group, Burnet Mary NRM and Fitzroy Basin Association for the research funding. Thanks also to Will Higham for organizing this project, and the Reef Partnership for facilitating the project.

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