Great Barrier Reef Catchment Area Research Articles

Pesticides in the Great Barrier Reef catchment area: Plausible risks to fish populations.

Waterways that drain the Great Barrier Reef catchment area (GBRCA) transport pollutants to marine habitats, provide a critical corridor between freshwater and marine habitats for migratory fish species, and are of high socioecological value. Some of these waterways contain concentrations of pesticide active ingredients (PAIs) that exceed Australian ecotoxicity threshold values (ETVs) for ecosystem protection. In this article, we use a "pathway to harm" model with five key criteria to assess whether the available information supports the hypothesis that PAIs are or could have harmful effects on fish and arthropod populations. Strong evidence of the first three criteria and circumstantial weaker evidence of the fourth and fifth criteria are presented. Specifically, we demonstrate that exceedances of Australian and New Zealand ETVs for ecosystem protection are widespread in the GBRCA, that the PAI contaminated water occurs (spatially and temporally) in important habitats for fisheries, and that there are clear direct and indirect mechanisms by which PAIs could cause harmful effects. The evidence of individuals and populations of fish and arthropods being adversely affected species is more circumstantial but consistent with PAIs causing harmful effects in the freshwater ecosystems of Great Barrier Reef waterways. We advocate strengthening the links between PAI concentrations and fish health because of the cultural values placed on the freshwater ecosystems by relevant stakeholders and Traditional Owners, with the aim that stronger links between elevated PAI concentrations and changes in recreationally and culturally important fish species will inspire improvements in water quality. Integr Environ Assess Manag 2024;00:1-24. © 2023 Commonwealth of Australia and The Commonwealth Scientific and Industrial Research Organisation. Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC).

Temporal variation of imidacloprid concentration and risk in waterways discharging to the Great Barrier Reef and potential causes

The widely used neonicotinoid insecticide imidacloprid has emerged as a significant risk to surface waters and the diverse aquatic and terrestrial fauna these ecosystems support. While herbicides have been the focus of research on pesticides in Australia's Great Barrier Reef catchment area, imidacloprid has been monitored in catchments across the region since 2009. This study assessed the spatial and temporal dynamics of imidacloprid in 14 waterways in Queensland, Australia over seven years in relation to land use and concentration trends. Imidacloprid could be quantified (i.e., concentrations were greater than the limit of reporting) in approximately 54% of all samples, but within individual waterways imidacloprid was quantified in 0 to 99.7% of samples. The percent of each catchment used to grow bananas, sugar cane and urban explained approximately 45% of the variation in imidacloprid concentrations and waterway discharge accounted for another 18%. In six waterways there were significant increases in imidacloprid concentrations and the frequency and magnitude of exceedances of aquatic ecosystem protection guidelines over time. Overall, the risk posed by imidacloprid was low with 74% of samples protecting at least 99% of species but it was estimated that upto 42% of aquatic species would experience harmful chronic effects. Potential explanations of the changes in imidacloprid were examined. Not surprisingly, the only plausible explanation of the increases was increased use of imidacloprid. While field-based measurement of the effects of imidacloprid are limited in the Great Barrier Reef Catchment Area (GBRCA) the risk assessment indicates that biological harm to aquatic organisms is highly likely. Action to reduce imidacloprid concentrations in the GBRCA waterways is urgently required to reverse the current trends and mitigate environmental impacts.

Pre-development denudation rates for the Great Barrier Reef catchments derived using 10Be

Understanding of the pre-development, baseline denudation rates that deliver sediment to the Great Barrier Reef (GBR) has been elusive. Cosmogenic 10Be in sediment is a useful integrator of denudation rates and sediment yields averaged over large spatial and temporal scales. This study presents 10Be data from 71 sites across 11 catchments draining to the GBR: representing 80% of the GBR catchment area and provide background sediment yields for the region. Modern, short-term, sediment yields derived from suspended load concentrations are compared to the 10Be data to calculate an Accelerated Erosion Factor (AEF) that highlights denudation “hot-spots” where sediment yields have increased over the long-term background values.The AEF results show that 58% basins have higher modern sediment yields than long-term yields. The AEF is considered a useful approach to help prioritise on-ground investments in remediation and the additional measured empirical data in this paper will help support future predictive models.

Modelled estimates of dissolved inorganic nitrogen exported to the Great Barrier Reef lagoon

Measuring stream pollutant loads across the Great Barrier Reef (GBR) catchment area (GBRCA) is challenging due to the spatial extent, climate variability, changing land use and evolving land management practices, and cost. Thus, models are used to estimate baseline pollutant loads. The eWater Source modelling framework is coupled with agricultural paddock scale models and the GBR Dynamic SedNet plugin to simulate dissolved inorganic nitrogen (DIN) generation and transport processes across the GBRCA. Catchment scale monitoring of flow and loads are used to calibrate the models, and performance is assessed qualitatively and quantitatively. Modelling indicates almost half (47%) of the total modelled DIN load exported to the GBR lagoon is from the Wet Tropics, and almost half of the total modelled DIN load is from sugarcane areas. We demonstrate that using locally developed, customised models coupled with a complementary monitoring program can produce reliable estimates of pollutant loads.

Land use change in the river basins of the Great Barrier Reef, 1860 to 2019: A foundation for understanding environmental history across the catchment to reef continuum

Land use in the catchments draining to the Great Barrier Reef lagoon has changed considerably since the introduction of livestock grazing, various crops, mining and urban development. Together these changes have resulted in increased pollutant loads and impaired coastal water quality. This study compiled records to produce annual time-series since 1860 of human population, livestock numbers and agricultural areas at the scale of surface drainage river basins, natural resource management regions and the whole Great Barrier Reef catchment area. Cattle and several crops have experienced progressive expansion interspersed by declines associated with droughts and diseases. Land uses which have experienced all time maxima since the year 2000 include cattle numbers and the areas of sugar cane, bananas and cotton. A Burdekin Basin case study shows that sediment loads initially increased with the introduction of livestock and mining, remained elevated with agricultural development, and declined slightly with the Burdekin Falls Dam construction.

Toxicity of Herbicide Mixtures to Tropical Freshwater Microalgae Using a Multispecies Test.

Agriculture within the Great Barrier Reef catchment area has contributed to pesticide contamination of adjacent freshwater ecosystems that flow into the Great Barrier Reef World Heritage Area. A novel multispecies toxicity test was used to assess the toxicity of diuron and hexazinone, 2 herbicides commonly detected within the Great Barrier Reef catchment area, to a community of 3 tropical freshwater microalgae: Monoraphidium arcuatum, Nannochloropsis-like sp., and Pediastrum duplex. Diuron was the most toxic herbicide, with 10% inhibition concentration (IC10) values of 4.3, 7.1, and 29 µg/L for P. duplex, M. arcuatum, and Nannochloropsis-like sp., respectively, followed by hexazinone, with IC10 values of 15, 18, and 450 µg/L, respectively Toxicity testing on 2 commercial formulations (Barrage, 13.2% hexazinone and 48.6% diuron; Diurex, 90% diuron) showed that additives in the commercial formulations did not significantly increase the toxicity of diuron. Direct toxicity assessments were carried out on water samples from the herbicide-contaminated Sandy Creek, which discharges to the Great Barrier Reef lagoon, and a clean reference site, Tully Gorge in the Tully River. Toxicity was observed in several Sandy Creek samples. Artificial herbicide mixtures were assessed in synthetic soft water and natural freshwaters, with toxic responses being observed at environmentally relevant concentrations. The present study successfully applied a novel multispecies tropical microalgal toxicity test, indicating that it is an effective tool for the assessment of herbicide toxicity in both natural and synthetic freshwaters. Environ Toxicol Chem 2021;40:473-486. © 2020 SETAC.

Toxicity of the herbicides diuron, propazine, tebuthiuron, and haloxyfop to the diatom Chaetoceros muelleri

Conventional photosystem II (PSII) herbicides applied in agriculture can pose significant environmental risks to aquatic environments. In response to the frequent detection of these herbicides in the Great Barrier Reef (GBR) catchment area, transitions towards ‘alternative’ herbicides are now widely supported. However, water quality guideline values (WQGVs) for alternative herbicides are lacking and their potential ecological impacts on tropical marine species are generally unknown. To improve our understanding of the risks posed by some of these alternative herbicides on marine species under tropical conditions, we tested the effects of four herbicides on the widely distributed diatom Chaetoceros muelleri. The PSII herbicides diuron, propazine, and tebuthiuron induced substantial reductions in both 24 h effective quantum yields (ΔF/Fm′) and 3-day specific growth rates (SGR). The effect concentrations, which reduced ΔF/Fm′ by 50% (EC50), ranged from 4.25 µg L−1 diuron to 48.6 µg L−1 propazine, while the EC50s for SGR were on average threefold higher, ranging from 12.4 µg L−1 diuron to 187 µg L−1 tebuthiuron. Our results clearly demonstrated that inhibition of ΔF/Fm′ in PSII is directly linked to reduced growth (R2 = 0.95) in this species, further supporting application of ΔF/Fm′ inhibition as a valid bioindicator of ecological relevance for PSII herbicides that could contribute to deriving future WQGVs. In contrast, SGR and ΔF/Fm′ of C. muelleri were nonresponsive to the non-PSII herbicide haloxyfop at the highest concentration tested (4570 µg L−1), suggesting haloxyfop does not pose a risk to C. muelleri. The toxicity thresholds (e.g. no effect concentrations; NECs) identified in this study will contribute to the derivation of high-reliability marine WQGVs for some alternative herbicides detected in GBR waters and support future assessments of the cumulative risks of complex herbicide mixtures commonly detected in coastal waters.

Pervasive Pesticide Contamination of Wetlands in the Great Barrier Reef Catchment Area.

Knowledge of the types and impacts of contaminants occurring in the freshwater wetlands of the Great Barrier Reef catchment area (GBRCA) is limited. The present study examined the presence and concentrations of pesticides occurring in 22 floodplain wetlands, situated in moderate to high-intensity land uses in the GBRCA. The dominant land use within 1 km of the wetlands was sugar cane for 12 wetlands, grazing for 6 wetlands, plantation forestry and conservation for 2 wetlands, and one with an equal mix of land uses. Fifty-nine pesticides and pesticide degradates were detected in the wetlands during 2 consecutive early wet seasons. These included 27 herbicides, 11 herbicide degradates, 11 insecticides, 8 fungicides, 1 nematicide, and 1 pesticide synergist. Each wetland sampled contained between 12 and 30 pesticides with an average of 21 pesticides detected per wetland sampling. Temporal differences existed in the number, types, and average concentrations of pesticides detected. No exceedances of Australian and New Zealand water guideline values were found during the first sampling season, while 10 wetlands had concentrations of at least 1 pesticide exceeding the guidelines during the following sampling season. For 1 wetland, concentrations of 4 pesticides were greater than the prescribed guideline values. Individually, the vast majority of aquatic species would be protected, but in some wetlands, diuron would affect 49% of species and atrazine up to 24% of species. Statistically significant correlations between the number of pesticides and the percentage of intensive land use, primarily sugar cane growing in a 1 km radius of the wetlands, were found. Integr Environ Assess Manag 2020;16:968-982. © 2020 SETAC.

Combining weed efficacy, economics and environmental considerations for improved herbicide management in the Great Barrier Reef catchment area

The current Australian sugarcane industry transition toward adoption of an ‘alternative’ herbicide strategy as part of improved environmental stewardship is increasingly complicated by recent farming system, regulatory and herbicidal product changes. This study quantified and compared the efficacy, economic costs and environmental risk profiles of a range of established, emerging, and recently registered pre-emergent herbicides across field trials in the Wet Tropics region of North Queensland. Several herbicides were effective on certain weed species, but lacked broad spectrum control. Better efficacy results from products with multiple active ingredients (i.e., imazapic-hexazinone) demonstrated the benefits of using mixtures of active ingredients to widen the spectrum of weed control efficacy. All tested pre-emergent herbicides behaved quite similarly in terms of their propensity for off-site movement in water (surface runoff losses generally >10% of active applied), with their losses largely driven by their application rate. Herbicides with lower application rates consistently contributed less to the total herbicide loads measured in surface runoff. Results demonstrated alternative choices from the more environmentally problematic herbicides (such as diuron) are available with effective alternative formulations providing between 4 and 29 times less risk than the traditional diuron-hexazinone ‘full rate’. However, considerable challenges still face canegrowers in making cost-effective decisions on sustainable herbicide selection. Additional research and effective grower extension are required to address information gaps in issues such as specific weed control efficacy of alternative herbicides and potential blending of some herbicides for more effective broad spectrum weed control, while also minimising environmental risks.

Social marketing’s role in improving water quality on the Great Barrier Reef

Purpose The purpose of this paper is to focus on the implications of claimed detrimental impacts for the agricultural activity of the Great Barrier Reef (GBR) ecosystem health in Queensland, Australia. The authors discuss the complex interaction of factors that have contributed to the decline in reef ecosystems and the challenges presented by multiple industries operating within the GBR catchment area. The authors then discuss measures employed to address agricultural run-off, claimed to be a significant factor in declining reef water quality. Design/methodology/approach Surveys of land managers were undertaken in partnership with two of the six natural resource management (NRM) organizations operating in areas adjacent to the GBR identified as having very high risk of natural and anthropogenic runoff. The sample population was obtained from a membership database within the two regions. Participants include land managers from the both regions who engaged in sugar cane production (Region 1 and Region 2, included in this paper) and cattle production (Region 2, to be reported later). Quantitative and qualitative data were analyzed including open-ended responses. Findings A large-scale study of land managers reveals several reasons for the lack of success at reducing agricultural run-off. The authors discuss the rationale for a move to a theory-grounded social marketing approach to encouraging land manager behavior change, highlighting barriers, and potential enablers of sustained behavior change. Originality/value This study is first of its kind that discusses the behavior of land managers in the GBR catchment area and highlights facilitators and impediments of land managers’ behavior change toward GBR protection actions.

Benthic diatoms as indicators of herbicide toxicity in rivers – A new SPEcies At Risk (SPEARherbicides) index

Benthic diatom communities are used widely as indicators of river health due to their rapid response to changes in water quality. The ability for diatom-based indices to detect eutrophication has been well documented; however, an index designed specifically to detect herbicide impacts is yet to be established. This is required as herbicide contamination of rivers is common in agricultural regions and poses a potential threat to aquatic ecosystems. This study developed a new biomonitoring index (SPEARherbicides) using benthic diatom communities to detect the toxic impacts of herbicides in rivers, and tested it across 14 rivers in the Great Barrier Reef catchment area, Australia. The new index uses diatom species traits to classify diatoms as either sensitive or tolerant to herbicides and calculates the fraction of sensitive taxa within a sample. The SPEARherbicides index showed a decline in herbicide sensitive diatoms with increasing herbicide toxicity of the sites. The impacts of herbicide toxicity on the diatom community were only apparent after the wet season when aqueous herbicide concentrations typically peak and diatoms were able to recover during the dry season when herbicide concentrations were lower. SPEARherbicides values had a negative relationship with the percentage of grazing and cropping in catchments but had a positive relationship with the percentage of conservation in catchments. SPEARherbicides also had a negative relationship with co-occurring potential stressors such as nutrients and total suspended solids.

Transcriptomic, lipid, and histological profiles suggest changes in health in fish from a pesticide hot spot

Barramundi (Lates calcarifer) were collected at the beginning (1st sampling) and end (2nd sampling) of the wet season from Sandy Creek, an agriculturally impacted catchment in the Mackay Whitsundays region of the Great Barrier Reef catchment area, and from Repulse Creek, located approximately 100 km north in Conway National Park, to assess the impacts of pesticide exposure. Gill and liver histology, lipid class composition in muscle, and the hepatic transcriptome were examined. The first sample of Repulse Creek fish showed little tissue damage and low transcript levels of xenobiotic metabolism enzymes. Sandy Creek fish showed altered transcriptomic patterns, including those that regulate lipid metabolism, xenobiotic metabolism, and immune response; gross histological alterations including lipidosis; and differences in some lipid classes. The second sampling of Repulse Creek fish showed similar alterations in hepatic transcriptome and tissue structure as fish from Sandy Creek. These changes may indicate a decrease in health of pesticide exposed fish.

Urea contributions to dissolved ‘organic’ nitrogen losses from intensive, fertilised agriculture

While nearly 60% of global nitrogen fertilizer use is in the form of urea, and urea is increasingly implicated in aquatic eutrophication, little is known of the scale or temporal dynamics of urea export from intensive agriculture. Annual paddock urea nitrogen exports in surfacewater runoff were quantified across multiple years from several sugarcane farms across Australia’s Great Barrier Reef catchment area. Study results suggest that runoff of undegraded urea can represent a significant proportion of the total ‘dissolved organic nitrogen’ pool leaving paddocks, and an important form of nitrogenous export from fertilised agricultural land uses. Situations where substantial rainfall or irrigation-driven surfacewater runoff occur within 1–2 weeks of fertiliser application particularly provide scope for major, and in some cases dominant, losses of undegraded urea from paddocks. Fertiliser derived urea can be a significant, bioavailable and anthropogenic form of dissolved ‘organic’ nitrogen export that warrants further attention in many field and catchment scale research applications.

Review and conceptual models of agricultural impacts and water quality in waterways of the Great Barrier Reef catchment area

Adequate conceptual frameworks that link land use to water quality and ecosystem health are lacking for tropical and subtropical freshwater systems, so we review here extensive water-quality research undertaken in the Great Barrier Reef catchment area (GBRCA) and present conceptual models synthesising the dynamics of agricultural pollutants and their ecological effects. The seasonal flow regime defines the following key periods of water-quality risk over the annual hydrological cycle for diverse GBRCA ecosystems: initial ‘pre-flush’ flows during the transition from the dry to the wet season; early wet-season ‘first flush’ flows; peak wet-season flood flows; and sustained base flow or periods of disconnection during the dry season. The level of seasonal contrast varies from the perennial systems of the wet tropics to the intermittent systems of the dry tropics. Major water-quality stressors may be catchment scale (e.g. in streams draining broad-scale agriculture) or more localised (e.g. cattle access, irrigation tail water). Water-quality stressors such as ammonia toxicity and hypoxia (due to organic or nutrient run-off and enhanced plant productivity) are of low relevance to downstream GBR ecosystems but are major threats to fresh waters. Similarly, whereas high contaminant loads in wet-season floods present the highest water-quality risk to marine ecosystems, the greatest risk in fresh waters is often from acute contamination during early wet-season ‘pre-flush’ flows into lentic waters, or continuous input of contaminants over long periods of base flow. Because of differences in the nature of risk periods, water-quality threats and pollutant-delivery mechanisms, the benefits of different management options to improve water quality can also differ among freshwater habitats and between freshwater and marine environments.

The potential benefits of herbicide regulation: A cautionary note for the Great Barrier Reef catchment area

Industry transitions away from traditional photosystem II inhibiting (PSII) herbicides towards an ‘alternative’ herbicide suite are now widely advocated as a key component of improved environmental outcomes for Australia's Great Barrier Reef and improved environmental stewardship on the part of the Queensland sugar industry. A systematic desktop risk analysis found that based on current farming practices, traditional PSII herbicides can pose significant environmental risks. Several of the ‘alternatives’ that can directly fill a specific pre-emergent (‘soil residual’) weed control function similar to regulated PSII herbicides also, however, presented a similar environmental risk profile, regardless of farming systems and bio-climatic zones being considered. Several alternatives with a pre-emergent residual function as well as alternative post-emergent (contact or ‘knockdown’) herbicides were, predicted to pose lower environmental risks than the regulated PSII herbicides to most trophic levels, although environmental risks could still be present. While several herbicides may well be viable alternatives in terms of weed control, they can still present equal or possibly higher risks to the environment. Imposing additional regulations (or even de-registrations) on particular herbicides could result in marginal, and possibly perverse environmental impacts in the long term, if usage shifts to alternative herbicides with similar risk profiles. Regardless of any regulatory efforts, improved environmental sustainability outcomes in pesticide practices within the Great Barrier Reef catchment area will hinge primarily on the continuing adoption of integrated, strategic pest management systems and technologies applied to both traditional and ‘alternative’ herbicides. One of the emerging policy challenges is ensuring the requisite technical and extension support for cane growers to ensure effective adoption of rapidly evolving farming system technologies, in a very dynamic and scrutinised herbicide management environment.

Monitoring, Predicting, and Managing One of the Seven Natural Wonders of the World

T he Great Barrier Reef (GBR) is a 2,000-km long reef and lagoon complex. It is a United Nations Educational, Scientific and Cultural Organization (UNESCO) World Heritage Site covering 347,800 km 2 , sandwiched between Australia’s northeast coastline and the Coral Sea. It includes an estimated 2,800 reefs, with an average depth of 35 m. One of the seven Natural Wonders of the World, the GBR is the world’s largest coral reef and a significant part of Australia’s natural heritage and economy. For example, a 2007 report by Access Economics for the Great Barrier Reef Marine Park Authority found that the total economic contributions to Australia from tourism, commercial fishing, and recreational activities in the GBR catchment area totaled more than $AUS 5.7 billion (approximately $US 5.9 billion) in 2005–06. The report also found that these activities accounted for a direct and indirect national contribution of 66,000 full-time equivalent jobs.

Distance Decay Functions for Iconic Assets: Assessing National Values to Protect the Health of the Great Barrier Reef in Australia

The aim of this study was to estimate the values to protect the health of the Great Barrier Reef (GBR) at the national level and to examine the effects of distance decay on valuation estimates. Two choice-modelling experiments were conducted in six locations: a regional town within the GBR catchment area (Townsville); Brisbane, the state capital approximately 450 km from the southern limit of the GBR; and four other capital cities (Sydney, Melbourne, Adelaide and Perth) ranging from nearly 1,000 km to over 4,000 km from Brisbane. Value estimates from a pooled model suggest that the average WTP across Australian households is $21.68 per household per annum for 5 years, and that those values are higher for respondents with higher levels of education and income, respondents who live in Queensland, respondents who live further away, and respondents who plan to visit more often in the future. For this nationally important iconic asset, apparent distance decay effects appear to be explained by variations in future usage and state responsibility, rather than proximity.

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.

Environmental impacts of irrigated sugarcane production: Herbicide run-off dynamics from farms and associated drainage systems

Irrigation is vital to most of the sugarcane produced in Australia's ecologically sensitive Great Barrier Reef catchment area, although little is known regarding pesticide losses under irrigated sugarcane production. This study determined the dynamics of off-site paddock-scale pesticide movement and subsequent concentrations in local receiving environments in fully irrigated sugarcane farming systems of the lower Burdekin floodplain region, the largest sugar producing area in Australia. Chemical movement (both mass and concentration) in paddock surface run-off followed a similar pattern across sites in the region for several of the commonly applied herbicides such as diuron, atrazine and ametryn. Highest losses (loads and event concentrations) occurred in the first irrigation run-off events following application, with subsequent irrigation losses tailing off rapidly. Significant losses could also occur during wet season rainfall run-off events from paddocks with recent pesticide applications. There was a strong seasonal signal evident in catchment monitoring results. Pesticide concentrations in nearby receiving creek systems were invariably an order of magnitude or more lower than values collected at paddock-scale, highlighting the considerable dilution that takes place over relatively short distances. While the concentrations found in receiving creek systems were considerably lower than direct paddock run-off, they regularly exceeded some ecological guidelines and results of pesticide risk modeling suggested concentrations, particularly under dry season conditions, posed considerable ecological risk to aquatic ecosystems.

Cost effectiveness of design-based water quality improvement regulations in the Great Barrier Reef Catchments

Coastal and marine ecosystems are adversely affected by diffuse source pollution from agricultural activities in coastal river catchments (or watersheds). To address this issue, government policy has been designed to increase adoption of improved land management practices that are known to minimise the runoff of sediments and nutrients into waterways. Government policy can be implemented through a spectrum of approaches ranging from voluntary to regulatory, and often occurs through a mix of approaches. In this paper we argue that it makes economic and ecological sense to focus investments on those parts of the landscape with greatest potential to reduce pollutants at least cost. To examine this, a financial economic analysis is combined with agricultural production analysis and catchment scale hydrological analysis using a Bayesian belief network model. We explore and compare the cost-effectiveness of two homogenous design based approaches, aimed at reducing diffuse source nutrient pollutants exports, between two biophysically and socio-economically heterogeneous catchments in the Great Barrier Reef Catchment Area, Australia. Sugarcane production is regarded as a major source of pollutants with the dominant pollutant being nitrogen. Our results show significant differences between catchments, both in relation to costs and benefits as well as water quality improvement potential, suggesting a spatially targeted approach may lead to more cost efficient pollutant reductions.