Improved understanding of biophysical and socio-economic connections between catchment and reef ecosystems: Wet and Dry Tropics case studies

MTSRF Synthesis Report

Compiled by Michelle Devlin and Jane Waterhouse

C2O Consultants, Townsville

ISBN 978-1-921359-58-3
Published December 2010

MTSRF Theme 5 - Enhancing Delivery
MTSRF Program 7 - Halting and Reversing the Decline of Water Quality

Executive Summary

Over the past thirty years an increasing amount of research and monitoring effort has been devoted to documenting and understanding the nature and importance of water quality issues for the Great Barrier Reef (GBR). Attention has become focused on land-based runoff as a primary source of pollutants into the GBR. This report reviews, synthesises and analyses the work carried out over the course of the Marine and Tropical Science Research Facility (MTSRF) in relation to our current understanding of the relationships between catchment processes, pollutant loads delivered to instream environments (including wetlands and estuaries) and the marine environment, and the impacts on instream environments and the near shore environment. Key sources of information for this synthesis of catchment to reef water quality research are largely from MTSRF Theme 3 'Halting and Reversing the Decline of Water Quality'. Case studies from MTSRF research conducted in the Dry Tropics (Burdekin) and Wet Tropics (Tully) of North Queensland are presented to demonstrate a more detailed understanding of catchment to reef relationships, and to highlight the advances made in our understanding of the broader relationships. The highlights of these case studies are summarised in Figure i and ii respectively.

This report highlights research results that have changed our understanding since the commencement of the MTSRF Research Program in 2006 and hence may be critical in revising the aims or priorities of water quality policy frameworks such as the Reef Water Quality Protection Plan, given the new understanding. The breadth and diversity of MTSRF funded research is presented in this report as well as other companion reports (Waterhouse, 2010; Waterhouse and Brodie, 2010; Martin, 2010). The success of the MTSRF model stems primarily from the research crossing over many science disciplines, including the social and economic sectors. The combination of these multiple strands of information has allowed a comprehensive approach to studying catchment-to-reef processes.

A number of key findings of the MTSRF in relation to the processes that connect the whole catchment to reef system are highlighted below:

• Conceptual biophysical models have been developed to identify appropriate heatlh indicators of freshwater ecosystems, including stream, floodplain lagoon and wetland health, while probable thresholds of concern, in terms of contaminant concentrations, ecological processes and biodiversity have also been investigated for these ecosystems. Indicators of freshwater ecosystems have been developed and are related to pressures that include patterns and types of land use, general water quality and contaminants, hydrological regime, channel and habitat structure, riparian vegetation condition and alien species of plants and fish. Measurement of spatial and temporal variability of biophysical indicators in floodplain wetlands of the Tully-Murray catchment have been correlated with those pressures.
• Connectivity between freshwater ecosystems is important for maintaining ecosystem health and has been studied using hydrological modelling in the Tully-Murray floodplain area. The degree of connectivity of different wetlands, ranging from those wetlands that are more permanently connected with streams and drains to those that are connected only when there are large overbank floods, varies with wetland location and flood magnitude. These results have important implications for (i) the movement and recruitment patterns of aquatic biota during and after flood events, (ii) wetland habitat characteristics and water quality, (iii) the biodiversity of individual wetlands over time, and (iv) the potential for wetland processes to influence the quality of water flowing to the GBR lagoon. As the hydrodynamic model is driven by daily rainfall it should also be possible to quantify the potential impacts of climate change on wetland connectivity, if the future changes in rainfall can be specified.
• Sediments, nutrients and pesticides are the priority pollutants for management of water quality in the GBR. Studies within the MTSRF have informed the refinement of knowledge of priority areas for pollutant generation and, hence, management in GBR catchments.
• In the Burdekin River catchment, sediment load is dependent on the catchment characteristics and size of flow event. However, regardless of flood event size in the other catchments, the upper Burdekin basin is always likely to be the dominant source (83-97%) of suspended sediment into the Burdekin Falls Dam. Total suspended sediment (TSS) load delivered over the dam spillway makes up a smaller proportion (20-50%) of the total load exported from the Burdekin River than the below dam catchment area, and it is estimated that 50-80% of the suspended sediment export ('bulk' suspended sediment) to the GBR lagoon has been sourced from the catchment area below the Burdekin Falls Dam. Thus, management efforts should be primarily focused on these lower catchments which make up only a small percentage of the overall Burdekin catchment area.
• Studies in the Burdekin catchment show that there are different delivery pathways between the bulk (heavier) sediment and the finer sediment. There is little deposition of the finer clay fraction as it is transported within the catchment compared to coarser size fractions (such as silts and sand) which are preferentially being deposited within the dam or during other opportunities for deposition. Size distribution shows the movement of the finer sediment from the upper catchments, through the dam and into the marine environment. These results are also relevant to other Dry Tropics catchments in the GBR. Further studies show that the finer fraction (<38 µm component) of the sediment is present in the turbid primary plume which is generally constrained closer to the coast but was not seen in the larger secondary plume as inorganic matter. These latest particle size results indicate that the finer clay fractions are being transported not only throughout the catchment, with little opportunity for deposition, but also within the marine environment via resultant flood plumes. It is this finer fraction which has been linked to the degradation of coral reef ecosystems and therefore may pose the greatest risk to receiving marine ecosystems.
• Building on this knowledge, receiving water models can be used to develop sediment budgets for areas within the GBR. For example, a hydrodynamic model has been developed for Cleveland Bay (receiving waters of the Burdekin River) which shows that the amount of riverine sediments settling on the bay may exceed the amount of sediment exported from the bay by 50-75%. Sediment is thus accumulating in the bay on an annual basis, with potentially negative effects on coral reefs. A net sediment outflow from the bay may only occur during years that experience a tropical cyclone. Thus the majority of the sediment accumulates in areas where it is frequently re-suspended by waves under trade winds, thus increasing the turbidity of the bay.
• In the Tully-Murray River catchments, estimates of nutrient loads being delivered during flood events to the GBR lagoon have been significantly underestimated in the past. Through MTSRF research, the flood contributions were found to increase the mean annual loads of phosphorus and nitrogen loads by 30-50% above previous river based estimates. These results indicate that there is therefore a clear need to obtain estimates of the contribution that floods make to marine loads in other GBR catchments.
• Comprehensive research on the impact of sediments and nutrients on the GBR ecosystems has been undertaken as part of the MTSRF, and the preceding CRC Reef Research Centre and Rainforest CRC joint 'Catchment to Reef' Program, and can be represented in a series of conceptual models. This work has led to the development of 'thresholds of concern' for several water quality variables and ecosystem components, which in turn have been used in the development of Water Quality Guidelines for the Great Barrier Reef Marine Park (GBRMPA, 2009). The research has also demonstrated a link between elevated concentrations of nutrients and the location and frequency of COTS outbreaks.
• Studies on the effects of herbicides on GBR ecosystems have shown that herbicides are being detected in many locations in the GBR, especially following rain events, and that increased exposure can potentially threaten ecosystems within the GBR. The herbicides most commonly detected in the GBR lagoon are designed to inhibit photosystem II in plants and so the risk of these herbicides should be considered additively. Previous studies have examined the risk of individual herbicides in isolation; recent monitoring studies show that 80% of the time when herbicides are detected, two or more herbicides are present in the GBR lagoon following wet season river discharge and, consequently, the area at risk to pesticide exposure increases when the additive risk is considered.
• Coral cores have been used to track change in material delivery to the GBR over long time periods. Coral Ba/Ca ratios in both short and long term coral core records display an increasing trend over time, particularly post European settlement (c. 1880) and in the last ~30 years, although peak values do not always coincide with river floods. In addition, the geochemical results from coral cores collected along a water quality gradient through the Whitsunday Islands have been useful in establishing local and regional patterns of terrestrial influence factors. These patterns correlate with an increased chronic terrestrial influence in the Whitsunday Islands. However, coral Y/Ca ratios typically lack long-term trends although peaks do generally relate to river discharge. Ba/Ca records from a long-lived coral (>100 years old) show a close correspondence with the generally annual river discharge peaks, providing further evidence that this approach provides a good proxy for changes in terrestrial inputs in the Wet and Dry Tropics.
• Recent publications presented for the Tully region (Kroon, 2009) showcased MTSRF supported research as a key component in the detailing of this ecosystem approach within the Tully catchment and marine region. In summary, this work included the estimate of the contribution of overbank (flood) flows to total pollutant loads, previously not taken into account in load estimates to the GBR (Wallace et al. 2009a,b). Maughan and Brodie (2009) provide a spatial model to visualise GBR exposure to land-sourced pollutants under current and changed land use regimes. Devlin and Schaffelke (2009) identified the transport and extent of pollutants in Tully flood plumes, and identified areas of high exposure and ultimately at high risk from the impacts of altered land use activities. This was reported as the number of marine biological systems that were frequently inundated by higher concentrations of sediment, nutrients and pesticides. The challenge to produce target estimates from catchment models with known levels of uncertainty, but robust enough for management purposes, was examined by Brodie et al. (2009a). The outcomes of these inter-related studies have contributed significantly to our capacity to understand and predict direct and indirect relationships between land use and management, impacts on water quality and flow on effects on marine biodiversity.