Project 3.2.1

Deriving ecologically relevant load targets to meet desired ecosystem condition for the Great Barrier Reef: a case study for seagrass meadows in the Burdekin region

Led by: Dr Catherine Collier, JCU

Project Summary

This project will derive ecologically relevant targets (ERTs) for water quality and sediment loads by: 1) defining seagrass desired state targets; 2) calculating water quality guidelines (light); and 3) calculating ERTs for terrestrially sourced sediment loads. Historical and new data collections will be used to derive ERTs via statistical models and eReefs (RECOM), and the seagrass sub-model in eReefs will be used to test ERTs. A framework for setting seagrass desired state will be developed for the entire GBR and water quality guidelines and sediment load ERTs will be quantitatively defined for the Burdekin basin where suspended sediments affect seagrass condition.

Project Publications

Project Webinar – 1st December 2020

Science Day, 20th November, Townsville

Problem Statements

Problem

Water quality in the GBR is directly linked to riverine discharge and land-based pollutants, which have increased since European settlement [1-3]. Discharge of excessive pollutants including sediments, nutrients and pesticides continue to impact ecosystem health in the GBR [4, 5].

The Reef 2050 Long Term Sustainability Plan (Reef 2050 Plan) has an overarching objective that “the quality of water entering the reef … has no detrimental impact on health and resilience of the GBR”. Thus, the Reef 2050 Plan hinges on ecologically relevant and measurable targets against which to assess progress towards meeting the plan’s tangible outcomes and objectives. Furthermore, a key component of the Reef 2050 Plan is the Reef 2050 Integrated Monitoring and Reporting Program (RIMReP); the effectiveness of which will be underpinned by having biologically relevant targets to provide a basis for monitoring and reporting objectives.

Targets for sustainable pollutant loads have been set in the Reef Water Quality Protection Plan 2013 (Reef Plan); however, these were primarily set on the basis of catchment management feasibility [6] and may not provide the required level of ecosystem protection [2]. Load targets that protect ecosystem condition in the GBR are termed Ecologically Relevant Targets (ERTs) [7]. ERTs are being reviewed in the Scientific Consensus Statement Update (SCSU) in 2016 but a statistically rigorous approach to developing ERTs is not possible within the timeframes for its delivery.

Seagrass is identified as a key ecosystem and a measure of ecosystem health, actions, targets and objectives in the Reef Plan. Seagrass meadows are also highly sensitive to water quality. Catchment-derived pollutants, particularly sediment loads, were linked to GBR-wide seagrass loss from 2008 to 2011 [8, 9]. Recent research defining acute light thresholds for seagrass provides a guiding document on their application, particularly in setting compliance targets [10]. However, water quality guidelines and load targets that protect seagrass meadows from long-term decline have not been established.

How Research Addresses Problem
The project will develop water quality guidelines and ERTs for end of catchment sediment load reductions by:

1. Defining seagrass desired state (i.e. seagrass health, resilience, and function targets) (EHA6, Reef 2050 Plan),
2. Calculating long-term water quality guidelines to meet seagrass desired state; and,
3. Calculating ecologically-relevant targets (ERTs) for terrestrially sourced sediment loads (WQA1, Reef 2050 Plan).

Development of regionally-relevant ecological ‘desired state’ for seagrass is a priority action in Reef 2050 Plan (EHA6). Targets for seagrass ‘desired state’ will be based on seagrass ecological function and resilience, set according to meadow type, and will consider temporal variability and data availability. Appropriate indicators for setting and evaluating desired state targets will also be considered. The definition of desired state is required to set ERTs.

The method for setting regionally-specific ERTs proposed by Brodie et al. (In press) will be tested for sediment loads to meet the seagrass desired state targets. For example, a preliminary exploration of how sediment loads affected seagrass distribution in coastal waters within Cleveland Bay reported that a 50% reduction in end of catchment sediment loads from the Burdekin River could increase the depth of the presence of Halophila species (IT2.5) from 4.8m to 6.6m.

The Burdekin Basin will be used as a case study because long-term data is available for setting seagrass desired state targets [12, 13]. Furthermore, terrestrially sourced sediment loads from the Burdekin River and their subsequent resuspension present a risk to seagrass meadows through the attenuation of light throughout the year [9, 14, 15]. In waters less than 15m, resuspension of sediment (with an organic component) that are sourced from the catchment is the principal cause of light attenuation in Cleveland Bay [1], and this reduces light across most of the visible spectrum. Thus, ERTs will be set considering re-suspension processes and residual sediment in the system (i.e. time-lags).

Historical and current data that documents both cause (increased loads and declining water quality, specifically increasing light attenuation) and effect (seagrass loss) can be used in dynamic statistical models [e.g. 16, 17] to set ERTs and end of catchment load reduction targets in the Burdekin Basin. With the recent implementation of the eReefs threedimensional hydrodynamic-biogeochemical model [18], which includes a detailed submodel for seagrass growth [19], the effect of catchment-derived sediment loads on seagrass can be tested. This has potential application for setting load reduction targets in data poor regions. Therefore, this project will test a protocol using both statistical models and eReefs to develop ERTs.

Desired state targets and preliminary ERTs for water quality and loads will be available for contribution to the Reef 2050 Plan review. Milestones in this proposal could be revised to facilitate timely delivery.

Alignment with NESP Research Priorities

2a) Undertake research for the Reef 2050 Integrated Monitoring and Reporting Program (RIMReP) to develop cost-effective indicators and metrics for key GBRWHA biophysical and human dimension values and identification of associated ecosystem thresholds and guidelines for grading scores, linked to specific objectives and targets in the Reef 2050 Plan.

Project Keywords

Sediment loads; Desired state; Water quality guidelines; Indicators; Reef plan target.

Project Funding

This project is jointly funded through JCU, UQ, CSIRO, GBRMPA and the Australian Government’s National Environmental Science Programme.

Project Map

https://eatlas.org.au/nesp-twq-3/eco-load-targets-seagrass-3-2-1