Estimating the Exposure of Coral Reefs and Seagrass Meadows to Land-Sourced Contaminants in River Flood Plumes of the Great Barrier Reef: Validating a Simple Satellite Risk Framework with Environmental Data

Abstract

River runoff and associated flood plumes (hereafter river plumes) are a major source of land-sourced contaminants to the marine environment, and are a significant threat to coastal and marine ecosystems worldwide. Remote sensing monitoring products have been developed to map the spatial extent, composition and frequency of occurrence of river plumes in the Great Barrier Reef (GBR), Australia. There is, however, a need to incorporate these monitoring products into Risk Assessment Frameworks as management decision tools. A simple Satellite Risk Framework has been recently proposed to generate maps of potential risk to seagrass and coral reef ecosystems in the GBR focusing on the Austral tropical wet season. This framework was based on a "magnitudeˆlikelihood" risk management approach and GBR plume water types mapped from satellite imagery. The GBR plume water types (so called "Primary" for the inshore plume waters, "Secondary" for the midshelf-plume waters and "Tertiary" for the offshore plume waters) represent distinct concentrations and combinations of land-sourced and marine contaminants. The current study aimed to test and refine the methods of the Satellite Risk Framework. It compared predicted pollutant concentrations in plume water types (multi-annual average from 2005-2014) to published ecological thresholds, and combined this information with similarly long-term measures of seagrass and coral ecosystem health. The Satellite Risk Framework and newly-introduced multi-annual risk scores were successful in demonstrating where water conditions were, on average, correlated to adverse biological responses. Seagrass meadow abundance (multi-annual change in % cover) was negatively correlated to the multi-annual risk score at the site level (R 2 = 0.47, p < 0.05). Relationships between multi-annual risk scores and multi-annual changes in proportional macroalgae cover (as an index for coral reef health) were more complex (R 2 = 0.04, p > 0.05), though reefs incurring higher risk scores showed relatively higher proportional macroalgae cover. Multi-annual risk score thresholds associated with loss of seagrass cover were defined, with lower risk scores (ď0.2) associated with a gain or little loss in seagrass cover (gain/´12%), medium risk scores (0.2-0.4) associated with moderate loss (´12/´30%) and higher risk scores (>0.4) with the greatest loss in cover (>´30%). These thresholds were used to generate an intermediate river plume risk map specifically for seagrass meadows of the GBR. An intermediate river plume risk map for coral reefs was also developed by considering a multi-annual risk score threshold of 0.2-above which a higher proportion of macroalgae within the algal communities can be expected. These findings contribute to a long-term and adaptive approach to set relevant risk framework and thresholds for adverse biological responses in the GBR. The ecological thresholds and risk scores used in this study will be refined and validated through ongoing monitoring and assessment. As uncertainties are red...

Potential Enhanced Survivorship of Crown of Thorns Starfish Larvae due to Near-Annual Nutrient Enrichment during Secondary Outbreaks on the Central Mid-Shelf of the Great Barrier Reef, Australia

“…These conditions of elevated phytoplankton biomass are seen both visually, and by water type analysis [44,64,65,67] and ascribed water types (right column panels of Figure 3), following the methodology of Devlin et al [44] and Petus et al [69,70].…”

Water Quality and River Plume Monitoring in the Great Barrier Reef: An Overview of Methods Based on Ocean Colour Satellite Data

“…Spatial maps of potential risk associated with water quality thresholds are also an important data visualization tool for communicating environmental risks to managers and providing information on prioritising land based management. Work is in progress to test and improve this approach [68].…”

“…This exercise is, however, challenging because the response of GBR ecosystems to an amount and/or duration of exposure to land-sourced contaminants (respectively or combined) in river plume waters is often unknown at a regional or ecosystem level [87]. Work in this area has progressed by using time series data of MODIS river plume water masses [33,68] to establish measures of frequency (as a proxy for intensity) (Figure 6a) with water quality gradients measured through the mapping of the 6CC's. These maps can help summarise the likelihood and magnitude of the river plume risk, by spatially clustering water masses with different concentrations and proportions of land-sourced contaminants against a risk framework (Figure 6d).…”

“…This framework produces river plume risk maps for seagrass and coral ecosystems based on a simplified risk matrix [33]. The "risk" of elevated nutrients and sediments needs to be closely linked to research identifying ecological thresholds to allow the qualitative framework expressed in Figure 6d to move to quantitative thresholds that can influence ecosystem decline and impact on GBR resilience [64,68,69,84]. Knowledge of these thresholds can identify ecosystems which may experience acute or chronic high exposure to contaminants in river plumes and help evaluate the susceptibility of GBR ecosystems to land-sourced contaminants.…”

Opportunities for Protecting and Restoring Tropical Coastal Ecosystems by Utilizing a Physical Connectivity Approach

“…Methods are already in use for local singular ecosystem-based monitoring but this could be expanded to determine how changes occur between one ecosystem and another. Remote and local data can then be integrated using satellite maps and spatial analysis to help create spatial explicit maps of fluxes across the seascape, which will cover mechanisms at the level of ecosystem extrapolated to the large-scale seascape (Brodie et al, 2010;Petus et al, 2016).…”