Connecting targets for catchment sediment loads to ecological outcomes for seagrass using multiple lines of evidence.

Catchment impacts on downstream ecosystems are difficult to quantify, but important for setting management targets. Here we compared 12 years of monitoring data of seagrass area and biomass in Cleveland Bay, northeast Australia, with discharge and associated sediment loads from nearby rivers. Seagrass biomass and area exhibited different trajectories in response to river inputs. River discharge was a slightly better predictor of seagrass indicators than total suspended solid (TSS) loads, indicating that catchment effects on seagrass are not restricted to sediment. Linear relationships between Burdekin River TSS loads delivered over 1-4 years and seagrass condition in Cleveland Bay generated Ecologically Relevant Targets (ERT) for catchment sediment inputs. Our predicted ERTs were comparable to those previously estimated using mechanistic models. This study highlights the challenges of linking catchment inputs to condition of downstream ecosystems, and the importance of integrating a variety of metrics and approaches to increase confidence in ERTs.

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.

Relating sediment impacts on coral reefs to watershed sources, processes and management: a review.

Modification of terrestrial sediment fluxes can result in increased sedimentation and turbidity in receiving waters, with detrimental impacts on coral reef ecosystems. Preventing anthropogenic sediment reaching coral reefs requires a better understanding of the specific characteristics, sources and processes generating the anthropogenic sediment, so that effective watershed management strategies can be implemented. Here, we review and synthesise research on measured runoff, sediment erosion and sediment delivery from watersheds to near-shore marine areas, with a strong focus on the Burdekin watershed in the Great Barrier Reef region, Australia. We first investigate the characteristics of sediment that pose the greatest risk to coral reef ecosystems. Next we track this sediment back from the marine system into the watershed to determine the storage zones, source areas and processes responsible for sediment generation and run-off. The review determined that only a small proportion of the sediment that has been eroded from the watershed makes it to the mid and outer reefs. The sediment transported >1 km offshore is generally the clay to fine silt (<4-16 µm) fraction, yet there is considerable potential for other terrestrially derived sediment fractions (<63 µm) to be stored in the near-shore zone and remobilised during wind and tide driven re-suspension. The specific source of the fine clay sediments is still under investigation; however, the Bowen, Upper Burdekin and Lower Burdekin sub-watersheds appear to be the dominant source of the clay and fine silt fractions. Sub-surface erosion is the dominant process responsible for the fine sediment exported from these watersheds in recent times, although further work on the particle size of this material is required. Maintaining average minimum ground cover >75% will likely be required to reduce runoff and prevent sub-soil erosion; however, it is not known whether ground cover management alone will reduce sediment supply to ecologically acceptable levels.

Fine sediment and nutrient dynamics related to particle size and floc formation in a Burdekin River flood plume, Australia.

The extreme 2010-2011 wet season resulted in highly elevated Burdekin River discharge into the Great Barrier Reef lagoon for a period of 200 days, resulting in a large flood plume extending >50km offshore and >100km north during peak conditions. Export of suspended sediment was dominated by clay and fine silt fractions and most sediment initially settled within ∼10km of the river mouth. Biologically-mediated flocculation of these particles enhanced deposition in the initial low salinity zone. Fine silt and clay particles and nutrients remaining in suspension, were carried as far as 100km northward from the mouth, binding with planktonic and transparent exopolymer particulate matter to form large floc aggregates (muddy marine snow). These aggregates, due to their sticky nature, likely pose a risk to benthic organisms e.g. coral and seagrass through smothering, and also by contributing to increased turbidity during wind-induced resuspension events.

Dynamics of herbicide transport and partitioning under event flow conditions in the lower Burdekin region, Australia.

This study examined the temporal variability in herbicide delivery to the Great Barrier Reef (GBR) lagoon (Australia) from one of the GBR catchment's major sugarcane growing regions. Annual loads of measured herbicides were consistently in the order of 200+kg. Atrazine, it's degradate desethylatrazine, and diuron contributed approximately 90% of annual herbicide load, with early 'first-flush' events accounting for the majority of herbicide loads leaving the catchment. Assessment of herbicide water-sediment partitioning in flood runoff highlighted the majority of herbicides were transported in predominantly dissolved form, although a considerable fraction of diuron was transported in particulate-bound form (ca. 33%). Diuron was also the herbicide demonstrating the highest concentrations and frequency of detection in sediments collected from catchment waterways and adjacent estuarine-marine environments, an outcome aligning with previous research. Herbicide physico-chemical properties appear to play a crucial role in partitioning between water column and sediment habitat types in GBR receiving ecosystems.

Assessing the additive risks of PSII herbicide exposure to the Great Barrier Reef.

Herbicide residues have been measured in the Great Barrier Reef lagoon at concentrations which have the potential to harm marine plant communities. Monitoring on the Great Barrier Reef lagoon following wet season discharge show that 80% of the time when herbicides are detected, more than one are present. These herbicides have been shown to act in an additive manner with regards to photosystem-II inhibition. In this study, the area of the Great Barrier Reef considered to be at risk from herbicides is compared when exposures are considered for each herbicide individually and also for herbicide mixtures. Two normalisation indices for herbicide mixtures were calculated based on current guidelines and PSII inhibition thresholds. The results show that the area of risk for most regions is greatly increased under the proposed additive PSII inhibition threshold and that the resilience of this important ecosystem could be reduced by exposure to these herbicides.

Herbicides: a new threat to the Great Barrier Reef.

The runoff of pesticides (insecticides, herbicides and fungicides) from agricultural lands is a key concern for the health of the iconic Great Barrier Reef, Australia. Relatively low levels of herbicide residues can reduce the productivity of marine plants and corals. However, the risk of these residues to Great Barrier Reef ecosystems has been poorly quantified due to a lack of large-scale datasets. Here we present results of a study tracing pesticide residues from rivers and creeks in three catchment regions to the adjacent marine environment. Several pesticides (mainly herbicides) were detected in both freshwater and coastal marine waters and were attributed to specific land uses in the catchment. Elevated herbicide concentrations were particularly associated with sugar cane cultivation in the adjacent catchment. We demonstrate that herbicides reach the Great Barrier Reef lagoon and may disturb sensitive marine ecosystems already affected by other pressures such as climate change.