3. Ecosystem health

The process of sediment exposure includes the transport of sediment into and throughout the Region, primarily from land-based runoff, but also through resuspension and increased turbidity and sediment settling on bottom-dwelling organisms. The inflow from land to sea, and subsequent dispersion, resuspension and consolidation of sediments, are natural processes that have affected water quality, and modified and shaped lagoon floor habitats (Section 2.3.6) since modern sea levels were reached about 7500 years ago.⁵⁸⁴ However, suspended sediments entering the Reef lagoon are estimated to have increased 1.4 to 5-fold since European settlement,⁴² which can be largely attributed to changes in land use.⁴² This increase is a result of soil erosion from rangeland grazing and cropping, urban development, deforestation, and mining. Some of the increase can also be attributed to climate change and associated changes in rainfall and land-based runoff.^42,585

Most new sediment is delivered to the Region during flood events (Figure 3.6). Amounts delivered vary between the 6 natural resource management regions and individual catchments.⁴² Modelling of land-derived total suspended solids indicates that the highest exposure levels from anthropogenic sediment loads are concentrated in inshore areas and the largest inputs come from the Burdekin River (Section 6.5.1).⁵⁷⁹ Most sediments delivered from river catchments settle and are retained along the coast, within floodplains, estuaries, close to river mouths and within the eastern sections of north-facing embayments.^42,586,587 Nearshore and inshore fringing coral reefs also host considerable proportions of land-derived sediments within their internal structures.⁴² Within the Reef lagoon, land-derived sediments are mostly contained within the inner shelf (from 0 to 20 metres depth).⁵⁸⁶  

The composition of suspended sediment is an important factor influencing its distribution and effects. Most fine sediments tend to clump together in a process called flocculation and settle in areas where fresh water and saltwater mix.⁵⁸⁸ However, some fine sediments can be carried long distances away from river mouths in suspension, and they occasionally reach mid-shelf and even outer-shelf areas of the northern and central Reef when very large riverine discharge events coincide with low wind speeds.^42,586 

Cross-shelf transport of land-derived sediments also occurs by a variety of other mechanisms.⁵⁸⁶ Fine sediment can be resuspended by wind and tidal currents in shallow waters and may persist in the water column for at least 6 to 8 months following river input.⁵⁸⁹ Delivery and resuspension of new sediments, and resuspension of legacy sediments can affect water quality year-round.⁴² Multiple lines of evidence from turbidity loggers, remote sensing, and modelling show that river discharge and associated sediment loads are linked to elevated and prolonged turbidity levels, resulting in longer periods of reduced light available for photosynthesis in certain inner and mid-shelf areas of the Reef lagoon (Section 3.2.7).^42,586

Sediment exposure has both positive and negative effects on ecosystems. For example, sufficient sediment supply is crucial for some habitats, such as low wooded islands, to keep pace with rising sea levels.⁵⁹¹ However, excessive sediment exposure can result in the degradation of coral reefs, soft-bottom habitats, seagrass meadows and freshwater wetlands, and can affect filter feeders and fish.^{39,592,593,594} Sediment exposure influences ecosystems in several ways such as through increased in water turbidity, by settling on and even burying organisms, remineralising into bioavailable nutrients that can drive productivity,⁵⁹⁵ and providing a vector for the spread of pathogens.⁵⁹⁶ There is mounting evidence that sediment exposure can suppress recovery from other disturbance events, such as marine heatwaves and cyclones.^42,39 The effects of sediment and particulate nutrients on freshwater wetlands and estuarine wetlands, such as mangroves, marshes and supratidal forests, in the Region is a key knowledge gap.^42,39

Inshore locations are frequently exposed to turbid waters that absorb light required for photosynthesis, affecting both quality and quantity of light reaching the seafloor.⁵⁹⁷ Seagrasses are especially vulnerable to reduced light availability.^598,599 Seagrass canopies capture suspended sediment, and loss of seagrass causes feedback loops that can result in persistent impacts on seagrass habitats ^600,601 and indirectly affect dependent species (such as dugongs).⁶⁰² Sedimentation and changes to the substrate are important for seagrass and reef organisms, especially for influencing settlement.⁴²

Some coral species and reefs live in turbid conditions,^227,603 and a high abundance and diversity of coral species can be found in shallow water in some turbid inshore settings with favourable hydrodynamics.⁴² However, excess loads of sediment and particulate nutrients can affect coral physiology and recruitment, and reef community composition (Section 6.5.2), and coral diversity is generally lower on inshore reefs than at offshore locations.³⁹ Coral assemblages on inshore reefs are dominated by turbidity-tolerant species and depend more on food capture by coral polyps. In contrast, offshore reefs have more turbidity-sensitive corals and a greater reliance on photosynthesis by symbiotic partners for food.⁶⁰⁴ Turbidity reduces the growth of macroalgae but they tend to be less sensitive than corals, and reefs chronically affected by sediment exposure often have higher cover of fleshy macroalgae than reefs in clear waters.^42,39 Settled sediment is energetically costly to remove and affects the suitability of the substrate for coral recruitment and growth.⁶⁰⁵ Settled sediment can also interfere with predation and directly or indirectly (through altering algal substrate) affect herbivorous grazing.⁶⁰⁶ 

Many other key species groups, including sponges, crustose coralline algae and large benthic photosynthetic foraminifera, are also negatively affected by sediment exposure.⁴² For fish communities, suspended sediments can affect the growth and time to metamorphosis of juvenile fish, juvenile gill morphology, foraging time, body condition and mortality. Suspended sediments can also interfere with the visual cues that juvenile fish use to settle into habitats, impairing their ability to: distinguish between live and dead coral; extend their settlement time; and alter feeding patterns.³⁹

In the 2022–23 water monitoring year (October to September), measurements of total suspended solids met guideline values in most regions where data were collected. While concentrations of total suspended sediments exceeded guideline values in the Mackay Whitsunday region, they have shown a recent trend of improvement.⁵⁷⁹ Additional information on sediment loads since 2019 is given in Section 6.5.1. Wet season river influence from 2018–19 to 2022–23 rarely extended seaward of inshore waters.⁵⁷⁹ Therefore, impacts to mid-shelf and offshore ecosystems from land-derived sediment exposure during this period were low, and this period has seen the recovery of previously affected seagrass meadows in several locations (Section 2.3.4). However, sediment exposure over the past 5 years was still considerably higher than pre-European levels. Across the Region, the relative influence of sediments sourced from river discharge compared to resuspension of sediments already existing in the ecosystem remains poorly understood.

Sediment loads delivered during flood events, and resuspension of legacy sediments continue to contribute to the poor state of many inshore coastal and inshore marine ecosystems. Since 2019, some areas have seen improvements in sediment exposure, but other inshore areas continue to be affected by resuspended and catchment-derived sediments.