2022 Scientific Consensus Statement: Question 3.2 What are the measured impacts of increased sediment and particulate nutrient loads on Great Barrier Reef ecosystems, what are the mechanism(s) for those impacts and where is there evidence of this occurring in the Great Barrier Reef?

Summary of evidence to 2022: Of all of the GBR ecosystems considered, the number of studies on coral reef habitats (coral, non- coral invertebrates, algae) was higher than for other ecosystems (142 on reefs, 91 on coral specifically), with a high diversity of study types (observational, experimental, modelling and review), and numerous that were of high relevance to the question leading to a Moderate to High level of consistency in the findings. The key findings included: • Suspended sediments and particulate nutrients attenuate light, settle on and smother corals and other reef organisms and increase nutrient supply. • Increased loads of sediment and particulate nutrients to inshore habitats have persistent impacts on reef composition, leading to greatly compressed depth zonations, reduced depth limit for reef development, lower coral diversity and suppression of many other sensitive species in favour of macroalgae. • High abundance of healthy persistent coral species can be found in 1-3 m water depth in some turbid inshore settings with favourable hydrodynamics. • Deeper down, low light levels can cause sublethal stress and partial mortality of corals after only short exposure (days to weeks). Here, communities are typically dominated by turbidity-tolerant species with high levels of heterotrophy and filter feeders, rather than phototrophic taxa. • Sediments and particulate nutrients are suppressing reef recovery following disturbances due to their strong negative effects on the early life stages of corals and coral recruitment. • For sponges, short-term exposure to high levels of suspended sediment affect growth, while long-term exposure reduces their abundance possibly limiting recruitment. • Many other sensitive calcifying species such as crustose coralline algae and large benthic photosynthetic foraminifera are also negatively affected by suspended particulates and water quality through a variety of mechanisms. Seagrass had a Moderate number of studies (45), only one that was highly relevant, and the majority were of Low to Moderate relevance to the study question because they were not specifically on particulates nor included a direct measure of particulates (many were about light). The key findings included: • The distribution, abundance and composition of seagrass species are impacted by particulate loads and change in light availability. • There is evidence of these impacts in the inshore areas of all Natural Resource Management (NRM) regions at different times, although to a lesser degree in Cape York. • Seagrass meadows are dynamic and will often recover, with recovery rates depending on the extent of decline and the local and regional conditions that follow. Protracted recovery has been observed in several locations. • Research on the mechanisms driving loss have focused on reductions in light caused by suspended particulate matter, and much less is known about the processes influencing recovery Fish studies were predominantly experimental and focused on damselfishes, with only one observational study and five reviews. The key findings included: • 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 visual cues for juvenile fish to settle into habitat, distinguish between live and dead coral, and settlement time. • Herbivorous fish communities are strongly shaped by water clarity, with flow-on effects on algal communities. • Settled sediment directly or indirectly affects herbivorous grazing by altering algal substrate palatability. Most of the evidence on freshwater systems is compiled from Australian and global sources. The limited information available indicates that aquatic organisms are generally able to tolerate short- term exposure to elevated sediments in pulsed runoff. Chronic turbidity has an impact on streams that are naturally clear but is less likely to affect rivers that permanently have low photic depths. This review highlights a critical knowledge gap on the effects of sediment and particulate nutrients on freshwater wetlands and estuarine wetlands such as mangroves, marshes and supratidal forests in the GBR. Recent findings 2016-2022: There were 94 new studies since the previous 2017 SCS and there have been considerable advancements in several key areas: • Fourteen new studies from the GBR addressed dredging-relevant influences of sediments, particulate nutrients and light reduction on corals, particularly on coral recruitment, and six on sponges. • There were nineteen new seagrass studies covering a range of topics including light thresholds, water quality and sediment loads, burial, reviews, data synthesis and modelling. • There were several new studies that used data from maturing long-term monitoring programs and those that applied eReefs or other advanced modelling.

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