8. Resilience

The Region is home to one of the world’s largest seagrass resources.²⁰⁵⁷ Seagrasses in the Region, particularly those in coastal habitats, act as a buffer between catchment inputs and reef communities.²⁰⁵⁸ Seagrass meadows are crucial feeding grounds for populations of large herbivores including dugongs and green sea turtles.⁵¹⁶ Seagrass meadows in the Region are dynamic ecosystems, and significant changes in distribution and abundance due to natural cycles are typical in some regions.²⁰⁵⁸ These cycles are often marked with decline in abundance and diversity followed by recovery. However, coastal development and the associated increase in land-based runoff are increasing the pressures on inshore seagrass meadows and affecting the frequency of the natural cycles of seagrass loss and recovery.¹⁴⁵ The significant proportion of seagrass meadows found in coastal areas are regularly exposed to threats associated with declines in water quality and available light. 

Low light levels associated with reduced water quality can limit the growth and recovery of coastal seagrasses, and they have likely contributed to the complete disappearance of seagrass at some sites in the Region in 2011.⁶⁹⁰ For seagrass meadows to be able to respond to acute and chronic disturbances, seagrasses must possess the necessary combination of characteristics that allows them to respond to changes. If they cannot respond quickly enough, these systems may be permanently lost.

Resilient seagrass meadows have the capacity to resist and recover from repeated, cumulative disturbances. Resilience in seagrass meadows is affected by:

• environmental features, such as good water quality and stable climate ²⁰⁵⁹ (Figure 8.1: the system’s broader environment and social space)
• ecological features, such as connectivity between ecosystems and trophic interactions 2060 and grazing pressures from large herbivores such as dugongs ¹⁹⁰³ (Figure 8.1: the system’s ecological processes)
• intrinsic characteristics of seagrass meadows, such as life history traits, species diversity and presence of seed banks ^{2058,2059,2061} (Figure 8.1: the system’s processes, with links to community and biodiversity structure).

Monitoring the extent, frequency and intensity of acute and chronic impacts, as well as the condition and trend of seagrass meadows, is essential to inform resilience-based management responses that assist seagrass meadows to remain productive.

Management

Seagrass meadows are important foraging grounds for large herbivores of cultural significance and nursery grounds for species of fisheries interest. Therefore, these ecosystems are managed and protected by proxy under various pieces of Commonwealth and Queensland legislation. The Fisheries Act 1994 (Qld) (Fisheries Act) protects all marine plants, including seagrasses. The Planning Act 2016 (Qld) manages the authorisation of ecologically sustainable development that involves the removal of or damage to marine plants. Other planning and assessment Acts and Regulations are used to protect seagrass meadows from potential development impacts, such as through powers to refuse or manage particular activities, to declare protected areas, or to place conditions on discharges or runoff.²⁰⁶² These tools include the Environment Protection and Biodiversity Conservation Act 1999 (Cth) (EPBC Act), the Environment Protection (Sea Dumping) Act 1981 (Cth), the Marine Park Act and the Planning Act 2016 (Qld). 

The primary focus of management actions to protect seagrass is to improve water quality entering the Region by implementing the Reef 2050 Water Quality Improvement Plan.¹⁸⁸³ Progress towards the plan targets is assessed through the Reef Water Quality Report Card. Understanding how the health and resilience of the Reef are affected by pressures such as declines in water quality is critical for evaluating the effectiveness of management actions. Information on the condition (and trend in condition) of inshore seagrass meadows is provided by the Great Barrier Reef Marine Monitoring Program, which also informs other initiatives, including regional report cards for Natural Resource Management regions. Seagrass rehabilitation methods are being explored, but outcomes are not yet available. 

Recently, a seagrass resilience metric was developed specifically to assess and monitor how resilience of inshore seagrass meadows changes in response to impacts. The metric was developed to provide a measurable and practical indicator informed by “a set of measurable biological characteristics that exemplify seagrass meadows’ resistance to pressures and essential mechanisms for recovery” as stated in the seagrass monitoring review by the Reef 2050 Integrated Monitoring and Reporting Program.²⁰⁶³ The resilience metric takes into consideration intrinsic characteristics of seagrass species shown to support the resilience of the meadow. The metric was built upon the long-term historical data on reproductive status and size of seed banks (capacity for recovery), and species functional traits that determine the meadow’s capacity to resist disturbance.¹⁵⁰ It uses a decision tree to assign site scores for resilience across three main categories: i) low resistance sites, ii) non-reproductive but high resistance sites, and iii) reproductive and high resistance sites.¹⁵⁰

Evidence for recovery or decline

In the Region, inshore seagrass meadows have been monitored regularly since 2005.¹⁴⁵ When monitoring started, seagrass meadows in most parts of the inshore Region were in a good state of health with abundant foundational species and seed banks. Resilience was high in most regions, and meadows were considered to be in good to very good condition. However, inshore seagrass meadows have been subjected to persistent chronic levels of stress, mainly due to increases in land-based runoff. As a result, seagrass meadows in some regions have been left in a weakened state with limited resilience and unable to recover from episodic disturbance, such as thermal stress (Figure 8.4). 

In late 2008, meadows in the northern Wet Tropics and Burdekin regions remained in a good state of health with abundant seagrasses and seed banks despite being affected by category 4 cyclone Larry in 2006.²⁰⁶⁴ In contrast, locations in the southern Mackay Whitsunday and Burnett Mary regions were in poor and moderate condition, respectively, having low abundance, reduced reproductive effort and small or absent seed banks.²⁰⁶⁵ 

The onset of La Niña in 2009 caused a steady decline in seagrass condition in the Burdekin and Wet Tropics regions, and in locations within the Fitzroy region.²⁰⁶⁶ This decline was likely driven by the above-average discharge from the catchments into the Reef.²⁰⁶⁷ The level of decline became substantial across all habitats and meadows, which were in a vulnerable condition leading up to 2011 when the Reef was severely affected by category 5 cyclone Yasi. Post cyclone Yasi, the abundances of inshore seagrass were the lowest recorded since monitoring started. In 2011, most monitoring sites remained classified as poor or very poor in abundance, and severe losses were reported from the Wet Tropics, Burdekin, Mackay Whitsunday and Burnett Mary regions.²⁰⁶⁶ 

By 2017, little or no recovery had occurred across most of the regions, and seagrass condition began to gradually decline.²⁰⁶⁵ Cumulative pressures, including severe climatic events, such as heat waves, continued to undermine the resilience of inshore seagrass meadows. Frequent and repeated disturbances seemed to be maintaining lower seagrass abundance at some locations, which in turn reduced the capacity of the seagrasses to expand and produce viable seed banks.²⁰⁶⁸ Since 2019, recovery was sustained across northern regions for a couple of years despite being affected by heatwaves in 2020 and 2022.¹⁴⁹ However, meadows in the southern regions of Fitzroy and Burnett Mary remain in poor condition and continue to decline, especially after the heavy floods in Burnett Mary catchments in early 2022.¹⁴⁹

Seagrass meadows in the Burdekin region have demonstrated exceptional resilience to cumulative disturbances. Inshore seagrass meadows in the Burdekin region are primarily structured by wind-induced turbidity (resuspension) in the short term and by episodic riverine delivery of nutrients and sediment in the medium term. Disturbance from wave action, sediment movement and elevated sea surface temperatures are also dominant influences. Rainfall in the Burdekin region is often lower than in other Reef regions.³⁶³ 

Like in the rest of the Region, seagrass meadows in the Burdekin region had high proportion of foundation species, high reproductive effort, and abundant seed banks at the beginning of monitoring in 2005 (Figure 8.5). Declines in condition started in 2008 due to large flood events and continued following cyclone Yasi, hitting their lowest point in the 2010–11 monitoring year.²⁰⁶⁶ From 2011, the seagrass rapidly recovered, and by 2013–14 seagrass abundance had nearly recovered to pre-2011 levels. Resilience was also high with high levels of foundational species and seed banks.²⁰⁶⁹However, recovery has varied between habitats, and seagrass abundance has progressively declined at intertidal and subtidal reef habitats since 2015. Nevertheless, overall seagrass condition remained good for four consecutive years despite two back-to-back marine heatwaves that affected the Region in 2016 and 2017. In 2017–18, coastal seagrasses increased to their highest abundance since 2001; however, this was immediately followed by large declines in 2018–19.²⁰⁶⁸

In 2018–19, the Burdekin region was affected by major acute events, such the high discharge event from the Burdekin River coupled with unusually large outputs from the smaller creeks and rivers discharging into Cleveland Bay in early 2019.²⁰⁶⁸ This led to declines in seagrass abundances across the region, the largest losses occurring in reef subtidal and coastal intertidal habitats. Sediment loads in the discharge, and wind-driven resuspension, elevated turbidity and reduced benthic light during the wet season, were a legacy from the Burdekin River floods. However, light availability quickly returned to seasonally expected levels in the dry season, during which rainfall and discharge were below average, allowing recovery at some sites.²⁰⁶⁸

In 2020–21, seagrass abundance recovered, and resilience increased slightly, but varied between sites. In coastal intertidal seagrass habitats, reproductive effort and seed banks increased to their highest level in 2 years ²⁰⁷⁰ but remained very low in reef intertidal and subtidal habitats. In all habitats, seed density was higher in the late wet season, indicating a possible late flowering and seed set. After a slightly cooler 2020–21 season, intertidal within-canopy temperatures increased in 2021–22 and were, on average, the warmest since monitoring was established.¹⁴⁵

For almost 2 decades, seagrass meadows of the Burdekin region have demonstrated high resilience, particularly through their capacity for recovery (Figure 8.5). The presence of a robust seed bank and high species diversity, which are characteristics known to confer resilience, may indicate a conditioning to acute disturbances and reflect the nature of the disturbances, which are episodic and dominated by wind events and Burdekin River flows.