Developing Guidelines for the Protection of GBR Seagrass Habitats

Seagrass meadows are vital habitat in tropical coastal ecosystems. However, ongoing monitoring indicates that seagrass meadows are in decline in the Great Barrier Reef where they are exposed to a range of stressors that reduce their capacity to recover after major disturbances.

Tropical Ecosystems Hub researchers used complementary laboratory and field experiments to investigate the cumulative impacts of exposure of seagrass meadows to diminished light, increased nutrients and decreased salinity, conditions that occur when water quality is reduced such as flooding and dredging.

The purpose of the research is the development of guidelines for the Great Barrier Reef Marine Park Authority (GBRMPA) for the protection of seagrass meadows, including guiding improvement to the existing Marine Monitoring Program.

The team from Project 5.3 Vulnerability of seagrass habitats in the GBR to flood plume impacts: light, nutrients and salinity tested seagrass vulnerability to water quality stressors that are typical during wet season runoff. The experiments tested environmental thresholds as well as lethal and sub-lethal indicators of stress. The tests were developed in conjunction with the GBRMPA.

This project team also mapped water quality using remote sensing, which can be used to update existing seagrass risk models. The work has provided robust and scientifically defensible environmental thresholds and has significantly enhanced the ability to detect and interpret responses to changes in water quality through high-resolution indicators testing.

The guidelines that will be developed from this project include four seagrass species and seasonally different temperatures. These will generate thresholds for species, temperature and time of light reduction. This will mean that for a specific time of exposure (e.g. duration of dredging program), the GBRMPA can state the light level required to meet that guideline. As an example, for 50% protection of seagrass density, being exposed to low light conditions for 3 months, 22.6% of surface irradiance or 7.4 mol photons m^-2 d^-1 is required for Z. muelleri at summer temperatures.

Large-scale water quality mapping can help define the type of seagrass communities and identify the main water types, which shape and drive seagrass response. Consequently, long term water quality data, both in-situ and through remote sensing, can provide measures of risk relative to the seagrass community health, including measures of seagrass biomass, cover and species.

This study has significantly advanced the certainty with which seagrass protection guidelines can be applied.