ENVIRONMENTAL
RESULTS
ABOUT THE ZONE
Situated in open coastal waters between Facing Island and Rodds Bay, the Outer Harbour is the largest of the 13 monitoring zones. Just over 50% of this zone lies within the Gladstone Port Limits. The south-western boundary consists of long sandy beaches and salt pans and mangroves around the entrance to Colosseum Inlet.
WATER & SEDIMENT
The Outer Harbour received an overall water quality score of 0.86 (A).
Sediment quality of the Outer Harbour was very good with an overall score of 0.97 (A).
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The following graphs compare the Water and Sediment scores reported for 2025 (top line) to those from 2024.
WATER
SEDIMENT
Very good (0.85-1.00)
Poor (0.25-0.49)
Good (0.65-0.84)
Satisfactory (0.5-0.64)
Very Poor (0.00-0.24)
No data available
Overall, Water quality in Outer Harbour scored 0.86 and received a Very Good (A) grade, similar to the 2024 Report Card (0.92, Very Good, A). This score was calculated by aggregating the three sub-indicator and associated measure scores (Physicochemical – pH and turbidity, Nutrients – total nitrogen, total phosphorus and chlorophyll-a, and dissolved metals – aluminium, copper, lead, manganese, nickel and zinc).
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Outer Harbour received a Very Good (A) grade for pH (1.00) and total phosphorus (0.87), a Good (B) for turbidity (0.67) and chlorophyll-a (0.75), and a Satisfactory (C) result for total nitrogen (0.56). All of the six of the dissolved metals received a Very Good (A) or Good (B) grade, indicating that average concentrations for these measures were within the guideline values for this zone.
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For Sediment quality, Outer Harbour received an overall score of 0.97 and a Very Good grade (A), a similar result to the previous year. This score was calculated by aggregating one sub-indicator and associated measure scores (Metal and metalloids – arsenic, cadmium, copper, lead, mercury, nickel and zinc). The results reflect that all concentrations were below the guideline values for this zone. Arsenic (0.78) received a Good (B) grade, reflecting its natural occurrence within the harbour and may not be associated with anthropogenic inputs.
HABITATS
CORAL - ZONE SCORE
The 2025 Gladstone Harbour Report Card scores for the Outer Harbour zone are based on data collected from two reefs: Seal Rocks North and Seal Rocks South. The overall score for Coral in the Outer Harbour zone declined slightly (2024: 0.12, 2025: 0.04) but retained a Very Poor (E). The Outer Harbour Coral score is exhibiting a negative trend over the past few years (2022: 0.18, 2023: 0.14, 2024: 0.12, 2025: 0.04).
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Coral cover scores at both Outer Harbour reefs remained unchanged from the previous year, with minor variations over the past eight years. Both reefs continue to score Very Poor (E), well below the 40% threshold for a satisfactory grade. Scuba surveys indicate the bio-eroding sponge Cliona orientalis is impacting corals across the harbour, particularly Turbinaria colonies at Seal Rocks South, likely limiting coral recovery.
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Macroalgal cover remained Very Poor (E) for the tenth consecutive year, dominated by Sargassum and Lobophora. Juvenile coral density also stayed Very Poor (E); fast-growing Acropora juveniles, which drove higher cover in 2009, are largely absent, restricting recovery. Hard coral cover returned to a Very Poor (E) grade, with overall low scores demonstrate that coral communities in Gladstone Harbour are still failing to meet expected recovery levels.
MANGROVE RESULTS
The 2025 Gladstone Harbour Report Card incorporates the 2024 Mangroves results. Mangrove condition has historically remained stable due to strong buffering capacity, and since 2019, monitoring has been undertaken every five years. In 2024, the overall grade for Mangroves at Outer Harbour maintained a Good (B) grade.
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The mangrove extent sub-indicator, which measures changes in canopy cover relative to saltmarsh and saltpan areas, retained a Good (B) grade. The canopy condition indicator, based on remote sensing of canopy density and health, improved from Satisfactory (C) to Good (B). Shoreline condition, assessed through aerial surveys of mangroves bordering Gladstone Harbour’s water quality zones, also improved, increasing from a Satisfactory (C) to a Good (B) in 2024.