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 0.92 (A).
Sediment quality of the Outer Harbour was very good with an overall score of 0.98 (A).
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The following graphs compare the Water and Sediment scores reported for 2024 (top line) to those from 2023.
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 the Outer Harbour zone (0.92) scored similarly to the 2023 report card, with overall grade 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 dissolved metals, and dissolved metals – aluminium, copper, lead, manganese, nickel and zinc).
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Within the Physicochemical sub-indicator, Outer Harbour received very good grades (A) for both pH (1.00) and turbidity (0.88), indicating that both measures were well within guideline value ranges.
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Within the Nutrient sub-indicator group, total nitrogen (0.56) declined to a poor grade (D), while chlorophyll-a (0.90) improved to a very good grade (A). Total phosphorus (1.00) also received a very good grade (A), indicating that this measure was well below the guideline value.
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All six Dissolved metals received high scores (1.00) and very good grades (A), indicating that these were well below the water quality guideline values.
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For Sediment quality, Outer Harbour received an overall score of 0.98 and a very good grade (A), a similar result to the previous year. This score was calculated by aggregating measure scores (Metal and metalloids – arsenic, cadmium, copper, lead, mercury, nickel and zinc).
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In 2024, six of the seven Metal and metalloid sub-indicator measures received very high scores (1.00) and very good grades (A), and one sub-indicator – arsenic – received a high score (0.84) and a good grade (B), indicating that these measures were below the guideline values for this zone.
HABITATS
CORAL - ZONE SCORE
CORAL - HARBOUR SCORE
The 2024 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 was
0.12 and graded very poor (E), indicating a very poor condition. The Outer Harbour Coral score has shown a negative trend over the past few years (2022: 0.18, 2023: 0.14, 2024: 0.12).
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Compared to the previous year, coral cover scores remained unchanged at both Outer Harbour reefs. Minor score variations have been observed over the past seven years; however, these scores are still substantially lower than the 40% threshold required to receive a satisfactory grade (C) with both harbour zones graded very poor (E) for coral cover.
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Scuba surveys indicated that the bio-eroding sponge Cliona orientalis continues to impact the coral community across the whole harbour and in particular, colonies of Turbinaria at Seal Rocks South. Sponge prevalence is likely contributing to a lack of coral cover recovery across the harbour.
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Both reefs in the Outer Harbour zone scored a very poor grade (E) for Macroalgal cover for the ninth consecutive year. Community composition at the two Outer Harbour reefs was stable, with communities consistently dominated by the two brown macroalgae genera, Sargassum and Lobophora.
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Both reefs remained with very poor grades (E) for juvenile density. High coral cover in 2009 was linked to abundant representation of fast-growing, branching corals Acropora juveniles; however, in 2024, this juvenile genus was notably lacking across the harbour. Recovery of corals may be limited until this family appears and survives.
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Changes in hard coral cover has been graded poor (D) in the Outer Harbour reefs for the past six years (2018-2023). In 2024, the change in hard coral cover at Seal Rocks North increased in score (0.50) and improved in grade to a satisfactory (C). However, in general, the ongoing low scores and very poor grades demonstrate negligible recovery of coral communities in Gladstone Harbour to date.
MANGROVE RESULTS
Outer Harbour has two Mangrove monitoring zones – one at Wild Cattle Creek, and another at Split End. Overall, Mangroves in the Outer Harbour zone maintained a good grade (B) in 2024. In 2024, mangrove extent remained a good grade (B) in the Outer Harbour zone. The grade for canopy condition in Outer Harbour improved from a satisfactory (C) in 2019 a good (B) in 2024. Shoreline condition of mangroves in the Outer Harbour zones improved in grade from a satisfactory (C) in 2019 to a good (B) in 2024.