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By Xanthe Rivett and Josh Coates, Marine Programs Officer

The importance of seagrasses as critical elements of coastal ecosystems is well recognised.

Seagrass/algae beds have been rated the third most valuable ecosystem globally (on a per hectare basis) for ecosystem services. They are highly responsive to water quality and are considered as excellent ‘canary in the coal mine’ early indicators of ecosystem health.

Seagrasses in Cairns are at their lowest distribution levels ever recorded, following three years of decline. According to Dr Rasheed, Principal Research Scientist in the Seagrass Habitats research group at TropWATER, “seagrass coverage around Cairns peaked between 2007 and 2009, and has declined each year since then.” The report links the decline to multiple above “average wet seasons and severe storm events” and states that the remaining patches of seagrasses are highly vulnerable as they are small remnant patches.  Similar declines have also been experienced around Mourilyan Harbour, Bowen and Townsville.

Seagrass meadows provide important ecosystem functions including critical nursery habitats for fish species and feeding habitat for dugong, turtles and wading birds.  They also help aid sediment trapping, nutrient cycling and carbon sequestration.  The meadows within Trinity Inlet are mostly within the Trinity Inlet Fish Habitat Area (encompassing 1200 ha of tidal waters with seagrass, mangrove and salt marsh habitats) but are still vulnerable to impacts including scouring, declining water quality and potential changes associated with port development and maintenance.

Implications of Trinity Inlet dredging

As we look at the implications of the proposed Cairns Shipping Development Project (CSDP) the release of the Cairns Seagrass Habitat Baseline Survey – 2012* provides us with some timely reminders of the possibleimpacts on one of our most sensitive and important local ecosystems.

The report concludes that “Results of the latest survey show that seagrass meadows in Cairns Harbour and Trinity Inlet were in a vulnerable state with the remnant patches of seagrass having a reduced resilience to further stressors. Future port activities and development should consider the state of seagrass resilience, and the vulnerability of these seagrasses as part of ongoing management strategies.”

And goes on to state that “…the scale and duration of the proposed CSDP have a much greater potential to impact seagrasses especially when combined with the current reduced state of resilience for most Cairns meadows.”

The proposed port expansion represents a large increase in dredging activity including a massive dredging campaign to increase the depth and width of the existing channel and a large increase in the annual maintenance dredging program required. Sediment stirred up and dumped at sea can release built up toxins and increase the turbidity (muddiness) of the water, cutting light to the seagrass and smothering seagrass beds.

Jennie Gilbert, Founder and Director of the Cairns Turtle Rehabilitation Centre, reports that the turtle rehabilitation centre is still getting turtles in suffering from starvation. “We expected to see less turtles suffering starvation with no extreme environmental conditions this year, however there are still a large number of starving and sick turtles coming into the centre”.

Seagrass before

Seagrass after

What needs to happen for the seagrass to recover

The capacity for seagrasses to recover is little understood but the JCU team believe it could be slow for many meadows.  Enhancements have been made to the seagrass monitoring program in the aim of better understanding some of the unknowns both in preparation for the Cairns Shipping Development Project and to inform actions that may be needed to protect the remaining seagrasses and facilitate recovery.

Research is now focusing on quarterly assessments and investigations of the seed bank to test seed viability and germination triggers.  Investigations of seed banks along the northern beaches and Mission Bay are also recommended by the report. There are no guarantees that lost seagrass beds can be artificially restored, or that we could afford the cost of attempting such action. Prevention is better than cure.

The question must be asked – can our already struggling seagrass beds cope with increased pressure? We know the resilience and ability to recover of this system is at an all-time low.

Consultation on the port expansion continues,and the release of the more detailed environmental impact statementis due soon. It is time to have a closer look at the costs to our fisheries and general ecosystem health and weigh them up against the uncertain economic benefits that are being promised by dredging proponents.

* Rasheed MA, McKenna SA, & Tol S. 2013. Seagrass habitat of Cairns Harbour and Trinity Inlet: Annual monitoring and updated baseline survey. JCU Publication, Centre for Tropical Water & Aquatic Ecosystem Research Publication 13/17, Cairns, 51 pp.