Seagrasses in the Great Barrier Reef can bounce back with good stewardship of our coastal environments

Seagrass meadows throughout the tropics are regularly exposed to destructive forces, such as cyclones, floods, poor water quality and coastal development. Nevertheless, there is evidence that they bounce back after major disturbances, typically within a couple of years.

What facilitates the rapid recovery of seagrass meadows? Our new research, published in Global Change Biology, shows that seagrass propagules are readily transported by water currents, supporting seagrass recovery in the central Great Barrier Reef.

Seagrasses of the Great Barrier Reef

The Great Barrier Reef is home to one of the world’s largest seagrass habitats, encompassing 15 species over some 40,000 km². Seagrasses are vital as food for green sea turtles and dugongs and are important nursery grounds for commercial, recreational and indigenous fisheries. They store carbon, stabilize bottom sediments and adsorb nutrients from coastal waters, helping to maintain good water quality that benefits other species, including corals.

Seagrasses are very unique. They grow in large, grassland-like meadows and are the only flowering plants that live underwater. Just like their land-based ancestors, seagrasses use flowers, fruits and seeds to reproduce, disperse and recruit.

A combination of cyclones, floods, poor water quality, dredging and coastal development have effected seagrasses along much of the Great Barrier Reef’s urban coast, from about Cooktown south.

An important mode of seagrass recovery is the replenishment of meadows by fragments, fruits and seeds (or propagules) from other locations.  All things being equal – the more connected you are to other seagrass meadows, the quicker you are to recover.

Predicting seagrass dispersal with oceanographic models

Measuring the dispersal of seagrass propagules is logistically difficult and expensive in such a large area as the Great Barrier Reef. An alternative to field measurements is predicting where propagules move using oceanographic models that describe the motion of waters.

With our colleagues at the Université catholique de Louvain in Belgium, we used an oceanographic model named SLIM to predict the movement of seagrass propagules in the central Great Barrier Reef (Hinchinbrook Island – The Whitsundays).

Our results were surprising. ‘Virtual’ seagrass propagules moved on average between 30 and 60 km, but distances of over 900 km also occurred. For example, we recorded ‘virtual’ propagules from Abbot Point moving to the remote waters off the Cape York Peninsula.

Seagrasses are highly connected, but some meadows are more important then others

We also found that seagrass meadows in the central Great Barrier Reef are highly connected to each other. However, three locations were particularly important sources of seagrass propagules in the central Great Barrier Reef: Abbot Point, Cleveland Bay and the deeper water meadows.

Cleveland Bay is adjacent to the large port and regional city of Townsville. Abbot Point is also an area that could be subject to human impacts if mining proposals in the Galilee Basin become reality.

Deep water meadows are highly connected to coastal meadows and may function as both a refuge and stepping stone from the south to the north, but only for those few species that inhabit both deep and coastal waters.

We found that the scale of disturbance required to disconnect and isolate all the seagrass meadows in the central Great Barrier Reef was enormous, > 245 kms. To put that in context, the very destructive winds of the 2011 Severe Tropical Cyclone Yasi covered ~150 km of coastline.

A few locations in central Queensland, such as Upstart Bay, are not well connected and may not recover after damage as well as other meadows in the region. However, these “disconnected” regions could still be replenished by an unusual source: the dugongs and turtles that feed on seagrass.

Dugongs and green sea turtles have the capacity to move viable seagrass seeds vast distances via their faeces. These animals almost certainly know where to look for seagrass meadows, further supporting seagrass replenishment and recovery in the region.

Implications for seagrass management in the Great Barrier Reef

There is always pressure to intervene and help seagrass recovery by replanting seagrass after major impacts. Our research suggests that would be money wasted except in very specific areas. Seagrasses take about 2-4 years to recover after major disturbance, about as long as it takes for the environment to stabilize and become a place where seagrass could grow. Simply waiting for propagules to arrive from distant meadows is all it takes for seagrasses to recover naturally.

The most important management action is improving environmental conditions that support natural seagrass recruitment and recovery. This necessitates mitigating the effects of poor water quality from agricultural runoff and dredging, and ensuring that our coastal developments are effectively managed.

Authors: Alana Grech (Coral CoE), Rob Coles and Samantha Tol (TropWater)

Acknowledgments: Funding for this research was provided by the Australian Research Council Centre of Excellence for Coral Reef Studies, James Cook University, Macquarie University, the Ian Potter Foundation, Sea World Research and Rescue Foundation Inc (SWR/6/15) and Wallonie‐Bruxelles International.