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Getting to the bottom of it - seagrass basics.

An often-overlooked part of the reef ecosystem, seagrass is a marine powerhouse that punches well above its weight. It cleans and oxygenates water flowing off our coast, and it feeds and shelters some of our most iconic marine species.  

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“It’s an exciting success and we are now ready to scale up to full scale meadow restoration. We need to get the area to a size where it can self-regenerate and to do that, we need more funding.

Landward origins 

Often confused with seaweed or algae, seagrasses are actually flowering plants that grow entirely underwater. They started off as land plants and moved into the water about 70 million years ago. Like their land-based relatives, they produce flowers and pollinate underwater, and send out underground stems with new shoots. A 180km-wide meadow was recently discovered off the Western Australian coast that turned out to be a single plant. 

Meadows needed 

Seagrass beds buffer the impacts of climate change by absorbing huge amounts of carbon dioxide from the ocean (which has in turn absorbed it from the atmosphere) and locking it away in the ocean floor.  

Seagrass is the preferred food of green sea turtles and the required food for dugongs. An adult can eat a whopping 40kg of seagrass a day! Recreational and commercial fishing industries also rely on seagrasses as they’re an important nursery habitat for things like prawns and fish.  

Growing conditions 

Like all plants, seagrasses need light to photosynthesise and make energy and oxygen. If the water is murky, which happens when rain and floods wash sediment into the sea, or cyclones stir up mud or bring a lot of cloud cover, the light can’t penetrate. Excessive nutrients from fertiliser also impact seagrasses and can lead to them being overgrown with algae.  

Seagrass can cope with short-term light reductions but if these last for too long or happen too frequently, the seagrass becomes unhealthy and can die. They’re also impacted by sediment plumes after flood events that persist for lengthy periods. 

Health check 

Meadows in the Wet Tropics have been recovering from some catastrophic years. In the lead up to 2011, multiple years of big rain, floods and cyclones culminated in huge losses of coastal seagrasses right across the Great Barrier Reef region. 

The good news is that meadows do bounce back when they’re given the opportunity and we’ve seen some strong recovery in Trinity Inlet, Cairns Harbour and along much of the Wet Tropics coast. Unfortunately, meadows in the Moresby Estuary and Mourilyan Harbour were completely wiped out, and without remnant plants there is no seedbank, which means the seagrass can’t grow back on its own. 

Restorative hope  

Dr Michael Rasheed leads a team that have successfully planted out new seagrass in Mourilyan Harbour. Technically difficult because of jellyfish and croc-filled waters and soft quick-sand like mud, the pilot project has seen the new seagrass grow and survive through two seasons. Surviving the tough conditions associated with wet season rains and floods is testament to the project’s success. 

“It’s an exciting success and we are now ready to scale up to full scale meadow restoration. We need to get the area to a size where it can self-regenerate and to do that, we need more funding,” says Michael.  

Local action 

Like coral, seagrass is resilient and can recover from disturbances so long as there is decent recovery time. Also like coral, strong climate action is at the heart of seagrass preservation because the biggest threats are caused by climate change-related extreme weather events. However, there are important things we can do locally to give seagrass a helping hand, minimising sediment running off the land into the ocean and keeping that water nice and clear. 

A shared love for the coast 

Something that humans and seagrass have in common is a preference for sheltered coastal environments. They’re where seagrass loves to grow, and where we like to build our cities and ports.  

Paul Doyle is the Sustainability Manager at Ports North, where a monitoring program has been mapping and assessing changes in seagrass meadows along the Wet Tropics coast for 28 years. 

“We’ve been able to do a lot of work to understand the light requirements that different seagrasses need to grow,’’ he says.  “Having those parameters means we can keep tabs on real-time data when essential activities like dredging are happening.”  

“This has been a great step forward in ensuring seagrasses and port activities can successfully co-exist and we were proud to be one of the first ports in Australia to manage dredging in real time to keep light at the required level for healthy seagrass growth.”  

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