See Grass No More?

Seagrass is being washed away by the waters of history, but what are the consequences?

There are many secrets still remaining in the Earth’s vast oceans. However, there is one secret that has been known for many years now, and that is the increasingly profound and detrimental impact humans are having on the planet. Marine ecosystems and, in particular, seagrass meadows are experiencing some of the most rapid degradation. Unfortunately, this is a trend that is likely to continue. Only recently has the first global assessment of seagrass abundance been examined in detail, revealing that between 1980 and 2006, seagrass meadows were disappearing at a rate of 110 square-kilometres per year and in many cases, seagrass habitats had completely vanished.

Weedy sea dragons are commonly found in seagrass meadows in Port Phillip Bay. Image: Cathy Cavallo

Weedy sea dragons are commonly found in seagrass meadows in Port Phillip Bay. Image: Cathy Cavallo

But what are seagrasses, what is their role and are they even worth saving?

Seagrasses are not equivalent to the ‘run-of-the-mill’ grass you may find in your backyard that also happens to grow underwater. It can’t just be cut, mowed, burnt, poisoned then cut again, magically re-appearing again after rain. In fact, seagrasses are much more fragile and serve a very different purpose. Seagrasses are part of a unique group of plants called angiosperms (flowering plants) that have adapted to an aquatic lifestyle and can be found in both temperate and tropical waters. Providing a multitude of vital services to the marine ecosystem, seagrasses are a primary habitat for many keystone species, including commercially important shellfish and fish species. They reduce the impact of waves and erosion, as well as improve water quality by filtering sediments. Additionally, seagrass beds provide services worth an estimated US$1.9 trillion in the form of nutrient cycling alone and indirectly support the livelihood of close to one billion people.

Evidently, they serve an incredibly important role in the Earth’s oceans and are very much worth protecting. In spite of this, the majority of the conservation effort concerning marine ecosystems has revolved around coral reefs. As a result, seagrass communities have received little attention and their ecological significance has been under-appreciated.

Despite Australia boasting the most abundant and species-rich seagrass habitats in the world, many of these communities still face heavy anthropogenic pressures in the form of dredging, overfishing, boat anchors, pollution, ocean acidification and global warming (examples shown below). These pressures have been linked to significant losses of seagrass communities in Australia with up to 50, 000 hectares lost since the 1990s. In particular, Western Port Bay has experienced significant losses of up to 18, 000 hectares of seagrass meadows over a 680 square-kilometre area.

So, how did all this seagrass magically disappear in such a small space of time?

Reduced light availability has been identified as a profound factor leading to seagrass loss. It is caused by a number of factors, such as suspended sediment in the water column, algal blooms derived from eutrophication, or even direct burial due to dredging or severe storms. While naturally occurring events rarely cause lasting impacts, human influence often tips the scale and renders the seagrass unable to recover.

While it has been important to identify specific events that cause seagrass mortality, many species have a variety of tolerances and coping mechanisms in regards to light limitations. Some species, such as Cymodocea for example, exist in a state of continual growth in response to direct burial. Other species possess a woody rhizome (stem) that allows them to withstand burial of up to 30cm and up to nearly a year of partial submersion. Furthermore, some species grow vertical rhizomes in order to ‘escape’ burial and reach sunlight.

Knowing the vast variability among seagrass species and their significance in the marine ecosystem, what could be done with all this information to ensure that management practices preserve existing communities in Port Phillip Bay?

To answer this question, a brief step back in time is necessary - three years to be precise. From stage left, a Bachelor of Science graduate enters who has had little success in obtaining a job in the ‘real world’. Without really knowing what they wanted to do, they ended up stumbling quite remarkably into a Master’s project answering the very question about how seagrass meadows in Port Phillip Bay might be affected by dredging (such as the dredging that occurred in 2008) and severe storm events, and how management practices might be able to reduce such events. As you have probably guessed, that student is myself - so I will briefly explain some of the good news from the project.

Firstly, I needed to work out how I could simulate a dredging or burial event under the water. This was a difficult task, but luckily my love of the water (even if that meant snorkeling in the middle of winter) made this task possible.

Snorkeling along the Bellarine Peninsula, Victoria. Image: Stephen McGain

Snorkeling along the Bellarine Peninsula, Victoria. Image: Stephen McGain

In order to simulate a potential dredging event, patches of seagrass were surrounded with plastic chambers (see below). Within the chambers, I buried the seagrass to specified depths to see how different levels of burial might impact survivorship. After analysis, it was revealed unsurprisingly that sustained burial indeed had a significant impact on survivorship - particularly at the higher levels of burial. At lower levels of burial however (<25% mean height), burial seemingly had negligible effects, suggesting that only significant storm and dredging events may impact this species.

Additionally, I was interested in the resilience of seagrass. That is, how they recover from the burial events, if at all. In order to test for this, I designed a similar experiment. After a 6-month period of regular monitoring, recovery was surprisingly evident in a number of the plots, suggesting that even if a major event occurred, recovery may be possible – this is great news for seagrass in Port Phillip Bay. We have also gone on to work out where seagrass in Port Phillip Bay is most threatened and how much burial might be needed to significantly reduce seagrass communities.

While the relationship between the environment and the resilience of seagrass communities is not completely understood, it is hoped that this research will be able to improve management practices with a particular emphasis surrounding intense human activity and climate driven changes.

Seagrass meadows are indeed amazing habitats that are teaming with life and are home to all manner of aquatic beauties. They also play a crucial role as nurseries for many species of locally caught fish, such as the King George Whiting (which may have also ended up on your dinner plate from time to time). As the weather warms, don’t miss out on experiencing as well as gaining an appreciation for a grass that just keeps on giving; they are indeed the aquatic angiosperms in your very own backyard!

 

Banner image courtesy of Parks Victoria. Bay images courtesy of Toorak College, Tim Glasby & John Carroll respectively.