Seagrass and sea level rise

A little clarity may save an endangered blue-carbon ecosystem

The amount of carbon dioxide sequestered by a hectare of seagrass is comparable to that absorbed by a hectare of Amazon rainforest. Unfortunately, seagrasses are one of our most threatened ecosystems. (Photo by Megan Saunders)

The amount of carbon dioxide sequestered by a hectare of seagrass is comparable to that absorbed by a hectare of Amazon rainforest. Unfortunately, seagrasses are one of our most threatened ecosystems. (Photo by Megan Saunders)

Like many important ecosystems, seagrasses will be hit hard by climate change. Some of those impacts relate to rising sea level. New research by a multi-disciplinary team at the University of Queensland has found that local action that improves water quality, and specifically its clarity, might go some way to compensating for rising sea level.

Seagrasses are marine plants that live in shallow coastal seas. They provide us with a range of valuable ecological services including the provision of habitat for fish and invertebrates, and food for turtles and dugongs. Seagrasses also filter the water as it runs off the land, thereby preventing sediments from smothering coral reefs, and they suck an astonishing amount of carbon dioxide out of the atmosphere. In fact, a hectare of seagrass sequesters a comparable amount of carbon dioxide as an equivalent area of Amazon rainforest. Protecting these ‘blue carbon’ ecosystems is important for our wellbeing on a number of scores.

Unfortunately, seagrasses are one of the most threatened ecosystems on Earth. Over the past several decades they have incurred staggering rates of loss. These losses have largely been due to local factors such as run off from land reducing water quality. But now seagrasses have the added stress of climate change, a ‘super wicked problem’ with the potential to affect seagrass at a global scale.

Local problems we have some capacity to deal with but climate change is another thing altogether. The big question is: can our management of local drivers of degradation offset the losses we might expect from climate change? To answer this question I worked with a multi-disciplinary team that included marine ecologists, engineers, remote sensors and modellers. We set out to discover how sea level rise in particular would affect seagrass in Moreton Bay, Southeast Queensland.

Watching the grass grow: A multi-disciplinary investigation on the seagrasses of Moreton Bay has found that if water clarity could be improved, seagrasses will cope better with rising sea level. (Photo by Megan Saunders)

Watching the grass grow: A multi-disciplinary investigation on the seagrasses of Moreton Bay has found that if water clarity could be improved, seagrasses will cope better with rising sea level. (Photo by Megan Saunders)

Moreton Bay is an internationally recognized Ramsar wetland site. It encompasses vast seagrass meadows, and supports a large number of vulnerable and threatened species, including green turtles and dugongs. It also lies right next to Brisbane, Queensland’s capital city and the fastest growing ‘mature’ metroplis in the world. Urban and agricultural pressures have historically caused the loss of seagrass in Moreton Bay; how the bay might fare with the added stresses of sea level rise is an important question for conservation and the region’s productivity.

Our study, published recently in the international journal Global Change Biology, used a species distribution model to predict the areas where habitat is suitable for seagrass now and where it would be suitable for seagrass in the future with a given amount of sea level rise.

We found that a sea level rise of 1.1 m by 2100 would result in a 17% reduction in area of seagrass in Moreton Bay. This was due to losses of seagrass at the deep edge of its range. As the water gets deeper, light decreases to the point where seagrass won’t grow. If water quality were to deteriorate further, for example from increased sediment in water flowing into the bay, the decline would be even more pronounced.

But it’s not all gloom. Local management could help stop, or even reverse, the expected decline in water quality. In this study, we found an improvement in water clarity of 30% could mitigate the losses of seagrass from 1.1 m sea level rise. The improvement would have to be greater for larger magnitudes of sea level rise. As a general rule of thumb, for each metre of sea level rise, water clarity, measured by secchi depth, would have to be increased by a similar amount to offset declines.

Water quality can be improved by upgrades to sewage treatment plants and by reducing the use of fertilizers. Reducing the pollutants and sediments that run-off into streams and beaches likewise benefits seagrass. For example, planting vegetation in riparian zones and maintaining vegetated buffer zones around waterways can benefit seagrasses living in the nearby sea.

“We found an improvement in water clarity of 30% could mitigate the losses of seagrass from 1.1 m sea level rise.”

Seagrass will also benefit from management policies that allow for the migration of marine plants into newly inundated regions. In a scenario where roads, houses and other hard infrastructure were removed from inundated coastal areas, we found the decline in seagrass was reduced to only 5%. In an era of rising seas we will need to design green belts and buffer zones which allow room for the migration of coastal ecosystems. This will minimize the coastal squeeze that occurs when ecosystems are blocked by hard infrastructure such as sea walls and levies.

All of this emphasises that there are financial, logistical, and social considerations associated with coastal retreat. In many instances individuals and councils will prefer to defend infrastructure rather than promote the migration of wetlands. These decisions could, however, have unforseen negative consequences such as loss of fisheries species which occupy coastal habitats. Identifying the trade-offs between different coastal adaptation policies should be a key priority for coastal management in the face of climate change.

More info: Megan Saunders m.saunders1@uq.edu.au

Dr Megan Saunders collaborates with members of CEED through her affiliation with the Australian Sea Level Rise Partnership (ASLRP). This group aims to find interdisciplinary solutions to the challenges of sea level rise in the 21st century. ASLRP is funded by an Australian SuperScience Fellowship and is based in the Global Change Institute at the University of Queensland.

Reference

Saunders MI, Leon J, Phinn SR, Callaghan DP, O’Brien KR, Roelfsema CM, Lovelock CE, Lyons MB, Mumby PJ (2013). Coastal retreat and improved water quality mitigate losses of seagrass from sea level rise. Global Change Biology http://onlinelibrary.wiley.com/doi/10.1111/gcb.12218/full

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