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At-risk seagrass records 6,000 years of climate and land-use change

seagrass920

Evidence from 6,000 years of climate change and farming developments can be seen in a threatened type of ancient Mediterranean seagrass, and help us understand how damaged coastal regions could be restored, research from Brunel University London has found.

Posidonia oceanica – otherwise known as Neptune grass or Mediterranean tapeweed – grows extremely slowly over millennia, maintains a diverse ecosystem, and is a vital absorbent of carbon dioxide from the atmosphere.

While its marine habitat is protected by the EU, seagrass meadows are now one of the world’s most threatened ecosystems. Loss has averaged 5% annually - at this current rate Neptune grass meadows would disappear in 20 years.

Researchers led by Dr Lourdes López-Merino at Brunel analysed the pollen, spores and microscopic plankton organisms contained in samples taken from a five-metre core of a 6,000-year-old seagrass mat located in the Portlligat bay on the Catalonian coast.

Samples were also tested at Brunel for their response to an applied magnetic field – their magnetic susceptibility – and for the quantity of charcoal fragments over time. At the University of Queensland, glomalin-related soil protein (GRSP) was extracted and measured from a previously studied seagrass mat located in the same bay.

Writing in the Journal of Ecology, the researchers explain that their results show how the seagrass mat sediments show an increase in fires in the western Mediterranean basin, particularly during Roman and Medieval times as human-induced fires helped open up land and expand areas of cultivation.

As land use changed, it had a huge impact on the continental landscape, and this can be seen in the results of pollen and microcharcoal analysis. Drops in GRSP content in the seagrass and the rise in land use change indicators corresponded with periods where crop production increased, especially since the Roman and Medieval periods. 

Magnetic susceptibility in a sample depends on its mineral make-up, and increases in susceptibility tend to be related to higher mineral content. The researchers found that increases in the mineral content in the seagrass mat corresponded to historical periods of more intense farming that triggered soil erosion, but also to periods of flooding and the rise and then stabilisation of sea levels during the Holocene.

This sequence of events warn what may happen in the near future if several impacts combine together and initiate ecological shifts in seagrass-dominated ecosystems.

The researchers explain that seagrass meadow loss has been significantly accelerated by modern chemical and mechanical factors, including trawl fishing and coastal development.

Dr López-Merino explains: “Many human pressures have been linked to recent decline in Posidonia meadows in the Mediterranean coastal zone and we can see that most of these anthropogenic impacts have occurred after the Industrial Revolution.

“Considering the seagrass ecosystem is an important marine carbon sink located in coastal areas, we wanted to find out if long-term agricultural or environmental changes, or a combination of the two, have an effect on the seagrass ecosystem.”

The authors found that thousands of years of on-land agricultural practices and climate change have also played a part in inducing seagrass meadow disturbance by impacting meadow health and how the seagrass accumulates carbon.

Dr López-Merino points out: “Adding a palaeoecological perspective to reconstruct environmental impacts on seagrass could inform the restoration and management of these very sensitive meadows now and in the future.”

An online preview of ‘A six thousand-year record of climate and land-use change from Mediterranean seagrass mats’ is available on the Journal of Ecology website.

(Image - Posidonia by Enric Ballesteros)