Sharing 40 years’ experience of working on mangroves – the generous forests of the tidal zone

Sharing 40 years’ experience of working on mangroves – the generous forests of the tidal zone

Discovering mangroves

My enthusiasm for mangrove ecology started in a long house in the Gulf Province in Papua New Guinea. This huge structure was made entirely of mangrove timbers and thatched with fronds of a mangrove palm. Here I listened as my colleague in the Office of Forests negotiated with the traditional land owners for permission to conduct fieldwork. Young men from the village were detailed to observe and assist our team that was to weigh a huge tree within the forest. That sweaty teamwork was my introduction to the idea of biomass – the weight of living matter within a forest. I wondered what the young men would tell the village elders about our activities, but was hooked on mangrove forests after that!

Mangroves are trees that live in tropical tidal waters, where the salt and daily submergence prevents establishment of almost all other trees. Confusingly, but understandably, people also refer to forests of these trees as mangroves. In the last quarter of last century, a large portion of mangrove forest cover was lost due to conversion of these areas for aquaculture of prawns and fish. Encouragingly, in the current century, the value of mangrove forests has come to be more widely appreciated and mangrove loss has slowed with some areas of forest being re-established.

Celebrating the importance of mangroves

Mangrove forests are a vital part of the carbon cycle that buffers us from climate change.  They draw down carbon dioxide from the atmosphere and store the carbon in the leaves, branches and trunks of the trees, but as leaves and woody parts of the trees are shed leaves, carbon is transferred to in the sediment in which they grow and into coastal waters. Remarkably these forests can contain as much carbon in the trees as in rainforests do, but they store much more carbon than rainforest do locked up in the soil in which they grow. Plant waste travelling out on the tides supplies food to coastal waters. The forests also act as nurseries for fish and prawns that are caught in waters offshore. Juvenile fish feed and develop among the protection of roots and move into offshore when able to fend for themselves. Some, such as groupers, move to coral reefs. Mangrove roots stabilise shore sediments and also break up inrushing waves.

The underwater life associated with mangroves – Bonaire Credit : Ewan Trégarot

Mangroves and the work of MaCoBioS

Mangroves range from the northern end of the Red Sea to the North Island of New Zealand and flourish on calm subtropical and tropical shorelines in between. Surf shores are not suitable for mangroves. Though mainly located in the waters of continental Europe, the MaCoBioS project extends into the Caribbean with particular case study sites in Bonaire, Martinique and Barbados, where mangroves play an important role in protecting and feeding juvenile reef fish. In the Caribbean, mangrove forests survive cyclones while protecting the shorelines. Scars of hurricane track are visible in these forests many years after the event. Though much smaller in height and area covered than the huge forests of the Gulf of Papua, these Caribbean forests serve the island communities in numerous ways. They are particularly closely linked with the health of nearby coral reefs, act as key stepping stones for migrating birds, are recreational areas and also destinations for ecotourists. The challenge is to ensure the future supply of these ecosystem services, by taking account of the needs of this generous ecosystem in coastal zone planning.

Strong connection with associated ecosystems such as seagrass beds and coral reefs – Bonaire Credit : Ewan Trégarot

One of the pleasures of working with mangrove ecosystems is that those who do are natural collaborators who are committed to the cause of protecting these ecosystems. I hand over here to Ewan Trégarot to talk about the mangrove component of the MaCoBioS project.

Our experts are studying what are the effects of climate changes and anthropogenic stressors on mangroves and how those multiple pressures interact with each other’s. How can we use remote sensing to monitor the ecological condition of mangroves and the ecosystem services provided? What would happen to mangroves in the Caribbean in 2050 or 2100 given the current climate change predictions? Many questions remained to be answered, and hopefully, interesting elements of response will come up soon. Accordingly, remedial work will be recommended to foster the return of mangroves through replanting, restoring tidal circulation and minimising undesirable threats from urbanisation. There need be no losers if remedies are well planned.  

The generous forest of the tidal zone – Martinique Credit Ewan Trégarot

Text by Prof. Simon Cragg & Ewan Tregarot

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Why are kelp forest important ?

Why are kelp forest important ?

Kelp forests and their importance for nature and society

Kelp forests are often regarded as “underwater rainforests”. Formed by the dense growth of several kelp species, they produce a three-dimensional habitat and a highly productive system. Usually found in water temperatures below 20 °C, kelps are large brown algae that attach to the seafloor (‘benthic’). Not only can kelps grow amazingly fast in the right conditions – up to 30 cm per day – they can also reach 45 m long for the giant kelp. As well as providing plenty of surfaces and nooks and crannies for other species to settle on or in and live, they shelter coastlines from storms and help sequester or absorb carbon from the atmosphere, making them incredibly important societal resources.

Laminaria hyperborea kelp, Norway

Kelps as important biodiversity and feeding grounds

The forests created by kelps provide a home for a huge variety of different species, from other benthic algae to invertebrates, fish and marine mammals. Investigations along the Norwegian coast have shown a maximum of almost 100 000 small invertebrates connected to one single stipe (main stem) of the species Laminaria hyperborea. With on average 10 plants per m2, this makes up a very high diversity and abundance of animals which form the base of food webs up to fish and mammals. Many of these fish are then caught for food by humans.

The three-dimensional habitat of kelp forests also provides shelter for many species and make great places to hide from possible enemies. The holdfasts of kelps anchor them to the seafloor, and their branched root-like structure means several different species use these as habitat. For example, the edible crab (Cancer pagurus) usually lives inside this holdfast when it is young, protected from predators. You will often see a large holdfast housing many species of worms, brittle stars, molluscs, and crustaceans. The stipe and the fronds (leaf-like structures) of kelps provide additional types of habitats to different species. Usually overgrown by epiphytes – algae and animals that grow on a plant – up to 50 or 60 different species of algae, consisting of mostly red algae, can be found on any one stipe. These epiphytes provide an additional dimension to the kelp forest and, in turn, support many other animals with shelter, food and raw materials. For instance, many smaller crustaceans, such as the shrimp-like amphipods, use these algae as a substrate to build the small tubes they live in.

Brittle stars in a kelp holdfast, Norway

Kelps also provide valuable spawning and nursery grounds for numerous species of fish and shellfish, which go on as adults to become the foundation for many commercial and recreational fisheries, such as the Atlantic cod, Gadus morhua. These smaller fish then attract larger predators like seals, sharks, and sea birds who hunt around the kelp canopies.

Threats and changes to kelp forests

Globally, kelp forests are increasingly threatened by a variety of human impacts, including climate change and fishing/hunting, harvesting, eutrophication.

Being a cold-water species, kelp forests are sensitive to elevated temperatures. As ocean temperatures increase as a result of greenhouse gas emissions, massive kelp forest die-offs are increasingly likely, with their return questionable. In some places, such as in Australia and Tasmania, we have already seen that kelps have not returned to areas they were once abundant.

Fishing through kelp forests using destructive methods like bottom trawling has also been implicated in dramatic declines of kelps, such as in the UK, while predator removal from fishing/hunting has likely changed ecosystem structure in many kelp forests. Few large animals graze on fresh kelps except for sea urchins; however, these animals can devastate a kelp forest, grazing until only denuded rocks, or barren grounds, are left. When urchins are removed, vegetation often rapidly returns, although the animals take longer. The reason for this overgrazing is still under debate but, in most cases, it is probably caused by predator removal leading to an increase in urchin populations. The most well-known example of this comes from the west coast of Canada and the United States, where sea otters were extensively hunted. As their population declined, urchin populations increased and grazed down the kelp forest. After the hunting of otters was stopped, the kelp forest returned.

Sea urchins (Strongylocentrotus droebachiensis) grazing kelp forest in Northern Norway.

Kelps in our daily lives

Kelps are more important to our daily lives than you might think. In particular, they produce alginates to allow their flexible branches to withstand the constant movement from waves. This substance is widely used in pharmaceutical products, like pill coatings or toothpaste, and food production, including ice cream or beer.

The role of kelp forests in climate regulation

Marine macroalgae, such as kelps, play an important role in reducing the effects of climate change. Like plants on land, kelps photosynthesise to grow, absorbing carbon dioxide in the process. Globally, marine macroalgae may sequester around 170 million tonnes per year (range 61-268 tonnes C year−1, Krause-Jensen & Duarte, 2016), equivalent to more than 600 million tonnes of CO2 or 2% of global emissions annually. Healthy kelp grows fast and exports much of its biomass, via frond shedding, to the deep sea. Because deep-sea sediments have little direct contact with human activities, this “blue carbon” is trapped and stored for centuries.

While there will be no substitute for rapid reductions in greenhouse gas emissions to mitigate climate change, kelp forests provide a valuable addition to the arsenal of tools for reducing its effects. Therefore, understanding the impact changing environmental conditions will have on kelp itself is key to predicting future changes in its distribution and functions, including its blue carbon role. In particular, within MaCoBioS, we focus on exploring the evidence for ‘tipping points’ for different kelp species in Europe and developing models to predict future changes and identify potential management options, including Nature-Based Solutions, to mitigate the further loss of kelp forests.

References

Krause-Jensen, D., & Duarte, C. M. (2016). Substantial role of macroalgae in marine carbon sequestration. Nature Geoscience, 9(10), 737-742.

Text by Stein Fredriksen, Bethan O’Leary, Ewan Trégarot

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