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.
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.
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.
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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