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Climate Change

Size Matters in Kelp Forests – Big, Dense Populations Are Better Equipped to Recover from Disturbance

A Kelp Forest (NOAA

Kelp are habitat-forming species, the “ecosystem engineers” of our coasts. Standing metres tall and sheltering coastal habitats from the full force of the ocean’s waves, kelp provide a refuge for a variety of marine animals and create a forest ecosystem similar to what you might find in the temperate latitudes on land. They are nursery grounds for squid, sharks, and other egg-layers, the hunting grounds of urchins, seals, and grazing snails, and they sequester massive amounts of carbon in their giant fronds while pumping out vital oxygen for the animals living among them. Kelp forests also support healthy dive-tourism industries in coastal nations around the world.

Like many terrestrial forests along the coasts, kelp forests seem to be controlled by changing levels of disturbance over time. Where I live, on the spruce-dominated Atlantic coast of Nova Scotia Canada, patches of native Acadian forest are opened up by wind action, which results in so-called early-succession species, like birch, popping up in the newly-formed sunny patches. Eventually, these are replaced by mid and late-succession species like spruce, oak, and hemlock, forming the final stage in the forest’s development (until the next wind-fall anyway). In the neighbouring kelp forest, storms and large waves act in the same way to remove individual kelp fronds and carve out new space for other kinds of algae, anemones, and sponges. Eventually, the kelp will return, but much like terrestrial old-growth forests, they need time to re-establish themselves.

Ecklonia radiata, on a beach in Murramarang National Park at low-tide (Donald Hobern

Facing both an increased frequency and severity of local disturbances due to climate change, on top of coastal pollution and other human influences, many researchers are concerned for the future of our kelp forests and the diverse ecosystems they support. It’s important that scientists understand the ecosystem dynamics that control kelp forests so that we can effectively work to conserve and restore them.

In Australia, researchers are looking at possible feedback mechanisms that might hinder kelp restoration.

Ecklonia radiata, or E. rad as I’m now calling it, is the most common kelp found in Australasia. It’s not as tall as some other species, but it does support a high level of biodiversity in this temperate reef system and, like other kelp, is under threat from climate change, grazing invasive species, urbanization, and pollution. Most research on the resilience of E. rad, and most kelp species, has focused on how they respond to very extreme or very frequent disturbances, with researchers diving down to the forest bed to open up experimental patches and watching for recovery. That’s important, because we need to know how many kelp will return after one or many extreme events. But, we also need to consider how populations respond as a whole, and how remaining adult kelp might help or hinder new kelp from establishing after a disturbance.

Layton et. al. from the Institute for Marine and Antarctic Studies at the University of Tasmania wanted to see how different sizes and densities of individual kelp might affect the odds of a forest surviving through disturbance.

Experimental kelp patches from Layton et. al.’s work off Maria Island, Tasmania. (Layton et. al., 2020)

They took young kelp fronds from natural patches close to their research site and planted them in experimental patches of differing sizes and densities, from low (about half that of the natural patch) to high (about double). After 90 days the researchers found significant differences in survival between the patches. Large patches, of more than 1m2, seemed to have greater overall survival, whereas smaller patches lost individual kelp to grazing animals or wave action. Denser patches, those with more individual fronds per square metre, also seemed to have greater individual survival. These large dense forests produced more new (baby) kelp, which were themselves more likely to survive due to the protection of the adults. Patches under 1m2  didn’t produce surviving babies, regardless of kelp density and even when the researchers tried planting the microscopic babies there, illustrating the need for healthy adult individuals to promote new kelp growth. The team also noted that while some planted babies did survive in the larger patches, large patches with low density of adult kelp showed no natural recruitment, meaning young kelp only established in these patches because of the researchers’ interventions.

Kelp need large and relatively dense populations if they’re going to grow and survive over time. Small gaps in the dark kelp canopy can promote the establishment of new baby kelp, just like how wind promotes the establishment of birch in my backyard, but large disturbances that create big barren patches will make it harder for kelp to return to these areas. Without the kelp, these patches of ocean floor can be taken over by other algae or grass species that don’t support the same diversity of marine life, with kelp re-establishment taking years.

Survival of adult kelp planted in experimental patches of differing sizes and densities (Layton et. al., 2020)

That is, if the forest doesn’t experience another significant disturbance…

As our kelp forests face increasing storm activity, rising ocean temperatures, and polluted runoff from the land, it’s unlikely these open patches will be given the opportunity to adequately recover their former kelp canopies, and this is likely a big reason behind why scientists are seeing kelp declines worldwide.

Research like Layton et. al.’s shows us that it’s not enough to just plant restored kelp beds. We need to work with the kelp’s natural ecology to ensure we’re actually conserving and promoting the growth of new individuals. By protecting existing large kelp forests and working with scientists to restore those impacted by human activities, we may still be able to save our coastlines, kelp-dependent fisheries, and unparalleled biodiversity of marine-life.

 

Article: Layton C, Shelamoff V, Cameron MJ, Tatsumi M, Wright JT, Johnson CR (2019) Resilience and stability of kelp forests: The importance of patch dynamics and environment-engineer feedbacks. PLoS ONE 14(1): e0210220. https://doi.org/10.1371/journal.pone.0210220

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