New research reveals that rising global temperatures could jeopardise today’s status quo for many kelp forests, with unknown implications for the productive ecosystems that depend on them.

In tropical waters corals dominate the coastal marine ecosystem, yet travel north or south from the paradisiacal tropics and you will find yourself in colder waters. Temperate seas can support corals both hard and soft, but the organisms that dominate the seabed here are the seaweeds, the largest and most complex of which are the kelps.

Kelps in many ways resemble terrestrial trees, towering above the smaller seaweeds and growing in dense aggregations. The aptly named ‘holdfast’ is the section of the kelp that firmly anchors to the rocky seabed that they inhabit while the ‘stipe,’ analogous to the stem or trunk in plants, elevates the kelp above the seabed. The fronds are the equivalent of leaves – long billowing ribbons that contain an array of photosynthetic pigments to harvest the light. The forests that they create give the impression of a great undersea beast laying on the seabed, the water and currents gently ruffling its long leathery hair.

Kelp forests are one of the most productive ecosystems on the planet and support a tremendously high level of biodiversity. Fish and crustacean species use the kelp forests as a nursery, an array of herbivores graze on the kelp tissue, and larger marine predators hunt through the gloom. From a human-centric point of view kelp forests represent areas vital for fisheries, a biological storm-protection system and an enormous carbon sink. In Asia, and increasingly in Europe, kelps are cultivated for human consumption providing a rich array of dietary nutrients.

It can be said that kelp are the foundation of the temperate coastal marine environment. However, with the onset of anthropogenic climate change, it has been unclear how kelps will respond to factors such as oceanic warming, and what this could mean for the ecosystems that they form and support. The northeast Atlantic in particular has shown a rapid increase in temperature of between 0.3-0.6⁰C per decade, with 2000 to 2010 being the warmest ever recorded in terms of sea temperature. A recent study of kelp species within this area, the South West of the UK, has shed light on how thermal stress in the form of ocean warming affects these kelps.

As waters warm, Laminaria digitata may be forced to seek a cooler climate farther north. © David Baird

As waters warm, Laminaria digitata may be forced to seek a cooler climate farther north. © David Baird

The dominant family of kelps in European waters are the Laminariales (Laminaria spp.). They resemble a hand, with long fingers fanning out from a spindly wrist. As such, the name of a dominant species Laminaria digitata seems fitting, its long finger-like digits undulating in the water column. This species is of a cold water origin with a range extending from the north of France well into the Arctic. Another species, L. ochroleuca, operates within warmer waters with a range from Morocco to the southern coast of the UK. So the southwest coast of England represents a unique study site, where these two similar kelps of different distributions overlap in population. A recent study used these two species to investigate how thermal stress might influence these habitat forming organisms.

For the study, individuals of these species were kept in a laboratory environment under a range of temperatures from current ambient sea temperature for the region (around 12⁰C) up to 18⁰C, a temperature that has been recorded in extreme heat wave events. At elevated temperatures the colder water species L. digitata showed a marked decline in growth rates and in photosynthetic efficiency. On the other hand, L. ocroleuca displayed no decline in growth or photosynthesis at 18⁰C; in fact, these higher temperatures seemed to be optimal for the warmth-adapted species. However, L. digitata  demonstrated an interesting response to heat stress: under higher temperatures the species showed an increase in the production of chemical defences, the compounds that discourage any potential grazers. L. ocroleuca did not display this ability.

Therefore it appears that the cold water kelp L. digitata is likely to decline under potential future oceanic warming, and it is probable that the warm water species L. ocroleuca will take its place. However, it is possible that the surprising ability for L. digitata to become more resistant to grazers in warmer conditions may compensate for any decreases in growth and photosynthetic ability that the species undergoes, although the mechanism for this is still unclear.

The phenomena exhibited by these two kelp species is in fact a classic response to a climate change scenario: the range of the species that cannot cope with the warming shuffles northwards towards the Pole, whilst the range of the warm water species expands northward as the conditions there become more favourable and replaces the extirpated native. What this means for the British marine ecosystem in particular is difficult to determine. It is possible that the kelp species may be able to rapidly adapt over generations to thermal stress, but it is also possible that the range of the cold water species L. digitata may shrink dramatically in the future.

The impacts of a shift in dominant kelp species on the UK coast are unclear, and the impact this may have on the ecosystem they support remains unknown. Yet what is abundantly clear is that global climate change will have far-reaching effects on the structures of many ecosystems both terrestrial and marine, and ongoing research into these changes is imperative if policymakers and conservationists are to prepare for a largely uncertain environmental future.

References
Hargrave, M. S., Foggo, A., Pessarrodona, A., & Smale, D. A. (2016). The effects of warming on the ecophysiology of two co-existing kelp species with contrasting distributions. Oecologia, 1-13.
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