What Is a Kelp Forest? Underwater Woodland Made of Algae

Kelps are large brown algae in the order Laminariales, and calling them seaweed is accurate while calling them plants is not, since they belong to an entirely separate branch of life from land plants and acquired their chloroplasts through a different evolutionary route. The body has three parts that resemble plant organs and work differently. The holdfast anchors it to rock and absorbs nothing, being purely a grip. The stipe supports the blades and is flexible rather than rigid, because resisting wave force is a losing strategy and bending is not. The blades photosynthesise, and since there is no vascular system every part takes nutrients directly from the surrounding water across its surface, which is why kelp needs cold nutrient-rich water and fails in warm nutrient-poor conditions. Many species carry gas-filled bladders that hold the blades up towards the light. Giant kelp reaches lengths comparable to tall trees and grows towards the surface at a rate that has been measured at up to about sixty centimetres a day in good conditions.

A kelp forest supports a dense and layered community for the same reasons a terrestrial forest does: it adds three-dimensional structure to a habitat that would otherwise be bare rock. The canopy shades the understory and creates distinct light zones. The holdfasts alone shelter enormous numbers of small invertebrates. Fish use the forest for shelter and spawning, and commercially important species depend on it in several regions. Sea otters, abalone, urchins, crabs, seals and many others occupy different parts of the system. The physical effect matters as much as the food: kelp attenuates wave energy, reducing coastal erosion, and it alters water chemistry locally by drawing down carbon dioxide during photosynthesis, which has been proposed as a buffer against ocean acidification at small scales. Kelp itself is heavily consumed indirectly rather than directly, since much of it breaks off, decomposes and enters the food web as detritus, some of it carried into deep water where it may contribute to carbon storage.

Kelp forests can flip abruptly into a completely different state, and the mechanism is one of the best documented cases of a trophic cascade:

• •Sea urchins graze on kelp, and at moderate densities they consume drift material without damaging the standing forest
• •When predators that control urchin numbers are removed, urchin populations increase and begin grazing living kelp directly, including attacking holdfasts
• •Once the kelp is gone the seabed becomes an urchin barren, a bare rocky surface covered in urchins, which is stable because urchins in a barren survive on encrusting algae at low condition and prevent any kelp from re-establishing
• •The classic case is the North Pacific, where hunting sea otters nearly to extinction for fur released urchins and destroyed kelp forests, with recovery following otter protection
• •Elsewhere the controlling predators are lobsters, wolf eels, sunflower sea stars or fish, and the collapse of the sunflower star from a wasting disease epidemic coincided with severe kelp loss along the North American west coast
• •Recovery is difficult because the barren state resists reversal, which is why restoration projects physically remove urchins from defined areas to allow kelp to return

Kelp requires cold water, so marine heatwaves have caused severe declines, with documented losses in Western Australia, Tasmania, California and elsewhere, and in several cases the forests have not returned because the replacement state, whether urchin barren or turf algae, holds. Nutrient supply matters too, and warm water is generally nutrient-poor, which compounds the temperature effect. Against that, kelp is increasingly farmed, both for food, where it is a long-established staple in East Asia, and for alginate, a gelling agent extracted from the cell walls and used across the food, pharmaceutical and printing industries. Cultivation requires no fresh water, no fertiliser and no arable land, which makes it attractive as a form of aquaculture, and proposals to use it for carbon removal by sinking biomass into the deep ocean are being examined with considerable scepticism about whether the carbon stays down and what the effect on deep-sea ecosystems would be.

Kelps are large brown algae rather than plants, with a holdfast that grips rather than feeds, a flexible stipe and blades that absorb nutrients directly from cold water, and the fastest grow around half a metre a day. The forests they build add structure that shelters fish and invertebrates and absorbs wave energy. Removing the predators that control sea urchins flips the system into a stable barren that resists recovery, as happened when sea otters were hunted out. Marine heatwaves have destroyed forests that have not returned.