Giant Kelp (Macrocystis pyrifera) forms one of the most productive and biologically diverse ecosystems on the planet, known as the kelp forest. This enormous brown algae, which is not a true plant, can grow at astonishing rates, sometimes up to two feet (60 cm) in a single day. The kelp’s structure features a root-like holdfast that anchors it to the rocky seafloor, a stem-like stipe, and numerous blades kept afloat by gas-filled sacs called pneumatocysts. The resulting underwater forest provides a three-dimensional habitat, offering shelter and a continuous source of energy that sustains a complex marine food web.
Organisms That Graze Living Kelp
A relatively small portion of the giant kelp’s overall biomass is consumed directly by grazers while the kelp is still alive and attached to the seafloor. These primary consumers are herbivores that actively feed on the blades, stipes, or the anchoring holdfast structure. The most significant direct consumers are the sea urchins, specifically the purple (Strongylocentrotus purpuratus) and red (Mesocentrotus franciscanus) species.
Sea urchins can be particularly damaging when they shift their feeding behavior from consuming detached kelp to actively grazing on the attached holdfast. By eating through this structure, the urchin severs the entire kelp plant from the substrate, causing it to drift away and die. This behavior can lead to the widespread destruction of kelp forests, resulting in “urchin barrens.”
Other invertebrates also graze on the live kelp blades, though their impact is less dramatic than that of the urchins. These grazers include various species of snails, such as turban snails, as well as isopods and small crustaceans. Certain herbivorous fish, like the opaleye, also directly consume the blades and can cause localized damage.
The Role of Drift Kelp in Deep-Sea Food Webs
The vast majority of the giant kelp’s productivity enters the marine food web not through direct grazing, but as detritus. This detached, decaying kelp, known as drift kelp or kelp wrack, is formed when blades and fronds break off due to natural senescence, strong storms, or holdfast grazing. This detrital pathway is an important mechanism for transferring kelp’s stored energy, as the material is tough for many organisms to digest.
A wide variety of detritivores and scavengers rely on this exported biomass, which is often deposited on the seafloor or washes up on shorelines. In the kelp forest itself, species like abalones, bat stars, and sea urchins primarily feed on this drift kelp that settles at the bottom. Sea urchins play a dual role as “shredders” by breaking the large, tough kelp pieces into smaller, more manageable particles.
This shredding activity creates fine detritus and nutrient-rich fecal pellets, which then become available to smaller benthic organisms. These secondary detritivores include:
- Deposit-feeders like sea cucumbers.
- Filter-feeders such as sponges.
- Small brittle stars, which consume the kelp material after it has been processed by urchins.
- Microbes, especially bacteria, which colonize the decaying kelp, increasing its nutritional quality for detritivores like amphipods.
The export of kelp detritus also provides energy to environments far from the kelp forest. Large pieces of kelp that sink into deeper waters become “kelp falls,” providing a temporary but rich feast for deep-sea organisms. Kelp wrack washed onto sandy beaches supports a food web of insects, crustaceans, and shorebirds.
Regulating the Forest: The Impact of Consumer-Predator Dynamics
The stability of the kelp forest ecosystem is governed by a complex balance known as a trophic cascade, which links predators, grazers, and the kelp itself. In this dynamic, the population size of the primary grazers—the sea urchins—is kept in check by their natural predators. When this control mechanism is active, the kelp forest is able to thrive and maintain high productivity.
The regulatory control involves the sea otter (Enhydra lutris), a marine mammal recognized as a keystone species in many kelp forests. Sea otters consume large quantities of sea urchins, preventing the urchin populations from growing unchecked and overgrazing the kelp. Their presence maintains the structural integrity of the kelp forest, which supports dozens of other species.
In areas where sea otters are absent, other predators help regulate the urchin population, such as the California sheephead (Semicossyphus pulcher) and spiny lobsters. The sheephead, a carnivorous fish, is effective at crushing and consuming sea urchins. The health of these predator populations directly determines the health of the kelp forest ecosystem.
When these predators decline or are removed, often due to factors like overfishing or disease, the trophic cascade is disrupted. The resulting explosion in sea urchin numbers leads to intense grazing pressure on the attached kelp. This unconstrained consumption results in the formation of “urchin barrens,” vast areas of seafloor devoid of kelp and other macroalgae. This shift illustrates the importance of top-down control in maintaining the kelp forest environment.