Kelp, a large brown algae, forms expansive underwater ecosystems known as kelp forests in cooler coastal waters around the world. These dense groupings of algae harness the sun’s energy through photosynthesis, converting carbon dioxide and water into oxygen and sugars. Kelp is a primary producer, meaning it forms the base of the marine food web, providing both habitat and a food source for a wide array of marine organisms. Its ability to grow rapidly, sometimes up to two feet per day, makes kelp forests some of the most productive and biodiverse ecosystems on Earth.
Consumers of Living Kelp
Organisms that directly feed on healthy, attached kelp are primary consumers in these underwater forests. Sea urchins are recognized grazers of living kelp. They use a specialized five-part jaw structure to efficiently scrape kelp from rocks or cut pieces of the blades. While sea urchins can consume drifting kelp, they are also known to actively graze on the kelp’s holdfast, the root-like structure that anchors kelp to the seafloor. This grazing behavior can cause the entire kelp body to detach, leading to significant damage to kelp forests.
Marine snails also graze on living kelp. Species like turban snails and sea hares feed on kelp forest algae. Certain fish species also consume kelp. The opaleye and halfmoon fish, for instance, are known to nibble on kelp blades and may also consume algal spores. They use their mouths to pluck and scrape small pieces. Kelp crabs, such as the northern kelp crab, are omnivores that primarily feed on kelp and other algae during warmer months when these are abundant. They can consume a notable amount of kelp, sometimes as much as 8-20% of their body mass per day.
Consumers of Decomposing Kelp
Not all kelp is consumed alive; a significant portion detaches and breaks down into detritus. This decaying organic matter feeds detritivores. When kelp fronds break off due to natural processes or storms, they are decomposed by bacteria, and the resulting detrital particles are then consumed by various bottom-dwelling invertebrates. Sea cucumbers are an example of organisms that ingest detritus from the sediment.
Some crabs, such as the masking crab, are generalist omnivores that feed on drift kelp. Kelp crabs, while also consuming living kelp, will readily eat detached drift kelp, especially when live algae is less available, such as in winter months. Amphipods are invertebrates that consume decaying kelp. These detritivores play a role in nutrient cycling by breaking down organic material and returning nutrients to the ecosystem.
The Role of Kelp Eaters in the Ecosystem
Kelp consumers regulate kelp growth and distribution within the ecosystem. Their grazing activities influence the overall structure and health of kelp forests. Kelp eaters serve as a link in the marine food web, transferring energy from kelp to higher trophic levels. For example, sea urchins consume kelp, and in turn, they are preyed upon by animals like sea otters and certain starfish species.
A balance exists between kelp growth and its consumption. When kelp consumer populations, particularly sea urchins, are not controlled by predators, overgrazing can occur. This can lead to the formation of “urchin barrens,” areas where kelp is almost entirely absent, significantly altering the ecosystem and reducing biodiversity. The presence or absence of these consumers can therefore have cascading effects throughout the entire kelp forest ecosystem, impacting the availability of habitat and food for numerous other marine species.