Why do we count the number of kelp plants and their stipes?

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Reef Check California — Why do we count the number of kelp plants and their stipes?

Giant kelp (Macrocystis pyrifera) is a large seaweed that grows from the rocky seafloor all the way to the surface of the ocean where it forms large canopies that can be seen from shore. By growing through the entire water column these algae provide habitat, shelter, and food for many of the fish and invertebrate species that live on these rocky reefs – just as trees do in a forest on land. Because giant kelp is so fundamental to the biological communities on rocky reefs along the California coast, Reef Check California divers count this species on their transects. But how do we insure that we capture in our data the important physical structure and biomass that the kelp provides?

In contrast to trees in a terrestrial forest where each plant has one trunk that supports the leafy canopy, kelp forests are structured differently. Each individual kelp plant* is made up of many ‘stipes’, or rope-like branches, that grow from the ‘holdfast’, a root-like structure that anchors the kelp to the reef. As a kelp plant grows, its stipes become longer and more numerous. Therefore, the older a kelp plant, the more structure and habitat it can provide to the reef community. If we were just to count the number of kelp plants on a reef we would not capture this important difference between small kelp plants and larger, older plants. In order to document the size, and hence the amount of habitat the kelp provides, we count the number of stipes of each kelp plant on our transects. At our survey sites, we encounter very differently structured kelp forests. Sometimes they are made up of many, many small individuals, which have only a few stipes each, and in other areas we find forests with fewer but larger plants with over a hundred stipes each. By counting the number of individuals as well as the number of their stipes, we can use our data to describe the important differences in the habitat structure that this forest-building algae provides to the rocky reef community.

* ‘Plant’ is a term that is regularly used and accepted when referring to kelps and other seaweeds. These organisms, however, are not plants belonging to the kingdom Plantae, but are rather algae within the kingdom Protista.