Life is arranged into a food chain, algae are at the bottom of the marine food chain. They use energy from the sun to produce organic material from inorganic. Microscopic phytoplankton float in the surface layers and can multiply in numbers incredibly fast if conditions are favourable. Seaweed are multicellular and much larger, they live attached to the bottom in shallow waters.
Seaweed, also called benthic algae, look like plants on land and in many ways have the same function in the sea. However, they are only distantly related. They can only be found on a narrow strip along the coast where sunlight can reach the bottom, usually not below 20 m depth. The total primary production by the benthic algae is therefore much smaller than that of phytoplankton that does not need to be anchored to the ocean bottom.
Benthic algae can furthermore only be found where there is hard substrate. They cannot attach themselves to sandy or muddy bottoms since they do not have proper roots. But they are often found even in such environments where they attach to shells and rocks lying in beds of soft sediment.
Seaweed is generally split into three main groups, green algae (Chlorophyta), brown algae (Phaeophyta), and red algae (Rhodophyta), named after the different combinations of pigments in these groups. Species from all groups occur at various depths but in general the green algae are most common in the upper part of the shore, brown algae in the lower part of the seashore and shallower part of the ocean. Below that the red algae are the most common. The brown algae are largest and most conspicuous.
Benthic algae are utilized by many organisms. However, only sea urchins and a few snail species can feed on it directly. Dead algae is broken down by microbes and through them channelled back into the marine food chain. Dead kelp also drifts to the open ocean and is a source of food for detritus and filter feeders in ecosystems further away. In this way, the productivity of the benthic algae in shallow waters directly or indirectly affects the efficiency of the entire marine ecosystem.
It is also of great importance that large brown algae species form kelp forests in shallow waters that are structurally similar to forests on land. The primary production per square mile of kelp forest is among the highest in the world, comparable to that of tropical rainforests. This is the most productive ecosystem in Iceland. As in forests on land the kelp forests also create a diverse three dimensional ecosystem where other species of animals and algae are able to thrive in between the large kelp.
The algae cover is notably zoned; different species adapt to different depths or are pushed by competition into marginal zones. In general, physical factors, such as air temperature, salinity, and draught, control what species can live in the upper parts of the seashore, while biological factors, such as competition and grazing, control the species composition further down in the ocean.