Benthic algae or kelp usually cover hard bottoms from the seashore down to 20 to 40 m depth, depending on the clarity of the ocean. Below that level, insufficient sunlight hampers their growth. Where the bottom is sandy or muddy the benthic algae cannot attach themselves as they have no roots.
 
Benthic algae are a source of food, energy and cover for many organisms. In fact there are not many animal species that can digest kelp, only sea urchins and a few snail species. But dead algae is broken down by microbes and through them channelled 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.
 
Benthic algae are generally split into three main groups, green algae (Chlorophyta), brown algae (Phaeophyta) and red algae (Rhodophyta), named after the different colouration caused by different 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.
 
Large brown algae species often form large kelp forests in shallow waters. The primary production per square mile of the kelp forests is among the highest in the world, comparable to the jungles of the tropics. This is the most productive part of the ocean around Iceland.
 
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 or draught, control what species can live in the upper parts of the seashore, but biological factors, such as competition and grazing, control the species composition further down in the ocean.
 
HV

Various kelp species on the seashore (photo Tryggvi Sveinsson) Various kelp species on the seashore (photo Tryggvi Sveinsson)

Kelp forest below the seashore (photo Erlendur Bogason) Kelp forest below the seashore (photo Erlendur Bogason)