Benthic algae

Benthic algae

3-Benthic_algea-(P)-Kelp_forest--(copyright-Erlendur_B)

Kelp forest in Þistilfjörður, N.E. Iceland

Photo: Erlendur Bogason

3-Benthic_algea-(P)-Cod_in_kelp_forest--(copyright-Erlendur_B)

A few cods in a kelp forest

Photo: Erlendur Bogason

Benthic algae 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. In some of these soft bottom areas, where the ocean is calm enough, marine grasses (Zoostera sp.) are able to live. These are not algae but higher plants as most plants on land. Apart from this, algae dominate the plant life in the ocean around Iceland.

Importance of benthic algae

Benthic algae are a source of food, energy and cover for many organisms. Dead algae also drift to the open ocean and are 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.

The primary production per m2 of the kelp forests is among the highest in the world, comparable to the jungles of the tropics. Therefore, kelp forests are the highest primary production ecosystems in Iceland per m2, on land or water. But as mentioned before, the kelp forests cover a relatively small area compared to the open ocean where the phytoplankton dominates, and therefore their contribution to total primary production is relatively small.

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.

Groups of benthic algae

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.

The brown algae are largest and most conspicuous. Almost all the large algae species found on the seashore and in shallow waters are brown algae, usually referred to as kelp. These form kelp forests in the ocean that are structurally similar to forests on land. As in forests on land, many other species of animals and algae are able to thrive in between the large kelp; many species also live on the kelp branches.

The dominant algae or seaweed species on the Icelandic seashore are knotted wrach (Ascophyllum nodosum) and several species of the genus Fucus. Below the seashore, much larger species of the genus Laminaria dominate the kelp forests of the North Atlantic (including Icelandic waters). Further down, where the large kelp species cannot live, coralline algae can cover the bottom; these are red algae. Over long time-scales dead coralline algae can form thick sediments of calcium which are called mearl beds.

Utilization of benthic algae

Benthic algae have been utilized in Iceland for centuries. They were used as sheep food, fertilizers and for burning in stoves. They are actually very bad for that, but general lack of other things to burn forced many Icelanders to use kelp for that purpose. Some species are also edible and provided an important food source in some areas. Benthic algae are rarely eaten today, except as rare exotic food and with sushi that has recently become popular in Iceland as well as in other western countries. Still about 20,000 t of knotted wrach and 4,000 t of oarweed kelp (Laminaria digitata) are harvested in Breiðafjörður, western Iceland, primarily to produce alginates, feed supplements and fertilizers. Geothermal energy is used in this process. Recently mearl sediments have been harvested for fertilizers.

References and further information

  • Gunnarsson, K., Jónsson, G., & Pálsson, Ó. K. (1998). Sjávarnytjar við Ísland (Marine resources in Icelandic waters). Reykjavík: Mál og Menning, 280 pp. (in Icelandic)
  • Munda, Ivka M. (1987). Distribution and use of some economically important seaweeds in Iceland. Hydrobiologia 151/152, 257-260
  • Munda, Ivka M. (1992). Gradient in seaweed vegetation patterns along the North Icelandic coast, related to hydrographic conditions. Hydrobiologia 242 (3), 133-147

Hreiðar Þór Valtýsson, University of Akureyri