Phytoplankton

Phytoplankton is the base of the marine food chain and therefore often called the grass of the sea. However, the phytoplankton of the ocean is fundamentally different from plants on land as it is predominantly made of single celled organisms and therefore microscopic in size. Some species of algae are large, multicellular and live on the ocean bottom, much like plants on land. However, in the larger context, these are insignificant players in the marine ecosystem compared to the phytoplankton as they only inhibit a narrow zone around the coast where enough sunlight reaches the bottom, usually to no more than 20 m depth.

Photosynthesis

Algae and other plants are able to photosynthesize, that is use the sun as an energy source to produce organic material from inorganic. This is, then, the fuel and building block for all animals higher up in the food chain. Almost all biomass in the ocean is thus derived from the phytoplankton and to a lesser extent the benthic algae. However, there is a fundamental problem phytoplankton in the open ocean have to face. They need both sunlight and nutrients (such as nitrate and phosphate) to be able to photosynthesize. Sunlight is only available in the uppermost layers. During photosynthesis, the nutrients are quickly used up by phytoplankton so they are not available for long periods in the upper layers under normal circumstances. This is indeed the case in tropical waters, and as a result they are very unproductive. To escape this problem the seawater needs to be mixed regularly to bring the nutrient rich deep waters up to the sunlight zone where the phytoplankton can grow. This is one of the reasons why areas where cold and warm currents meet are very productive; the collision of currents causes mixing. Furthermore, in surroundings where atmospheric temperatures are often colder than oceanic temperatures, the top layers of the ocean are cooled by the atmosphere. This increases the density of the surface waters and causes them to sink and therefore causes mixing. Both of these factors play a role in Icelandic waters, resulting in the very productive ocean environment around Iceland.

Phytoplankton production around Iceland

Phytoplankton blooms around Iceland occur in early spring and fall. The spring bloom is driven by longer lasting daylight and warming of the surface layers which leads to stratification. This allows the phytoplankton to stay in the surface waters. By summer, the rapid growth of the phytoplankton renders the surface waters deficient in nutrients and photosynthesis declines to a low level. A bloom in the fall is aided by vertical mixing caused by temperature differential at the air-sea interface and increases autumn winds. Plankton growth is low in the winter due to intense mixing in the water column (the phytoplankton cannot stay long enough in the upper layers) and lack of sunlight. However, this mixing is essential for the spring bloom because this brings the nutrients from the deep to the surface where they can be used by the phytoplankton during the spring bloom.

The primary productivity on the Icelandic shelf is rather high by global standards or between 150-300 g C m^-2 yr^-1, higher over the shelf than offshore. The total primary production in Icelandic waters has been estimated to be around 55 million tonnes carbon annually (450 million tonnes wet weight) on the continental shelf, but 122 million tonnes within the entire EEZ.

Productivity is generally greater in the waters south and west of the country, where blooms occur earlier and autumn blooms are also more prominent. Productivity is also particularly high in areas where cold and warm ocean currents meet, NW and SE of the country. There are marked changes in the spring development of phytoplankton from one year to another, depending on local atmospheric conditions, but spring blooms may start as early as mid-March, rather than the more usual mid-April. Particularly on the shelf, primary productivity appears to have been trending upward since the 1970s, but year-to-year variation has been as much as 3- to 4-fold during that period. Cold years, with less influence of North Atlantic Current waters, tend to have lower primary productivity than warmer years.

Species of phytoplankton

The most prominent groups of phytoplankton are dinoflagellates and diatoms. The diatoms (themselves dominated by the genera Thalassiosira spp. and Chaetoceros spp.) are generally quicker to respond when enough sunlight is available, but they, however, need relatively high amounts of nutrients. They dominate the spring blooms when both nutrients and sunlight are available. During the summer, when limited nutrients are available, the dinoflagellates (such as Ceratium spp. and Protoperidinium spp.) tend to be more common.

References and further information

References: (Ástþórsson et al., 2007), (Guðmundsson, 1998), (McQuatters-Gollop, Raitsos, Edwards, & Attrill, 2007), (Stefánsson & Ólafsson, 1991), (Þórðardóttir & Guðmundsson, 1998).

For full citation and further information on the ecosystem in general see this page

Hreiðar Þór Valtýsson, University of Akureyri