The beginning of the marine food chain, and indeed all life on Earth, is the single-cellular microscopic algae called phytoplankton. Despite being invisble to the naked eye, phytoplankton is found nearly everywhere in the surface layers of the ocean. Like plants on land, phytoplankton only needs sunshine, carbon dioxide and a few nutrients (such as nitrogen and phosphates) to photosynthesize and grow. When photosynthesising, they convert inorganic molecules in the water into organic molecules, using the energy in sunlight. Animals cannot do that and therefore have to eat plants, algae or other animals to live. The organic molecules that the algae in the ocean make are therefore the basis for the entire food chain in the ocean.
Sunlight is only available in the uppermost layers of the ocean while nutrients are mostly in the deeper ocean. This means that many parts of the ocean are unproductive due to lack of nutrients despite having plenty of sunlight. An annual cycle solves this problem in colder latitudes.
Due to low levels of sunlight on Iceland's waters in winter, plankton growth is low. However, there is an intense mixing going on in the water column. This mixing is because the uppermost layers of the ocean are cooled down by the cold air. They therefore become denser and sink, warmer nutrient rich deep water is brought up instead. This subsequently cools down and sinks, causing constant mixing of the water column.
In early spring, as the days become brighter and the now nutrient rich upper layers of the ocean become warmer and more stable the phytoplankton wakes up and begins multiplying rapidly. This spring bloom is by far the most productive season for the plankton growth. This would not be possible if the upper layers of the ocean would not be nutrient rich after the winter mixing.
During the spring bloom the nutrients are quickly used up by the phytoplankton and the stratification of the ocean prevents new nutrients to be brought up. Growth is therefore low during the summer despite plenty of sunlight. Now the air is warmer than the ocean and the surface waters becomme warmer and therefore less dense. Mixing therefore stops, nutrients are used up and phytoplankton growth stops.
The most common phytoplankton groups in Icelandic waters are dinoflagellates and diatoms. The main difference between these groups is that dinoflagellates have a skeleton made from chitin, while in diatoms it is made of silicates. Diatoms cannot move and are therefore entirely dependent on oceanic currents. They do also often form chains. On the other hand, dinoflagellates have taile-like flagella so they can move; they rarely form chains.
Diatoms are quickest to respond when conditions are favorable, but they need plenty of nutrients. Therefore, they dominate the spring bloom when both sunlight and nutrients are plentiful. During summer when nutrients are generally lacking the dinoflagellates can be more common.
A few other groups can be found off northern Iceland but they are very small and usually inconspicuous. Some, like coccolithophores can, under favorable conditions, multiply enormously and color the ocean milky white. This effect can be seen from space.