Phytoplankton needs both sunlight and nutrients to be able to photosynthesize. Sunlight is only available in the uppermost layers while nutrients are mostly in the deeper ocean. This is a fundamental problem in the ocean that makes many parts of the ocean unproductive despite plenty of sunlight. A certain thing happens annually in cold waters, like Icelandic, that causes nutrients to be regularly brought up to surface waters leading to very productive waters.
 
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. This mixing is because uppermost layers of the ocean are cooled down by the cold air temperatures. They therefore become more dense and sink, warmer deep water is brought up instead. This also cools down and sinks, causing constant mixing of the water column during winter.
   
In early spring, as the sun rises and the upper layers become warmers and more stable the phytoplankton blooms. This of course would not be possible it the upper layers of the ocean would not be nutrient rich after the winter mixing.
 
Phytoplankton blooms in Eyjafjörður usually start in late March while outside the fjord, in Grímsey channel it is later or around end of April of beginning of May. This is because of earlier stratification in the fjords so that the algae can stay in the upper layers.
 
During photosynthesis, 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. Furthermore the zooplankton is also more active eating up the phytoplankton. Production increases somewhat during the autumn due to more wind driven mixing with nutrient rich deep waters and also cooler air temperatures.
 
 Diatoms are quickest to respond when conditions are favourable, but they need plenty of nutrients. Diatoms dominate the spring bloom when both sunlight and nutrients are plentiful. During summer when nutrients are generally lacking the dinoflagellates can be more common. They can move, which is good when nutrients are lacking in the upper layers. During the night they simply swim down into nutrient richer deep waters while during the day they absorb sunlight in the upper layers.  
 
HV

Styrkur nitrats og magn þörunga á Grímseyjarsundi og við Hrísey. Styrkur nitrats og magn þörunga á Grímseyjarsundi og við Hrísey.

Árstíðabundnar breytingar í magni blaðgrænu eftir dýpi á Akureyrarpolli Árstíðabundnar breytingar í magni blaðgrænu eftir dýpi á Akureyrarpolli