Diatoms — Phylum Diatomae
Members of the phylum Diatomae include the golden-brown and yellow-green algae called diatoms. Diatoms thrive in cold waters and they are common members of both freshwater and marine phytoplankton communities in the Arctic. There are 192 freshwater species in the eastern Arctic alone! All diatoms are small, unicellular, and are characterized by shells made of silica. The varying geometric patterns of these shells are not only strikingly beautiful, but they are also useful in differentiating species. One adaptation of diatoms to aquatic life is their development of elaborate horns and other projections. These structures increase their surface area-to-volume ratio, slowing down the rate at which they sink and helping them to remain in the well-lit surface waters. In fact, some species of diatoms sink less than 2 mm per day!
Diatoms make up most of the phytoplankton in cold arctic lakes and are ultimately the most important source of food for freshwater fish and invertebrates. It is not unusual to find more than one million diatoms in a litre of water!