WHAT ARE DIATOMS?
Diatoms are microscopic algae living in both fresh and salt water. They are unicellular organisms with heavily silica impregnated cell walls. Living diatoms are amongst the most abundant forms of plankton and represent an essential part of the food chain in the ocean. Once dead, their shells accumulate on the seabed and eventually form siliceous sediment deposits. (Eureka 2002)
HOW DO THEY GROW AND REPRODUCE?
Diatoms are almost all photosynthetic. Diatoms represent a large fraction of the marine biomass and thus are responsible for a large proportion of the total energy production of the oceans, possibly as much as half. The few diatoms that don’t photosynthesize live on dissolved nutrients from rich organic matter. Under the right conditions, diatoms can reproduce very rapidly and a population of a small species can double each day. Diatoms normally reproduce by binary fission, where one ‘mother cell’ splits into two daughter cells. (Eureka 2002)
WHERE DO THEY LIVE?
Given that diatoms are photosynthetic algae, they are restricted to the euphotic zone during the productive phase of their live cycle. Therefore, they are highly sensitive to any environmental changes such as light availability, temperature, salinity etc. In general, diatoms prefer cold, nutrient rich waters. Diatoms are most abundant under these conditions and therefore, large diatom biomasses are found in coastal areas and upwelling zones, particularly around Antarctica in the Southern Ocean. (Stoermer and Smol 1999)
WHAT EATS THEM?
Diatoms are important sources of food for the smallest planktonic animals, especially when concentrations of nutrients are fairly high. At certain times (for example, the well known ‘spring bloom’) they are probably the major source of new energy into the ocean ecosystem. It is possible that sinking diatoms are an important source of energy for deep ocean ecosystems. (Stoermer and Smol 1999)
HOW DOES THIS RELATE TO SEA-ICE?
Ice has an interesting and complex structure. Sea-ice consists of a mixture of ice crystals and salt channels (also known as brine channels) that form in small spaces between ice crystals. An ice-associated community has adapted to such variable conditions found in the ice matrix. In 1852, Sutherland was the first to describe life in Arctic sea-ice as "minute vegetable forms of exquisite beauty.” The American trans-polar research (1994) demonstrated high biological activity in the ice as well as the water column. Unicelluar algae are the main primary producers in sea ice. More than 200 diatom species are known to grow in Arctic ice. Melosira arctica may grow within the brine channels, but also attaches to the bottom of ice floes (Bartsch 1989).
Two thirds of sea-ice biota in floes from Weddell Sea consist of diatoms (Garrison et al. 1986). Some of the prominent sea-ice related diatom taxa are:
(Garrison et al. 1986; Gersonde 1986; Wilson et al. 1986; Burkle et al. 1987; Bartsch 1989; Tanimura et al. 1990; Kang et al. 1993; Palimisano & Garrison 1993; Gleitz et al. 1998) The oceanic silica cycle is very efficient at recycling the biogenic silica produced in the surface ocean, which results in less than 3% preservation in the sediment floor (Broecker 1982). Lightly silified taxa produced in the surface ocean tend to be dissolved more efficiently before reaching the seafloor. In order to quantify the abundance of various taxa reaching the seafloor and to establish diatoms as a sea-ice proxy, Gersonde et al. (2000) use a time-series of diatom flux in sediment traps placed in the Southern Ocean in conjunction with satellite sea-ice extent data. They find that Fragilariopsis cylindrus and Fragilariopsis curta are the most well preserved and often dominate the diatom assemblage in the sediment traps. These diatom taxa are restricted to very cold waters of <1oC and have relatively thick silica shells. They also find that the siliceous hardparts of Chaetoceros are preserved. However, these remains are not easily determined to the spieces level and thus not well suited as a proxy.
The general trend of increased F. cylindrus + F. curta seems to be correlated with winter sea-ice cover (as seen in figure to the left). Presence of Fragilariopsis obliquecostata seems to be correlated with summer sea-ice extent. Relative abundance of 3% (F. cylindrus & F. curta) of the total assemblage is considered the threshold for presence of summer sea-ice and year round open water. Values of 1-3% represent maximum winter sea-ice extent.