The dynamics of diatoms and nitrate

In this movie we show simultaneously the dynamics of diatoms and nitrate in an east-west section at 56^o N in a vertical profile from the sea surface to the bottom. In winter when phytoplankton production is low a slow increase in nitrate is seen due to regeneration from the sediment and input from the ocean boundaries. After the start of the diatom bloom a fast decrease of the nitrate is seen. Subsequently at the end of the spring bloom the diatoms sink to the sediment and become accessible as food for the benthic filter-feeders.

At the same time due to input of light energy the temperature of the upper layers rapidly increases; this results in a thermal stratification of the water column, effectively separating the upper and lower layers (See movie 'E-W Transect: Temperature' under the section 'Physics'). The regenerated nutrients from the sediment and in the lower water column can not be mixed over the entire water column. This leads to a high nitrate concentration in the bottom layers and to a low diatom concentration in the upper layers. Only when the stratification (due to a wind event) is eroded a short algal growth peak can be seen in the shallow parts of the slice due to the event-driven mixing exchange between upper and lower layers. In autumn the surface water cools again and the stratification of the water column breaks down. This may lead to small algae blooms which utilise new nutrients in the upper layer.