By Annette Wilson & Minna Hovi |
Greetings from the RV Polarstern! As we approach the equator, obvious changes in the ocean colour can be seen from the deck and through remote sensing satellites. We have also seen changes on a smaller scale through the microscope in the phytoplankton species that cause these colours.
We began our survey in the coastal waters of the English Channel, where the waters are green and nutrient rich. In temperate regions, the uneven warming of the sun and seasonal weather conditions cause mixing of the surface waters. During the winter, in temperate regions, nutrients are mixed up from 200-300 m depth. This allows for cycles in phytoplankton growth. In contrast, the tropical waters we are sailing through now are deep blue and lacking nutrients. The constant sun – which we are all enjoying – leads to a shallow mixed layer in which nutrients are rapidly exhausted through uptake by phytoplankton.
Our first phytoplankton samples, collected with 20 μm (0.002 centimetre) nets from the most northern stations (~48° N) were full of life. We have seen many species common to the coastal and shelf seas, particularly “the pill box” like phytoplankton known as diatoms. We have also observed toxic dinoflagellates such as Dinophysis acuminata. As we sailed off the Celtic sea shelf and into deep waters (~3-4 kilometre deep) to the south, we have seen a dramatic change in species composition. For example different species of the dinoflagellate genus Ceratium occur at different latitudes. Ceratium is easily recognised by its three horns.
The first signs of tropical influence were seen with the presence of the sun-like diatom Planktoniella sol in the Bay of Biscay. This beautiful cell became more common as we approached the Ampère Seamounts (~ 35 °N). Although we have seen more elaborate and pretty species in the tropics -we have also observed less, which is linked to the nutrient poor conditions.
Around the Cape Verde islands (~16 °N) we found many species common to coastal waters particularly diatoms. Special to this area was a small Thalassiosira species that forms dense “blooms” and appeared as brown gooey stuff in our plankton nets.
Generally for all organisms, species diversity increases towards the tropical latitudes. This is also true for phytoplankton. At the beginning of our journey we observed Emiliania huxleyi, a coccolithophorid that commonly blooms in artic and temperate regions. Now in the tropics, we are seeing a variety of species in this group, with differences in their external plate structures called liths. These calcified liths are used to identify the coccolithophorid species.
Our further route through the tropical latitudes of the Southern Atlantic Ocean has not been sampled as extensively as the coastal and temperate zones of Europe. We are all very excited to discover what lies ahead when we travel further south!
Slán go fóill & Terveisiä maailmalta!
Annette and Minna on behalf of the students onboard RV Polarstern