PS94 Weekly Report No. 7 | 28. September till 4. October 2015
Ice-free diet for copepods
How will the Arctic ecosystems react on climate change – how will they change as a consequence of the ongoing summer sea ice decrease? These are the questions moving our biologists and therefore they study flora and fauna in the sea ice, the water and at the sea floor. Hence our transects through the Arctic contain also net hauls and a considerable part of the water from the rosettes goes to the biologists.
At almost each station, several water samplers of the rosette are closed in the upper water layers and the obtained water is filtered for phytoplankton. Phytoplankton (marine drifting plants) thrives in the uppermost hundreds of meters since there it finds light and nutrients. The Arctic sea ice somewhat hampers sun light to penetrate far in the water – only when the ice is thickly covered with snow, eventually it is getting really dark.
Also the nutrient concentration is affected by the state of sea ice. In summer, ice melt adds fresh water to the uppermost layer and with the thereby reduced salinity the density is reduced as well. This hampers mixing of the top layer – where phytoplankton happily consumes nutrients during summer - with the salty and thus dense waters from deeper layers that still contain a lot of nutrients. Eventually the nutrients in the top layer are gone and phytoplankton growth comes to a halt. Now, this scheme may be transferred to longer time scales in that more Arctic warming might cause more summer ice melt and thus nutrient depletion. The slow decrease in nutrients available might favor smaller species on the account of larger ones and thus lead to a change in species composition.
To find out the truth, our phytoplankton team aims to determine the composition of sizes and of species in the different regions and conditions of the Arctic. Hence they pour their water samples first through a coarse filter to capture the largest plankton. They specify the taxa by inspecting it through the microscope, and weigh the samples: how many microgram diatoms with siliceous shells, how many algae with calcareous skeletons, and so on. They continue pouring the water through finer and finer filters until eventually the organisms, e.g. bacteria, are too small to be identified under the microscope; at this point the analysis will be have to rely on genetic tools to identify the species.
Other than the phytoplankton, the zooplankton lives in all water depths. It feeds from remnants of phytoplankton and from excrements which were produced by zooplankton colleagues living higher up in the water column and that slowly sink downward. To capture the vertical distribution of zooplankton we use a multinet and LOKI. The multinet consists of nine nets which are dragged vertically through the water, each opening and closing in a different depth. LOKI stands for „Light frame On-site Key species Investigation“, a device which takes photos (27 per second) of the zooplankton while heaved.
Also the zooplankton is expected to react on the changing diet provided by the reduced size phytoplankton. Presumably it will also undergo a size shift towards more small species and less large ones. Altogether this has the potential to sum up to less biomass. To test such hypotheses we have currently way too little data. In order to distinguish between inter-annual and regional variability and long-term trends we need measurements at least with annual resolution which in the harsh conditions of the Arctic remains a challenge.
On Saturday we safely delivered our patient in Tromsoe and we are glad to hear that he is in good care. Now we steam towards the Greenland Sea to recover two gliders and thereafter return to the Barents Sea for a final short section.
Best regards from Polarstern,
With support from Juliane Riedel, Imke Petersen, Nicole Hildebrandt, Ksenia Kosobokova