Objectives

Scientific Objectives:

This cruise will investigate the following interacting parameters and processes

  • The production and fate of phytoplankton cells entrained in winter convective cells;
  • The depth of diapause of Calanus finmarchicus in relation to deep convection;
  • Timing of the response of the phytoplankton community to stratification in different hydrographic regimes in the North Atlantic;
  • The size, structure and taxonomic composition of plankton and particles in relation to the transition period from winter convection to spring stratification, subsequent response of the zooplankton community and particle aggregation;
  • The vertical distribution, taxonomy and size structure of phytoplankton, zooplankton, and particles during the transition from the winter convective regime to the spring bloom regime;
  • Individual interactions between zooplankton consumers and sinking particles. In particular encounter feeding rates and residence times of the key zooplankton species on different types of aggregates during the transition from convective regime to spring bloom situation;
  • Data and parameterizations developed will be employed in coupled 1-D vertical biological-physical models to assess the mechanisms controlling the timing, intensity, duration and fate of phytoplankton biomass in the convective and spring bloom regimes as well as for the development of an adaptive 1-D model of Calanus finmarchicus emergence from diapause based on the interaction between abiotic and biotic agents;
  • Assess the spatial variability in trophic biomass transfer efficiencies in the convective and bloom regimes;
  • Identify the key predators consuming Calanus finmarchicus during diapause and its ascent to the surface mixed layer;
  • Assess the potential impacts of climate change on biomass transfer efficiencies and biogeochemical fluxes due to changes in plankton communities.

Work Programme:
The study region of the 2012 EURO-BASIN “Deep Convection” Cruise has previously been surveyed, and based on this earlier study a hypothesized relationship between phytoplankton and convection was developed (Backhaus et al. 2000, Backhaus et al. 2003). That survey with RV ‘Valdivia’ showed a pronounced difference in both convection depth and plankton biomass between Norwegian Sea and Icelandic Basin. These regions are separated by the Polar Front which is aligned with the Greenland-Scotland Ridge. In the northern Icelandic Basin convection depths were in excess of 800 m whereas in the Norwegian Sea convective penetration rarely exceeded a depth of 500 m. We have devised a triangular station pattern for the cruise that encompasses the Faeroe Islands. Each corner of the triangle represents a different hydrographic regime. Icelandic and Norwegian Sea, the two western corners represent two differing oceanic regimes, whereas the third lies on the shelf, east of the Shetland Islands. Steaming around the triangle and doing repeated and extensive observations at each corner, we will resolve the temporal evolution from winter to spring conditions in the three regimes.
The three stations in the eastern North Atlantic will be sampled three times with the following gears to enable a temporal resolution of the data. A Video Plankton Recorder will be towed undulating down to a depth of 200 m to allow insights into the spatial distribution of zooplankton in the euphotic zone. The MOCNESS is equipped with twenty nets of 333μm mesh size. Oblique hauls down to a depth of 1000 m will catch overwintering stages of Calanus finmarchicus and other zooplankton. The vertical nets will be used to catch animals and phytoplankton (Apstein net) for biochemical analyses. The CTD with rosette will deliver oceanographic data and water samples. Stand-alone-pumping-system (SAPS) can be attached to the CTD cable. These pumps will filter large amounts of water for biogeochemical analyses. The Remotely Operated Vehicle Mohawk of the University of Hamburg will be used to make high resolution pictures of the zooplankton in the water column. An attached 1200 kHz ADCP will deliver high resolution acoustic data. Additionally the ship’s ADCP’s will be used continuously. Drifting sediment traps will be used to sample the export flux out of the euphotic zone during the station time.
We expect information on how convection in differing hydrographic regimes affects the lower trophic levels of the food web, and biogeochemistry. Whereas convection in the oceanic regimes is confined to the (upper) water column, it penetrates down to the seabed on the shelf. Here, benthic communities have the potential to be affected by convection as opposed to oceanic regimes. The close proximity of the regimes around the Faeroe Islands forms an ideal setting for our investigation in terms of science but also in terms of logistics. The Faeroes offer ideal shelter in heavy weather which is common during the study period. Furthermore, their close proximity allows replacement or exchange of broken gear and transfer of personnel. One stop around the 6th of April 2012 is planned in Thorshavn, Faeroes, for bunkering and an exchange of some personnel.

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