Plankton Ecology and Biogeochemistry in a Changing Arctic Ocean (PEBCAO)

Antje Boetius, Alexandra Cherkasheva, Alfred-Wegener-Institut Estelle Kilias, Ilka Peeken, Olivia Serdeczny

Not on board: Eva - Maria Nöthig Objectives

The project PEBCAO (Plankton Ecology and Biogeochemistry in a Changing Arctic Ocean) is focused on studying plankton and microbial processes relevant for biogeochemical cycles in the Arctic Ocean. In order to understand and track

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consequences of climate change for the pelagic ecosystem, both long-term field observations and experimental work with Arctic plankton species and communities are needed to gain knowledge about their feedback potential in the future Arctic Ocean.

Biogeochemistry, phytoplankton & vertical particle flux

Recent investigations indicate that rising temperatures and freshening of polar surface waters promote a shift in the phytoplankton community towards a dominance of smaller cells. A change in size of the primary producers could have significant consequences for the entire food web and for the cycling and sequestering of organic matter. An increase in ice-free areas as well as CO₂- and temperature-related changes will also affect plankton and the carbonate chemistry of the ocean;

even small changes in the biological pump could significantly affect atmospheric CO₂ concentration. In such a scenario, picoplankton can comprise a large pool of biomass and can attain high abundances. Therefore, we particularly need to understand how environmental parameters influence the diversity, occurrence and distribution of the picoplankton.

In this context, results collected during this cruise will be compared with results obtained during former cruises to answer questions like: Is there a shift of the phytoplankton community towards smaller cells? Are there any intrusions of Atlantic species into the Arctic Ocean? How does the phytoplankton community differ between different water masses? Do observed changes in the pelagic realm influence the vertical particle flux of organic matter?

Primary production & phytooptics

At high latitudes ocean color satellite data have a sparse coverage due to the presence of sea ice, clouds and low sun elevation angles. This is the main cause that satellite ocean colour algorithms which achieve large-scale information on primary production perform poorly in these regions. Therefore, our main aim for TransArc was to collect in-situ data that allow us to adopt satellite global primary production algorithms (as far as the satellite reaches northern latitudes) for the Arctic Ocean. Furthermore, in-situ optical measurements and the photosynthetic characteristic of phytoplankton in the central part of the Arctic Ocean were sampled because so far only a few data of phytoplankton characteristics on larger temporal and spatial scales than obtained from discrete water samples exist.

Work at sea

Samples have been collected along two transects; some additional stations were sampled in the outer Laptev Sea. In addition, surface water was sampled along the ship’s route (see Fig. 7.7).

Fig. 7.7: Stations sampled by the PEBCAO group

Biogeochemistry, phytoplankton & vertical particle flux

Samples for phytoplankton ecology investigations were taken from the CTD-rosette at 87 stations at 5 to 8 depths according to the water mass structure (at 66 stations the upper 70 m were sampled, at 21 stations the upper 200 m were sampled). On 10 of the stations, under-ice plankton was sampled at 0.5, 1 and 5 m, respectively. Ocean surface water was sampled every 20-60 miles from the ship’s flow through membrane pump at 97 spots. At all 87 stations and at the 97 spots, samples have been taken for analysing biogeochemical and biooptical parameters such as chlorophyll a, pigments (HPLC), absorption of particulates, CDOM and phytoplankton and CDOM fluorescence. Samples to determine seston, particulate organic carbon (POC), particulate organic nitrogen (PON), and particulate biogenic silica (PbSi), phytoplankton and protozooplankton were taken at selected stations.

The water was filtered through pre-combusted Whatman GF/C glass-fiber filters, polycarbonate and, cellulose acetate filters and stored deep-frozen at -20°C or -80°C for later analyses in the home laboratory. The abundances of autotrophic pico- and nanoplankton, and small microplankton were determined with the flow cytometer directly on board. Microscope identification and enumeration of larger nanoplankton and microplankton will be carried out later in the home laboratory at AWI. Samples are preserved and stored at a cool and dark place.

Due to the small size and thus complicated phenotypic characterization the smallest phytoplankton fraction, picoplankton (0.2-2.0 µm), is difficult to detect and assess.

However, a proper identification down to the species level can be achieved by the application of molecular methods. Thus, in order to investigate the genetic diversity of picoeukaryotes special filter samples for DNA analyses were taken during the cruise. Additionally some water samples were taken into culture for later microscopy analyses. Surface water samples were taken twice a day from the membrane pump and 3 depth were sampled from 58 CTD casts. According to the fluorescence probe profile, one sample was taken from the surface layer, one from the chlorophyll maximum and one from a depth of approximately 50 m. For the DNA analyses a size fractionation was performed. Two liters of the water sample

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were first filtered through a membrane filter (Millipore) with a pore size of 10 µm.

Afterwards the flow trough was filtered through a second membrane filter with a pore size of 3 µm and finally the flow trough of the previous filtration was filtered through a membrane filter with a pore size of 0.4 µm. All in all, three different phytoplankton size fractions were achieved from every water sample taken, above 10 µm, between 3 and 10 µm and between 0.4 µm and 3 µm. After filtration the samples were stored immediately at -80°C.

Vertical particle flux of particulate matter under the almost permanent ice cover will be investigated by means of sediment traps which were deployed in two moorings near the Gakkel Ridge (two in the Nansen Basin and two in Amundsen Basin; both deployed at ~200 m & ~150 m above the sea floor) for one year (see oceanography, chapter 4). The traps were equipped with 20 sampling jars each and pre-programmed individually. They will be recovered in summer 2012 during the Polarstern expedition “IceArc” (ARK-XXVII/3).

Primary production & phytooptics

In-situ radiance and irradiance measurements at high spectral resolution down to 110 m have been carried out using a TriOS RAMSES radiometer. Every measurement includes spectra of downwelling irradiance (sensor 5038), upwelling irradiance (sensor 81EA) and upwelling radiance (sensors 82D6 and 81E6). These data will later be used to validate radiative transfer modeling through the water column.

The photosynthetic characteristic of phytoplankton has been investigated by measurements of the variable fluorescence in the upper 110 m. A FastTracka Fast Repetition Rate Fluorimeter (FRRF) exposes phytoplankton to a series of flashes of blue light at 200 kHz repetition rate and then records the fluorescence signal. From this signal the efficiency of the photochemical conversion during photosynthesis of the observed algal population was calculated. The data will be later processed with e.g. "Submersible FRRF Data Reduction – FRS1" software. These estimates will be used to validate one of the key parameters in satellite primary production modeling – the photosynthetic yield.

Preliminary/expected results

All samples have to be finally analyzed in the home laboratory at AWI. Results obtained with the flow cytometer showed distinct differences between Atlantic and Pacific influenced water masses with much higher cell concentrations in the ‘Atlantic part’ of the Arctic Ocean with highest counts at the beginning of the cruise in early August. To investigate the community structure and the diversity of picoeukaryotes, ribosomal fingerprinting technology (ARISA) as well as pyrosequencing will be applied. We speculate to find less biomass and a trend towards smaller cells in comparison with results obtained during former cruises.

Combined with the data collected for chlorophyll a and phytoplankton specific absorption the optical measurements will serve as ground truth data for the adaptation of the global satellite primary production algorithm to the Arctic Ocean.