Ice Conditions, Irradiance and Fluorescence Imaging

The ice thickness varied from 1.07 m to 3.00 m amongst the stations sampled, with no trends in ice thickness along the transect of the cruise (Table 20). Snow thickness ranged from 5 to 15 cm, and temperatures at the bottom of the ice varied from -1.9ºC to -2.6ºC. A few cores had visible signs of recent ice growth, with very even, crystalline bases, but most cores had apparently older lower surfaces which had been stable or even thawing prior to collection. Almost all cores were annual sea ice rather than multi-year ice, and Figure 88

shows a typical example with variation in vertical structure due to short-term temperature differences during the ice growth season.

Station ID

Table 20. List of all 37 ice sampling stations including latitude and longitude. Ice thickness is the range of core lengths (n = 2 – 4) each day. The minimum sampling program (see “Field Sampling”) was performed on all days. Additional sampling was performed as noted. Samples for ice-active substances and cores for S. Rysgaard (SR) were frozen for return to Denmark, and picoeukaryote samples for N. Sørensen (NS) were filtered on-board and frozen for return to Denmark.

* = Additional field sampling on Day 247 with journalists Martin Breum and photographer Kenneth Sorrento. Under-ice video recordings made at Stations 246-255.

Figure 88. Photo of a typical ice core, with variation in structure including denser ice (darker layers) and less dense, crystalline layers. Bottom of the ice to the left. Scale bar = 1.60 m.

The incident irradiance at the sampling stations varied considerably from day to day due to daily weather conditions, but was generally low, ranging from 100 µmol m^-2 s^-1 up to 200 µmol m^-2 s^-1 on the clearest, sunniest days. Light attenuation in the snow-ice column reduced PAR to just 1–10 µmol m^-2 s^-1 immediately under the ice. The spectral distribution was also clearly affected, as seen in Figure 89, an example of change in spectral distribution of light between air and below sea ice, as recorded with the TRIos spectroradiometer. The figure shows that not only is PAR reduced to < 10% of the surface value, but also that the snow-ice column removes all of the red and infrared light from the spectrum, leaving primarily blue light. This further reduces the light energy available to photosynthesis, as chlorophyll has its maximum absorption in the red part of the spectrum.

Figure 89. Light attenuation with depth and change in spectral composition on Day 227, as recorded with the TRIos spectroradiomenter. Note differences in y-axis scales in right-hand graph.

Ice algae were present in cores at every station, but the amount of algal development and its distribution in cores varied considerably. In many stations the algal development was too low to be visible or was only scarcely visible to the naked eye, but occasionally large irregular patches of high algal biomass were present (Figures 86 & 90). These patches were encountered at random intervals throughout the cruise, with no clear trend in space or time. Generally they were restricted to the lower 20 mm of the ice, but occasionally isolated patches were found higher in ice cores.

Spatial variations in algal biomass and Fv /Fm within these patches were easy to document with fluorescence imaging. Figure 90 shows both surface and cross-sections of a typical visible patch, showing the high biomass but limited penetration of algae up into the core. Fv

/Fm was very low in most patches (< 0.1), but rose to 0.2 – 0.4 when measured in the Phyto-PAM after thawing of the core slice; this was still considerably lower than Fv /Fm in seawater phytoplankton (Fv /Fm > 0.5).

Figure 90. Examples of images of F0 (minimum variable fluorescence) obtained in the field with the Imaging-PAM fluorometer, from ice cores with high biomass patches. Colour scales from red (low biomass) to blue (high biomass), but is not comparable between images due to different instrument settings. Left: Surface view of bottom of an ice core with a high biomass patch.

Middle: Cross-section through a patch, showing algal development limited to the lower 15 - 20 mm of the ice. Right: Cross-section of a patch isolated ca. 5 – 7 cm from the bottom of a core, in which the lower 5 cm lacking algae appeared to be recent ice growth. Circles are AOIs (‘Areas of Interest’) used to quantify aspects of algal photobiology at specific areas of the image. All pictures 30 x 23 mm.

Unlike the large visible patches, the ‘background’ algal community, almost invisible to the naked eye but usually distributed very evenly across the width of cores, showed a strong trend during the cruise. Biomass was initially high during transit across the Gakkel Ridge and into the south-western corner of the Amundsen Basin, fell to low levels over the western Lomonosov Ridge and past the North Pole, and then rose dramatically again in the eastern Lomonosov Ridge stations. The distribution of this type of algal development is shown in Figure 91, in which the algal material is visible in its brine channels, amongst the non-fluorescing ice crystal structure, and again is primarily restricted to the lower 10 – 15 mm of the ice core. Although high sensitivity settings were required in fluorescence imaging to detect algae in the stations with lowest biomass, the method successfully resolved both the spatial and vertical distribution of these populations at all stations.

Figure 91. Examples of images of F0 (minimum variable fluorescence) obtained in the field with the Imaging-PAM fluorometer in cores without large visible patches. Colour scales from red (low biomass) to blue (high biomass). (Left) Surface view of a core showing extensive algal development throughout. Circles are AOIs (‘Areas of Interest’) used to quantify aspects of algal photobiology at specific areas of the image. (Right) Cross-section of the bottom of a core, with the depth of algal development identified in rectangular box. All pictures 30 x 23 mm.