Solar irradiance over and under seasonal land-fast sea ice off Barrow, Alaska, in 2010

The amount of solar radiation transmitted through Arctic sea ice is determined by the thickness and physical properties of snow and sea ice. Light transmittance is highly variable in space and time since thickness and physical properties of snow and sea ice are highly heterogeneous on variable time and length scales. We present field measurements of under-ice irradiance along transects under undeformed land-fast sea ice at Barrow, Alaska (March, May, and June 2010). The measurements were performed with a spectral radiometer mounted on a floating under-ice sled. The objective was to quantify the spatial variability of light transmittance through snow and sea ice, and to compare this variability along its seasonal evolution. Along with optical measurements, snow depth, sea ice thickness, and freeboard were recorded, and ice cores were analyzed for chlorophyll a and particulate matter. Our results show that snow cover variability prior to onset of snow melt causes as much relative spatial variability of light transmittance as the contrast of ponded and white ice during summer. Both before and after melt onset, measured transmittances fell in a range from one third to three times the mean value. In addition, we found a twentyfold increase of light transmittance as a result of partial snowmelt, showing the seasonal evolution of transmittance through sea ice far exceeds the spatial variability. However, prior melt onset, light transmittance was time invariant and differences in under-ice irradiance were directly related to the spatial variability of the snow cover.

Physicochemical sea ice and water characteristics near Point Barrow 2005-2006

Photosynthetic parameters of phytoplankton and sea ice algae from landfast sea ice of the Chukchi Sea off Point Barrow, Alaska, were assessed in spring 2005 and winter through spring 2006 using Pulse Amplitude Modulated (PAM) fluorometry including estimates of maximum quantum efficiency (Fv/Fm), maximum relative electron transport rate (rETRmax), photosynthetic efficiency (alpha), and the photoadaptive index (Ek). The use of centrifuged brine samples allowed to document vertical gradients in ice algal acclimation with 5 cm vertical resolution for the first time. Bottom ice algae (0-5 cm from ice-water interface) expressed low Fv/Fm (0.331-0.426) and low alpha (0.098-0.130 /(µmol photons/m**2/s)) in December. Fv/Fm and alpha increased in March and May (0.468-0.588 and 0.141-0.438 /(µmol photons/m**2/s), respectively) indicating increased photosynthetic activity. In addition, increases in rETRmax (3.3-16.4 a.u.) and Ek (20-88 µmol photons/m**2/s) from December to May illustrates a higher potential for primary productivity as communities become better acclimated to under-ice light conditions. In conclusion, photosynthetic performance by ice algae (as assessed by PAM fluorometry) was tightly linked to sea ice salinity, temperature, and inorganic nutrient concentrations (mainly nitrogen).

Climate and soil characteristics and soil arthropod abundance on Anchorage, Signy and Falkland Islands

Over a 2-year study, we investigated the effect of environmental change on the diversity and abundance of soil arthropod communities (Acari and Collembola) in the Maritime Antarctic and the Falkland Islands. Open Top Chambers (OTCs), as used extensively in the framework of the northern boreal International Tundra Experiment (ITEX), were used to increase the temperature in contrasting communities on three islands along a latitudinal temperature gradient, ranging from the Falkland Islands (51°S, mean annual temperature 7.5 °C) to Signy Island (60°S, -2.3°C) and Anchorage Island (67°S, -3.8°C). At each island an open and a closed plant community were studied: lichen vs. moss at the Antarctic sites, and grass vs. dwarf shrub at the Falkland Islands. The OTCs raised the soil surface temperature during most months of the year. During the summer the level of warming achieved was 1.7 °C at the Falkland Islands, 0.7 °C at Signy Island, and 1.1 °C at Anchorage Island. The native arthropod community diversity decreased with increasing latitude. In contrast with this pattern, Collembola abundance in the closed vegetation (dwarf shrub or moss) communities increased by at least an order of magnitude from the Falkland Islands (9.0 +/- 2 x 10**3 ind./m**2) to Signy (3.3 +/- 8.0 x 10**4 ind./m**2) and Anchorage Island (3.1 +/- 0.82 x 10**5 ind./m**2). The abundance of Acari did not show a latitudinal trend. Abundance and diversity of Acari and Collembola were unaffected by the warming treatment on the Falkland Islands and Anchorage Island. However, after two seasons of experimental warming, the total abundance of Collembola decreased (p < 0.05) in the lichen community on Signy Island as a result of the population decline of the isotomid Cryptopygus antarcticus. In the same lichen community there was also a decline (p < 0.05) of the mesostigmatid predatory mite Gamasellus racovitzai, and a significant increase in the total number of Prostigmata. Overall, our data suggest that the consequences of an experimental temperature increase of 1-2°C, comparable to the magnitude currently seen through recent climate change in the Antarctic Peninsula region, on soil arthropod communities in this region may not be similar for each location but is most likely to be small and initially slow to develop.