Phytoplankton & bio-optics under the ice in the Arctic Ocean
|Enlarge ImageDeployment of an Ice-Tethered Profiler with bio-optical suite. Field engineers rig a tripod to lower a 850m cable through the ice with a weight attached to the end. The ITP is attached to the cable and a final check is performed to ensure that the profiler's program is correct. The bio-optical sensor suite for measuring phytoplankton abundance and other optical properties is visible at the top of the profiler.
|Enlarge ImageThe optical sensors on this Ice-Tethered Profiler are just about to submerge beneath the Arctic Ocean for the first time. These profilers can last over a year in the ocean, making daily vertical measurements of ocean physics and biology. (Steve Lambert (WHOI))
|Enlarge ImageThe bio-optical suite developed for the ITP includes a new self-calibration/self-characterization system which incorporates optical feedback between sensors. This feedback is done by a rigid image conduit embedded in the shutter, which directs excitation light from the triplet fluorometer to the PAR sensor. Decreases in this level of feedback over time indicate a loss in coupling, due to increases in biofouling and/or decreases in the fluorometer output.
One of the hardest places to study phytoplankton ecology is in ocean regions that are perennially ice-covered, such as much of the central Arctic Ocean. In the Laney Lab we work on new technology for measuring the distribution, seasonality, and interannual variability in phytoplankton assemblages that live under perennial ice cover in polar oceans.
Over the past three years eight Ice-Tethered Profilers have been deployed in Arctic under-ice ecosystems with a special suite of bio-optical sensors developed in the Laney Lab. These profilers travel along 800m wire tethers which are anchored in the surface ice and move vertically up and down this tether, collecting physical and bio-optical information and relaying this to satellite, bringing under-ice data to our lab in near-real-time. In August 2012 one of these profilers (ITP48) became the first to measure an entire year of phytoplankton distributions with daily resolution, under Arctic ice cover in the Transpolar Drift. More information on the initial findings from these deployments can be found in our 2014 paper:
Assessing algal biomass and bio-optical distributions in perennially ice-covered polar ocean ecosystems. Laney, S. R., R. A. Krishfield, J. M. Toole, T. R. Hammar, C. J. Ashjian, and M.-L. Timmermans, 2014. Polar Sci. 8, 73-85.
The units deployed were:
ITP48 Transpolar Drift, deployed 9 Sept 2011, 14 month time series of profiles
ITP52 Beaufort Sea, deployed 5 Aug 2011
ITP60 Transpolar Drift, deployed 8 Sept 2012
ITP64 Beaufort Sea, deployed 28 Aug 2012
ITP65 Beaufort Sea, deployed 27 Aug 2012
ITP68 Beaufort Sea, deployed 26 Aug 2013
ITP69 Beaufort Sea, deployed 27 Aug 2013
ITP72 Transpolar Drift, deployed 30 Aug 2013
The bio-optical sensor suite for the ITPs include a WETLabs 'triplet' fluorometer custom-engineered to work at 2000m depth, a Satlantic PAR sensor, and an electromechanical shuttering system that protects the optical faces of these two sensors (Satlantic Bioshutter-II). The sensors are mounted on the top endcap of the ITP, adjacent to the standard SBE41CP condutivity-temperature-depth (CTD) sensor. An integrated dissolved oxygen sensor is also part of the measurement complement. Low-power control electronics inside the endcap interface this suite to the ITP main controller and provide an easy serial interface for polling and controlling this suite of sensors and actuator. The mechanical engineering and design of this compact bio-optical sensor suite was done by Terry Hammar's group in AOPE. One of the most critical design requirements was that the entire system fit down the standard 11" hole drilled for ITP deployments.
At present five of these bio-optical ITPs have been deployed in the Arctic in 2011 and 2012: three in the Beaufort Sea (ITP52, ITP64, ITP65; deployed by WHOI/Canadian teams - see a slideshow of an ITP deployment) and two in the Transpolar Drift (ITP48 and ITP60; deployed by German colleagues at AWI). Ice movement from the Transpolar Drift took ITP48 on an over 3000km ride toward the Canadian Archipelago, then toward Greenland. Dots in the below plot indicate the location where each ITP was initially deployed.
These bio-optical sensors allow ITPs to monitor seasonal trends in phytoplankton biomass, optical scattering (an index of particulate concentration), the concentration of colored dissolved organic material (CDOM), and the under-ice light field. The plots below show the 14-month history of chlorophyll biomass and CDOM concentration as measured by ITP48 from August 2011 to November 2012 (along the longest track in the above map);
In these data the seasonal trends in phytoplankton biomass can be clearly seen in the top 50m beginning in June. The strong discontinuity in December (seen most clearly in the CDOM data) reflects a change in water mass as the surface ice moved across the ocean below.
Variability in bio-optical properties within a given profile can also provide valuable insight into important ecological phenomena, such as the export of organic material from the euphotic zone. Within a given profile it is common to see individual 'spikes' (below, in chlorophyll fluroescence) that are likely to indicate aggregate particles of phytoplankton. These spikes are seen well below the euphotic zone, but only at certain times of the year when phytoplankton in the euphotic zone are becoming senescent and 'dying'. The four profiles below are from ITP48 and show the difference in the number of spikes observed in chlorophyll from 220 to 800m, before and during the summer growing season shown above (profile dates & numbers shown above each plot).
Time series of the number of spikes observed deep under the euphotic zone can be compared to time series of phytoplankton biomass within the euphotic zone, to compare the timing of growth in phytoplankton biomass with the timing of export of this material to depth.
More data from ITP48 and the rest of the bio-optical ITPs can be found on the ITP data portal. Please note that the data on this portal are raw instrument transmissions only, and should not be used for any posters, publications, or other analyses. Researchers interested in using ITP bio-optical data in their research should contact Sam Laney for more information about data format and opportunities to collaborate with these data.