Chief Scientist Synopsis- Cruise NBP04-08
Week 4: November 17 – 23.
Week four of NBP04-05 finally provided the first opportunity to conduct our standard station operations. While an open water polynya continued to elude us, in situ fluorescence monitoring indicated that we were entering phytoplankton bloom conditions as we traveled through areas dominated by new ice. This new station allowed the Jeffrey/Neale groups (B200/B203) to begin to monitor phytoplankton and
bacterioplankton response to UV in a fully Phaeocystis dominated community. Biomass varied between low and moderate at the beginning of the station, but increased dramatically towards the end of the station. Our approach is built around a six day sampling cycle with two days focused on determination of Biological Weighting Functions (BWF) for phytoplankton photosynthesis and bacterioplankton growth. Another two days target experiments to quantify growth inhibition by solar UVR and subsequent repair and recovery. Additional primary experiments include deployment of an array to monitor in situ effects of solar irradiance on phytoplankton photosynthesis and bacterioplankton growth. In this case, the array was quickly engulfed in pancake ice during the majority of its deployment. Nevertheless, we observed light effects to 4 meters below the surface. Our final experiment is a diel/depth profile experiment where we monitor solar UV effects over a 24 hr period in surface and waters down to 20 meters. Critical parameters are production and DNA damage and it is this experiment, in conjunction with the Gargett project (B208), where we most closely combine biological effects with measures of water column mixing. Preliminary results on phytoplankton quantum yield suggest a simultaneous build up of phytoplankton biomass and transition of the phytoplankton community from an iron sufficient to iron limited status. A preliminary experiment indicated that bacterioplankton were also iron limited as well as carbon limited for growth. The opportunity to measure UV responses during this transition motivated the extension of the station period to a seventh day.
OPP post doctoral Fellow Brook Nunn (B200) continued depth profile sample collection for trace metal analysis, organic carbon (DOC and TOC), nutrients, particulate trace metal fractions and dissolved proteins. Additional samples were collected via zodiac to monitor trace metals in sea ice slush and melt waters. Ice slush with visibly high algae concentrations were sampled to analyze the size fractionation of particulate iron (and other metals) and differential protein expression in the ice algae. Biomass was concentrated via plankton net tows for trace metal analyses and protein expression.
The Gargett team (B208) continued to run yo-yo CTD casts for overturning scale analysis several times a day. In the mainly ice-covered environment of operations during this week, most profiles contained only small (<1m) overturns. However one yo-yo session sampled a 40m overturn across the mixed layer base. This feature was present only on a single up-cast and the succeeding down-cast, suggesting a surprisingly small spatial scale for such massive overturns. We continued to work on the HTI two-frequency towed backscatter sonar system. The high frequency (120KHz) transducer is definitely inoperable, and shallow operating depths in the study area produce multiple bottom echoes in the 38KHz return that can obscure the desired near-surface measurement range. In addition, trying to measure small signatures of ocean processes in the presence of ship-generated turbulence has been a major challenge. However working with the crew and
officers of the NBP, we discovered a successful deployment technique that allowed uncontaminated measurements of the backscatter features associated with Langmuir circulations. Distinctive backscatter clouds (of either air micro- bubbles and/or grease ice particles) occur under the “ice lines” that form on the surface of even small (< 1 mile) open water areas in windy conditions. We interpret these lines of grease ice as the manifestations of surface convergences associated with the downwelling limbs of Langmuir cells. With 20 Kt winds, the associated backscatter clouds extend to a minimum depth of 10m, revealing the minimum extent of active mixing driven directly by this surface wind/wave process.
The Gast/Caron group (B-207) continued to collect and process water samples, examining surface water and depth profiles for changes in microbial community composition during the time that we were on station. Ice samples were collected on the second to last day of the station from potentially multi-year floes trapped in younger ice. Six cores total were taken, 4 for nucleic acid based analysis and two for mixotroph/bacterivory experiments. These experiments have continued for water and ice samples, and indicate that mixotrophy is consistently present even this relatively early stage of the phytoplankton bloom. Single cell isolations and DNA-based analyses are working well, particularly on net tow samples and slush/ice samples
The Goes group (B206) began a new long term experiment to examine the
combined effects of UVR and iron enrichment on phytoplankton growth rates, production of fatty acids, amino acids, carbohydrates and MAAs of phytoplankton communities associated with sea ice melt. An additional goal of this experiment is to examine whether iron enrichment could lead to significant differences in the composition of phytoplankton community structure in the presence and absence of UVR. Experiments were also conducted with plankton net collected samples to examine the wavelength dependency of carbon incorporation into fatty acids, amino acids and carbohydrates. Samples for these experiments were collected by means of a plankton net. A final experiment was initiated to examine the effects of salinity changes on the growth of phytoplankton communities associated with slush ice.
The Kiene/Kieber group (B002/B266) conducted several seawater incubations to assess the day-long effects of UV-B, UV-A, and visible light on: 1) the biological consumption of DMS and DMSP, 2) the biological production of DMS, and 3) the photochemical loss of DMS. A long-term sulfur transformation experiment (8 day incubation) was also initiated exposing 2 L aliquots of 20 m seawater to sunlight with and without UV. Ultraviolet light doses in the incubators were monitored by chemical actinometry. Polychromatic irradiations were also conducted to resolve the wavelength dependence for DMS photolysis and consumption, and the biological loss of DMSP and production of DMS. CTD casts were deployed to determine concentrations of DMS, dissolved and particulate DMSP and DMSO were determined as a function of depth in the water column. Ancillary measurements included CDOM absorbance, nutrients, chlorophyll, pigments, and PAM fluorometry to quantify Fv/Fm. Two over-the-side deployments were made with the Biospherical PRR to characterize the downwelling and upwelling irradiance of the six visible SEAWIFS channels in the upper 30 m of the water column. Finally, several plankton, krill and particulate samples were collected for carbon and nitrogen stable isotope analysis.
This week finally provided an opportunity for all projects to proceed full ahead. Several technical and methodological hurdles were overcome in time to work in the Pheaocystis dominated waters expected during this time of year in the Ross Sea. Our work was greatly facilitated by the excellent assistance of both RPSC and ECO staff and crew. We are particularly impressed with their tireless efforts during the 24 hr diel experiments and we thank them for their efforts.
Wade H. Jeffrey and Patrick Neale