Date: Tue, 2 Dec 1997 19:48:31 -0500
Weekly situation report: NBP 97-08 AESOPS Process IV Cruise This past week has been an extremely busy and productive week for all AESOPS Process IV participants. The week began with a successful deployment of the LDEO pumping SeaSoar along the 76(30'S transect. The SeaSoar and the underway measurements of pCO2 and fluorescence suggested substantial mesoscale variability. These variations are seemingly superimposed on the variations induced by the seasonal growth of phytoplankton in the Ross Sea. The reprocessed SeaWIFS image also corroborated these variations and provided a challenge to successfully characterizing the Ross Sea carbon dynamics. On November 26 we transited to the Ross Ice Shelf, celebrated Thanksgiving, and occupied stations northward back to the 76(30'S line. These stations showed the clear presence of supercooled water suggestive of flow from under the ice shelf. On November 28 we began an intensive transect from Site Orca in the east to Minke in the west, with stations located ca. 50 km apart. In addition to the observational program, an extensive experimental program was continued by many projects. The Pumping SeaSoar was deployed in the afternoon of 25 November from Station A and towed the package eastward beyond Station P where heavy brash ice forced us to recover the instrument. The SeaSoar 'flew' reproducibly between depths as shallow as 15 and as deep as 205 m, while delivering a constant supply of water to the ship's lab of over 4 liters per minute. Sensors on the fish (oxygen, PAR, chlorophyll, temperature, and salinity) performed well, allowing us to accurately and consistently place the time (and hence position) when water arriving at the ship was originally sampled in situ. We took several samples from the outlet of the SeaSoar for independent assessment of the in situ salinity, oxygen, and chlorophyll measurements. We are currently interpreting the results of the shipboard measurements of the chemistry of the water delivered from the SeaSoar. Nutrient analyses (nitrate measurements made every 12 seconds and silicate and phosphate measured every 24 seconds) show that nitrate varies between concentrations slightly less than 25 uM in the mixed layer to slightly greater than 31 uM at 200 meters. Silicate and phosphate measurements roughly follow nitrate (with perhaps better coupling between nitrate and phosphate than between silicate and either of the former), varying from 72-83 and 1.4-2.2, respectively, between the mixed layer and 200 meters. PCO2 measured with the new fast-response beergasser equilibrator showed dramatic variations as the SeaSoar dives and climbs, and measurements of water sampled by the SeaSoar in the mixed layer agree with those made by our surface underway equilibrator to better than 2 uatm. To date 18 stations have been processed for DOC and TOC. Stations P, O and Z have demonstrated elevated levels of TOC in the surface mixed layer with concentration exceeding 53 uM. This value is up from 41 uM at the low biomass stations. Despite the elevated TOC levels, DOC level remain very close to deep water background levels (i.e., 41 uM) with a slight elevation to 43 uM at stations P and O. Ice core profiles were conducted at station Minke which demonstrate elevated concentrations of DOC in the ice, with concentrations exceeding 300 uM at the bottom-most depth in the ice. In addition to field sampling, several DOC production and mineralization experiments are progressing well. Phytoplankton biomass and productivity continued to increase over much of the entire region, although this increase is complicated by the mesoscale variations in fluorescence that are observed both between and within stations. Chlorophyll concentrations reached close to 5 ug/l at one location, and primary productivity exceeded 1 g C/m2/d at numerous others. Photosynthesis-irradiance experiments showed that vertical stratification was occurring within the water column of particular stations. We also observed notable variations in species composition in space. In contrast to previous years, the phytoplankton assemblages was not completely dominated by Phaeocystis antarctica, but had substantial contributions by diatoms and dinoflagellates, in addition to Phaeocystis. Large aggregates were observed at several stations where mixed communities of Phaeocystis colonies and diatoms coexisted. At these stations, the ectoenzymatic activities of attached bacteria constituted 21- 64 % of the total ectoproteasic activity and 50-64 % ectoglucosidasic activity. Full nitrogen productivity experiments from six depths within the euphotic were completed at Station 'O' (twice) and Station Orca. At Station Orca we also conducted an ammonium uptake kinetics experiment, and an experiment designed to evaluate the potential inhibition of nitrate uptake by increasing concentrations of ambient ammonium. In addition, we initiated a week-long experiment to determine the potential effect of sustained ultraviolet irradiation on the nitrogen (nitrate, ammonium) uptake capabilities of phytoplankton. Measurements of photosynthesis-irradiance relationships, bacterial productivity, bacterial biomass and DNA content, DOC breakdown and phytoplankton pigments are also included. Throughout the week slight increases wee noted in bacterial production in the surface waters along the 76(30'S line. Samples 100m and deeper display rates of bacterial production which often fall below detection. Overall rates of bacterial production remain low. Early estimates of bacterial abundance along 76(30'S indicate that cell numbers have increased by as much as 25% in the surface waters, but that deeper waters show only slight increases in bacterial abundance. The microplankton group has run dilution experiments for herbivory and vertical profiles of FLB grazing for bacterivory at all JGOFS major stations visited to date. Net phytoplankton plankton growth rates from the 4 experiments analyzed earlier in the cruise range from 0 - 0.1 per day, with the highest growth rate at Station O and the lowest at Station Sei. The coming week will involve a second SeaSoar tow along the transect line, some extensive station work at selected sites, a mesoscale survey using the SeaSoar, and the occupation of all stations along 76(30'S. After the completion of this transect on Dec. 13, all science operations will cease and the Ross Sea AESOPS Process Cruise IV will begin preparations for the return of samples and personnel for the Christmas season. Respectfully submitted, Walker Smith, Chief Scientist