Date: Fri, 12 Dec 1997 08:02:43 -0500 Final Sitrep NBP 97-08 WEEKLY SITUATION REPORT; RVIB N.B. PALMER, DECEMBER 12, 1997 The final stages of the Process IV cruise of the Antarctic Environment Southern Ocean Process Study (AESOPS), which in turn is part of the Joint Global Ocean Flux Study (JGOFS), has been completed. As of this date we are occupying our last station, after which we will begin packing our samples for retrograde and completing final ship-board analyses. The past week has been exceptionally busy. We completed the following: 1) a SeaSoar transect along the 76°30'S line until ice forced its retrieval, 2) experimental, high biomass stations, 3) a mesoscale SeaSoar survey to investigate mesoscale variability, 4) an intense (to put it mildly) transect of water collection along the same 76°30'S line, and 5) a surface map of mesoscale variability within the middle of the transect. The initial results of many of these are described within individual reports of each group. In general we found increasing phytoplankton biomass throughout the transect, but this was superimposed on extremely large mesoscale variations within the region. Understanding these variations and their influence on chemical and biological properties and processes will be a challenge to each of the projects. Ice conditions were somewhat surprising during this period. We had expected from satellite composites that the ice would rapidly melt and be advected to the north; however, we found that ice had in fact advected from the east to the west and into regions where we had not previously encountered it. Ice therefore increased in the eastern portions of our study area, and decreased in the west. Ice was encountered in the northern reaches of our mesoscale surveys (ca. 12 nm), and did not appear to change much during the past week. Surface water temperatures were somewhat elevated (from -1.2 to -1.5C), so melting is occurring through much of the region. Winds have been modest throughout the week, allowing uninterrupted work. We have also had a number of clear, sunny days, which have allowed satellite observations of the region and surface validation of remote sensing algorithms. Surface pCO2 during the past week has continued to be drawn down as the bloom progresses near the western end of the Orca-Minke line, with in-situ values of just under 200 uatm at stations A and Minke. From station E to Orca there has been relatively little change during the same period. Surface pCO2 variability in the area of the mesoscale (surface) grid was significantly less than that found in the area of the SeaSoar grid just to the west. This probably reflects the much less dramatic depletions seen between stations O and Z during all of our occupations of the line. Nutrient concentrations remained fairly high, although a nitrate concentration of 15.5 uM was detected at the station with the highest particulate matter concentrations. All TOC and DOC samples collected on the first two transect lines have been processed We have observed elevated TOC at the western extent of the second transect, with concentrations exceeding 75 uM C in the surface 40 m of several stations. In contrast, we have observed only slight accumulations of DOC in these same waters, with concentration reaching 45 uM C. A second ice core was sampled at station Minke. We observed mM DOC accumulation near the bottom of ice core. Several DOC production and consumption experiments are still in progress and will continue until the end of the cruise. Phytoplankton biomass and productivity have continued to increase. Within the transect we have found chlorophyll levels reaching 9.8 ug/l, although most values remained near 2-3 ug/l. Intense variations in fluorescence (ca. 6-fold changes) were noted over the scale of 20 km. The biomass changes were reflected in the community composition, which also changed dramatically. In general the phytoplankton assemblage showed a much greater diversity than in previous years, with more diatoms and dinoflagellates relative to Phaeocystis. Phaeocystis did dominate at a number of stations, particularly those in the western portion of the transect, with many large, spherical colonies being observed. The suggestion of serving the colonies on toast for mid-rats was rejected. Primary productivity exceeded 2.5 g C/m2/d early in the transect, and is expected to more than double in the western portions. At the "high biomass" stations dominated by Phaeocystis, large aggregates were not observed. However, the ectoenzymatic activities of bacteria were 10 times higher than those measured at the beginning of the cruise. The ratio between ectoproteasic and ectoglucosidasic activities aren't significant different at the stations dominated by Phaeocystis than those of the other stations. Bacterial production continues to rise since our first occupation of the 76°30'S line. Surface waters show elevated rates at every station thus far sampled., but rates in deeper waters (below 100m) remain near zero. Bacterial abundance continues to rise, with some stations revealing a possible subsurface maxima (20m). Observations of bacterial attachment to particles is more frequent, particularly at higher biomass stations. The microplankton group has completed 10 grazing experiments for herbivory and bacterivory at the major stations visited during the cruise. Net phytoplankton growth rate at these stations ranged from 0 - 0.37 per day with the highest rates at Station O and Minke during the later part of the cruise. Samples for nano and microplankton abundance in the upper 100M were collected at most stations. The Pumping SeaSoar was deployed three times: twice surveying the 76°30'S line from Station A to between Stations P and S (2 and 8 December), and once in a mesoscale grid survey over a 30 nm square centered on the line and covering the area between Stations E and Z (5 December). We observed much greater depletion of surface nutrients and pCO2, along with higher chlorophyll levels, in the 2 December transect along the 76°30'S line than we had observed in our 25 November transect, indicating that the bloom had begun in earnest. Accompanying these greater depletions were larger spatial variations along the line-- surface nutrient depletions and fluorescence levels were large between A and Z, slightly less in the middle of the line near Station O, and large again near Station P. In contrast to the differences between the 2 December and 25 November surveys, the 8 December survey was not dramatically different from that of 2 December, as if the bloom had not progressed as rapidly in that time. While a few points, notably near Station E, showed increased surface nutrient reductions consistent with continued bloom conditions, most of the line seemed fairly similar to the previous transect. With the help of some SeaWIFS, we targeted a high biomass area between Stations E and Z for a mesoscale survey on 5 December. We towed the SeaSoar for over 30 hours, completing a 'square-wave' grid pattern with five 30 nm legs running north and south across the 76°30' connected by four 7 nm east-west legs, thus covering the E-Z portion of the line. In this grid, we saw the greatest surface nutrient drawdowns in a region north of the line between E and Z, and lower productivity to the south of the line, consistent with the SeaWIFS images. The optics group continued detailed characterization of the spectral apparent and inherent optical properties, including spectral reflectance, backscattering and absorption coefficients that are critical to understanding ocean color algorithms. Lab experiments have been carried out to improve methods in measurement of ocean absorption coefficients. In situ measurements reveal 20-fold variations in particulate beam attenuation for surface waters in the study area (e.g. from 0.05 to 1.0 m-1 at 660 nm). Particulate absorption have shown similar large dynamic range, but soluble absorption coefficients have increased by a factor of 2 at the most. Coulter Counter analysis of the particle size distribution has revealed an overall increase in particle numbers within the size range 2-60 um, with the appearance of a significant population in the 15-20 um range that had been rare early in the cruise. We accomplished one detailed calibration-validation station during simultaneous SeaWiFS overpass under perfect clear skies, calm ice-free sea state and chlorophyll concentrations of about 4 ug/l.. The group has coordinated receipt of SeaWiFS data which revealed a mesoscale chlorophyll feature centered on Station E with a diameter of approximately 100 km. The imagery was used to help specify the best location to conduct the mesoscale survey with the SeaSoar. The NASA SeaWiFS Project is gratefully acknowledged for their transmission of imagery to the ship. The last station of the cruise is now being occupied. This station has a euphotic zone depth (i.e., the depth of the 1% isolume) of 9 m, which is the shallowest euphotic zone any of the investigators have ever seen. The CTD and other instruments disappear from view within meters of the surface, and the underway fluorescence trace is off-scale. It represents the last of 37 locations we have visited and sampled, using 63 trace metal rosette casts and 88 CTD casts. These casts, in addition to the underway sampling and SeaSoar activities, are testament to the hard work of the entire scientific party. Thus the last station of AESOPS continues the tradition of providing tantalizing hints of processes which can only begin to understand. Despite the entreaties of the eager scientists, science operations will cease by 0800 of December 13, and ship-board analyses will soon be complete. The mood of the scientists, crew and ASA personnel remain remarkably positive, and we all look forward to the completion of the cruise and the return of many of us for the holiday season. Respectfully submitted, Walker Smith Chief Scientist