Date: Thu, 20 Nov 1997 02:14:11 GMT AESOPS Polar Front Survey - RV Revelle - Work Week 3
Cruise Report - November 20, 1997 Antarctic Environment Southern Ocean Process Study (AESOPS) Polar Front Survey I R/V Roger Revelle, Expedition KIWI, Leg 06 We continue to define the mesoscale distribution of physical, chemical, and biological properties in and around the Polar Front. We have had no serious delays due to weather during this past week, and now have completed all our mooring, mapping, and station operations. We are enroute to Lyttleton for our November 24 port call. Mesoscale mapping We completed a 5-day SeaSoar survey grid in the early morning hours of Nov 12. The frontal boundaries had moved south and east about 20-30km from their position on November 1. The portion of the front within our grid contains a meander with zonal scale of 200km and meridional scale of 100km. The drifters deployed last week are tracking that meander, with velocities of approximately 30 cm/s. As reported last week, a large iceberg, also moving at 30 cm/s, was found within our survey area. We have continued to encounter the same iceberg over the past several days and it appears to have passed within 2 nautical miles of one of the optical/current meter moorings. We won't know until late March if the iceberg snagged the mooring. We completed a second SeaSoar survey on Nov 19 of 3-day duration which focussed on a smaller region of the Polar Front than our first survey. We found that the meander translated 25-40km to the east over a 5 day interval. The phytoplankton biomass signal, as detected from fluorescence and absorption, has increased three-fold since the first SeaSoar survey, with extracted chlorophyll values of about 0.8 micrograms per liter in the mixed layer. We have observed at least two distinct patterns of enhanced phytoplankton biomass in the frontal region. One is on the "warm" side of the boundary where the mixed layer temperature is above zero, 0.8 to 1.2 C. The second major biomass band is found on the "cold" side in a temperature interval between 0.0 and -0.6 C. Surface taxonomic samples, along with supporting chemical data collected along the SeaSoar survey grid, will help us define these populations. We deployed a second group of 10 WOCE GPS drifters (five with optical sensors) in this second SeaSoar survey and are receiving daily position updates for those drifters, in addition to the ten drifters deployed during the first SeaSoar survey. We have been impressed by the rate of eastward translation of Polar Front features. Our maps suggest that large features, such as meanders, move to the east at 5-10 km/day, while the mean flow through those features is about 30 cm/s. We have found extensive intrusive features along the frontal boundaries that appear to have scales of 10-30 km, and penetrate from south to north below the mixed layer. These intrusive features may be significant in cross-front transport. Station Work Mixed layer depths remain deep in and around the Polar Front, extending from 80-200m. Bottle samples show that mixed layer chlorophyll has increased from 0.3 micrograms per liter to 0.8 in the front, but remains at 0.3 just 40km north of the front. Primary production measurements within the front show integrated production values that range from 500 - 600 mgC per square meter per day. Total organic carbon values are now 45-50 micromoles per liter in the upper 50m, decreasing to 35-40 through the rest of the water column. Mixed layer pCO2 values remain near or above equilibrium values. Our final sampling station at 60 46' S, 168 9' W was conducted within three miles of the large iceberg which has moved through our survey area during the past two weeks. This one-mile long, 200' high iceberg made an impressive backdrop for station activities. Samples at this station showed relatively high phytoplankton biomass (0.7 - 0.9 microgram chl per liter), and we observed many single cells of Phaeocystis in this assemblage. Nutrient concentrations do not yet show any indication of significant drawdown due to phytoplankton growth along the southern boundary of the front, with mixed layer nitrate and silicate concentrations of 28 and 26 micromoles per liter, respectively. We completed 22 stations with CTD casts and discrete sampling for nutrients, chlorophyll, microplankton, dissolved gases, total organic carbon, particulate matter, and trace metals. Six of these station were complete stations - three north of the front, two within the front, and one south of the front. At these stations, rate measurement experiments were conducted for phytoplankton growth, microzooplankton grazing, silica-32 uptake, and carbon-14 uptake. Profiles examining particle flux were done at each complete station, as were profiles for trace metal analysis. Iron enrichment experiments were conducted to examine the state of iron limitation on phytoplankton growth north and south of the Polar Front boundary. Mooring Deployments We completed the deployment all 12 current meter/optical sensor moorings within the Polar Front region between 60S and 61S. The final three mooring deployments were conducted in better weather than earlier in the cruise, with the final deployment done in "travel brochure" conditions - sunshine, calms seas, and many birds. All moorings will be recovered in late March 1998 from the RV Nathaniel B. Palmer. The only dark note of the trip was the loss of the Trace Metal rosette on Nov 13. The JGOFS program, Scripps Institution of Oceanography, Antarctic Support Associates, Moss Landing Marine Laboratory, and NSF are working together in an effort to replace this sampling system for the subsequent Polar Front cruises. We have had a productive cruise. Our science operations have gone smoothly, and we are grateful for the support we have received, often under challenging weather conditions, from Captain Desjardins, the ship's officers and crew, and the resident technical staff. The Revelle has been a solid, comfortable platform for work in the Southern Ocean, and we have been able to deploy instruments when weather conditions would have restricted operations on other vessels. Respectfully submitted, Tim Cowles Chief Scientist