Date: Sun, 2 Nov 1997 18:33:31 -0500 AESOPS Polar Front Survey - RV Revelle - Work Week 1
Cruise Report - November 1, 1997 Antarctic Environment Southern Ocean Process Study (AESOPS) Polar Front Survey I R/V Roger Revelle, Expedition KIWI, Leg 06 The primary objective of this leg of the Southern Ocean JGOFS program is to provide mesoscale resolution of the physical, chemical, and biological properties in and around the Polar Front. To this end, we have a full complement of scientists (37) measuring a wide range of parameters. We sailed from Lyttleton New Zealand at 0300 Oct 19 (GMT) and proceeded to our first station at 57S, 170W. During the transit, Chris Measures' trace metal group from Univ of Hawaii (Sue Vink, Kendra McDonough) deployed their towed water collection system and began monitoring surface concentrations of iron and aluminum. Millero's group from RSMAS (Xiagong Zhu, Elizabeth Degler) monitored nutrients, pCO2, total CO2 from the underway system. Surface pCO2 concentrations were below atmospheric equilibrium (~365 ppm) from New Zealand to 47S, then increased to 380-400 ppm from 47S to 60S. The members of the hydro group (Bob Williams and Stacey Morgan of Scripps ODF, and Calvin Mordy from PMEL and UW) collected discrete underway nutrient samples and found that nitrate and silicate concentrations were 3-5 micromoles per liter and 15-20 micromoles per liter, respectively, from 45S to 53S. From 53S to 58S surface nitrate increased to 24, while silicate increased to 11 micromoles per liter. Catherine Goyet (WHOI) and Thomas Kirchlechner (Univ of Otago) provided underway assesment of TOC, and found that surface values decreased from approximately 70 micromoles per liter just east of New Zealand to about 45 micromoles per liter at 57S. We conducted our first station at 57S, 170W where weather conditions were less than optimal. Hydro team members Mike Realander (UW) and Mark Cook (NCSU), with the considerable assistance of Bob Wilson, resident technician (SIO), helped us through some challenging deployments of the CTD system and the Trace Metal rosette. We were able to deploy the TM rosette in winds of 30 knots and seas of 12-15 feet. CTDs to 250m and 2000m revealed a 100m mixed layer (4.2 C, 34.04 salinity) with chlorophyll concentrations of approximately 0.25 micrograms per liter in the upper 20m, and chl values near 0.20 to 160m (Scott Polk, Univ of Tennessee). Optical measurements of downwelling spectral irradiance and upwelling spectral radiance were made by Mark Abbott (Oregon State University), and provided an optical estimate of upper mixed layer chlorphyll concentration of 0.26 micrograms per liter. The Moss Landing group (Kenneth Coale, Sara Tanner, Michael Gordon) was able to rinse the trace-metal clean 30-l GoFlo bottles for later collection of samples for analysis of iron, zinc, and other metals. They deployed and recovered Goflo bottles in winds exceeding 40 knots and with snow and sleet in their faces. The thorium team from Woods Hole (Larry Ball, Glenn Crossin) obtained a profile of dissolved and particulate thorium for estimation of particle flux. The microplankton group (Mike Landry, Sue Brown (Univ of Hawaii) and Karen Casciotti(SIO)) conducted dilution experiments and population sampling (live large volume flow cytometry and preserved samples and slides for pico-, nano- and microplankton). Dilution experiments conducted for the mixed layer (10 and 21% of surface irradiance) at 57 deg gave community (chl) growth rates of about 0.25 per day and microzooplankton grazing rates of 0.33 per day. The net negative rates reflect the fact that the incubation period included a snow storm, during which light was severely attenuated. Total organic carbon measurements (Goyet and Kirchlechner) yielded values of 40-45 micromoles per liter in the upper 160m, and about 40 down to 250m. The physiological condition of the phytoplankton was assessed in photosynthesis versus irradiance experiments by Brian Kulka and John Meadows (Duke Univ). Nitrate and silicate concentrations in the mixed layer were 23 and 8 micromoles per liter, respectively. Winds and high sea states forced us to cancel TM rosette collections for productivity experiments and to cancel deployment of the MOCNESS for zooplankton collections. We had to cancel the planned deployment of SeaSoar at 57S due to the sea state. We proceeded to 58S in hopes of better conditions for deployment. At Station 2 (58S, 170W) we sent the CTD to 2000m in preparation for the launch of the SeaSoar vehicle for a transit across the Polar Front. The upper 170m was well-mixed, with temperature of 2.7 C and salinity of 33.97. Mixed layer chlorophyll concentrations were about 0.20 micrograms per liter, while nitrate and silicate were 25 and 11 micromoles per liter, respectively. Total organic carbon values had decreased slightly from the station at 57S, with mixed layer values between 35-40 micromoles per liter. Weather conditions had improved and we launched the SeaSoar to map the frontal features along 170W. The SeaSoar is a towed, undulating vehicle (Cowles, Barth, Richman, Oregon State Univ, technical support from Marc Willis, Linda Fayler, and Bob O'Malley) instrumented with a CTD, bio- optical sensors, and an optical plankton counter for assessing zooplankton biomass in different size classes (Huntley). The SeaSoar tow across the frontal region provided 2km horizontal resolution of the upper 350m between 58S and 62.3S, and revealed several steep horizontal gradients in properties between 59.6S and 61.2S. A steep temperature gradient from 1.6 to 0.4 C water was evident across a 10km boundary at 60.4S, while another temperature gradient from 0.0 to -1.0 C occurred across a 15km interval at 60.8S. South of 61.2S the mixed layer temperature was -1.6 C. There was little to no vertical stratification within the upper 100-150m along the entire 480km track. Between 200 and 350m, below the surface front near 60.4 S, a 25 km band of cold, fresh water from the upper water column south of the front was observed beneath warm, salty water above. The cold, fresh water appears to be sliding down the steeply sloping isopycnals beneath the front. The SeaSoar grid surveys planned for later in the cruise should help establish to what extent this process is three-dimensional. Acoustic Doppler Current Profiler (ADCP) data reveal ENE flow between 58.5S and the ice edge at velocities of 10-40 cm per sec. Peak easterly flow of 40 cm/s occurred between 60.1 and 60.5S, with 40 cm/s flow extending the surface to 200m. Optical sensors on the SeaSoar showed an increase in phytoplankton concentration south of the Polar Front - from approximately 0.2 micrograms chl per liter north of the front to 0.35 in the mixed layer south of the front. Underway nutrient sampling showed that surface nutrient values increased across the front, with nitrate increasing to 28 while silicate jumped from 13 at 59S to 35 micromoles per liter at 61S. Hourly sampling for microplankton populations during the Seasoar transect revealed that the northern end of the transect is dominated by small nanoplankton (prymnesiophytes and small pennate diatoms), while the southern end (near the ice margin) has abundant and very healthy-looking large diatoms. Heterotrophic dinoflagellates seem to be the most conspicuous "microzooplankton" consumers. The frontal zone did not show a marked transition in populations from waters to the north. The Brzezinski/Nelson group, represented by Valerie Franck (UCSB), tracked silicon uptake rates, particulate silica and dissolved silicic acid along the SeaSoar transect. Additional data on property changes will be available later. Following the recovery of the SeaSoar vehicle, we began a 24 hr station at 62.3 S 170W, our most southerly position during this survey cruise. The CTD cast revealed a 60m deep mixed layer, with a temperature of -1.63 C and salinity of 33.93. Mixed layer chlorophyll concentrations were between 0.37 and 0.40 micrograms per liter. Nitrate and silicate were 30 and 45 micromoles per liter, respectively. Although sea state required us to cancel our first TM rosette deployment, improving weather conditions permitted us to complete nearly all of our activities at this southerly station. In addition to sampling from hydro and TM casts, we completed thorium casts (Ball and Crossian), tethered radiometer measurements (Abbott), bio- optical profiles (Cowles and Wingard), a MOCNESS tow (Gonzales), primary productivity experiments (Kulak and Meadows), incubations to examine iron limitation (Coale, Tanner, Gordon), silicon uptake and dissolution experiments (Franck), assessment of microplankton growth and microzooplankton grazing (Landry), and characterization of particulate matter (Stillman for Mark Altabet, Polk, and others). Mooring Deployments We completed the deployment of the first four current meter/optical sensor moorings (Abbott, Richman, Root, OSU) within a 50 hr interval ending at 0330 31 Oct (GMT). We are in the middle of a 24 hr sampling station at 60.5S 169.0 W to give the mooring team a day of rest and to sample within the frontal boundary. The remaining eight moorings will be deployed within the next six days. Ship Operations The new Trace Metal van for Moss Landing Marine Laboratory group has performed well. It is placed on the starboard side of the main deck, just forward of the hydrographic staging bay. This van replaces the one damaged last year on the Palmer. We have had some difficulties with CTD operations due to cable and winch problems. With the help of Revelle engineers and the experienced marine technicians aboard during this leg, we have resolved most of the problems. We also are investigating several different approaches to solving the complicated deployment problems created by having the hydro rosette and the trace metal rosette in the Revelle staging bay. Captain Desjardins, the ship's officers and crew, and the resident technical staff have been extremely helpful in dealing with science operations under challenging conditions. Weather permitting, we expect the next three weeks of operations around the Polar Front to be productive. Respectfully submitted, Tim Cowles, Oregon State Univ. Chief Scientist