3/28/95 Cruise Update from R/V Thompson
We finished the oligotrophic station (10N) a few days ago and are beginning the transect back into the coast. The seas are calm, the winds light, and the sun strong. Like Process-1, we are running the stations clockwise. We completed the northeastern transect in good time, and collected good data. There were only a few hangups, but Mike Grogan and Tony Burke did double-duty keeping operations on track and troubleshooting equipment problems. A mysterious intestinal virus put several of the crew and scientific party to bed for a few days, and although it seemed hard to keep up the pace of stations, we managed to complete all the work. Some of the first results we saw were from Guy Rosinbaum, collecting aerosol samples. He found a tremendous decrease in the amount of particulate matter in the atmosphere compared to Process-1. The general trend going south was oligotrophy: increasing stratification, clearer water, and increasing water temperatures. Nitrate declined from 5-6 uM in the surface waters near the Gulf of Oman to about 0.1-0.2, and then by station 11 became undetectable. By station 8, ammonium had also vanished from the surface layer. Chlorophyll declined from 0.5-0.9 ug/l in the north to 0.1 at station 11 the end of the NE transect. Near-surface maxima in chlorophyll gave way to a sharp subsurface maximum at a depth of about 2-3% of surface irradiance. The subsurface maximum is also a particle maximum, thus unlike many open ocean areas, it is not only a photoadaptation phenomenon. Opposite the general offshore trends noted above, Dennis Hansell and Nancy Hayward say that DOC concentrations _increase_ offshore, from 75 to 85 uM in the mixed layer, reflecting higher nutrients and upwelled water closer to shore. pCO2 and TCO2 declined offshore, in the case of pCO2, from 400 to 370 ppm, according to Dan O'Sullivan and Esa Peltola. Anne Gauzens and Juanita Urban report many more herbivorous copepods to the north (Eucalanus, Oithona), than on Process1, while further south the community consists of smaller forms. They say that the copepods are producing fecal pellets at the rate of 1 per hour, each pellet having a carbon concentration of 0.6 ug, or 1.4 ugC per day per copepod (lower than EqPac). Anne also notes that in her copepod grazing experiments, chlorophyll increases only when copepods are present (not in the e controls). This is the same result as on Process 1, and she and Mike Roman have hypothesized that the copepods crop the protozoans allowing the nanophytoplankters to increase. Rates of herbivory and bactivory are significant, although somewhat lower than on Process-1, according to Dave Caron and Peter Countway. The largest fraction of phytoplankton is < 5um, and loss rates are up to 0.7 per day, declining with depth. Similar rates and trends hold for bactivory. The rate of primary production (Mike Hiscock, Carol Knudson, John Marra) is very high, around 2 gC/m2/d, and similar to Process1, although the rate declines to less than half that value by the station at 10 N. Still these are very high rates, perhaps driven by high values of nutrients, temperature and irradiance. Nitrogen production experiments are proceeding, and Ray Sambrotto and Bonnie Mace also note high levels of urea and large utilization of this nitrogen species. Bacterial production done by Hugh Ducklow and Matt Church, ranges over 200-400 mgC/m^2/d in surface waters. We conducted a simple "grow-out" experiment (incubating a sample of near-surface water from station 2 for a few days), and produced a fairly heavy culture of Phaeocystis (about 50 colonies per 0.3 l) and Rhizosolenia after two days. We recently began an experiment to see if common zooplankters eat the Phaeocystis. At station 08 (about 1000 km out), we noticed a secondary fluorescence maximum at 150 m. At station 11 (The "Twin Peaks" station) Jeff Kinder managed to trip a bottle in the maximum. Mikel Latasa (UH) noticed that his filter for this sample was a light blue-green. Dave Caron said that the secondary max was also seen on Process-1, and identified as consisting of a population of Prochlorococcus. Dave confirmed that assesment for this sample using his epifluorescence microscope. Chuck Trees made an absorption spectrum of the filtered material, and said that it was unusual in having a double-peak at 440-500 (with one maximum at 490), and a broad peak in the red region of the spectrum. Mikel also noted that the concentration of chlorophyll measured by the Turner was no different than zero because of a very high Fa (acid) value, indicating that a substantial fraction of the pigment was chl-b. We incubated a sample for 24h with 14C at 1%Io, but Carol said the uptake was very small. Samples for nutrients, gases, POC, DOC, and bacteria, were, of course, also taken for later analysis. The secondary fluorescence peak also coincides with a salinity minimum. The literature suggests that the salinity minimum is a water mass formed along the coast of India during the NE monsoon, and spreads at depth in the eastern Arabian Sea. The cells in the layer certainly appear viable (although their PS rate must be very low), so it is uncertain whether they are a remnant of subduction, or growing happily at a few milliEin m^-2 d^-1. The oxygen minimum zone extends from about 200-1000 m, and the concentrations, according to Ron Patrick of the Hydro team, are near zero. Still, Anna Fahrenkopf ((UDel) says there are large quantities of iodide in the OMZ, 50x greater than the Atlantic or Pacific, and from comparable stations during the SW monsoon. Iodate reduction may be greater than nitrate reduction and experiments are underway to isolate the responsible agents. Among nitrogen species, however, nitrite went "off-scale" according to Kathy Krogslund of the Hydro team, at one station reaching concentrations of 6 uM. The nitrite max is in the upper part of the OMZ, between 250-350m. Marcia Gowing and Karen Wishner collected a great set of MOCNESS samples at various oxygen levels in the OMZ and noted substantial faunal differences layers-to-layer. Becky Reitmeyer (UH) reports that Fe and Al concentrations are 0.5-1 nm in the surface layers. They are lower than on Process-1, which is what she and Chris Measures expected (and agrees with Guy's dust samples, and decrease going offshore. Mary-Jo Richardson-Gardner has an ac-3 absorption-meter/transmissometer in-line with the ship's uncontaminated seawater line. Notable in the data is a diurnal signal in transmittance of about 1.5-2% in amplitude. Brent Lewis has been collecting samples for metals (in the OMZ), and John Andrews is doing the same for the Thorium series, to be analyzed in their respective labs after the cruise. The CTD team (Jeff Kinder, Richard Korman, Mike Grogan and Tony Burke) continue to produce high quality hydrographic data and, and the same goes for the chemical analysis team of Ron Patrick, Kathy Krogslund, Dennis Guffy, Walter Schmitt, Howard Rutherford and Rhonda Kelly. The deck and sampling operations are smooth, consistent, and professional, and much appreciated by the PI's. The data is on-line very soon after the water is collected, and we have had the opportunity to visualize some of the data collected thus far. With lots of help from Cheng Ho back at LDEO, we have been able to produce depth-distance sections of the CTD and nutrient data, using THOMPSON's Sparc station and GMT system. The sections give us a good overall picture of the water column and how it has changed going south. Overall, it appears that water at depth is moving north, and the temperature, salinity and density contours all suggest upwelling near Oman. The NE section shows about the same increase in surface temperature going south seen in the AVHRR images that are sent to the ship by Mark Luther. The nutrient data also support upwelling; the sections showing the offshore decreasing gradient in surface nitrate. The fluorescence data (roughly, chlorophyll biomass) show a maximum at about 600 km out from Oman, corresponding to stations 5 and 6. We have observed some human impacts. During a transit, Eric (the 3rd mate) on his watch saw a large sea turtle swimming with its hind leg entangled in a drift net, and dragging an also entangled dead shark. Haven't seen any marlin, yet, although sharks (mostly white-tip) and squid have been hanging out at night in the ship's lights, and tuna schools have been passing by. A color front zipped by as we transited to to Station 13, and crossing it may have put us in a gradient region near the North Equatorial Current. According to THOMPSON Captain Glenn Gomes, our drifter traveled 8 nm WNW in 12h on 24 March. We've all enjoyed the tremendous support, assistance, friendliness, and good cheer of the Captain and crew of the THOMPSON. We're running smoothly and we're ready for the second transect of stations. John Marra is Chief Scientist on Process Cruise 2.