Show these notes with Cruise Track included

4 Apr 1995 19:00:19 +0000 (GMT)

Cruise Update from Chief Scientist, John Marra

We're now ready to begin the "final four" stations; the one we just departed, station 26 at about 17N/60E, a designated LONG one. The other day we swung by the WHOI buoy, and it looked good. The Omani flag was still attached, and the anemometer was spinning. The sea around the mooring was "boiling" with fish (Captain Gomes' description). We are seeing the expected gradient toward more productive waters as we go away from the oligotrophic stations to the south. Chuck Trees said that the euphotic zone decreased from 90 to 60 m in less than a degree of latitude, accompanied by a change in the color of the water. Now the euphotic zone is < 50 m in blue-green water. We thought the fluorescence maximum would shoal but that hasn't happened much. Instead it has become broader, the increase beginning at about 30 m and extending down to 100 m with a peak at 40 m of about 2.5-4.5 Volts (scale = 0-5), depending on time of day, and corresponding to about 0.5-1.0 ug Chl per liter. We've also begun to notice an oxygen maximum in the top part of the fluorescence maximum. Primary production continues high. At 10N (the oligotrophic station) we went below 1 gC per m^2 per day (that's about 0.1 mol C per m^2 per day for the conversionally-challenged), but is back up to about 2. Karen Wishner devised a means to sample some of the neuston that we are seeing on occasion, and caught an unidentified siphonophore, a snail, and a barnacle. All, by one mechanism or another, inhabit the air-water interface. She and Marcia are noticing _Calanoides_ in diapause ("hibernating" at depth), evidenced by the presence of oil in their bodies. Sharon has studied Calanoides, and the net-tow results may help her explain more of its life cycle with respect to the southwest monsoon. Anne Gauzens and John Marra devised an experiment to see what eats Phaeocystis (grown earlier), and Anne picked out several volunteers from among the copepod community. One in particular, _Undinula_, Anne thought might be a good candidate, and after the experiment, out of five species, only Undinula showed signs of having ingested the Phaeo colonies. Another experiment with Undinula showed a large production of fecal pellets ('fresh' according to Juanita Urban). A 14C-labelling experiment is currently being analyzed. Dave Caron finds abundant Sarcodines (Forams, Acantharians, particularly), and even saw in a microscope sample an Acanth release swarmers. Dennis is finding that, overall, distribution of DOC is greater (by 5 uM) in the offshore stations, but was lowest at the oligotrophic station at 10N. There appears to be a broad maximum at 13-15N, with lower values north and south of this zone. As we get closer to the coast, Guy is finding the dust concentrations picking up. We continue to chase down the nitrite story. Kathy Krogslund sees three separate maxima (with high-resolution bottle trips), a small one in the chlorophyll maximum (perhaps related to light-limited nitrate uptake), and two at 250-400 m in the oxygen minimum zone at greater than 4 uM. The nitrite maxima mirror minima in nitrate, implicating denitrification. The CTD team continues to provide excellent data and support, as does the Hydro Team. The TM-rosette is working great, mistrips are about 0.2% of the total. In a few hours we'll arrive at station 27, and after a MOC, initial CTD and TM-rosette casts, we'll have two Slurps, a Bongo, a Monster, and a NoonShot. John Marra is