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Date:  Mon, 10 Jul 1995 15:13:37 +0000 (GMT)

Cruise Summary -- SeaSoar #3

The South West monsoon SeaSoar leg (TTN-048) left Muscat June 21, and is due to return July 13. The objective of the cruise is to document physical, chemical and biological conditions in the upper ocean during the sustained monsoon conditions. Groups on board include representatives from Bigelow Laboratory (Phinney, leader), NRL/Stennis (Arnone and Young, leaders), University of Oregon (Wood, leader), University of South Florida (Coble, leader), the University of Southern California (Jones, leader) and WHOI (Brink, leader). Smaller scale efforts on thorium (Benitez), aerosols (Seymour), primary productivity (Toon) are also taking place. Working conditions in Muscat were difficult, with deck temperatures reaching 100 deg before 0700, and increasing to about 115 by mid morning. After sailing, deck temperatures were typically in the 80s and conditions were humid. Winds in the Arabian Sea proper have been about 25-35 kts (sustained), except weaker on one day relatively close to shore. Seas have generally been about 10-15 feet, but disproportionately rough because they are almost always mixed (i.e., never coming from just a single direction). The ship has rolled at least as much as 25 degrees, and 10 degrees is very common. CTD/rosette casts have been possible throughout the cruise (using tie lines), while HIDEX (bioluminescence) casts have been difficult and occasionally cancelled. Vertical net tows have not presented any operational problems. The overall sampling approach called for SeaSoar measurements at the outset of the cruise, followed by hydrographic stations. Stations were carried out along the southern JGOFS line from offshore onto the shelf. Well-resolved sections were made across the filament and across the shelf break (into water as shallow as about 50 m)in an active upwelling area. Additional stations are being made along a zig-zag pattern tending alongshore. In all, 40 stations have been made to date, and about 10 more are expected. Shortly after leaving Muscat, the SeaSoar was deployed to measure temperature, salinity, depth, fluorescence, light levels, light transmission and accoustic zooplankton estimates. This vehicle is towed at about 8 kts, and cycles between roughly 5m and 300m every 10 minutes. The SeaSoar track ran along the coast until, south of Masirah Island, a distinct filament was found extending offshore from the coast. This feature was distinguished by cool surface temperatures (25 vs. 28 degrees), high nutrients, high chlorphyll, and high offshore velocities (up to 1.5 m/sec). The feature continued offshore, and then curled southward across the JGOFS sampling line at about 17 deg N, 58.5 deg E. If sampled incompletely, this feature could easily be mistaken for offshore (Findlater jet-driven) upwelling, rather than the coastal feature that it is. Upon mapping the filament, SeaSoar was recovered to allow a well-resolved (15 km spacing) hydrographic transect across the feature. Following the transect, SeaSoar was redeployed at about 17.5 deg N, 58.25 deg E. Two "radiator" patterns were then conducted, as well as a "bowtie" near the moorings (See recent JGOFS newsletter article about our February cruise). A tendril from the filament was found to pass north to south through the inner radiator. Farther offshore, near-surface conditions were consistently warm (SST about 28 deg), and the mixed layer depth ranged between 40 and 70m (inshore of the upwelling front, and within the filament, the mixed layer tends to be substantially shallower). In the warm waters, a subsurface chlorophyll maximum (as evidenced by light transmission and fluorescence) was always present below the base of the mixed layer. SeaSoar was recovered July 4, after 11 successful days. The only operational problem was that the faired cable was fouled on its first recovery by both a fish net and a long line, leading to a painfully slow recovery and many sunburns. The Phinney group has been measuring chlorophyll and particulate absorption spectra underway, and these plus flow cytometry on stations. In the filament, surface chlorphyll concentrations reached 14 ug/l in the coldest waters (23 deg). The nearest-to-shore line was highly structured, but the feature became smoother farther offshore. On the intermediate cross-filament line where a section was made, surface concentrations reached 2.5 ug/l, and the subsurface chlorophyll maximum, where present, reached values as high as 4 ug/l. In radiator number 1, where the filament passed through, surface chlorphyll reached 1.5 ug/l, while in all other offshore waters, surface chlorophyll concentrations ranged between 0.2 and 0.4 ug/l. Results from the section along the JGOFS line were consistent in that offshore, surface chlorophyll concentrations were low, and the subsurface chlorophyll maximum (20-80m depth) was always < 1 ug/l. The ratio of chlorophyll-containing to total particles was 40% with total particle concentrations less than 1000/ml in the size range of 3-30 um. Closer to shore on the same line, the cold surface waters (21 deg) had chlorophyll concentrations of 3.8 ug/l, the ratio of chlorophyll to total particles reached 65% and the total particle concentration was > 3000/ml. The NRL group has set up a shipboard satellite receiving and processing station for collecting AVHRR imagery. We are currently receiving about 9 passes a day (NOAA 9,12,14), and these are automatically processed and archived. The imagery has been very useful in tracing the evolution of filaments and planning data collection. A movie loop demonstrated the extension of coastal waters in a filament eastward to at least 61 E. The data are also being used to validate real-timne numerical models being run at Stennis. In addition, spatial variability in apparent and inherent optical properties and associated remote sensing reflectance are being characterized by continuous flow measurements and vertical profiles at each station. Optical properties similar to those of the filament have been found to extend well offshore (61 E) and confirm expectations from the AVHRR imagery. These optical properties are being used to test bio-optical algorithms and to estimate a light budget for driving numerical models. The NRL group has also been measuring bioluminescence intensity, number of events and kinetics at depths down to 200m using the HIDEX (High Intake Defined EXcitation) bathyphotometer. Tenative findings are that bioluminescence intensities are highest (comparable to November-December 1994) in coastal waters, especially freshly upwelled waters. Intensities are about two orders of magnitude lower offshore, reflecting low apparent biomasses in this area. As during Fall 1994, bioluminescence intensities fall off quickly below the base of the mixed layer and only occasional flashes of long duration were found in the oxygen minimum layer. This pattern suggests zooplankton bioluminescence. The upper water column is typified by fast flash kinetics suggestive of dinoflaggelates, consistent with the results of Wood's group. Wood's group found that net phytoplankton and picoplankton (Synechococcus sp.) abundance is closely correlated with smaller scale physical variability. Early in the cruise, diatoms were less abundant than expected, although the diatom Rhizosolenia was consistently found in samples from newly upwelled water and in the filament. Dinoflagellates, particularly Ceratium were common in the filament and were associated with upwelled water having a low Si:N ration. Dinoflagellates also dominated those offshore waters and coastal waters having high surface temperatures. Coastal samples collected later in the cruise have been dominated by diatoms (eg., intense blooms of Rhizosolenia and Thalassionema). Synechococcus abundance is higly variable near the coast (ranging over three orders of magnitude), but is low offshore (more than an order of magnitude less abundant than during December, 1994). Pigment markers suggest genetic differences in Synechococcus nearshore over small spatial scales, the boundaries usually occuring a physically defined fronts. Coble's group has been measuring colored dissolved organic matter (CDOM) fluorescence and absorbance, remote sensing reflectance, chlorophyll and dissolved organic carbon (DOC). Flourescence was measured with the SeaSoar, the underway pumping system and discrete samples. CDOM concentrations are low in the mixed layer, as expected, but CDOM varies directly with salinity and chlorphyll unlike in most coastal regions. The most likely explanation is that upwelling is providing deep water with high DOM to the surface. Below about 500m, the CDOM:salinity relationship changes signs. Coble is also coordinating with the NASA P-3B aircraft Airborne Oceanic Lidar (AOL) team (Hoge), which is presently flying both passive and remote sensors for CDOM, chlorophyll, temperature and other pigments, as well as Lewis's SeaWiFS simulator. The aircraft flew its first mission on July 4, and has flown three more times since then, including coverage of the entire JGOFS region. Their results show good agreement with in situ measurements and also show high chlorophyll and other pigment concentrations along the northern JGOFS line. Specifically, aircraft observations show high chlorophyll and other pigment concentrations within the filament. Another aircraft mission is planned for July 12. Jones's group has been measuring nutrients both underway and from bottle casts. Surface nutrients have been detectable both near the coast and within the filament. Within the filament, surface nitrate ( > 5 uM) and ammonia levels ( > 1uM in some cases) were high, but silicate was surprislingly low ( <0.5 uM). Ammonia concentrations increased to as much as 2.5 uM at 50 m depth in the core of tthe filament, suggesting active regeneration. Nutrient information suggests some subtle evidence for subduction within the filament. As in past cruises, denitrification seems to be quite active in the entire region surveyed (one unfortunately leaky bottle from 750m depth off Muscat even smelled of hydrogen sulfide). Along the main JGOFS line, highest nitrite concentrations ( > 5 uM in two cases) were found at the outer part of the line. Suboxic conditions continue at depth. At most stations, oxygen concentrations < 0.1 ml/l are observed at 300m and deeper, but deep casts show that higher concentrations are found below about 1200m. This has been a trying but rewarding cruise. The crew, as always, has been excellent. We had a contest to fill out the following: 'How rough was it? It was so rough that ....' There were lots of entries. Any errors or mis-statements in the above are my responsibility. Ken Brink
From a subsequent Brink e-mail: The winner of our contest was Will Peterson: How rough was it? It was so rough that 7 graduates students changed majors to terrestrial ecology, one professor went into administration, another went into numerical modeling, and 10 crew members moved to North Dakota. Ken