NOAA GLOBEC ACTIVITIES IN THE ARABIAN SEA

Principal Investigator: Peter B. Ortner
Collaborating scientist(s):

Sharon Smith (RSMAS), Van Holliday (Tracor)

Objective: The objectives of the GLOBEC cruise were to:
(1) to collect data on plankton and mesopelagic animal communties, and how they are affected by monsoonal forcing
(2) to deploy satellite-tracked Langrangian drifters and collect ancilliary data on the interconnection of Somali coastal waters and the central basin of the Arabian Sea,
(3) to support the acoustic sampling being conducted at the ONR central mooring site and across the Arabian Sea as part of the Joint Global Ocean Flux (JGOFS) and ONR/ARI (Accelerated Research Initiative) and
(4) to support the OACES (Ocean-Atmosphere Carbon Exchange Study) and RITS (Radiatively Important Trace Species) programs in extending their geographic and temporal coverage.

Rationale: Among the strongest atmospheric forcing of the upper water column observable anywhere in the world ocean, recurs in the Arabian Sea. Seasonally reversing monsoonal winds drive strong currents, produce complex eddy fields, deepen the mixed layer and induce both coastal and open ocean upwelling. These physical changes directly affect primary production processes but also may be expected to influence the abundance, distribution and diversity of the animals dependent upon that production. US.JGOFS, ONR and WOCE all planned studies in the Arabian Sea in FY95. These provided GLOBEC investigators a chance to study zooplankton and mesopelagic fish in an exhaustively detailed physical, chemical and biological context that will be provided from research cruises, satellite remote-sensing, long-term mooring and modelling activities conducted by other U.S. Global Change Program research components.

Method: While underway the epipelagic and mesopelagic animal distribution were continuously sampled using hull-mounted transducers (12 and 100KHz) as well as a acoustic doppler current profiler (ADCP) by P. Ortner, AOML, S. Smith, RSMAS, and V. Holliday, TRACOR. At the same time the atmospheric boundary layer was continuously sampled (0

and CO) as well as surface waters (salinity, temperature, chlorophyll fluorescence, pCO

, pH, NO

, DIC) by R. Wanninkhof, AOML, and the epipelagic plankton community (optical particle size, 880nm backscatter, acoustic backscatter from 256khz to 30000khz and real- time video imagery) by P. Ortner, AOML. The boundary layer was sampled by an air sampler mounted on the bridge, surface waters by a continuous clean pump system mounted amidships and surface plankton by instruments mounted aboard the AOML towed aquashuttle vehicle. A continuous pump system was intermittently sampled for nutrients by J-Z Zhang, AOML, extracted plant pigment determination and, zooplankton. At least daily along the transect (and at most of the demarcated station positions) the plankton and mesopelagic fish communties were sampled with a 1m MOCNESS (multiple-opening-closing-net system) by S. Smith, RSMAS, and a 10m MOCNESS by L. Madin, WHOI et al., respectively. At each MOCNESS deployment station the AOML aquashuttle was bounced to 150m while maneuvering prior to the launch. Instrumented drifters (temperature, fluorescence) were deployed off the Somali coast by G. Hitchcock, RSMAS and regular (at least daily) CTD casts made by RSMAS and NIOZ investigators to obtain baseline information on temperature, salinity, chlorophyll, nutrient and oxygen distribution and to obtain material for microbiological experiments other academic collaborators made additional net tows throughout the cruise to obtain live animals for experimental purposes. These included both neuston tows and vertical net tows. As near as possible to the ONR mooring site a series of shallow (<150m) MOCNESS tows were made by P. Ortner et al. every three hours with a specially configured 1m MOCNESS equipped with the high frequency acoustic system from the aquashuttle. Weather permitting small-boat dive operations were conducted by L. Madin, WHOI and P. Kremer, UC. The objective of this effort was to collect live animals for experiments and to observe their in-situ behavior. Additional physical measurements included the deployment of ARGOS-tracked lagrangian drifters (M. Bushnell, AOML) and continuous collection of ADCP data (D. Wilson, AOML) as well as logging the standard meteorological data.

Accomplishment: Two cruises were made, one in May prior to the true onset of the SW Monsoon and another in August toward the end of the monsoon aboard the NOAA Ship MALCOLM BALDRIDGE. The effect of the monsoon is readily apparent in AVHRR imagery from each of the cruises ( (figure 2). Incipient Upwelling along Somalia has become full blown coastal upwelling by August while overall temperatures have cooled throughout the basin. The primary responsibility of the NOAA/AOML group was analysis of optical and acoustic data in relation to vertical and along track hydrography. These data included continuous along track data, Acoustic Doppler Current Profiler and 12/100kHz hull transducers as well as profiles with a Tracor multifrequency (256kHz - 3.0mHz) acoustic plankton sampler (TAPS) and an optical plankton counter (OPC) mounted both upon a MOCNESS net system and an Aquashuttle. To date we have focused primarily upon the offshore station Central Basin at the center of the mooring ar ray near the axis of the Findlater wind jet.

Hydrographic data confirms a substantial mixing as a result of the winds of the southwest monsoon (figure 3). The mixed layer had deepened and the subsurface chlorophyll maximum had been eroded along with the subsurface oxygen peak indicative of the productivity maximum. Nutrients now available in the near surface permitted a considerable increase in mixed layer chlorophyll values. However the data are not unambiguous since they also indicate the presence of considerably fresher surface waters. Remote-sensing and mooring data are being carefully examined to determine the source of this water.

A multiple-opening/closing net system (MOCNESS) was used to sample zooplankton and larval fishes at the Central Basin station on both cruises. Displacement volume estimates made from the actual net catches of replicate tows indicate a more than five fold increase in biomass in August as compared to May (figure 4). Using acoustic sensors can negate avoidance problems. Data from a TAPS mounted on this MOCNESS confirm this difference as well as a particular increase in larger forms (figure 5).

ADCP backscatter intensity is used to estimate animal biomass. Data from the second cruise is depicted as a curtain plot following the cruise tract. Particularly offshore diel vertical migration is the dominant signal resulting in higher biomass in the surface each night. Looking more closely at some data from the Central Basin station we can compare our observations in May and August. Considerably higher biomass was evident in August. Compare figure 6 with figure 7. Fitting these data to a line representing the center of mass we can calculate the speed of vertical migration. It approached 10cm/sec suggesting that these migrators were substantial in size, most likely decapods and mesopelagic fishes (figure 8).

The Aquashuttle developed at AOML for the GLOBEC cruises housed an integrated plankton sampler including a CTD, fluorometer, IR backscatter, multifrequency acoustic subsystem, optical plankton counter and in-situ video. Optical plankton counter data were taken at ca. the same time of day at the Central Basin station on each cruise (compare figure 10 and figure 11). They indicate not only a substantial increase in overall abundance in August but also a shift toward larger organism size.

By an odd coincidence the first NOAA GLOBEC cruise with its focus upon faunal change rather than the JGOFS cruises that focused more directly upon upwelling per se. was the only cruise to sample the Somali upwelling, the first to sample the Omani coastal upwelling and, perhaps, to have documented open ocean upwelling as well. The effects of oceanic upwelling, eddy passage and perhaps the offshore advection of coastal upwelling plankton populations are apparent in our data. MOCNESS net tow displacement volume, OPC, ADCP, and TAPS data are consistent in confirming a more than fourfold increase in animal abundance in August. This difference exceeds any observed by researchers aboard the RV/Thomas Thompson which sampled the same site just before our first and after our second cruise. Combining the data from the THOMPSON and BALDRIDGE cruises we will yield a rigorous understanding of the biological response to the monsoon.

Key reference:
U.S. GLOBEC Report Nine: Implementation Plan and Workshop Report for U.S. GLOBEC studies in the Arabian Sea, May 1993.

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