Patterns and Scales of Variability in the Optical Properties of Georges Bank Waters, with Special Reference to Phytoplankton Biomass and Production
 
Principal Investigator: Heidi M. Sosik
Sponsoring Agency: Office of Naval Research
 

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SeaWiFS image from October 8, 1997 showing chlorophyll a concentration in the Gulf of Maine/Georges Bank region.  Image provided by SeaWiFS Project, NASA/Goddard Space Flight Center.

Long Term Goals
The long term goals of this work are to contribute to a fundamental understanding of the sources of optical variability in coastal ocean systems.  Particular focus is on applications useful for studying important ecological processes and the links between phytoplankton properties and physical processes in coastal regions.

Objectives
 The initial objectives of this project are focused on making measurements of time series and spatial distributions of both apparent and inherent optical properties in the waters on and around Georges Bank.  These observations will be used to identify spatial and temporal patterns of variability and contribute to defining the dominant sources of variability in optical and phytoplankton properties in the region.

Approach
  The approach we are pursuing is to integrate and deploy commercially available spectral radiance, irradiance, absorption and scattering sensors on existing oceanographic platforms with widely different spatial and temporal sampling regimes.  Two platforms are being specifically adapted, a profiling oceanographic mooring and a towed underwater vehicle.  In coordination with the GLOBEC Georges Bank study, these sampling platforms will be used to construct an observational data set for the waters on and around the bank, with temporal scales spanning hours to seasons and spatial scales of meters to hundreds of kilometers.  This will be accomplished by combining measurements conducted from the mooring and towed vehicle with remotely sensed surface ocean optical properties from global ocean color missions (e.g., SeaWiFS) and with conventional ship-based sampling.

Work Completed
BIOMAPER II Adaptation and integration of sensors into the BIOMAPER II (Bio-Optical Multifrequency Acoustical and Physical Environmental Recorder) towed vehicle has been fully completed and successful observations have begun (Fig. 1).  BIOMAPER II, designed and built at the Woods Hole Oceanographic Institution primarily for acoustic research (Wiebe et al. 1997), now routinely carries two spectral radiometers (OCI/OCR-200 series, Satlantic, Inc.) and two ac-9 in situ absorption and attenuation meters (Wet Labs, Inc.), one sampling whole water and the other sampling material less than 0.2 um.  Integration of these sensors required construction of a data acquisition assembly which takes advantage of the optical fiber and network communication systems already active on the vehicle (Fig. 2).  The upgraded vehicle also now carries a Video Plankton Recorder (VPR) designed to observe plankton of mm size or larger.  On recent GLOBEC program cruises to the Gulf of Maine and Georges Bank (R/V Endeavor cruise #307, October 8-17, 1997 and R/V Oceanus cruise # 332, October 19-30, 1998), the new vehicle configuration and optical sensor acquisition system were successfully used to collect observations nearly continuously throughout the water column.  During these cruises, BIOMAPER II was towed behind the ship and manually controlled (based on ship speed and winch operation) to produce tow-yo flight patterns.
 
 
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Figure 1.  Top and side views of the new BIOMAPER II vehicle with exterior panels cut away to show the complete interior layout, included optical sensors integrated as part of this project.  Two ac-9s, associated pumps and the optical system electronics assembly are mounted in the interior of the vehicle, the irradiance sensor is located on top of the stabilizing tail fin and the radiance sensor is supported by a specially constructed rear-mounted frame intended to lower vehicle shadow effects.  Other sensors carried on the vehicle include an aray of up- and down-looking acoustic transducers, a CTD system, chlorophyll fluorometer, and Video Plankton Recorder.  The length of the main body of the vehicle is approximately 6 m.
 

 
Figure 2.  Block diagram of the acquisition system for the optical sensors integrated into BIOMAPER.  The data acquisition system (designed and assem-bled at WHOI) for these instruments is based on a sub-surface PC-104 and includes 2 serial ports, a 16-channel/16-bit A/D converter, an 8 Mb flash disk and an ethernet adapter for communication with the BIOMAPER Lantastic network.  Through network access this system enables storage of data files to a desktop PC on board the ship.

AVPPO We have also recently completed adaptation and integration of sensors into the Autonomous Vertically Profiling Plankton Observatory (AVPPO).  AVPPO is a mooring system for operation in coastal environments, designed and constructed at the Woods Hole Oceanographic Institution (Gallager et al. 1998, Thwaites et al. 1998).  The AVPPO consists of a combination of a buoyant sampling vehicle and a trawl-resistant bottom-mounted enclosure, which holds a winch, the vehicle (when not sampling) and batteries.  The AVPPO is set to sample at preprogrammed times; the vehicle is released and floats to the surface, with power and data connection maintained through the winch cable, and is then returned to the bottom with the winch.  High-resolution vertical sampling can be conducted on the up- and/or downward profiles and on scales of minutes to weeks and months, limited by power and data capacities.  The primary sampling system on the original vehicle is a dual camera VPR, but it also carries accessory environmental sensors (including conductivity, temperature, pressure, chlorophyll fluorescence and beam transmission).  As part of this project, we have integrated the same suite of optical sensors as on BIOMAPER II (except with only one ac-9) into the AVPPO sampling vehicle (Fig. 3).  The new optical sensor data acquisition system includes power and network connections to the main vehicle systems and on-board data storage (Fig. 4).

The new AVPPO configuration has been tested using both shore link and autonomous modes in waters off Woods Hole, MA.  Hydrographic, optical and video data were successfully recorded during hourly profiles over periods from days to 1-2 weeks.  Following further testing, an approximately 3-month deployment on Georges Bank is planned for later this year.  This deployment will coincide with collection of SeaWiFS ocean color imagery and will encompass a planned BIOMAPER II survey cruise (December 1-13, 1998).
 

 
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Figure 3.  View of the upgraded AVPPO showing the optical sensor system integrated into the profiling vehicle.  The ac-9 and optical system electronics case are mounted on top of the vehicle body and the up- and down-looking radiometer heads are positioned on the inside edges  for protection during landing.  CTD sensors and a single wavelength transmissometer are also visible on the vehicle;  the VPR sensing system is housed in the nose of the vehicle.  The winch in the bottom-mounted housing is not visible.
 
 
 


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Figure 4.  Block digram of the acquisition system for the optical sensors integrated into AVPPO.  The data acquisition system (designed and asembled at WHOI) for these instruments is based on a subsurface PC-104 and includes 2 serial ports, a 16-channel/16-bit A/D converter, two 2-Gb hard disks and an open collector SAIL network adapter for communication with the AVPPO main network.  This system enables storage of data files on redundant hard drives on board the profiling vehicle.  Download is achieved post-deployment by ftp protocol after connecting a temporary Ethernet adapter in the laboratory.
 
SeaWiFS In conjunction with the current and planned BIOMAPER II and AVPPO sampling, we have begun processing and interpretation of SeaWiFS ocean color images for the Georges Bank/Gulf of Maine region.  On the most recent BIOMAPER II cruise, imagery was used to guide observations and sampling strategy ensuring that a variety of optical water types were surveyed.
 
Preliminary Results
Integration of the optical sensor system into the BIOMAPER II towed vehicle was completely successful and the test cruise yielded an excellent set of observations collected during surveys of the deep basins of the Gulf of Maine (Fig. 5).  The vehicle was operated in tow-yo mode for most of the 10-day cruise, allowing vertically and horizontally resolved sampling.  Preliminary analysis of some of the data collected during a transect across Wilkinson Basin shows spatial variations in scattering and absorption coefficients associated with water column structure and distributions of optically active material (Fig. 6).  Highest values were found near the bottom and in conjunction with phytoplankton patches in the top 50 m of the water column.  Absorption by dissolved material exhibited somewhat less patchiness than found for particles, but this component was consistently a major source of absorption in the blue region of the spectrum at the mid-water depths.  The results shown here were acquired during an approximately 12 h period and represent only a small portion of the total data set collected.  We are currently processing the complete set of observations and exploring methods for visualizing and interpreting the results.  Water samples for pigment analysis and high spectral resolution absorption by particulate and soluble material were collected daily and will be used to evaluate the performance of the in situ instruments and to aid in data interpretation.

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Figure 5.  A successful test cruise with the newly configured BIOMAPER II was carried out in the Gulf of Maine during October 1997.  The vehicle was operated in "tow-yo" mode to sample horizontal and vertical distributions of acoustic and optical properties throughout most of the 10-day cruise.  The cruise track, station locations and regional bathymetry are indicated.
 
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Figure 6.   Vertical structure observed during a tow-yo section through Wilkinson Basin, Gulf of Maine on October 14, 1997.  Absorption and attenuation coefficients have been corrected for instrumental temperature dependence in the ac-9s and for differences in temperature and salinity from the calibration water.  Preliminary corrections for residual scattering effects on absorption estimates have also been applied and some adjustments have been made in the as measurements to account for time lags caused by the in-line particle filter.
 
New Observations from BIOMAPER II were collected too recently (October 19-30, 1998) to be available at this time.  Preliminary analysis of results at sea, however, confirm that excellent data was collected and a variety of water types were sampled, including relatively high pigment (3-5 mg m-3) shallow waters (50-70 m) on and around Georges Bank and stratified waters of the deep (200-300 m) basins in the Gulf of Maine.
 
The AVPPO system has been successfully tested in autonomous mode in waters off Woods Hole.  High quality optical data were collected with the ac-9 and the radiometers.  With hourly sampling, diel changes and trends over several days were evident in both hydrographic and optical properties (Fig. 7).
 

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Figure 7.  Time series of hydrographic and optical properties collected from the AVPPO mooring during autonomous profiling tests off Woods Hole, MA.   Hourly vertical profiles allowed diurnal, tidal and weekly variations to be observed.

Impact/Applications
This research will contribute to a fundamental understanding of the sources of optical variability in coastal ocean systems.  This in turn has implications for better understanding of ecological processes in these regions, since there are strong connections between optical characteristics and plant biomass and primary production.  These connections span scales from single cells to the global ecosystem and optical techniques provide the potential for measurements that cover this range.  This work will also contribute to the development of approaches and methods for merging information from widely different observational perspectives to obtain consistent and unbiased views of how large natural systems function.  We anticipate that complementary spatial and temporal information will contribute to better understanding of the sources and mechanisms leading to optical variability in an important region of the coastal ocean.

References
Gallager, S. M., F. T. Thwaites, C. S. Davis, A. M. Bradley and A. Girard.  1998.  Time series measurements in the coastal ocean: The Autonomous Vertically Profiling Plankton Observatory (AVPPO).  Sea Technology.  Submitted.

Martin Traykovski L. V. and H. M. Sosik.  1998.  Optical classification of water types based on ocean color.  In: Ocean Optics XIV, S. G. Ackleson (ed.).  In press.

Sosik, H. M., R. E. Green and R. J. Olson.  1998.  Optical variability in coastal waters of the Northwest Atlantic.  In: Ocean Optics XIV, S. G. Ackleson (ed.).  In press.

Thwaites F. T., S. M. Gallager, C. S. Davis, A. M. Bradley, A. Girard and W. Paul. 1998.  A winch and cable for the autonomous vertically profiling plankton observatory. Proc. "Oceans 98".  In press.

Wiebe, P. H, T. K. Stanton, M. C. Benfield, D. Mountain and C. H. Greene.  1997.  Acoustical study of the spatial distribution of plankton on Georges Bank and the relationship between volume backscattering strength and the taxonomic composition of the plankton.  IEEE J. Oceanic Eng.  22: 445-464.
 

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