September 6, 2008
Innovations in Optics for Coastal And Open-Ocean Mooring Applications
By Chang, Grace Honda, Makio; Nencioli, Francesco
Optical Sensors for Research and Long-Term Monitoring of the Underwater Environment The utility of ocean optics for environmental applications is based on decades of theoretical research and the development of technologies for underwater measurements of fundamental optical properties.The coupling between optical theory and technological advancements has resulted in widespread use of optical sensors for the monitoring and understanding of interdisciplinary properties and processes.
Long-term sustained monitoring of ocean processes on timescales from minutes to years is performed most effectively from moorings.
This article discusses technological innovations in moored underwater optical sensors: spectral resolution, optical scattering and anti-biofouling. Applications that have used moored observations of optical properties in coastal and open-ocean environments are presented, followed by an example of novel optical techniques for biogeochemical research.
Innovations in Optical Sensors
Hyperspectral Technology. Optical technology has seen a lot progress in recent decades, due in part to increasing sophistication in optical applications. Optical parameters that were once obtainable only by laborious laboratory techniques can now be measured in-situ on moorings at unprecedented temporal scales and multiple depths, as well as at hyperspectral wavelength resolution. Hyperspectral is defined here as continuous spectral coverage over a broad wavelength range with a better than 10-nanometer resolution.
Hyperspectral technology has been applied to sensors for moored measurements of inherent optical properties (IOPs) - optical properties that depend only on the aquatic medium. WET Labs (Philomath, Oregon) has developed the ac-spectra, a hyperspectral absorptionattenuation meter that is now commercially available. This sensor uses a linear variable filter scanning spectrometer to provide in-situ absorption, attenuation and scattering by difference at better than five-nanometer-wavelength resolution in the visible. The ac-spectra has been deployed on moorings to identify and characterize ecosystem dynamics and biogeochemical properties and processes, including river plumes and sediment resuspension.1
Radiometric properties can also be measured at hyperspectral wavelength resolution. These properties are important for computation of the apparent optical properties (dependent on the IOPs and the geometry of the light field) for relating i?-water optics to remotelysensed images and for in-situ estimates of primary production. Satlantic (Halifax, Canada) has developed commercially available radiometers with an approximate 3.5-nanometer-wave-length resolution, from the near ultraviolet (350 nanometers) to near infrared (800 nanometers). The hyperspectral capability is made possible by a 256-channel silicon photodiode array, analog-to- digital converters and an adaptive gain for selectable integration time. Hyperspectral radiometers have been deployed on moorings for remote sensing validation, characterization of biogeochemical processes and computation of upper-ocean radiant heating rates.1
Optical Scattering. Optical theory suggests that light scattering by particles depends on characteristics such as particle size, shape and composition. Thus, spectral scattering measured in natural waters can provide information about the nature of the particles contained within it. Quantification of scattering in the backward direction is important for interpretation of ocean color measurements; backscattering is directly related to reflectance. These optical applications have provided the impetus for the development of in-situ sensors for measurements of spectral and angular scattering of oceanic environments.
WET Labs has developed a series of user-configurable and compact sensors for in-situ observations of backscatter- ing at a wide range of temporal and spatial scales and, importantly, at multi- ple wavelengths and angles. Eco sen- sors are designed such that the light emitting diodes and detectors occupy the same flat face. Eco- backscattering sensors provide measurements of the backscattering coefficient at up to nine different wavelengths at 117[degrees] or threeangle volume scattering function (VSF) meters (100, 125 and 150[degrees]) at one to three wavelengths. The VSF quantifies the angular distribution of scattered light, in this case in the backward direction.
Near-forward angle scattering measurements are now routinely made in-situ on moored platforms using Sequoia Scientific's (Mercer Island, Washington) Laser In-Situ Scattering and Transmissometry (LISST) sensors. LISST sensors measure a weighted form of the VSF, which is geometrically corrected to derive the optical VSF. Angular ranges covered by LISST sensors are from 0.017 to 20[degrees]. The wavelength of measurement is typically 670 nanometers. Particle diffraction theory allows the measurement of scattering at multiple angles to be mathematically inverted to derive the particle size distribution in log-spaced size bins. The LISST sensors are capable of characterizing and quantifying sediment resuspension, land-ocean runoff and size-dependent settling of suspended particles. In-situ VSF sensors that encompass both the forward and backward angles do exist, but they are still in development and currently primarily deployed on shortterm measurement platforms or used in the laboratory.
Anti-Biofouling. With the advent of compact data storage systems and energy-efficient devices, biofouling is now the major limiting factor for long-term underwater deployments of optical sensors, particularly in the coastal ocean. Therefore, active and passive anti-biofouling systems are being developed for open-faced and flow- through optical sensors. The use of copper for anti-biofouling has been widely adopted by the optics community. Active copper shutters or bio-wipers coupled with passive copper faceplates are available for many flat-faced optical sensors. These shutters and wipers are preprogrammed to open or wipe the optical window just prior to obtaining a measurement and then close following sensor operation. Copper tubing has been successfully employed as passive biofouling mitigation on flow-through systems. Flowthrough systems are also being fitted with active injection systems to deter biological growth. Preliminary chlorine injection tests show promise for extending mooring deployment durations of optical sensors.
Ocean Optics Applications
Coastal Ocean. A suite of optical properties was obtained on a mooring deployed for about three months in 25meter-deep water. Hyperspectral IOPs and radiometric quantities were obtained, as well as multispectral backscattering, chlorophyll fluorescence and ancillary chemical and physical data. Optical proxies indicated that four different optical water types were present over the time series, ranging from relatively clear and dominated by biogenic particles to extremely turbid with high concentrations of inorganic particles (the result of a storm-induced river plume).1 The distinction between particle types was enabled by technological developments in scattering sensors.
An optical classification technique, the spectral angle mapper (SAM), was applied to hyperspectral time series of reflectance, downwelling irradiance and absorption in an effort to distinguish plume waters from other optical types.' The SAM's success in isolating plume-like conditions from other optical water types was due to changes in the spectral shape of IOPs and radiometric properties, which were captured by novel optical sensors. Innovations in optics, in conjunction with techniques such as the SAM, are extremely valuable for scientific and coastal management applications, such as contaminant detection and tracking, harmful algal bloom identification and monitoring of storm-induced beach erosion and runoff.
Open Ocean. The time series of downwelling irradiance (Ed) and chlorophyll fluorescence were obtained using copper-shuttered optical sensors deployed at 40 meters' water depth on a deep-sea mooring in the Western Pacific Subarctic Gyre. The ratio E,i(555):Ed(443) correlated well with the integrated chlorophyll concentration (Chl) within the euphotic layer.-2 Optical observations were therefore used to estimate integrated biomass and primary productivity (PP). The model results compared extremely well with laboratory-based measurements of PP. Data from various time periods and locations in the North Pacific Ocean were applied to this model with great success, thereby extending the utility of these measurements and methods. Copper-shutter anti-biofouling technology enabled mooring deployments that extended beyond 400 days and, thus, sustained observations of the long-term biomass and PP variability as affected by the underwater light field. Quantification of open-ocean productivity is essential for understanding the biological pump and the global carbon cycle.
Looking Toward the Future
The E-Flux study was a recent shipbased research program designed to investigate the role eddies play in openocean productivity and biogeochemistry. Innovative optical sensors were used to obtain vertical profiles of hyperspectral absorption and attenuation coefficients, along with chlorophyll fluorescence and particle backscattering. Optical properties were collected within the cyclonic mesoscale eddy Opal during the E-Flux III field experiment in the lee of the Hawaiian Islands in March 2005.1 Opal was associated with upwelling of nutrient-rich waters and an increase in phytoplankton concentration in the deep chlorophyll maximum (DCM) at the center of the eddy. Microscopy revealed that the bloom was dominated by diatoms, with a layer of unhealthy diatoms and empty frustules above the DCM. The biological layers were observed in optical observations; vertical profiles of the attenuation coefficient exhibited a broader peak that extended to shallower depths (i.e., the layer with empty frustules) compared to the peak in ChI, which was confined to a narrower depth range (i.e., the DCM). The relationship between the backscattering ratio (bbp/bp) and the ratio of ChI to the attenuation coefficient (Chl/Cp) was used to optically characterize the water column into four distinct layers: surface (low values of Chl/Cp and high bbp/bp, dominated by small sized phytoplankton), 50 to 60 meters' depth (particle concentration and Chl increased with depth, bbp/bp and Chl/Cp inversely related, unhealthy diatoms and empty frustules), DCM (bbp/bp roughly constant, whereas Chl/Cp reached maximum values and increased ChI within frustules) and below DCM (higher values of bbp/bp for Chl/ ci. values similar to the surface layer and particle size reduction from degradation and remineralizaron of sinking cells).1
These optical characterizations were consistent with results obtained from laboratory analyses.
The results from the ?-Flux shipbased study indicate that long- term continuous observations of ecological and biogeochemical processes are necessary for understanding and quantifying enhanced biological production and carbon export. Spectral and angular backscattering properties show great potential for providing detailed information about phytoplankton populations and particle characteristics in general.
Importantly, backscattering properties must be known in order to accurately interpret synoptic-scale remotelysensed ocean color measurements. Together, these provide incentives for the continued development of in-situ autonomous backscattering sensors, including those for measurement of the backscattering coefficient at hyperspectral wavelength resolution and quantification of scattering at high angular resolution over the entire VSF.
Innovations in optical sensors are improving the ability to quantify biogeochemical variability in coastal and open-ocean environments. Newly developed observational platform technologies (e.g., moored autonomous profilers, autonomous underwater vehicles IAUVs]) equipped with optical sensors show promise for providing observations of ecosystem dynamics on relevant temporal and spatial scales as well as in real time.
Fixed-depth or profiling moorings, deployed in concert with AUVs, traditional ship-based methods and satellite remote sensing, would enable longterm, sustained observations and advanced scientific understanding of phenomena affecting the biogeochemistry of the world's oceans.
(A) Optical sensors. (B) Wife versus Chl. Four optical water types indicated (red circles=plume conditions). (C) Plume conditions (black) separated.
(A, B) Copper shutters. (C) Seasonal variability of PP. Yellow=derived, white=measured, orange=bio-optically modeled, blue=satellite estimated.
(A) Opal. (B) cp(650) and ChI versus depth. (C) bbp/bp versus Chl/ cP. Backscattering data are uncalibrated; variability is accurate, but magnitude is high.
Innovations in optical sensors are improving the ability to quantify biogeochemical variability in coastal and open-ocean environments."
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1. Chang, G.C., A.H. Barnard, S. McLean, P.J. Egli, C Moore, J.R.V. Zaneveld, TD. Dickey and A. Hanson, "In-Situ Optical Variability and Relationships in the Santa Barbara Channel: Implications for Remote Sensing," Applied Optics, vol. 45, pp. 3,593- 3,604, 2006.
2. Honda, M.C., H. Kawakami, K. Sasaoka, S. Watanabe and T. Dickey, "Quick Transport of Primary Produced Organic Carbon to the Ocean Interior," Geophysical Research Letters, rei. no. 33, Ll 660, doi:10.1029/2006GL026466, 2006.
3. Benitez-Nelson, C.R., et al., "Mesoscale Eddies Drive Increased Silica Export in the Subtropical Pacific Ocean," Science, vol. 18, pp. 1, 017-1, 021, 2007.
By Dr. Grace Chang
University of California, Santa Barbara
Santa Barbara, California
Dr. Makio Honda
Mutsu Institute for Oceanography
Japan Agency for Marine-Earth Science and Technology
University of California, Santa Barbara
Santa Barbara, California
Dr. Grace Chang's primary research interest is the use of optical properties for inferring physical and biogeochemical processes. She is also active in the areas of sensor development and testing and observatory systems.
Dr. Makio Honda, a chemical oceanographer, has been conducting biogeochemical time series observations with autonomous sensors, such as optical sensors, water samplers and sediment traps in the North Pacific Ocean, over the last decade.
Francesco Nencioli is a Ph.D. student at the University of California, Santa Barbara. The focus of his dissertation is physical and biogeochemical modeling of mesoscale eddies in the lee of the Hawaiian Islands.
Copyright Compass Publications, Inc. Aug 2008
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