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Intensifying pressure on global aquatic resources and services due to population growth and climate change is inspiring new surveying technologies to provide science-based information in support of management and policy strategies. One area of rapid development is hyperspectral remote sensing: imaging across the full spectrum of visible and infrared light. Hyperspectral imagery contains more environmentally meaningful information than panchromatic or multispectral imagery and is poised to provide new applications relevant to society, including assessments of aquatic biodiversity, habitats, water quality, and natural and anthropogenic hazards. To aid in these advances, we provide resources relevant to hyperspectral remote sensing in terms of providing the latest reviews, databases, and software available for practitioners in the field. We highlight recent advances in sensor design, modes of deployment, and image analysis techniques that are becoming more widely available to environmental researchers and resource managers alike. Systems recently deployed on space- and airborne platforms are presented, as well as future missions and advances in unoccupied aerial systems (UAS) and autonomous in-water survey methods. These systems will greatly enhance the ability to collect interdisciplinary observations on-demand and in previously inaccessible environments. Looking forward, advances in sensor miniaturization are discussed alongside the incorporation of citizen science, moving toward open and FAIR (findable, accessible, interoperable, and reusable) data. Advances in machine learning and cloud computing allow for exploitation of the full electromagnetic spectrum, and better bridging across the larger scientific community that also includes biogeochemical modelers and climate scientists. These advances will place sophisticated remote sensing capabilities into the hands of individual users and provide on-demand imagery tailored to research and management requirements, as well as provide critical input to marine and climate forecasting systems. The next decade of hyperspectral aquatic remote sensing is on the cusp of revolutionizing the way we assess and monitor aquatic environments and detect changes relevant to global communities.

Hyperspectral data can be exploited to retrieve multiple biophysical parameters of the environment. We present results from applying a spectral optimization approach to retrieve bottom characteristics in a coral reef environment using airborne hyperspectral data.

Ocean color measured from satellites provides daily global, synoptic views of spectral water-leaving reflectances that can be used to generate estimates of marine inherent optical properties (IOPs). These reflectances, namely the ratio of spectral upwelled radiances to spectral downwelled irradiances, describe the light exiting a water mass that defines its color. IOPs are the spectral absorption and scattering characteristics of ocean water and its dissolved and particulate constituents. Because of their dependence on the concentration and composition of marine constituents, IOPs can be used to describe the contents of the upper ocean mixed layer. This information is critical to further our scientific understanding of biogeochemical oceanic processes, such as organic carbon production and export, phytoplankton dynamics, and responses to climatic disturbances. Given their importance, the international ocean color community has invested significant effort in improving the quality of satellite-derived IOP products, both regionally and globally. Recognizing the current influx of data products into the community and the need to improve current algorithms in anticipation of new satellite instruments (e.g., the global, hyperspectral spectroradiometer of the NASA Plankton, Aerosol, Cloud, ocean Ecosystem (PACE) mission), we present a synopsis of the current state of the art in the retrieval of these core optical properties. Contemporary approaches for obtaining IOPs from satellite ocean color are reviewed and, for clarity, separated based their inversion methodology or the type of IOPs sought. Summaries of known uncertainties associated with each approach are provided, as well as common performance metrics used to evaluate them. We discuss current knowledge gaps and make recommendations for future investment for upcoming missions whose instrument characteristics diverge sufficiently from heritage and existing sensors to warrant reassessing current approaches.

Coral reefs are biologically diverse and economically important ecosystems that are on the decline worldwide in response to direct human impacts and climate change. Ocean color remote sensing has proven to be an important tool in coral reef research and monitoring. Remote sensing data quality is driven by factors related to sensor design and environmental variability. This work explored the impact of sensor noise, defined as the signal to noise ratio (SNR), on the detection uncertainty of key coral reef ecological properties (bottom depth, benthic cover, and water quality) in the absence of environmental uncertainties. A radiative transfer model for a shallow reef environment was developed and Monte Carlo methods were employed to identify the range in environmental conditions that are spectrally indistinguishable from true conditions as a function of SNR. The spectrally averaged difference between remotely sensed radiance relative to sensor noise, &epsilon, was used to quantify uncertainty in bottom depth, the fraction of benthic cover by coral, algae, and uncolonized sand, and the concentration of water constituents defined as chlorophyll, dissolved organic matter, and suspended calcite particles. Parameter uncertainty was found to increase with sensor noise (decreasing SNR) but the impact was non-linear. The rate of change in uncertainty per incremental change in SNR was greatest for SNR &lt, 500 and increasing SNR further to 1000 resulted in only modest improvements. Parameter uncertainty was complicated by the bottom depth and benthic cover. Benthic cover uncertainty increased with bottom depth, but water constituent uncertainty changed inversely with bottom depth. Furthermore, water constituent uncertainty was impacted by the type of constituent material in relation to the type of benthic cover. Uncertainty associated with chlorophyll concentration and dissolved organic matter increased when the benthic cover was coral and/or benthic algae while uncertainty in the concentration of suspended calcite increased when the benthic cover was uncolonized sand. While the definition of an optimal SNR is subject to user needs, we propose that SNR of approximately 500 (relative to 5% Earth surface reflectance and a clear maritime atmosphere) is a reasonable engineering goal for a future satellite sensor to support research and management activities directed at coral reef ecology and, more generally, shallow aquatic ecosystems.

The biodiversity and high productivity of coastal terrestrial and aquatic habitats are the foundation for important benefits to human societies around the world. These globally distributed habitats need frequent and broad systematic assessments, but field surveys only cover a small fraction of these areas. Satellite-based sensors can repeatedly record the visible and near-infrared reflectance spectra that contain the absorption, scattering, and fluorescence signatures of functional phytoplankton groups, colored dissolved matter, and particulate matter near the surface ocean, and of biologically structured habitats (floating and emergent vegetation, benthic habitats like coral, seagrass, and algae). These measures can be incorporated into Essential Biodiversity Variables (EBVs), including the distribution, abundance, and traits of groups of species populations, and used to evaluate habitat fragmentation. However, current and planned satellites are not designed to observe the EBVs that change rapidly with extreme tides, salinity, temperatures, storms, pollution, or physical habitat destruction over scales relevant to human activity. Making these observations requires a new generation of satellite sensors able to sample with these combined characteristics: (1) spatial resolution on the order of 30 to 100-m pixels or smaller; (2) spectral resolution on the order of 5 nm in the visible and 10 nm in the short-wave infrared spectrum (or at least two or more bands at 1,030, 1,240, 1,630, 2,125, and/or 2,260 nm) for atmospheric correction and aquatic and vegetation assessments; (3) radiometric quality with signal to noise ratios (SNR) above 800 (relative to signal levels typical of the open ocean), 14-bit digitization, absolute radiometric calibration temporal resolution of hours to days. We refer to these combined specifications as H4 imaging. Enabling H4 imaging is vital for the conservation and management of global biodiversity and ecosystem services, including food provisioning and water security. An agile satellite in a 3-d repeat low-Earth orbit could sample 30-km swath images of several hundred coastal habitats daily. Nine H4 satellites would provide weekly coverage of global coastal zones. Such satellite constellations are now feasible and are used in various applications.

An autonomous surface vehicle instrumented with optical and acoustical sensors was deployed in Kane'ohe Bay, HI, U.S.A., to provide high-resolution, in situ observations of coral reef reflectance with minimal human presence. The data represented a wide range in bottom type, water depth, and illumination and supported more thorough investigations of remote sensing methods for identifying and mapping shallow reef features. The in situ data were used to compute spectral bottom reflectance and remote sensing reflectance R_(rs,λ) as a function of water depth and benthic features. The signals were used to distinguish between live coral and uncolonized sediment within the depth range of the measurements (2.5 m to 5 m). In situ R_(rs,λ) were found to compare well with remotely sensed measurements from an imaging spectrometer, the Airborne Visible and Infrared Imaging Spectrometer (AVIRIS), deployed on an aircraft at high altitude. Cloud cover and in situ sensor orientation were found to have minimal impact on in situ R_(rs,λ), suggesting that valid reflectance data may be collected using autonomous surveys even when atmospheric conditions are not favorable for remote sensing operations. The use of reflectance in the red and near infrared portions of the spectrum, expressed as the red edge height 〖REH〗_λ, was investigated for detecting live aquatic vegetative biomass, including coral symbionts and turf algae. The 〖REH〗_λ signal from live coral was detected in Kane'ohe Bay to a depth of approximately 4 m with in situ measurements. A remote sensing algorithm based on the 〖REH〗_λ signal was defined and applied to AVIRIS imagery of the entire bay and was found to reveal areas of shallow, dense coral and algal cover. The peak wavelength of 〖REH〗_λ decreased with increasing water depth, indicating that a more complete examination of the red edge signal may potentially yield a remote sensing approach to simultaneously estimate vegetative biomass and bathymetry in shallow water.

There is a pressing need to globally inventory and assess coastal and inland aquatic habitats; extremely valuable and productive regions that are vulnerable to global anthropogenic pressures and climatic change. Basic information about sessile communities (wetlands, coral reefs, and sea grasses) includes mapping their extent and distribution, which can be gleaned from spectral surface reflectance imagery at high spatial resolution, but moderate temporal resolution. Moderate to high temporal resolution is also required for detailed observations of sessile community change (e.g., phenology, disturbance) and high temporal resolution is required for environmental changes in the surrounding water, phytoplankton concentration and composition, and concentrations of sediment or chromophoric dissolved organic matter (CDOM). Current and upcoming satellite missions and technology could meet spatial and spectral challenges. Multiple orbiting and airborne platforms, along with a network of in situ measurements, could provide a more complete picture of how these vital resources are changing. This paper provides an overview of these resources.

Spectral variability in the visible, near-infrared and shortwave directional reflectance factor of beach sands and freshwater sheet flow is examined using principal component and correlation matrix analysis of in situ measurements. In previous work we concluded that the hyperspectral bidirectional reflectance distribution function (BRDF) of beach sands in the absence of sheet flow exhibit weak spectral variability, the majority of which can be described with three broad spectral bands with wavelength ranges of 350-450 nm, 700-1350 nm, and 1450-2400 nm. 1 Observing sheet flow on sand we find that a thin layer of water enhances reflectance in the specular direction at all wavelengths and that spectral variability may be described using four spectral band regions of 350-450 nm, 500-950 nm, 950-1350 nm, and 1450-2400 nm. Spectral variations are more evident in sand surfaces of greater visual roughness than in smooth surfaces, regardless of sheet flow.

Understanding how turbulence impacts marine floc formation and breakup is key to predicting particulate carbon transport in the ocean. While floc formation and sinking rate has been studied in the laboratory and in-situ, the breakup response to turbulence has attracted less attention. To address this problem, the breakup response of bentonite clay particles flocculated in salt water was studied experimentally. Flocs were grown in a large aggregation tank under unmixed and mixed aggregation conditions and then subjected to turbulent pipe flow. Particle size was quantified using microscope imaging and in-situ measurements obtained from standard optical oceanographic instruments; a Sequoia Scientific LISST-100X and two WET Labs ac-9 spectrophotometers. The LISST instrument was found to capture the breakup response of flocs to turbulent energy, though the resulting particle size spectra appear to have underestimated the largest floc lengthscales in the flow while overestimating the abundance of primary particles. Floc breakup and the resulting shift towards smaller particles caused an increase in spectral slope of attenuation as measured by the ac-9 instruments. The Kolmogorov lengthscale was not found to have a limiting effect on floc size in these experiments. While the flocs were found to decrease in overall strength over the course of the two-month experimental time period, repeatable breakup responses to turbulence exposure were observed. Hydrodynamic conditions during floc formation were found to have a large influence on floc strength and breakup response. A non-constant strength exponent was observed for flocs formed with more energetic mixing. Increased turbulence from mixing during aggregation was found to increase floc fractal dimension and apparent density, resulting in a shift in the breakup relationships to higher turbulence dissipation rates. The results suggest that marine particle aggregation and vertical carbon transport concepts should include the turbulence energy responsible for aggregate formation and the resulting impact on floc strength, density, and the disruption potential.

Until recently, optical processes in shallow water, where a large portion of solar photons penetrate to the ocean floor, has received little attention outside of a relatively small number of modeling and remote sensing investigations. In the open ocean, scales of variability in relation to optical attenuation length often permit the treatment of the inwater light field as a one-dimensional, depth-dependent problem. In shallow waters hosting productive benthic ecosystems, such as coral reefs or seagrasses, the in-water light field is often three-dimensional in character. In the past decade, quantitative investigations of benthic optical properties and the resulting shallow-water light field have been conducted, fueled by a variety of new sensors designed specifically to address the shallow water problem. Recent publications, as well as the papers contained in this volume, illustrate the rich diversity and interdisciplinary nature of shallow-water optical problems and highlight important issues that should attract closer attention in the future. When sunlight penetrates the ocean surface and propagates down into the water column, portions of the electromagnetic energy are absorbed and scattered at rates that are determined by, in addition to pure water, the concentrations of colored dissolved and particulate matter that make up the water mixture. The ocean radiative transfer problem and, to a lesser extent, the nature of optically important matter in ocean water is well understood in areas where the depth of the ocean floor is greater than the depth of sunlight penetration. In this situation, variability in the subsurface light field under a specified illumination condition and surface wave field (Walker 1994) is largely determined by the distribution of optically important matter dissolved and suspended in the water column, and light intensity generally decreases with depth in accordance with Beer’s Law (Jerlov 1976; Kirk 1983; Mobley 1994). In shallow water, where the depth is much less than the potential for light to penetrate, a large fraction of the subsurface light reaches the ocean floor, where portions of the light energy are absorbed, reflected back into the overlying water column, or re-emitted as fluorescence. The subsurface light field in shallow water is not only a function of the properties of the water mixture, but also of the depth and properties of the ocean floor. Depending on water depth and benthic optical properties, light intensity might decrease more rapidly than expected, remain constant throughout the water column, or even increase with depth (Maritorena et al. 1994). Although the fundamental radiative transfer processes do not change in response to water depth, the environmental context does, and this affects the assumptions and boundary conditions necessary to solve the radiative transfer problem. In the ocean volume, it is most often appropriate to treat the water column as plane-parallel and infinite in horizontal extent because the geometric scales of constituent variability are much greater than the length scales of optical propagation. This is typically the case in simulations of ocean pri1

Suspended particulate matter (SPM) is a key environmental indicator for rivers, estuaries, and coastal waters, which can be calculated from remote sensing reflectance obtained by an airborne or satellite imager. Here, algorithms from prior studies are applied to a dataset of in-situ at surface hyperspectral remote sensing reflectance, collected in three geographic regions representing different water types. These data show the optically inherent exponential nature of the relationship between reflectance and sediment concentration. However, linear models are also shown to provide a reasonable estimate of sediment concentration when utilized with care in similar conditions to those under which the algorithms were developed, particularly at lower SPM values (0 to 20 mg/L). Fifteen published SPM algorithms are tested, returning strong correlations of R2>0.7, and in most cases, R2>0.8. Very low SPM values show weaker correlation with algorithm calculated SPM that is not wavelength dependent. None of the tested algorithms performs well for high SPM values (>30 mg/L), with most algorithms underestimating SPM. A shift toward a smaller number of simple exponential or linear models relating satellite remote sensing reflectance to suspended sediment concentration with regional consideration will greatly aid larger spatiotemporal studies of suspended sediment trends.

THE ocean has considerable spatial and temporal heterogeneity in biomass and productivity owing in part to the effects of ocean circulation and mixing1,2. Water mass boundaries (fronts) in coastal waters are well-known sites of enhanced biological activity3,4. Comparatively little is known of open-ocean fronts, and one of the few biological studies of an oceanic front showed phytoplankton biomass at only slightly higher densities than in surrounding waters5. Here we present photographs and measurements from satellites, aircraft, ships and the Space Shuttle Atlantis which show dramatic biological responses to circulation and mixing processes associated with an open-ocean front. Breaking waves (whitecaps) caused by water turbulence and mixing, and very dark green water caused by extremely high concentrations (>20 mg of chlorophyll a per m3) of buoyant diatoms (Rhizosolenia sp.) made a distinct line in the sea visible for hundreds of kilometres. The line traced the northern edge of a westward-progagating (50 km per day) tropical instability wave (1,000-km wavelength) delineating the boundary between cold, upwelled waters and warmer waters to the north. High phytoplankton biomass and primary production associated with the extensive diatom patches may explain anecdotal observations of high animal abundance along this frontal boundary.

We present results of oceanographic surveys of visually turbid blooms of the coccolithophore Emiliania huxleyi in the Gulf of Maine during the summers of 1988, 1989 and 1990. In each year, hydrographic stations within the blooms could be distinguished from non-bloom stations on a temperature-salinity diagram. In 1988 and 1989 the blooms were confined to the surface waters of the central western Gulf of Maine; T-S analyses showed they occurred in higher salinity surface waters at stations characterized by a well-defined upper mixed layer overriding a sharp pycnocline. Nutrients (not measured in 1988) were near depletion in the surface waters of both bloom and non-bloom stations in 1989, with surface phosphate being lower in the bloom waters (0.02–0.16 μM in the top 15 m) than in non-bloom waters (0.21–0.49 μM). Phosphate was not as low in the surface waters of the 1990 bloom. The bloom that year was much smaller in areal extent than in 1988 or 1989, and was limited to the northern part of the Great South Channel and western Georges Bank area of the Gulf of Maine. T-S analyses indicated significant mixing of different water masses in the area of the bloom in 1990, with the bloom being confined to those stations having less dense surface waters, of lower salinity, than the non-bloom stations. There also was evidence of a subsurface salinity minimum beneath the bloom waters in 1990. Blooms of E. huxleyi with surface expressions of visually turbid waters do not occur every year in the Gulf of Maine, and we discuss possible causative factors, specifically as related to the age or maturity of surface waters and macro- and micro-nutrient levels, that could facilitate bloom formation and which could vary between years.

[1] We report high spatial resolution observations of optical proxies for suspended particles and dissolved matter measured at the boundary of the Connecticut River plume (CRP) in Long Island Sound (LIS) in April 2000 when river discharge was near the annual maximum. The magnitude of beam attenuation, cp, backscatter, bb, and absorption at short wavelengths indicated higher concentration of suspended particles and colored dissolved matter within the LIS relative to the adjacent CRP. The fractional backscatter from particles indicated relatively higher organic matter fraction within the LIS. An absorption feature centered at 429 nm (aPB), indicated the presence of pigmented heterotrophic bacteria unique to the LIS and the strongest signals were observed at locations closest to shore. The spectral slope of beam attenuation associated with particulate matter (γ) indicated the presence of relatively larger particle assemblages within the LIS. Strong linear relationships between γ and salinity were observed within the energetic CRP boundary region, within 30 m of the front location. Regression residuals indicated a shift to smaller particles and were greatest at the front and decreased with distance toward the plume interior with a length scale similar to previous reports of the kinetic energy dissipation rate. At the same time, the magnitude of cp and bb remained uniform. These results are consistent with the disruption of low fractal dimension particle assemblages due to enhanced turbulence and mixing. The residuals in γ were weakly correlated with salinity and aPB suggesting that aggregate disruption was primarily associated with entrained Long Island Sound water.

Flow cytometric measurements of light scatter, near-forward light scatter (theta = 1.5-19 degrees ) and side scatter (theta = 73-107 degrees ), from individual marine particles is modeled using Mie theory. Particles are assumed to be homogeneous and nearly spherical in shape. Uniform polystyrene microspheres and oil suspensions are used to estimate model accuracy. Within the particle diameter range of 1-32 microm, the mean error for near-forward scatter and side scatter is 16.9% and 30.1%. respectively. The model is used to estimate size and refractive index of several nannoplankton species and the results are compared with microscopic measurements of cell size and published values of phytoplankton refractive index. Within the refractive-index range of 1.01-1.1, the model may be inverted to yield refractive index with an absolute error of between 0.01 and 0.003. Measurements of particle size distributions in clear ocean water indicate this range accounts for 99% of all nannoplankton and 39% of all particles within the size range from 1 to 10 microm.

As a result of the 1991 Persian Gulf war, between mid-January and June 1991, the Persian Gulf was contaminated with an estimated 4 to 6 million barrels of crude oil, released directly into the Gulf from refinement facilities, transhipment terminals, and moored tankers along the coast of Kuwait, and precipitated from oil fire smoke plumes. To assess the environmental impact of the oil, an international team of marine scientists representing 14 nations was assembled under the auspices of the United Nations International Oceanic Commission and the Regional Organization for Protection of the Marine Environment to conduct detailed surveys of the Persian Gulf, the Strait of Hormuz, and the Gulf of Oman, including hydrographic, chemical, and biological measurements. To supplement the field surveys and to serve as an aid in data interpretation, astronauts aboard the Space Shuttle Atlantis photographed water features and coastal habitats in the Persian Gulf during mission STS-45 (24 March to 02 April 1992). The astronauts collected 111 hand-held, color photographs of the Gulf (72 70-mm photographs and 39 5-inch photographs) from an altitude of 296 km (160 n.mi.). The photographs reveal distributions in water turbidity associated with outflow from the Shatt-al-Arab and water circulation along the entire coast of Iran and the Strait of Hormuz, coastal wetlands and shallow-water habitats, and sticks appearing in the sunglint pattern, which appear to be oil.

A description is presented of the activities and results of the Space Shuttle mission STS-45, known as the Mission to Planet Earth. Observations of Mount St. Helens, Manila Bay and Mt. Pinatubo, the Great Salt Lake, the Aral Sea, and the Siberian cities of Troitsk and Kuybyshev are examined. The geological features and effects of human activity seen in photographs of these areas are pointed out.

THE spring phytoplankton bloom in temperate and boreal waters represents a pulsed source of organic carbon that is important to ecosystem productivity1 and carbon flux2 in the world ocean. It is widely accepted that the seasonal development of a thermocline, in combination with increasing solar elevation in spring, is requisite for the development of the bloom in shelf and open ocean environments3–7. Here we report results for the offshore waters of the Gulf of Maine which suggest that the spring bloom can precede the onset of vertical water column stability, and may even be a contributing factor in the development of the thermocline. Deep penetration of light in relatively clear, late-winter waters, and weak, or absent, wind-driven vertical mixing, appear to support cell growth rates that overcome the vertical excursion rates in the neutrally stable water column, leading to a bloom. Phytoplankton forms typical of a spring bloom, including gelatinous colonies and chains, may contribute to the cells' ability to maintain a vertical position in a water column lacking stability.

Two coccolithophore blooms in the Gulf of Maine were studied in 1988 and 1989. Each bloom was about 50,000 km* in area and confined to the top 20 m of the water column. Maximal cell concentrations were -2,000 cells ml-* and coccolith densities of 3 x lo5 ml-’ were observed. The coccolith : cell ratio was highest in the bloom center (region of most intense reflectance) and lowest at the bloom periphery, an indication of varying organic vs. inorganic C production. Chlorophyll concentrations were generally low within the bloom and no relation could be observed between major nutrients and coccolithophore abundance. Backscattered light was profoundly affected by coccolith density and was slightly wavelength-dependent. We calculated total backscattering as well as backscattering (bh) caused exclusively by coccoliths and derived the algorithm relating coccolith density to backscattering. Although cells were efficient light absorbers, coccoliths showed negligible light absorption. Diffuse attenuation was lowest in the green and blue-green part of the visible spectrum. At the center of the bloom, coccoliths contributed ~75% of the backscattering signal and > 50% of the beam attenuation signal. The most accurate way to estimate coccolith concentrations via remote sensing is to measure water-leaving radiance in the green wavebands. The coccolithophore Emiliania huxleyi (Lohm) Hay et al. Mohler (class Prymnesiophyceae) is thought to be the most abunI Present address: Lockheed Engineering and Sciences, P.O. Box 58561, Houston, Texas 77258. Acknowledgments Many thanks to Capt. Donald Bradford and the crew of the RV Argo Maine for ship handling and help with sampling. Shiptime for leg 1 of the 1988 cruise was provided by Charles S. Yentsch. Howard Gordon provided the light scattering photometer as well as the software for calculating total backscattering. Robert Evans and Jody Splain arranged for the transfer of AVHRR data from Miami to Bigelow Lab. Janet Campbell and Thor Aarup analyzed the satellite data during the cruise and relayed the information to the ship. Stephen Groom calculated the visible band reflectance from the AVHRR data. Jeffrey Brown, Tracy Skinner, and Albert Chapin were instrumental in completing many measurements at sea and in the laboratory. Dave Townsend coordinated the CTD measurements. Elin Haugen and R. R. L. Guillard provided an inverted microscope for the 1988 cruise. Christodant calcifying organism on earth (Westbroek et al. 1985). Of all coccolithophore species, E. huxleyi is numerically dominant and can be found from tropical to subarctic regions of the Atlantic, extending into waters with temperatures

[1] We report observations of the structure of the front that surrounds the plume of the Connecticut River in Long Island Sound (LIS). Salinity, temperature, and velocity in the near-surface waters were measured by both towed and ship-mounted sensors and an autonomous underwater vehicle. We find that the plume front extends south from the mouth of the river, normal to the direction of the tidal flow in LIS and then curves to the east to parallel the tidal current. The layer depth at the front and the cross-front jumps in salinity and near-surface velocity all tend to decrease as distance from the source increases. This is qualitatively consistent with the prediction of layer models. In the across-front direction, the plume layer depth increases from zero to the asymptotic value within a few times the plume depth (∼5 m). Vertical motion is generated in this zone, and there is evidence of overturning. Farther from the front, the high-frequency salinity standard deviation decays exponentially with a length scale of 30 m. Assuming that the salinity fluctuations are a consequence of turbulence, we find that the rate of turbulent kinetic energy dissipation decreases exponentially in the across-front direction with a decay scale LG ≈ 15 m. Estimates based on AUV-mounted shear probes are consistent with this estimate. We present an explanation of the physics that determines LG and provide a simple formula to guide the choice of resolution in models that are designed to resolve the frontal structure.

The optical properties of North Sea phytoplankton (refractive index and fluorescence-specific scattering) were measured using flow cytometry in July 1987 during a research cruise sponsored by the Plymouth Marine Laboratory and the Environmental Research Council of the UK. Refractive index and associated size distributions of chlorophyll-containing cells were quite broad and did not provide unique descriptions of North Sea water masses. Fluorescence-specific scattering was found to increase within northern and central North Sea waters. AVHRR channel 1 data from satellite overpasses prior to and during the initial stages of the cruise indicate that a large coccolithophore bloom was developing within the northern portion of the North Sea and within Norwegian coastal waters. This bloom may account for the large values of fluorescence-specific scattering.

[1] The magnitude of beam attenuation attributed to suspended sediments, cp, and the slope of the log-transformed attenuation spectrum, γ, were used to investigate the properties and dynamic nature of matter suspended in the waters of western Long Island Sound (LIS) and the adjacent Connecticut River plume (CRP). Within the LIS, cp and γ indicate a robust relationship between sediment concentration and particle size distribution (PSD). As concentration increased, the PSD shifted to larger particles. The highest concentrations and particle sizes were found in a nepheloid layer adjacent to the sound floor. Within the adjacent CRP, sediments were observed to shift towards smaller particles at the lateral plume boundary, where current shear stress may have disrupted large particle aggregates, relative to sediments in the more quiescent plume interior. A strong linear correlation between γ and salinity was also found, indicating that mixing between the two water masses may also have altered the PSD of the plume sediments. A suspended sediment attenuation model based on Mie theory, a power law form of the PSD, and a single sediment source indicates that the observed changes in LIS cp and γ are consistent with sediment removal as particles settle with size-dependent rates. In contrast, within the CRP, the model supports the hypothesis that turbulence-induced aggregate disruption at the lateral plume boundary is responsible for the observed variability in cp and γ. However, mixing between the LIS and CRP particle populations would violate the single source requirement of the model and necessitate a more complicated set of particle dynamics in order to correctly interpret the observed variability in particulate attenuation.

It is well-known that in the ocean, where the depth of the ocean floor is large compared with the attenuation length of irradiance, the diffuse attenuation coefficients for vector and scalar irradiance (K-functions) are not affected by the optical properties or proximity of the ocean floor. This is the case of an optically deep ocean where the attenuation coefficients are determined solely by the inherent optical properties of the water and the distribution of radiance. Since, within optical-deep water, variability in the K- functions due to radiance distribution are small relative to the effects of the inherent optical properties of water, K- functions have been treated as quasi-inherent optical properties. Furthermore, when the depth of the ocean floor is shallow enough so that it becomes illuminated by down- welling irradiance, i.e. when the ocean is optically shallow, the in-water light field is modified by the optical properties of the ocean floor. The effect decreases with increasing depth and distance from the ocean floor. It is not generally appreciated, however, that the associated K- functions will also be affected by both the optical properties and the proximity of the ocean floor and, therefore, cannot be treated as quasi-inherent optical properties. If these effects are neglected, large errors, exceeding 25 percent in some cases, can result from modeling the optically shallow scalar irradiance profile as a function of a constant diffuse attenuation coefficient.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The papers appearing in this book comprise the proceedings of the Ocean Optics XIII Conference. They reflect the authors' opinions and are published as presented and without change, in the interests of timely dissemination. Some of the topics include: Radiative Transfer Theory, Optical Properties and In-Situ and Remote Sensing Sensors Systems.

In modeling the optical properties of and radiative transfer within ocean water, a common assumption is that the optical properties of the various dissolved and particulate constituents per unit concentration are constant. While this assumption is probably valid for non-living materials, it is frequently not the case with phytoplankton. Living cells are dynamic in their size, shape, and composition and respond readily (time scale of minutes to hours) to changes in the environment. Since each of these factors plays a role in determining the light scatter and absorption properties of the cell, the assignment of optical constants to phytoplankton cells, especially within Case I waters, may introduce significant errors in any theoretical treatment of radiative transfer and may result in erroneous interpretations of bulk optical measurements. Single-cell light scatter and beam attenuation of several species of marine phytoplankton, Thalassiosira pseudonana (clone 3H), an unidentified prasinophyte (clone OMEGA 48-23), Paviova sp. (clone NEP), Emiliania huxleyi (clone BT6), and an unidentified cryptomonad (clone 1D2), respond rapidly to increased light intensities. Flow cytometric determinations of cell refractive index n, measured at 488 nm, was found to decrease under high light conditions while cell size D, expressed as equivalent spherical diameter, increased. Beam attenuation c, measured at 660 nm, was generally found to increase when cultures were exposed to high light intensities. With a constant cell concentration, the observed change in n tends to decrease beam attenuation, while an increase in D tends to increase c. The magnitude and sign of dc/dt will depend upon the relative change in mean cell refractive index ii and the mean cell size i In response to bright light, i5f 3H and NEP was found to increase more rapidly than particulate organic carbon (POC). POC normalized to total cell volume increased within NEP and decreased within 3H', while at the same time, decreased within both cultures, suggesting that the cells swelled. As a result of light induced changes in i and ! optical properties of cells can change independently of biomass.

In 1991, Optics & Photonics News published a review article on ocean optics1 that discussed the field of ocean optical research. Many advances have occurred during the past five years. The emphasis of much of the work has also shifted from the deep clear ocean to the coastal environment. The present status and future directions of the field are discussed.

A coupled atmosphere and ocean radiative transfer model, the Gulf of Maine (GOM) model, was developed to simulate water-leaving radiance from a vertically stratified ocean containing a bloom of the coccolithophore Emiliania huxleyi. The model is based largely on atmospheric and ocean data representing the Gulf of Maine. The atmospheric submodel simulates direct sunlight and diffuse skylight illuminating the sea surface and is adjusted to account for seasonal changes in atmospheric aerosols. The optical properties of E. huxleyi, required by the ocean submodel, are derived from measurements collected in Gulf of Maine coccolithophore blooms occurring in 1989 and 1990. The modeled response of volume reflectance to the combined effects of chlorophyll and particulate calcite compares favorably with field measurements of E. huxleyi cell abundance and coastal zone color scanner (CZCS)-derived volume reflectance representing a coccolithophore bloom in the northeast Atlantic Ocean. The GOM model was used to investigate the response of normalized water-leaving radiance, modeled for visible CZCS and sea viewing wide field of view sensor (SeaWiFS) bands, to particulate calcite and chlorophyll a. Ranges in the concentrations of particulate calcite, chlorophyll a, and colored dissolved organic material (CDOM) are selected to represent conditions reported for coccolithophore blooms. Water-leaving radiance increases with increasing particulate calcite concentration, primarily because of a disproportionately large amount of backscatter from detached coccoliths (about an order of magnitude larger than is predicted using Mie theory). As a result, CZCS plant pigment algorithms based upon radiance ratios may be corrupted more severely than previously estimated. As an alternative to radiance ratio-based algorithms, an iterative procedure (also referred to as optimization) is used to invert the GOM model in order to simultaneously retrieve particulate calcite and chlorophyll a concentrations. The approach uses normalized water-leaving radiance computed for all visible CZCS or SeaWiFS bands. Tests of the approach suggest that independent of errors associated with instrument calibration and atmospheric correction, errors in the retrieved concentrations are small, even when high concentrations of CDOM and vertical structure within the water column are neglected, i.e., with the assumptions that CDOM concentration is small and the water is vertically homogeneous. However, since there are no data sets of contemporaneous chlorophyll a concentration, particulate calcite concentration, and CZCS imagery, a rigorous test of the model and inversion technique must wait for the launch of new ocean color scanners such as the NASA SeaWiFS.

Images of distinctive slicks in the Gulf of Mexico collected by two orbital platforms are presented. An analytical expression is derived for the formation of floating slicks based on a parameterization of seafloor flow rate, downstream movement on the surface, half-life of floating oil, and threshold thickness for detection. From application of this equation to the lengths of the observed slicks, it is suggested that the slicks in the Atlantis photograph and in the TM image represent seepage rates of 2.2-30 cu m/1000 sq km s and 1.4-18 cu m/1000 sq km d, respectively. Generalizing to an annual rate suggests that total natural seepage in this region is of the order of at least 20,000 cu m/yr (120,000 barrels/yr).

Landsat MSS and TM imagery, obtained simultaneously over Guinea Marsh, VA, as analyzed and compares for its ability to detect submerged aquatic vegetation (SAV). An unsupervised clustering algorithm was applied to each image, where the input classification parameters are defined as functions of apparent sensor noise. Class confidence and accuracy were computed for all water areas by comparing the classified images, pixel-by-pixel, to rasterized SAV distributions derived from color aerial photography. To illustrate the effect of water depth on classification error, areas of depth greater than 1.9 m were masked, and class confidence and accuracy recalculated. A single-scattering radiative-transfer model is used to illustrate how percent canopy cover and water depth affect the volume reflectance from a water column containing SAV. For a submerged canopy that is morphologically and optically similar to Zostera marina inhabiting Lower Chesapeake Bay, dense canopies may be isolated by masking optically deep water. For less dense canopies, the effect of increasing water depth is to increase the apparent percent crown cover, which may result in classification error.

A Coulter EPICS 741 Flow Cytometer was used at sea to measure distributions in phytoplankton refractive index and size within the North Sea. The optical data gathered by the flow cytometer, multi-band fluorescence and multi-angle light scatter, agreed well with coincident measurements of in vivo fluorescence. By gating on fluorescence, unique subpopulations of phytoplankton may be isolated optically from all other particles within a natural population. At several coastal stations, a unique subpopulation of phytoplankton was found which appeared to contain both chlorophyll and phycoerythrin. Using Mie scatter theory and flow cytometric measurements of light scatter in the near-forward and side directions, this population was found to be composed of cells of refractive index between 1.07 and 1.095 and spherical diameter between 7 and 9 pm (verified using Coulter volume measurements).

When solar radiation penetrates the sea, it is both scattered and absorbed at rates which depend upon the concentration of dissolved organic substances and suspended particulate matter present in the water column. Both scattering and absorption act in unison to produce a rate of attenuation. It is well known that as a transformation from clear oceanic waters to more turbid coastal waters occurs, short wavelengths of light are attenuated more quickly while yellow light penetrates the deepest. This phenomenon is termed “Yellow Shift.” Light penetration measurements at selected wavelengths (430, 470, 514.5, 540, and 600 nm ± 10 nm) taken in August, 1977 in the upper 200 km of Chesapeake Bay from the mouth of the Susquehanna River, suggest that there may be an estuarine “Orange Shift.” It was found for 50% of all light penetration measurements that orange light (600 nm) exhibited the smallest average attenuation coefficient (i.e., orange light penetrated the deepest). Beam transmission volume attenuation coefficients (α) calculated for stations from the mouth of the Susquehanna River to just offshore (6.3 km) from the mouth of Chesapeake Bay indicate that Chesapeake Bay can be divided into three turbidity zones: Estuarine Turbidity Maximum (α values of 3.50 to 4.77 ln/m); Estuarine Turbidity Normal (2.79 to 2.91); Estuarine Turbidity Minimum (2.73 to 2.76). The Turbidity Maximum of northern Chesapeake Bay is examined by comparing the relationship of contours of attenuation coefficient, turbidity, suspended sediment, salinity and chlorophyll a.

Data collected in controlled laboratory experiments indicate that nutrient concentration can induce changes in the singlecell light scatter properties, as measured using flow cytometry, and beam attenuation of the marine phytoplankton Emiliania huxleyi. Nutrient-induced optical variability is due primarily to the production of external scales called coccoliths, which are composed almost entirely of calcium carbonate. Coccolith production appears to be inversely related to the concentration of nitrate. Cultures maintained in low nitrate conditions are heavily plated and ten to twenty times as many individual coccoliths are found suspended within the media. The result is an increased single-cell light scatter in both the forward and side directions. Cells grown in high nutrient conditions are either lightly plated or completely nude and fewer detached coccoliths are found within the media. As a result, single-cell light scatter is decreased. In either case, beam attenuation at 670 nm appears to be dominated by light scatter from coccoliths rather than absorption by chlorophyll.

Procedures that may be used to evaluate the operational performance of a wide spectrum of geophysical models are introduced. Primarily using a complementary set of difference measures, both model accuracy and precision can be meaningfully estimated, regardless of whether the model predictions are manifested as scalars, directions, or vectors. It is additionally suggested that the reliability of the accuracy and precision measures can be determined from bootstrap estimates of confidence and significance. Recommended procedures are illustrated with a comparative evaluation of two models that estimate wind velocity over the South Atlantic Bight.

A two-flow model is developed to simulate a light field composed of both collimated and diffuse irradiance within natural waters containing a canopy of bottom-adhering plants. To account for the effects of submerging a canopy, the transmittance and reflectance terms associated with each plant structure (leaves, stems, fruiting bodies, etc.) are expressed as functions of the ratio of the refractive index of the plant material to the refractive index of the surrounding media and the internal transmittance of the plant structure. Algebraic solutions to the model are shown to yield plausible physical explanations for unanticipated variations in volume reflectance spectra. The effect of bottom reflectance on the near-bottom light field is also investigated. These results indicate that within light-limited submerged aquatic plant canopies, substrate reflectance may play an important role in determining the amount of light available to the plants and, therefore, canopy productivity.