Microscale Quantification of the Absorption by Dissolved and Particulate Material in Coastal Waters with an ac-9

Measuring coastal and oceanic absorption coefficients of dissolved and particulate matter in the visible domain usually requires a methodology for amplifying the natural signal because conventional spectrophotometers lack the necessary sensitivity. The WET Labs ac-9 is a recently developed in situ absorption and attenuation meter with a precision better than ±0.001 m−1 in the raw signal, which is sufficient to make these measurements in pristine samples. Whereas the superior sensitivity of the ac-9 has been well documented, the accuracy of in situ measurements for bio-optical applications has not been rigorously evaluated. Obtaining accurate results with an ac-9 requires careful attention to calibration procedures because baselines drift as a result of the changing optical properties of several ac-9 components. To correct in situ measurements for instrument drift, a pressurized flow procedure was developed for calibrating an ac-9 with optically clean water. In situ, micro- (cm) to fine- (m) scale vertical profiles of spectral total absorption, at(λ), and spectral absorption of dissolved materials, ag(λ), were then measured concurrently using multiple meters, corrected for drift, temperature, salinity, and scattering errors and subsequently compared. Particulate absorption, ap(λ), was obtained from at(λ) − ag(λ). CTD microstructure was simultaneously recorded. Vertical profiles of ag(λ), at(λ), and ap(λ) were replicated with different meters within ±0.005 m−1, and spectral relationships compared well with laboratory measurements and hydrographic structure

Temporal variability of suspended matter in Astoria Canyon

Suspended matter in Astoria canyon was monitored by means of an in situ nephelometer and by means of light-scattering and particle concentration measurements performed aboard ship on water samples. Nephelometer profiles obtained along the axis of the canyon in February and April 1973 indicate that the canyon is divided into two distinct zones: a nearshore zone in which the suspensoid distribution undergoes large changes and an offshore zone in which the distribution varies to a much less degree. A 15-hour time series of light-scattering and particle concentration profiles at a depth of 1100 m in the canyon shows extensive and rapid changes in suspended matter concentration at several depths. The effect of nonsteady state distributions of suspended matter on calculations of the coefficient of eddy diffusivity is examined and shown to be an important consideration in a submarine canyon

Optical and hydrographic observations of the Cromwell Current between 92°00'W and the Galapagos Islands

Optical and hydrographic observations were made at two meridional sections across the Cromwell Current at 92°00’ and 91°40’W during February 1969. The distribution of hydrographic and optical properties near the Galapagos Islands is described. Beneath the thermocline the Cromwell Current is characterized by deepening of the light-scattering isolines. Above the thermocline, shallowing of the light-scattering isolines results in a surface minimum in light scattering directly above the core of the current. Splitting of the core of the current into a northern and less clearly indicated southern branch is inferred from the horizontal distributions of temperature, oxygen content, and light scattering just beneath the depth of maximum horizontal velocity

Distribution of suspended matter in the Panama basin

The distribution of suspended matter in the Panama basin was determined by means of light scattering and Coulter counter measurements on water samples collected at 50 hydrographic stations. The observed distribution indicates three probable sources of suspended matter: (1) the surface waters throughout the basin; (2) erosion and runoff from the continents; and (3) bottom erosion at two sites on the Carnegie Ridge. The distribution of suspended matter also confirms in most respects the pattern of abyssal circulation proposed by Norman P. Laird (1971)

Global relationships of the inherent optical properties of the oceans

We have collected data during eight separate research cruises from open ocean to estuarine oceanic environments. Inherent optical property data collected during these cruises were incorporated into a large database totaling 1914 samples. The range of each inherent optical property within this database spans over 2 orders of magnitude. Using this database, we examine the spectral relationships of each of the inherent optical properties based on the measurements made at 488 nm. The results of this study show that there are dependencies in the individual inherent optical properties (IOP) spectra that are linearly related. The information from the regression models s used to explain the linear dependencies observed in the global data set. A separate data set collected from a recent cruise is used to compare regional relationships with the global. The implication of this research is that over a diversity of ocranic regimes, there are fundamental, first-order relationships in the individual IOP spectra. These relationships can provide an estimate of the individual IOP spectral relationships when no information about the IOP is available, as is often the case in ocean color remote sensing. More detailed models, however, may be necessary in order to more accurately predict the IOP spectral relationships on regional scales where the expected range of variability is small

Microscale Quantification of the Absorption by Dissolved and Particulate Material in Coastal Waters with an ac-9

Measuring coastal and oceanic absorption coefficients of dissolved and particulate matter in the visible domain usually requires a methodology for amplifying the natural signal because conventional spectrophotometers lack the necessary sensitivity. The WET Labs ac-9 is a recently developed in situ absorption and attenuation meter with a precision better than ±0.001 m⁻¹ in the raw signal, which is sufficient to make these measurements in pristine samples. Whereas the superior sensitivity of the ac-9 has been well documented, the accuracy of in situ measurements for bio-optical applications has not been rigorously evaluated. Obtaining accurate results with an ac-9 requires careful attention to calibration procedures because baselines drift as a result of the changing optical properties of several ac-9 components. To correct in situ measurements for instrument drift, a pressurized flow procedure was developed for calibrating an ac-9 with optically clean water. In situ, micro- (cm) to fine- (m) scale vertical profiles of spectral total absorption, a[subscript]t(λ), and spectral absorption of dissolved materials, a[subscript]g(λ), were then measured concurrently using multiple meters, corrected for drift, temperature, salinity, and scattering errors and subsequently compared. Particulate absorption, a[subscript]p(λ), was obtained from a[subscript]t(λ) − a[subscript]g(λ). CTD microstructure was simultaneously recorded. Vertical profiles of a[subscript]g(λ), a[subscript]t(λ), and a[subscript]p(λ) were replicated with different meters within ±0.005 m⁻¹, and spectral relationships compared well with laboratory measurements and hydrographic structur

Characteristics, Distribution and Persistence of Thin Layers Over a 48 Hour Period

The biological and physical processes contributing to planktonic thin layer dynamics were examined in a multidisciplinary study conducted in East Sound, Washington, USA between June 10 and June 25, 1998. The temporal and spatial scales characteristic of thin layers were determined using a nested sampling strategy utilizing 4 major types of platforms: (1) an array of 3 moored acoustical instrument packages and 2 moored optical instrument packages that recorded distributions and intensities of thin layers; (2) additional stationary instrumentation deployed outside the array comprised of meteorological stations, wave-tide gauges, and thermistor chains; (3) a research vessel anchored 150 m outside the western edge of the array; (4) 2 mobile vessels performing basin-wide surveys to define the spatial extent of thin layers and the physical hydrography of the Sound. We observed numerous occurrences of thin layers that contained locally enhanced concentrations of material; many of the layers persisted for intervals of several hours to a few days. More than one persistent thin layer may be present at any one time, and these spatially distinct thin layers often contain distinct plankton assemblages. The results suggest that the species or populations comprising each distinct thin layer have responded to different sets of biological and/or physical processes. The existence and persistence of planktonic thin layers generates extensive biological heterogeneity in the water column and may be important in maintaining species diversity and overall community structure

Hyperspectral temperature and salt dependencies of absorption by water and heavy water in the 400-750 nm spectral range

The temperature and salt dependencies of absorption by liquid water (H₂O) and heavy water (D₂O) were determined using a hyperspectral absorption and attenuation meter (WET Labs, AC-S). Sodium chloride (NaCl) was used as a proxy for seawater salts. There was no significant temperature (ψₜ) or salt (ψₛ) dependency of absorption at wavelengths 550 nm, ψₜ exhibited peaks at ~604, 662, and 740 nm. A small negative trough in ψₛ occurred at ~590 nm, followed by a small positive peak ~620 nm, a larger negative trough at ~720 nm, and a strong positive peak at ~755 nm. The salt dependency of absorption by heavy water, ψₛᴴ, exhibited a negative power-law shape with very low ψₛᴴ, at wavelengths >550 nm. Our experiments with NaCl, clean open ocean seawater, and artificial seawater support the hypothesis that salts modify the absorption spectra of seawater by modifying the molecular matrix and vibrations of pure water

Spectral particulate attenuation and particle size distribution in the bottom boundary layer of a continental shelf

Spectral attenuation and absorption coefficients of particulate matter and collocated hydrographic measurements were obtained in the Mid-Atlantic Bight during the fall of 1996 and the spring of 1997 as part of the Coastal Mixing and Optics experiment. Within the bottom boundary layer (BBL) the magnitude of the beam attenuation decreased and its spectral shape became steeper with distance from the bottom. Concurrently, the slope of the particulate size distribution (PSD) was found to increase with distance from the bottom. Changes in the PSD shape and the magnitude of the beam attenuation as functions of distance from the bottom in the BBL are consistent with particle resuspension and settling in the BBL, two processes that are dependent on particle size and density. For particles of similar density, resuspension and settling would result in a flattening of the PSD and an increase in the beam attenuation toward the bottom. In both fall and spring the magnitude of the particle attenuation coefficient correlates with its spectral shape, with a flatter shape associated with higher values of the attenuation. This observation is consistent with idealized optical theory for polydispersed nonabsorbing spheres. According to this theory, changes in the steepness of the particle size distribution (particle concentration as a function of size) will be associated with changes in the steepness of the attenuation spectra as a function of wavelength; a flatter particle size distribution will be associated with a flatter attenuation spectrum. In addition, the observed ranges of the beam attenuation spectral slope and PSD exponent are found to be consistent with this theory