Variability In Inherent Optical Properties Research Articles

Bathymetry Retrieval Algorithm Based on Hyperspectral Features of Pure Water Absorption From 570 to 600 nm

Current efforts for improving the hyperspectral optimization processing exemplar (HOPE) model include further testing of remote-sensing reflectance (Rrs) features containing useful information for bathymetry retrieval via the minimization of the interference stemming from the variability in inherent optical properties and benthic reflectance. In this paper, we found a novel feature originating from the pure water absorption within the narrow spectral region of 570–600 nm. In most coastal regions of clear water, for example, in a coral reefs environment, pure water accounts for the majority of the total absorption in this spectral range. In addition to the depth variation, the spectral behavior of Rrs(570–600) is primarily dominated by a steep increase in pure water absorption with wavelength, whereas the influence of other optical properties such as phytoplankton/CDOM absorption, particle backscattering, and benthic reflectance can be simplified using the spectrally constant shape model. A HOPE pure water (HOPE-PW) algorithm using this feature was developed based on Rrs measurements with a spectral resolution of near 3.5 nm, wherein only four unknown parameters must be resolved. The validation from LiDAR data and comparison with HOPE-BRUCE using PRISM data at 15 sites located in five distinct regions of Palau, Guam, Great Barrier Reef, Hawaiian Islands and Florida Key, confirmed that the HOPW-PW yielded a considerable performance and provided adequate transferability to other sites with varying bottom and water environments. The sensitivity analysis based on Hydrolight-simulated datasets showed that HOPE-PW was less effected by bottom type variations but still had limitations in retrieving water optical properties.

Impact of Thermohaline Conditions on Vertical Variability of Optical Properties in the Gulf of Finland (Baltic Sea): Implications for Water Quality Remote Sensing

Vertical variability of inherent optical properties (IOPs) affect the water quality retrievals from remote sensing data. Here, we studied the vertical variability of IOPs and simulated apparent optical properties (AOPs) in the Gulf of Finland (Baltic Sea) under three characteristic (non)stratification conditions. In the case of mixed water column, the vertical variability of optically significant constituents (OSC) and IOPs was relatively small. While in case of stratified water column the IOPs of surface layer were three times higher compared to the IOPs below the thermocline and the IOPs were strongly correlated with the physical parameters (temperature, salinity). Measurements of IOPs in stratified water column showed that the ratio of scattering (b(440)) to absorption (a(440)) changed under the thermocline (b(440)/a(440) < 1) i.e., absorption became the dominant component of attenuation under thermocline while the opposite is true for the upper layer. Simulated (from IOPs) spectral irradiance reflectance (R(λ)) and spectral diffuse attenuation coefficient (Kd(λ)) from deeper layers (below thermocline) have significantly smaller magnitude and smoother shape. This becomes relevant during upwelling events—a common process in the coastal Baltic Sea. We quantified the effect of upwelling on surface water properties using simulated AOPs. The simulated AOPs (from IOPs measurements) showed a decrease of the signal up to 68.8% and an increase of optical depth (z90(λ)) from 2.3 to 4.3 m in the green part of the spectrum in case upwelled water mass reaches the surface. In the coastal waters a vertical decrease of Kd(λ) in the PAR region (400–700 nm) by 6.8% (surface to 20 m depth) was observed, while vertical decrease of chlorophyll-a (Chl-a) and total suspended matter (TSM) was 31.7 and 42.1%, respectively. The ratio R(490)/R(560)≥0.77 indicates also the upwelled water mass. The study showed that upwelling is a process that, in addition to biological activity, horizontal transport of OSC, and temperature changes, alters the optical signal of surface water measured by a remote sensor. Knowledge about the vertical variability of IOPs and AOPs relation to upwelling can help the parametrisation of remote sensing algorithms for retrieving water quality estimates in the coastal regions.

Inherent Optical Properties‐Reflectance Relationships Revisited

AbstractUnderstanding the relationship between remote sensing reflectance, Rrs(λ) and the inherent optical properties (IOPs) of natural waters is potentially a key to improving our ability to determine biogeochemical constituents from radiometric measurements. These relationships are usually described as a function of absorption, a(λ), and backscattering, bb(λ), coefficients, with the literature providing various forms of equation operating on either bb(λ)/a(λ) or bb(λ)/[a(λ)+bb(λ)] to represent the impact of variations in light field geometries and changes in sea‐water composition. The performance of several IOP‐reflectance relationships is assessed using HydroLight radiative transfer simulations covering a broad range of Case 1 and Case 2 water conditions. While early versions of IOP‐reflectance relationships assigned variability to associated proportionality factors (e.g., f/Q) or low‐order polynomial functions, recent studies have demonstrated relationships between Rrs(λ) and bb(λ)/[a(λ)+bb(λ)] are well‐characterized by nonlinear (high‐order polynomial), monotonic functions. This study demonstrates that this approach is also valid for relationships operating on bb(λ)/a(λ) and that there is no intrinsic benefit to functions operating on bb(λ)/[a(λ)+bb(λ)] compared to bb(λ)/a(λ) for Case 2 waters, contrary to recent suggestions in the literature. In all cases it is necessary to carefully consider the performance of best fit relationships across the full range of variability of IOPs and Rrs(λ), with higher order polynomials required to enable equivalent performance across the range of natural variability. The analysis further demonstrates insignificant wavelength sensitivity across the visible region, limited sensitivity to changes in solar zenith angle and extends to relationships for below surface remote sensing reflectance, rrs(λ).

Spatial and temporal variability of inherent and apparent optical properties in western Lake Erie: Implications for water quality remote sensing

Lake Erie has experienced dramatic changes in water quality over the past several decades requiring extensive monitoring to assess effectiveness of adaptive management strategies. Remote sensing offers a unique potential to provide synoptic monitoring at daily time scales complementing in-situ sampling activities occurring in Lake Erie. Bio-optical remote sensing algorithms require knowledge about the inherent optical properties (IOPs) of the water for parameterization to produce robust water quality products. This study reports new IOP and apparent optical property (AOP) datasets for western Lake Erie that encapsulate the May–October period for 2015 and 2016 at weekly sampling intervals. Previously reported IOP and AOP observations have been temporally limited and have not assessed statistical differences between IOPs over spatial and temporal gradients. The objective of this study is to assess trends in IOPs over variable spatial and temporal scales. Large spatio-temporal variability in IOPs was observed between 2015 and 2016 likely due to the difference in the extent and duration of mid-summer cyanobacteria blooms. Differences in the seasonal trends of the specific phytoplankton absorption coefficient between 2015 and 2016 suggest differing algal assemblages between the years. Other IOP variables, including chromophoric, dissolved organic matter (CDOM) and beam attenuation spectral slopes, suggest variability is influenced by river discharge and sediment re-suspension. The datasets presented in this study show how these IOPs and AOPs change over a season and between years, and are useful in advancing the applicability and robustness of remote sensing methods to retrieve water quality information in western Lake Erie.

In situ high frequency long term measurements of suspended sediment concentration in turbid estuarine system (Seine Estuary, France): Optical turbidity sensors response to suspended sediment characteristics

The aim of this study is to investigate the complex response of optical turbidity sensors (side- and back-scattering sensors) to Suspended Particle Matter (SPM) characteristics and the consequences when investigating SPM dynamics from long-term high frequency monitoring networks. Our investigation is based on the analysis of a unique dataset of monthly 12 h cycle measurements of SPM characteristics such as turbidity, concentration, floc size distribution, floc density and organic matter content in the macrotidal Seine Estuary (France) between February 2015 and June 2016. Results reveal that despite calibration to a Formazin standard, turbidity sensor response to SPM concentrations (in the range of 7–7000 mg L−1) are strongly variable, from the tidal scale to the annual scale and in different compartments of the Seine Estuary. The variability in the calibration relationships is related to changes in the sensor sensitivity according to (i) the sensor intern technology (mainly due to optical geometry) and (ii) the variability in inherent optical properties (IOP) of SPM.Side-scattering optical instruments (measuring scattering at 90°) provide at the annual scale a more stable optical response than backscattering sensor (measuring scattering at angles larger than 100°) for a wide variety of floc size and density in the estuarine environment, while at the tidal scale the backscatter sensors are the most accurate. Sensor sensitivity is strongly affected by floc characteristics, i.e. their median size D50, dry density ρ and the scattering efficiency Qb. Results highlight that the median particle diameter contribute to modify the scattering efficiency Qb as well as the dry density: Qb increases with increasing floc size, and for a given floc size, Qb increases with floc density.In this study, the results are next applied to turbidity data from the long-term automated monitoring network in order to estimate SPM concentrations and estimate the related uncertainties.

Second‐order variability of inherent optical properties of particles in Bohai Sea and Yellow Sea: Driving factor analysis and modeling

AbstractThis study explores second‐order variability of inherent optical properties (IOPs) of particles caused by particle intrinsic attributes, including the mean particle size (DA) calculated by a LISST measurement restricted to a size range of 2.5–500 μm, mean apparent density (ρa), bulk refractive index (ñp), and composition (using a ratio of chlorophyll a to total suspended matter concentrations, Chl a/TSM, as the delegate), obtained from five field cruise observations collected from 2013 to 2015 in Bohai Sea (BS) and Yellow Sea (YS). Large variations in magnitude were observed in terms of mass‐specific beam attenuation ( ), scattering ( ), and backscattering ( ), indicative of optically complex water conditions. and showed similarities in their change and driving mechanisms, in which DA explained the majority (35%), both ñp and ρa separately contributed variability levels of approximately 25%, and Chl a/TSM accounted for the lowest proportion (15%). Most (approximately 38%) of the variability in was found to be attributed to ñp when the other factors were responsible for the rest of the variation to different degrees of 9–29%. We also model , , and with direct linkages to DA, ρa, ñp, and Chl a/TSM using a multivariate linear regression method. The established , , and models generated very high goodness of fit results, with determination coefficients (R2) of above 0.970 (p < 0.001) and with favorable predictive errors (a mean absolute percentage error, MAPE, of approximately 30% for the and models and of approximately 40% for the model). Overall, the findings of this study further knowledge on the nature of the inherent optical properties in BS and YS, and are very helpful for understanding many marine biogeochemical processes.

Spatial variability of absorption coefficients over a biogeochemical gradient in a large and optically complex shallow lake

AbstractIn order to improve robustness of remote sensing algorithms for lakes, it is vital to understand the variability of inherent optical properties (IOPs) and their mass‐specific representations (SIOPs). In this study, absorption coefficients for particulate and dissolved constituents were measured at 38 stations distributed over a biogeochemical gradient in Lake Balaton, Hungary. There was a large range of phytoplankton absorption (aph(λ)) over blue and red wavelengths (aph(440) = 0.11–4.39 m−1, aph(675) = 0.048–2.52 m−1), while there was less variability in chlorophyll‐specific phytoplankton absorption (a*ph(λ)) in the lake (a*ph(440) = 0.022 ± 0.0046 m2 mg−1, a*ph(675) = 0.010 ± 0.0020 m2 mg−1) and adjoining wetland system, Kis‐Balaton (a*ph(440) = 0.017 ± 0.0015 m2 mg−1, a*ph(675) = 0.0088 ± 0.0017 m2 mg−1). However, in the UV, a*ph(350) significantly increased with increasing distance from the main inflow (Zala River). This was likely due to variable production of photoprotective pigments (e.g., MAAs) in response to the decreasing gradient of colored dissolved organic matter (CDOM). The slope of CDOM absorption (SCDOM) also increased from west to east due to larger terrestrial CDOM input in the western basins. Absorption by nonalgal particles (aNAP(λ)) was highly influenced by inorganic particulates, as a result of the largely mineral sediments in Balaton. The relative contributions to the absorption budget varied more widely than oceans with a greater contribution from NAP (up to 30%), and wind speed affected the proportion attributed to NAP, phytoplankton, or CDOM. Ultimately, these data provide knowledge of the heterogeneity of (S)IOPs in Lake Balaton, suggesting the full range of variability must be considered for future improvement of analytical algorithms for constituent retrieval in inland waters.

Adaptive semianalytical inversion of ocean color radiometry in optically complex waters

To address the challenges of the parameterization of ocean color inversion algorithms in optically complex waters, we present an adaptive implementation of the linear matrix inversion method (LMI) [J. Geophys. Res.101, 16631 (1996)], which iterates over a limited number of model parameter sets to account for naturally occurring spatial or temporal variability in inherent optical properties (IOPs) and concentration specific IOPs (SIOPs). LMI was applied to a simulated reflectance dataset for spectral bands representing measured water properties of a macrotidal embayment characterized by a large variability in the shape and amplitude factors controlling the IOP spectra. We compare the inversion results for the single-model parameter implementation to the adaptive parameterization of LMI for the retrieval of bulk IOPs, the IOPs apportioned to the optically active constituents, and the concentrations of the optically active constituents. We found that ocean color inversion with LMI is significantly sensitive to the a priori selection of the empirical parameters g0 and g1 of the equations relating the above-surface remote-sensing reflectance to the IOPs in the water column [J. Geophys. Res.93, 10909 (1988)]. When assuming the values proposed for open-ocean applications for g0 and g1 [J. Geophys. Res.93, 10909 (1988)], the accuracy of the retrieved IOPs, and concentrations was substantially lower than that retrieved with the parameterization developed for coastal waters [Appl. Opt.38, 3831 (1999)] because the optically complex waters analyzed in this study were dominated by particulate and dissolved matter. The adaptive parameterization of LMI yielded consistently more accurate inversion results than the single fixed SIOP model parameterizations of LMI. The adaptive implementation of LMI led to an improvement in the accuracy of apportioned IOPs and concentrations, particularly for the phytoplankton-related quantities. The adaptive parameterization encompassing wider IOP ranges were more accurate for the retrieval of bulk IOPs, apportioned IOPs, and concentration of optically active constituents.

Influence of suspended particle concentration, composition and size on the variability of inherent optical properties of the Southern North Sea

Suspended particles play an important role in coastal waters by controlling to a large extent the variability of the water inherent optical properties (IOPs). In this study, focused on the complex waters of the Southern North Sea, the relationships between the concentration, composition and size of suspended particles and their optical properties (light absorption, and attenuation in the visible and near-infrared spectral regions) are investigated. Over a one-year period, field measurements were carried out along regular transects from the Belgian to the English coasts to cover a wide gradient of water masses. Results show that the area can be divided into three geographical zones, each one having specific biogeochemical and optical properties: Scheldt coastal zone (SCZ), Middle of the Southern North Sea (MSNS) and Thames coastal zone (TCZ). Concentrations of organic (inorganic) particles were always higher in the SCZ (TCZ). The MSNS was characterized by a high proportion of organic particles in low concentration. The spectral shape of particle attenuation reveals a wide range from negative to positive slopes. Particle size distributions reveal a power-law shape along the coasts (especially in the TCZ) and a bimodal distribution in the MSNS notably during the spring phytoplankton bloom. This bimodal size distribution and more precisely a size peak around 7μm results in an unexpected negative spectral slope of the particle attenuation coefficient. Variations in the particulate mass-specific IOPs between the three regions were observed to predominate over seasonal variations. The implications in terms of inversion of IOPs into biogeochemical parameters, such as chlorophyll a and total suspended matter, in coastal waters are discussed.

Characterization of the bio-optical anomaly and diurnal variability of particulate matter, as seen from scattering and backscattering coefficients, in ultra-oligotrophic eddies of the Mediterranean Sea

Abstract. The variability of inherent optical properties is investigated in the ultra-oligotrophic waters of the Mediterranean Sea sampled during the BOUM experiment performed during early summer 2008. Bio-optical relationships found for ultra-oligotrophic waters of the three anticyclonic gyres sampled significantly depart from the mean standard relationships provided for the global ocean, confirming the peculiar character of these Mediterranean waters. These optical anomalies are diversely related to the specific biological and environmental conditions occurring in the studied ecosystem. Specifically, the surface specific phytoplankton absorption coefficient exhibits values lower than those expected from the general relationships mainly in relation with a high contribution of relatively large sized phytoplankton. Conversely, the particulate backscattering coefficient, bbp, values are much higher than the mean standard values for a given chlorophyll-a concentration, TChl-a. This feature can presumably be related to the relevant influence of highly refractive submicrometer particles of Saharan origin in the surface layer of the water column. The present measurements also show that the Mediterranean Sea is greener than TChl-a alone indicates, as already stressed in previous studies. This color anomaly is partly explained by the estimated colored dissolved organic matter and submicrometer particles absorption coefficients, and to a greater extent by the high bbp/TChl-a values assuming that these particles backscatter light similarly in the green and blue parts of the visible spectrum. The diel variation of both the particulate matter attenuation and backscattering coefficients were also investigated specifically. Despite some differences in the timing and the magnitude of the daily oscillations found for these optical parameters, potential for the backscattering coefficient daily oscillation to be used, similarly to that for the attenuation coefficient, as a proxy for estimating carbon community production budget has been highlighted for the first time. This result is particularly relevant for present and future geostationary spatial ocean color missions.

Satellite retrieval of inherent optical properties in the turbid waters of the Yellow Sea and the East China Sea

In situ-satellite match-ups of radiometric data are established in the turbid waters of the Yellow Sea and the East China Sea. Inherent optical properties (IOPs) are retrieved by match-up radiometric data and multi-band quasi-analytical algorithm (QAA). By comparing in situ spectra-retrieved IOPs with the satellite ones of moderate resolution imaging spectroradiometer (MODIS) and medium resolution imaging spectrometer (MERIS), the accuracy of satellite-derived IOPs is quantified. The median of the absolute percentage difference is found to be approximately 20% for the total absorption coefficient at(\lambda) at green and blue-green bands, and 30% for particulate material backscattering coefficient bbp(\lambda) throughout the visible bands. The spatial pattern and temporal variability of IOPs along the eastern coast of China are clarified based on satellite images and the QAA model.

Optical closure in a complex coastal environment: particle effects

An optical dataset was collected on a mooring in the Santa Barbara Channel. Radiative transfer modeling and statistical analyses were employed to investigate sources of variability of in situ remote sensing reflectance [r(rs)(lambda,4 m)] and the f/Q ratio. It was found that the variability of inherent optical properties and the slope of the particle size distribution (xi) were strongly related to the variability of r(rs)(lambda,4 m). The variability of f/Q was strongly affected by particle type characteristics. A semianalytical radiative transfer model was applied and effects of variable particle characteristics on optical closure were evaluated. Closure was best achieved in waters composed of a mixture of biogenic and minerogenic particles.

Bio‐optical consequences of viral infection of phytoplankton: I. Experiments with the cyanobacterium, Synechococcus sp.

Four strains of the cyanobacteria, Synechococcus, were infected with viruses isolated and purified from coastal waters of the Gulf of Maine. Changes in host and virus concentration were observed in time course experiments, along with associated variability in inherent optical properties. There were strong optical shifts before and after lysis in inherent optical properties on time scales commensurate with the viral infection process. Specifically, both backscattering and absorption (particularly in visible wavelengths) increased slightly after infection, but decreased markedly after lysis. The most rapid optical change was seen in in vivo chlorophyll fluorescence, with a slight increase within 2 d after initial infection. Such changes likely reflect fundamental changes in host photophysiology after infection. After host lysis, there was a major drop in bulk Chl a fluorescence, synchronous with a major increase in the ratio of free viruses per host cell. The net effect of the changes in fluorescence and host concentration was an increase in the fluorescence per cell. These optical changes were accompanied by major changes in the submicron particle size spectra as cells were lysed, releasing their intracellular contents.

Inherent optical properties of the Irish Sea and their effect on satellite primary production algorithms

Three cruises were conducted in the Irish Sea during May, June and July 2001 to determine the variability in inherent optical properties (IOP), photo physiological parameters and primary production (PP) and to assess the effect of IOP on satellite PP algorithms. The absorption coefficients of phytoplankton (aph), coloured dissolved organic material (aCDOM) and nonalgal particles (aNAP) were higher during May than June and July. A radiative transfer model was used to model the in-water light field based on aph (case 1) and aph, aCDOM and aNAP (case 2). When PP was compared using these light fields, there was a 46% difference in estimates. The case 2 in-water light field was coupled to a wavelength resolving satellite model of PP (PPcase2) and had a low root mean square error (RMS) (0.27 log10PP) compared with in situ PPcase2. IOP absorption, especially aCDOM, had a significant effect on the performance of this algorithm, but scattering of light by suspended particulate material had a small effect. A look-up table was generated from the in situ aph, aCDOM and aNAP measurements, which can be used in conjunction with satellite products to produce satellite maps of PP. There was <25% difference between in situ PPcase2 and the satellite PP maps, which suggests that they could be produced routinely and accurately to monitor PP in the Irish Sea and other coastal and estuarine areas.

Bio-optical and biogeochemical properties of different trophic regimes in oceanic waters

To examine the source and magnitude of the variability of bio-optical properties in open ocean, we simultaneously measured inherent optical properties (IOPs) and biogeochemical quantities during late summer from the eutrophic waters of the Moroccan upwelling to the oligotrophic waters of the northwestern Mediterranean and the ultraoligotrophic waters of the eastern Mediterranean. Vertical distributions of spectral absorption and attenuation coefficients were measured with a high-resolution in situ spectrophotometer (WETLabs ac9) together with biogeochemical measurements that included phytoplanktonic pigments and particulate organic carbon concentrations, particle size distributions, and picoplankton abundance. The variability in specific IOPs (i.e., per unit of biogeochemical constituent concentration) was examined, and an optical index of particle size was derived. The fine-scale vertical distributions of various biogeochemical properties were thus described from ac9 profiles. Particle attenuation and carbon budgets, estimated from a combination of optical and biogeochemical measurements, underlie a major contribution of nonalgal stocks in oceanic waters. We show that first-order variations in IOPs in oceanic waters are explained by the trophic state (i.e., chlorophyll a concentration) and that second-order variations are the result of changes in the composition of phytoplankton assemblage, the balance between algal and nonalgal stocks, and lightrelated processes (colored dissolved organic material photo-oxidation and algal photo-adaptation).

Seasonal variability in inherent optical properties in a western Norwegian fjord

We present measured seasonal variations in the inherent optical properties (the absorption and scattering coefficients) of water in a deep silled fjord (Samnangerfjorden) in western Norway. These were based on measurements taken at monthly intervals during an annual cycle. The measurements also include concentrations of chlorophyll a and yellow substance, which were assumed to dominate the behaviour of the absorption and scattering coefficients. The stations were at three fixed locations, one being placed in the innermost part of the fjord where there is little mixing of fjord water with water from the coastal current. The other two stations were placed at different distances from the mouth of the fjord, so that the water masses are characterized by different amounts of mixing between fjord water and coastal current water. Our data set shows how the absorption and scattering coefficients vary in a Norwegian fjord during an annual cycle, and how they depend on the concentrations of chlorophyll a and yellow substance. Values of the absorption coefficient at 412 nm varied between 0.1 and 2.0 m , and scattering coefficients were also found to vary within this range. Little variation over the spectral range was found for the scattering coefficients, but the absorption coefficient had larger spectral variations. The chlorophyll a concentrations varied from 0.01 to 6.3 mg m , and the concentration of yellow substance, as expressed by its absorption coefficient at 310 nm, was within the range 0.7–7.8 m .

Coastal optical properties estimated from airborne sensors: Reply to the comments by Hu and Carder

Gould and Arnone [Remote Sens. Environ. 61 (1997) 290] characterized the spatial and temporal variability of inherent optical properties (water absorption and scattering coefficients) in a coastal region using in situ shipboard and moored optical data, and high-resolution aircraft ocean color imagery. Following atmospheric correction of the aircraft imagery, they applied a bio-optical model [J. Geophys. Res. 104 (C3) (1999) 5403] to the image data to produce surface maps of absorption and scattering coefficients. The results from the model were compared with in situ measurements and adjustments were made to two model parameters to force the model results to more closely match the in situ measurements. Hu and Carder [Remote Sens. Environ. (2001)] raise questions concerning (a) the atmospheric correction applied to the aircraft data and (b) the reason for adjusting the parameters of the bio-optical model. Here we respond to their comments.