Non-algal Particles Absorption Research Articles

Estimation of chromophoric dissolved organic matter and non-algal particulate absorption coefficients of seawater in the ultraviolet by extrapolation from the visible spectral region.

Extending the capabilities of optical remote sensing and inverse optical algorithms, which have been commonly focused on the visible (VIS) range of the electromagnetic spectrum, to derive the optical properties of seawater in the ultraviolet (UV) range is important to advancing the understanding of various optical, biological, and photochemical processes in the ocean. In particular, existing remote-sensing reflectance models that derive the total spectral absorption coefficient of seawater, a(λ), and absorption partitioning models that partition a(λ) into the component absorption coefficients of phytoplankton, aph(λ), non-algal (depigmented) particles, ad(λ), and chromophoric dissolved organic matter (CDOM), ag(λ), are restricted to the VIS range. We assembled a quality-controlled development dataset of hyperspectral measurements of ag(λ) (N = 1294) and ad(λ) (N = 409) spanning a wide range of values across various ocean basins, and evaluated several extrapolation methods to extend ag(λ), ad(λ), and adg(λ) ≡ ag(λ) + ad(λ) into the near-UV spectral region by examining different sections of the VIS as a basis for extrapolation, different extrapolation functions, and different spectral sampling intervals of input data in the VIS. Our analysis determined the optimal method to estimate ag(λ) and adg(λ) at near-UV wavelengths (350 to 400 nm) which relies on an exponential extrapolation of data from the 400-450 nm range. The initial ad(λ) is obtained as a difference between the extrapolated estimates of adg(λ) and ag(λ). Additional correction functions based on the analysis of differences between the extrapolated and measured values in the near-UV were defined to obtain improved final estimates of ag(λ) and ad(λ) and then the final estimates of adg(λ) as a sum of final ag(λ) and ad(λ). The extrapolation model provides very good agreement between the extrapolated and measured data in the near-UV when the input data in the blue spectral region are available at 1 or 5 nm spectral sampling intervals. There is negligible bias between the modeled and measured values of all three absorption coefficients and the median absolute percent difference (MdAPD) is small, e.g., < 5.2% for ag(λ) and < 10.5% for ad(λ) at all near-UV wavelengths when evaluated with the development dataset. Assessment of the model on an independent dataset of concurrent ag(λ) and ad(λ) measurements (N = 149) yielded similar findings with only slight reduction of performance and MdAPD remaining below 6.7% for ag(λ) and 11% for ad(λ). These results are promising for integration of the extrapolation method with absorption partitioning models operating in the VIS.

Bio-optical properties of the Brazilian Abrolhos Bank’s shallow coral-reef waters

The Abrolhos Bank harbors the richest coral reef ecosystem in the South Atlantic Ocean. It exhibits unique geomorphologic structures, is localized in shallow depths, and is divided into two reef regions with an inner arc close to the coast (3–20 m depth) and an outer deeper arc (5–30 m depth). This study aims to describe some bio-optical properties of the Abrolhos Bank waters and to evaluate the performance of the inversion Hyperspectral Optimization Processing Exemplar (HOPE) model, developed to retrieve optical properties in shallow waters, in the region. To this end, measurements at 75 stations during two field campaigns conducted during the 2013 and 2016 wet seasons were analyzed, and the HOPE model was applied to both in situ remote sensing reflectance (Rrs) spectra and PRecursore IperSpettrale della Missione Applicativa (PRISMA) imagery. Significant differences in optical and biological properties were found between the two arcs. The empirical relationships between chlorophyll-a concentration (Chl-a) and absorption coefficient of phytoplankton at 440 nm (aphy(440)) diverged from Bricaud’s models, suggesting differences in phytoplankton diversity and cell size. In both arcs, total non-water absorption coefficient at 440 nm (aT-w(440)) was dominated by colored dissolved organic matter (CDOM) by ∼60%. Absorption coefficient by CDOM (acdom) presented a higher variability within the outer arc, with the lowest contribution from non-algal particles (NAPs), and the spectral slopes of aCDOM resembled those of the inner arc. The spectral slopes of the NAP absorption coefficient suggested a dominance by organic rather than mineral particles that probably originated from biological production. The HOPE model applied to in situ Rrs performed satisfactorily for depth in the Abrolhos Bank waters, although retrievals of aphy(440), CDOM plus NAP (adg(440)) and aT-w(440) were underestimated with a relative bias of −27.9%, −32.1% and −45.8%, respectively. The HOPE model retrievals from the PRISMA image exhibited low aphy(440) values over the whole scene and the highest adg(440) values in the Caravelas river plume. Very shallow depths (≤3 m), bottom substrate reflectance used as input in the HOPE model, model parametrization associated with the water complexity in the study site, and uncertainties associated to Rrs measurements used as input might be responsible for differences found when comparing HOPE retrievals with in situ measurements.

Seasonal and decadal variations in absorption properties of phytoplankton and non-algal particulate matter in three oceanic regimes of the Northwest Atlantic

Seasonal and inter-annual absorption properties of phytoplankton and non-algal particulate matter were studied in relation to phytoplankton biomass, as indexed by chlorophyll-a concentration, and presence of diatoms, as indexed by fucoxanthin concentration, using a 20-year time series of in situ data collected in the Northwest Atlantic. We found significant differences in the spatiotemporal variations of the bio-optical properties for three oceanic regimes: mesotrophic (Scotian Shelf), oligotrophic (Northwest Atlantic Basin, NAB), and subartic (Labrador Sea). The Scotian Shelf and NAB exhibited similar phenology with the spring and autumn blooms associated with low phytoplankton specific absorption, while only relatively high fucoxanthin concentration occurred in spring. The NAB showed a smaller seasonal variation than the Scotian Shelf in agreement with its oceanic conditions. The Labrador Sea showed a single phytoplankton bloom in spring followed by a continuous decrease in biomass the rest of the year. The relationship between phytoplankton absorption coefficient at 443 nm and chlorophyll-a concentration was consistent with other studies with coefficients that were region-dependent. Absorption by non-algal particulate matter remained between 5% and 60% of phytoplankton absorption with a mean of 15%. The slope of the non-algal particulate absorption varied with seasons and regions and appeared to depend on the trophic status with high values (i.e., up to 0.04) occurring during bloom conditions. We also introduced a new index, the phytoplankton apparent absorption wavelength (PAAW), a wavelength-weighted sum of absorption expressed in nanometers that provides information on the phytoplankton biomass and assemblage in a simple manner. Time series analysis of the PAAW revealed a decrease of this property in spring on the Scotian Shelf, NAB, and Labrador Sea and an increase in autumn on the Scotian Shelf and NAB, suggesting a shift in these ecosystems.

A novel semianalytical remote sensing retrieval strategy and algorithm for particulate organic carbon in inland waters based on biogeochemical-optical mechanisms

The estimation of particulate organic carbon (POC) concentrations from satellite images can provide crucial spatiotemporal continuous observation data for the carbon cycle and ecological environmental governance. Here, we developed a novel inversion algorithm for deriving POC in inland water based on remote sensing and geochemical isotopes, which is summarized as follows. First, we developed empirical relationships between the phytoplankton absorption coefficient and endogenous POC concentration (CendPOC) and between the nonalgal particulate absorption coefficient and terrestrial POC concentration (CterPOC). Second, based on the valid relationships, semianalytical retrieval models were established to estimate CendPOC and CterPOC. Third, the proportions of endogenous POC (RendPOC) and terrestrial POC (RterPOC) to the total POC concentration (CPOC) were derived using a three-band empirical model. Finally, CPOC was obtained by dividing CendPOC by RendPOC (RendPOC ≥ 0.5) or dividing CterPOC by RterPOC (RendPOC < 0.5). Validation with field data shows that our proposed algorithm can accurately derive CPOC (0–20 mg/L), with a root mean square deviation (RMSD), median bias (MB), median absolute percent difference (MAPD), and median ratio (MR) of 1.15 mg/L, −0.05 mg/L, 24%, and 0.98, respectively. Synchronous validation based on Sentinel-3/OLCI images confirmed the accuracy, with RMSD, MB, MAPD, and MR values of 0.41 mg/L, −0.16 mg/L, 28%, and 0.91, respectively. The algorithm was applied to ocean and land color sensor (OLCI) images to reveal the temporal and spatial variations in POC in Lake Taihu.

Seasonal evolution of particulate and dissolved absorption coefficients in a subtropical estuary

The seasonal evolution of colored dissolved organic matter (CDOM), phytoplankton, and non-algal particulate absorption coefficients were measured for the first time along a salinity gradient within the Paranaguá Estuarine Complex (PEC), in southern Brazil. CDOM (mostly of autochthonous origin) inside the estuary exhibited a conservative behavior that dominates the absorption budget at lower wavelengths (~380 nm). On average, absorption by non-algal particles was more important than absorption by phytoplankton throughout the year and across all wavelengths, except for the maximum absorption band of chlorophyll-a (665–675 nm), where the algal component was predominant year-round. Phytoplankton optical properties were generally consistent with variability in phytoplankton cell counts, particularly during bloom periods. Simultaneous condition of high suspended matter concentrations and low turbidity conditions associated with sampling of mucilaginous materials released by centric diatoms exemplified the complex relationships between biogeochemical and optical properties in this region. The relationship between chlorophyll-a concentration and phytoplankton absorption exhibited seasonal oscillations related to the variation of algal assemblages, ultimately determining the magnitude of the packaging effect. Finally, separation of the signals of these three components in the visible domain is possible only in the near-ultraviolet range (380 nm), associated with CDOM dominance, and at 665 nm where the phytoplankton signature can be isolated.

A global compilation of in situ aquatic high spectral resolution inherent and apparent optical property data for remote sensing applications.

Light emerging from natural water bodies and measured by radiometers contains information about the local type and concentrations of phytoplankton, non-algal particles and colored dissolved organic matter in the underlying waters. An increase in spectral resolution in forthcoming satellite and airborne remote sensing missions is expected to lead to new or improved capabilities for characterizing aquatic ecosystems. Such upcoming missions include NASA's Plankton, Aerosol, Cloud, ocean Ecosystem (PACE) mission; the NASA Surface Biology and Geology designated observable mission; and NASA Airborne Visible/Infrared Imaging Spectrometer - Next Generation (AVIRIS-NG) airborne missions. In anticipation of these missions, we present an organized dataset of geographically diverse, quality-controlled, high spectral resolution inherent and apparent optical property (IOP-AOP) aquatic data. The data are intended to be of use to increase our understanding of aquatic optical properties, to develop aquatic remote sensing data product algorithms, and to perform calibration and validation activities for forthcoming aquatic-focused imaging spectrometry missions. The dataset is comprised of contributions from several investigators and investigating teams collected over a range of geographic areas and water types, including inland waters, estuaries, and oceans. Specific in situ measurements include remote-sensing reflectance, irradiance reflectance, and coefficients describing particulate absorption, particulate attenuation, non-algal particulate absorption, colored dissolved organic matter absorption, phytoplankton absorption, total absorption, total attenuation, particulate backscattering, and total backscattering. The dataset can be downloaded from https://doi.org/10.1594/PANGAEA.902230 (Casey et al., 2019).

An investigation into the relationship between light absorption budget and trophic status in inland waters

The optical properties of the water can give comprehensive information about the in-water constituents which is directly related to concentration, size and composition, all essential for the development of bio-optical algorithms. Aiming to provide new observations about optical properties of water in two distinct reservoirs situated along the cascade system of Tietê River: Barra Bonita (BB) and Nova Avanhandava (Nav), this study described and discussed their inherent optical properties (IOPs) in terms of the absorption contribution. BB showed a dominance in the phytoplankton fraction at all visible wavelengths, and Nav by nonalgal particle absorption (aNAP), mainly at the blue and green wavelengths. The mass-specific absorption coefficient of phytoplankton at 443 nm observed in Nav and BB corroborated with the assumption that the trophic state decreases from oligotrophic to hypereutrophic. The aNAP(λ) did not show a seasonal pattern between field campaigns and reservoirs. The absorption coefficient of particulate enhanced the strong influence of phytoplankton pigments along the visible spectrum in BB, whereas Nav only displayed the peak at 675 nm represented by chlorophyll a absorption and high contribution of non-algal particle and colored dissolved organic matter in the blue region. These findings recommend the use of different modelling approaches to estimate water quality parameters from both sites when the inputs of the models consider the specific information about the IOPs and their relation with the in-water constituents.

Human-induced eutrophication dominates the bio-optical compositions of suspended particles in shallow lakes: Implications for remote sensing

Suspended particulate matter (SPM) is generally divided into inorganic (SPIM) and organic (SPOM) parts; they come from different sources, and have different impacts on the optical properties and/or water quality of lake. However, in a specific remote sensing process, they are not retrieved separately. Using in-situ data of 59 lakes along the middle and lower reaches of the Yangtze River (MLR-YR) in dry season (April) and wet season (August) in 2012, we first studied the absorption properties and sources of different SPM. On this basis, we proposed a workflow for simultaneously estimating SPIM and SPOM from satellite data. Our results are as follows: Bio-optical compositions of SPM in these eutrophic shallow lakes tempo-spatially varied greatly and were dominated by human-induced eutrophication. Phytoplankton contributed 18.42 ± 18.92% of SPIM and 26.22 ± 19.24% of SPOM in April 2012, but 30.4 ± 23.41% of SPIM and 47.03 ± 18.1% of SPOM in August 2012. The trophic state index explained 42.84% of SPOM variation in April 2012, and 54.64% in August 2012. Moreover, there were strong linear relationships between SPIM concentration and non-algal particle absorption coefficient (Pearson's r = 0.73; p < 0.01) and between SPOM concentration and phytoplankton absorption coefficient (r = 0.76; p < 0.01). Based on these results, SPIM and SPOM concentrations in the lakes along the MLR-YR could be retrieved from OLCI/Sentinel-3A satellite data, respectively. This study has a great significance for real-time monitoring and managing aquatic environment in various eutrophic and/or shallow lakes as a group.

Diurnal Variation of Light Absorption in the Yellow River Estuary

Considering the influence of river discharge and strong winds, the diurnal variability of ocean optical absorption properties in the Yellow River Estuary (YRE) is quantified, using in-situ measurements. The study finds that terrestrial sources due to the Yellow River discharge can cause high diurnal variation of water absorption because of the movement of river plume in the YRE, but such an influence diminishes far away from the Yellow River plume. The diurnal variability of water absorption, affected by strong winds, is found to be strengthened with a rapid increase of particles and colored dissolved organic matter (CDOM) arising from re-suspended sediment induced by wave forcing. The diurnal variability of particle absorption is controlled by non-algal particle absorption in the YRE, and the ratio of non-algal particle absorption (aNAP) and total particle absorption for most wavelengths is more than 0.56. The diurnal variation of spectral slope of non-algal particle absorption (SNAP) is found to vary within a narrow range, although large variability in the aNAP spectrum is observed. The CDOM is correlated negatively with salinity, and such negative correlation becomes weaker with the decreasing influence of riverine input. The spectral slope of CDOM absorption (Sg) may reflect the formation and constituents of CDOM with weak relationship to its concentration, and its relationship with the absorption of CDOM at 440 nm may be associated with the source of CDOM. The value of Sg, which is affected by re-suspended bottom sediment, is much lower than that derived from CDOM affected by Yellow River runoff. Disregarding the absorption of pure water, the diurnal variability of total water absorption stems principally from changes in non-algal particle matter rather than CDOM and Chl-a. By the observations of hourly GOCI (Geostationary Ocean Color Imager) data, the major diurnal variations of remote sensing reflectance at 680 nm are observed in near-coastal waters and the estuary of the Yellow River, which are mainly influenced by the flow discharge of Yellow River and strong winds. Finally, the seasonal differences of diurnal variations of water absorption caused by strong winds and river discharge are determined.

Particulate absorption properties in the Red Sea from hyperspectral particulate absorption spectra

This paper aims to describe the variability of particulate absorption properties using a unique hyperspectral dataset collected in the Red Sea as part of the TARA Oceans expedition. The absorption contributions by phytoplankton (aph) and non-algal particles (aNAP) to the total particulate absorption coefficients are determined using a numerical decomposition method (NDM). The NDM is validated by comparing the NDM derived values of aph and aNAP with simulated values of aph and aNAP are found to be in excellent agreement for the selected wavelengths (i.e., 443, 490, 555, and 676 nm) with high correlation coefficient (R2), low root mean square error (RMSE), mean relative error (MRE), and with a slope close to unity. Further analyses showed that the total particulate absorption coefficients (i.e., ap(443)average = 0.01995 m−1) were dominated by phytoplankton absorption (i.e., aph(443)average = 0.01743 m−1) with a smaller contribution by non-algal particles absorption (i.e., aNAP(443)average = 0.002524 m−1). The chlorophyll a is computed using the absorption based Line Height Method (LHM). The derived chlorophyll-specific absorption ((a*ph = aph(λ)/ChlLH)) showed more variability in the blue part of spectrum as compared to the red part of spectrum representative of the package effect and changes in pigment composition. A new parametrization proposed also enabled the reconstruction of a*ph(λ) for the Red Sea. Comparison of derived spectral constants with the spectral constants of existing models showed that our study A(λ) values are consistent with the existing values, despite there is a divergence with the B(λ) values. This study provides valuable information derived from the particulate absorption properties and its spectral variability and this would help us to determine the relationship between the phytoplankton absorption coefficients and chlorophyll a and its host of variables for the Red Sea.

Inherent Optical Properties of the Baltic Sea in Comparison to Other Seas and Oceans

In order to retrieve geophysical satellite products in coastal waters with high coloured dissolved organic matter (CDOM), models and processors require parameterization with regional specific inherent optical properties (sIOPs). The sIOPs of the Baltic Sea were evaluated and compared to a global NOMAD/COLORS Reference Data Set (RDS), covering a wide range of optical provinces. Ternary plots of relative absorption at 442 nm showed CDOM dominance over phytoplankton and non-algal particle absorption (NAP). At 670 nm, the distribution of Baltic measurements was not different from case 1 waters and the retrieval of Chl a was shown to be improved by red-ratio algorithms. For correct retrieval of CDOM from Medium Resolution Imaging Spectrometer (MERIS) data, a different CDOM slope over the Baltic region is required. The CDOM absorption slope, SCDOM, was significantly higher in the northwestern Baltic Sea: 0.018 (±0.002) compared to 0.016 (±0.005) for the RDS. Chl a-specific absorption and ad [SPM]*(442) and its spectral slope did not differ significantly. The comparison to the MERIS Reference Model Document (RMD) showed that the SNAP slope was generally much higher (0.011 ± 0.003) than in the RMD (0.0072 ± 0.00108), and that the SPM scattering slope was also higher (0.547 ± 0.188) vs. 0.4. The SPM-specific scattering was much higher (1.016 ± 0.326 m2 g−1) vs. 0.578 m2 g−1 in RMD. SPM retrieval could be improved by applying the local specific scattering. A novel method was implemented to derive the phase function (PF) from AC9 and VSF-3 data. b ˜ was calculated fitting a Fournier–Forand PF to the normalized VSF data. b ˜ was similar to Petzold, but the PF differed in the backwards direction. Some of the sIOPs showed a bimodal distribution, indicating different water types—e.g., coastal vs. open sea. This seems to be partially caused by the distribution of inorganic particles that fall out relatively close to the coast. In order to improve remote sensing retrieval from Baltic Sea data, one should apply different parameterization to these distinct water types, i.e., inner coastal waters that are more influenced by scattering of inorganic particles vs. open sea waters that are optically dominated by CDOM absorption.

High colored dissolved organic matter (CDOM) absorption in surface waters of the central-eastern Arctic Ocean: Implications for biogeochemistry and ocean color algorithms.

As consequences of global warming sea-ice shrinking, permafrost thawing and changes in fresh water and terrestrial material export have already been reported in the Arctic environment. These processes impact light penetration and primary production. To reach a better understanding of the current status and to provide accurate forecasts Arctic biogeochemical and physical parameters need to be extensively monitored. In this sense, bio-optical properties are useful to be measured due to the applicability of optical instrumentation to autonomous platforms, including satellites. This study characterizes the non-water absorbers and their coupling to hydrographic conditions in the poorly sampled surface waters of the central and eastern Arctic Ocean. Over the entire sampled area colored dissolved organic matter (CDOM) dominates the light absorption in surface waters. The distribution of CDOM, phytoplankton and non-algal particles absorption reproduces the hydrographic variability in this region of the Arctic Ocean which suggests a subdivision into five major bio-optical provinces: Laptev Sea Shelf, Laptev Sea, Central Arctic/Transpolar Drift, Beaufort Gyre and Eurasian/Nansen Basin. Evaluating ocean color algorithms commonly applied in the Arctic Ocean shows that global and regionally tuned empirical algorithms provide poor chlorophyll-a (Chl-a) estimates. The semi-analytical algorithms Generalized Inherent Optical Property model (GIOP) and Garver-Siegel-Maritorena (GSM), on the other hand, provide robust estimates of Chl-a and absorption of colored matter. Applying GSM with modifications proposed for the western Arctic Ocean produced reliable information on the absorption by colored matter, and specifically by CDOM. These findings highlight that only semi-analytical ocean color algorithms are able to identify with low uncertainty the distribution of the different optical water constituents in these high CDOM absorbing waters. In addition, a clustering of the Arctic Ocean into bio-optical provinces will help to develop and then select province-specific ocean color algorithms.

Effects of rainstorm on the spectral absorption properties of chromophoric dissolved organic matter and particles in Lake Qiandao

为了研究暴雨事件对千岛湖有色可溶性有机物（CDOM）和颗粒物吸收光谱的影响，利用2016年暴雨前（3月1-6日）和暴雨后（4月6-11日）采集的水样，对暴雨前、后千岛湖水体CDOM、浮游藻类和非藻类颗粒物的吸收光谱特征进行分析，探讨暴雨事件对其造成的影响.结果表明：千岛湖作为典型的深水型内陆湖泊，其CDOM、浮游藻类颗粒物和非藻类颗粒物的吸收强度较太湖等浅水型湖泊弱.暴雨前，CDOM光谱吸收系数a_CDOM（λ）值在0~0.6 m^-1范围内变化，其光谱拟合系数S_CDOM的均值为0.0158±0.00145 nm^-1.暴雨前浮游藻类光谱吸收在总颗粒物中占主导，a_ph（λ）在0~0.35 m^-1范围内变化，非藻类颗粒物光谱吸收系数a_NAP（λ）在0~0.15 m^-1范围内变化，其光谱拟合系数S_NAP均值为5.62±0.57 μm^-1；暴雨后CDOM光谱吸收系数a_CDOM（λ）值在0~1.6 m^-1范围内变化，其光谱拟合系数S_CDOM的均值为0.0157±0.00101 nm^-1.暴雨后浮游藻类光谱吸收系数a_ph（λ）在0~2.5 m^-1范围内变化，非藻类颗粒物光谱吸收在部分区域已占据主导地位，a_NAP（λ）在0~0.8 m^-1范围内变化，其光谱拟合系数S_NAP均值为5.72±0.68 μm^-1.由CDOM吸收特征值相对分子质量M值得出，暴雨前、后千岛湖不同区域CDOM组成都以富里酸为主，且暴雨前M值分布较均匀，暴雨后M值呈现从新安江向缓冲区、东南区递增的趋势，这说明西北区随暴雨输入的腐殖酸增加了CDOM的相对分子质量.暴雨对S_NAP值影响较大的区域为西北区、西南区、东北区，对西南区影响最小.本研究为使用光学手段深入探讨暴雨事件对千岛湖水环境的影响提供重要依据.;To study the influence of heavy rainfall on absorption characteristics of chromophoric dissolved organic matter (CDOM) and particles in Lake Qiandao, the samples were collected on March 1-6, 2016 (before the rainstorm) and April 6-11, 2016 (after the rainstorm). The spectral absorption of CDOM, phytoplankton and non-algal particles (NAP) in the lake were analyzed and the influence of rainstorm was discussed. The results showed that Lake Qiandao, as a typical deep inland lake, its spectral absorption coefficients of CDOM, phytoplankton and NAP is small, compared with shallow lakes like Lake Taihu. The CDOM spectral absorption coefficients varied in the range of 0-0.6 m^-1, the mean value of the spectral fitting coefficient of CDOM(S_CDOM) was 0.0158±0.00145 nm^-1 before the rainstorm. The spectral absorption of phytoplankton before the rainstorm dominated the absorption of total particles, varied in the range of 0-0.35 m^-1. The spectral absorption coefficients of NAP varied in the range of 0-0.15 m^-1, and the mean value of spectral fitting coefficient of NAP(S_NAP) was 5.62±0.57 μm^-1 before the rainstorm; The CDOM spectral absorption coefficients varied in the range of 0-1.6 m^-1 S_CDOM was 0.0157±0.00101 nm^-1 after the rainstorm. The spectral absorption of phytoplankton varied in the range of 0-2.5 m^-1. NAP absorption dominated the absorption of particles in some regions after the rainstorm, a_NAP(λ)varied in the range of 0-0.8 m^-1 and S_NAP was 5.72±0.68 μm^-1. The M value showed that CDOM composition of the different regions of the lake is mainly fulvic acid both before and after the rainstorm, the spatial distribution of M value is relative even before the rainstorm. However, the M value showed an increasing trend from the Xin'anjiang to the buffer area and southeastern area after the rainstorm, which indicates that humic acids increased the relative molecular mass of CDOM in the northwestern area. Rainstorm had the greatest impact on the northwestern, southwestern and northeastern areas regarding the value of S_NAP, with the smallest impact on the southwestern area. This study provides an important basis for the use of optical means to further explore the impact of heavy rainfall on aquatic environment of Lake Qiandao.

Variations in Spectral Absorption Properties of Phytoplankton, Non-algal Particles and Chromophoric Dissolved Organic Matter in Lake Qiandaohu

Light absorption by phytoplankton, non-algal particles (NAP) and chromophoric dissolved organic matter (CDOM) was investigated at 90 sites of a clear, deep artificial lake (Lake Qiandaohu) to study natural variability of absorption coefficients. Our study shows that CDOM absorption is a major contributor to the total absorption signal in Lake Qiandaohu during all seasons, except autumn when it has an equivalent contribution as total particle absorption. The exponential slope of CDOM absorption varies within a narrow range around a mean value of 0.0164 nm−1 ( s d = 0.00176 nm−1). Our study finds some evidence for thIS autochthonous production of CDOM in winter and spring. Absorption by phytoplankton, and therefore its contribution to total absorption, is generally greatest in spring, suggesting that phytoplankton growth in Lake Qiandaohu occurs predominantly in the spring. Phytoplankton absorption in freshwater lakes generally has a direct relationship with chlorophyll-a concentration, similar to the one established for open ocean waters. The NAP absorption, whose relative contribution to total absorption is highest in summer, has a spectral shape that can be well fitted by an exponential function with an average slope of 0.0065 nm−1 ( s d = 0.00076 nm−1). There is significant spatial variability present in the summer of Lake Qiandaohu, especially in the northwestern and southwestern extremes where the optical properties of the water column are strongly affected by the presence of allochthonous matter. Variations in the properties of the particle absorption spectra with depths provides evidence that the water column was vertically inhomogeneous and can be monitored with an optical measurement program. Moreover, the optical inhomogeneity in winter is less obvious. Our study will support the parameterization of the Bio-optical model for Lake Qiandaohu from in situ or remotely sensing aquatic color signals.

Variability of light absorption properties in optically complex inland waters of Lake Chaohu, China

Abstract Absorption coefficients of phytoplankton, colored detrital matter (CDM), non-algal particles (NAP), colored dissolved organic matter (CDOM), and their relative contributions to total non-water absorption ( a t − w ) are essential variables for bio-optical and radiative transfer models. Light absorption properties showed large range and variability sampled at 194 stations throughout Lake Chaohu between May 2013 and April 2015. The a t − w was dominated by phytoplankton absorption ( a ph ) and NAP absorption ( a d ). The contribution of CDOM absorption to a t − w was lower than 30%. Phytoplankton and NAP were the primary sources of spatial and vertical variability in absorption properties. Light absorption by CDOM, though significant in magnitude, was relatively constant. CDM absorption ( a dg ) was dominated by NAP. The spatial variation of the absorption coefficients from each of the optically active constituents were driven by several main inflow rivers in the western and middle part of Lake Chaohu. Algal blooms and bottom resuspension contributed to vertical variability as observed by phytoplankton and NAP profiles. Specific absorption of phytoplankton had significant spatial and seasonal variations without vertical variation. The spectral slope of absorption showed no significant spatial variability ( p  > 0.05). Variations of absorption affected different ranges of remote sensing reflectance ( R rs ) spectrum, thereby increasing the difficulty of applying the remote sensing algorithm in optically complex waters. Parameters and relationships presented in this study provide useful information for bio-optical models and remote sensing of lakes similar to Lake Chaohu in terms of optical properties.

Spatiotemporal Characterization of Chromophoric Dissolved Organic Matter (CDOM) and CDOM-DOC Relationships for Highly Polluted Rivers

Spectral characteristics of CDOM (Chromophoric dissolved organic matter) in water columns are a key parameter for bio-optical modeling. Knowledge of CDOM optical properties and spatial discrepancy based on the relationship between water quality and spectral parameters in the Yinma River watershed with in situ data collected from highly polluted waters are exhibited in this study. Based on the comprehensive index method, the riverine waters showed serious contamination; especially the chemical oxygen demand (COD), iron (Fe), manganese (Mn), mercury (Hg) and dissolved oxygen (DO) were out of range of the contamination warning. Dissolved organic carbon (DOC) and total suspended matter (TSM) with prominent non-homogenizing were significantly high in the riverine waters, but chlorophyll-a (Chl-a) was the opposite. The ternary phase diagram showed that non-algal particle absorption played an important role in total non-water light absorption (&gt;50%) in most sampling locations, and mean contributions of CDOM were 13% and 22% in the summer and autumn, respectively. The analysis of the ratio of absorption at 250–365 nm (E250:365) and the spectral slope (S275–295) indicated that CDOM had higher aromaticity and molecular weight in autumn than in summer, which is consistent with the results of water quality and the CDOM relative contribution rate. Redundancy analysis (RDA) indicated that the environmental variables OSM (Organic suspended matter) had a strong correlation with CDOM absorption, followed by heavy metals, e.g., Mn, Hg and Cr6+. However, for the specific UV absorbance (SUVA254), the seasonal values showed opposite results compared with the reported literature. The potential reasons were that more UDOM (uncolored dissolved organic matter) from human sources (wastewater effluent) existed in the waters. Terrigenous inputs simultaneously are in relation to the aCDOM(440)-DOC relationship with the correlation coefficient of 0.90 in the summer (two-tailed, p &lt; 0.01), and 0.58 in the autumn (two-tailed, p &lt; 0.05). Spatial distribution of the CDOM parameters exhibited that the downstream regions focused on dry land have high CDOM molecular weight and aromatic hydrocarbon. Partial sampling locations around the cities or countries generally showed abnormal values due to terrigenous inputs. As a bio-optical model parameter, the spectral characteristic of CDOM is helpful in adjusting the derived algorithms in highly polluted environments. The study on organic carbon and pollutants in highly polluted waters had an important contribution to global carbon balance estimation and water environment protection.

Light-absorbing properties of mineral particles in the Great Lakes

Abstract The light-absorbing attributes of minerogenic particle populations are presented for 32 sites from five areas of the Laurentian Great Lakes based on laboratory measurements of non-algal particulate absorption along with a particle size distribution (PSD) metric and total projected area of minerogenic particles per unit volume of water (PAV m ) determined with an individual particle analysis (IPA) technique. IPA established clay mineral and calcite dominance as the primary and secondary composition cases, respectively, for the sampled populations. Estimates of spectral absorption properties of these populations are presented, including the mean absorption efficiency ( Q a,m >), the imaginary part of the refractive index ( n ′), and the mean package effect index ( Q a ⁎ ,m >). Substantial differences in these attributes among the samples are documented; those for Q a,m > and Q a ⁎ ,m > were explained well by the effects of optical thickness, which represent the multiplicative effects of n ′ and a PSD metric. Greater package effect is attributed to increases in n ′. Mie theory calculations, based on the n ′ estimates and the IPA characterizations, depict substantial decreases in the spectral backscattering coefficient ( b b,m ) for the minerogenic particle populations from the red to blue spectral regions, relative to the case of non-absorption. Related recommendations for mechanistic remote sensing initiatives to represent the effects of minerogenic particles are presented, including (1) use of PAV m as the optically active constituent metric, (2) use of Q a,m > as the corresponding cross-section (interim value of 0.33 or 0.39), and (3) representation of the spectral character for b b,m reported here.

Brazilian inland water bio-optical dataset to support carbon budget studies in reservoirs as well as anthropogenic impacts in Amazon floodplain lakes: Preliminary results

Abstract. This work presents ongoing efforts and preliminary results for building a dataset that represents the first and most comprehensive bio-optical information available on Brazilian inland waters to support the development of remote sensing algorithms for monitoring aquatic systems. From 2012 to 2014 optical and limnological data was gathered along thirteen field campaigns in five Brazilian reservoirs, in an irrigation and domestic water supply reservoir located in semi-arid northeast of the country and in Amazonian floodplain lakes, thus covering the diversity of Brazilian inland waters. At each site 20 stations, on average, were sampled to acquire profiles of the following optical variables: absorption, attenuation, scattering, and backscattering coefficients and radiances/irradiances spectra above and in-water. Alongside these measurements, water samples were collected for determining concentrations of chlorophyll-a (Chl-a), Total Suspended Solid (TSS), Total Dissolved Carbon (TDC) and its organic/inorganic fractions, CDOM absorption, phytoplankton specific absorption [aph*] and Non-Algal Particles absorption [aNAP*]. Preliminary results show that Chl-a concentrations ranged from 0.6 to 243μg/L in reservoirs and 0.90 to 92μg/L in Amazonian lakes, while TSS concentrations ranged from 0.3 to 31mg/L in reservoirs and 0.5 to 162mg/L in Amazonian lakes. In situ beam attenuation coefficients ranged from 1.4 to 16m-1 in reservoirs and 12.5 to 38m-1 in Amazonian lakes, while diffuse attenuation coefficients of downwelling irradiance over the Photosynthetically Active Radiation (Kd(PAR)) extended from 0.35 to 4.5m-1 in reservoirs and 1.69 to 13.30m-1 in Amazonian lakes. Our research group is building this dataset anticipating future demands for algorithm validation regarding OLI/Landsat8 data and ESA Sentinel missions to be launched as of 2015.

Absorption properties of in-water constituents and their variation among various lake types in the boreal region

We studied the absorption properties of in-water constituents among 15 boreal lakes belonging to various lake types. One of the lakes was also monitored frequently during its growth season to reveal the seasonal variation of absorption properties. Within the studied lakes, CDOM absorption aCDOM(442) varied from 0.4 to 15m−1. CDOM was clearly dominating the absorption signal at low wavelengths, but even at the red-end of the spectra its contribution to total absorption was notable, varying from 48 to 99% of total absorption (excl. water) at 442nm and from 5 to 86% at 665nm. Slope of the CDOM absorption spectra SCDOM(400–700) varied from 12.9 to 19.3μm−1. Higher slopes were obtained when the wavelength range was extended to the UV-range (350–700 and 300–700nm). Non-linear relationships were noted between aCDOM and SCDOM calculated over various wavelength ranges. Despite the significant relationship between DOC concentration and aCDOM, almost five-fold variation was noted in DOC-specific CDOM absorption at 442nm. This variation was clearly associated with concurrent changes in SCDOM. Non-algal particulate absorption varied from 0.037 to 1.8m−1 at 442nm, contributing <1 to 34% of total absorption at 442nm and <1 to 27% at 442nm. Slope of the NAP absorption spectra SNAP(380–700) varied from 7.6 to 12.8μm−1. Despite of the apparent covariation between weight of suspended particulate matter (SPM) and aNAP, six-fold variation was observed in SPM-specific NAP absorption at 442nm, which was mostly associated with organic content of the particle pool. Contribution of phytoplankton pigments ranged from 1 to 28% of total absorption at 442nm, and 13 to 86% at 665nm, respectively. Chlorophyll a (Chla) specific absorption of phytoplankton pigments varied from 0.012 to 0.038m2 (mg Chla)−1 at blue peak, and from 0.008 to 0.020m2 (mg Chla)−1 at red peak. Observed values cluster around the low-end of the previously reported range, indicating a high package effect of phytoplankton cells in lakes. Observed variation was modeled as function of Chla (ranging from 1.8 to 95mgm−3). Parameters and relationships presented in this study provide useful information for remote sensing of lakes, as well as for various ecological and geochemical applications.

Classification of Inland Waters Based on Bio-Optical Properties

Multiple bio-optical measurements of optically active substances were conducted in Taihu Lake, Chaohu Lake, Three Gorges Reservoir, and Dianchi Lake (China). A hierarchical cluster analysis was applied to remote sensing spectra (Rrs(λ)), by which those waters were clustered into three optically distinct types (Type I, Type II, and Type III). Absorption coefficients of phytoplankton were simulated using the linear function and the coefficients of the functions varied in different water types. The slope of colored dissolved organic matter (CDOM) absorption could be clustered into two classes, one class of high slope for Types I and II, whereas the other class of low slope for Type III. A uniform model was used for parameterization of the nonalgal particles absorption coefficients spectral between different water types. The power-law function was used to parameterize the scattering coefficient and the slope of scattering spectra could be classified into two groups of high slope for Types I and II, low slope for Type III. The composition of particles, particulate scattering, and nonalgal particulate absorption are the factors controlling the variability in Rrs(λ) for corresponding type waters. Those results support such a fact that it is indispensable to classify waters before developing a model to estimate water quality parameters for multi-type optical waters.