Optically Active Substances Research Articles

Suitability of different in-water algorithms for eutrophic and absorbing waters applied to Sentinel-2 MSI and Sentinel-3 OLCI data

Optically complex waters present significant challenges for remote sensing due to high concentrations of optically active substances (OASs) and their inherent optical properties (IOPs), as well as the adjacency effect. OASs and IOPs can be derived from atmospheric correction processors’ in-water algorithms applied to data from Sentinel-2 MultiSpectral Instrument (S2 MSI) and Sentinel-3 Ocean and Land Color Instrument (S3 OLCI). This study compared S3 OLCI Level-2 in-water products for Case-2 waters with alternative in-water algorithms derived from ACOLITE, POLYMER, C2RCC, and A4O. Fifty in-water algorithms were evaluated using an extensive match-up dataset from lakes and coastal areas, focusing particularly on small lakes with high colored dissolved organic matter absorption at 442 nm (up to 48 m-1). The Chl a band ratio introduced by Gons et al. (2022) applied to data processed by ACOLITE performed best for S3 OLCI Chl a retrieval (dispersion = 23%, bias = 10%). Gons et al. (2022) band ratio also showed consistent agreement between S3 OLCI and S2 MSI resampled data (intercept of 6.27 and slope of 0.83, close to the 1:1 line); however, lower Chl a values (<20 mg/m3) were overestimated by S2 MSI. When estimating errors associated with proximity to land, S2 MSI Chl a in-water algorithms had higher errors close to the shore (on average 315%) compared to S3 OLCI (on average 150%). Chl a retrieved with POLYMER had the lowest errors close to the shore for both S2 MSI and S3 OLCI data (on average 70%). Total suspended matter (TSM) retrieval with C2RCC performed well for S2 MSI (dispersion 24% and bias −12%). Total absorption was most accurately derived from C2RCC applied to S3 OLCI L1 data (dispersion < 43% and bias < −39%), and it was better estimated than its individual components: phytoplankton, mineral particles, and colored dissolved organic matter absorption. However, none of the colored dissolved organic matter absorption in-water algorithms performed well (dispersion > 59% and bias < −29%).

Short-term time-series observations of phytoplankton light-absorption and productivity in Prydz Bay, coastal Antarctica

The optical characteristics of coastal Antarctic waters exhibit complexity due to the dynamic hydrography influenced by meltwater intrusion, which alters nutrient levels, thermohaline structure, and optically active substances (OAS) regimes. Studies on bio-optical variability and its implications on phytoplankton productivity (PP) are scanty in coastal polar regions. On this backdrop, time-series measurements (72 h at 6 h intervals) of bio-optical properties such as phytoplankton biomass (chlorophyll-a), absorption (aph), and total suspended matter (TSM) concurrently with PP were measured to understand their interplay and variability in relation to the ambient physicochemical settings in the under-sampled Prydz Bay, coastal Antarctica. Our findings revealed thermohaline stratification within the bay, likely attributed to the inflow of less saline meltwater from nearby glaciers and minimal wind activity. The consistent presence of sub-surface chlorophyll maximum (SCM) beneath the stratified layer underscored the light-acclimatization response of shade-adapted phytoplankton. Surface waters exhibited higher TSM compared to deeper layers, indicating glacial melt influence, while the depth of the sunlit layer remained relatively stable, suggesting limited water mass movement and/or variability in OAS at the study site. An inverse relation between chlorophyll-a and chlorophyll-a-specific phytoplankton light absorption (a*ph(λ)) manifested ‘pigment package effect’ within the prevailing phytoplankton community, implying reduced light-absorption efficiency and consequent lower PP. Compared to chlorophyll-a, the phytoplankton light absorption (aph(λ)) emerged as a better proxy for explaining PP variability. Nutrient availability was not limiting, which was conducive to micro (large) phytoplankton growth. Classification of phytoplankton size classes (micro, nano, and pico) based on the B/R ratio (aph at Blue (443 nm)/Red (676 nm) region) confirmed the dominance of larger (micro) phytoplankton that are more susceptible to package effect, thus have implications on reduced PP potential of this polar marine ecosystem.

Preliminary examination of influence of Chlorophyll, Total Suspended Material, and Turbidity on Satellite Derived-Bathymetry estimation in coastal turbid water

The satellite-Derived Bathymetry (SDB) has been researched for almost the past five decades, but the application of SDB for operational use is still in its initial stages. The higher concentration of suspended material is a major barrier in assessing bathymetry accurately with remote sensing data. The influence of various optically active substances (OAS) needs to be quantified to develop a more robust SDB algorithm. The goal of the study is to improve the accuracy of SDB by understanding the spatial distribution of Chlorophyll (Chl), Total Suspended Material (TSM), and Turbidity in coastal water. A preliminary investigation using the numerical analysis between Landsat-7 & 8 spectral bands, OAS parameters, and bathymetry is presented in this paper. SDB has been derived using three machine learning algorithms; Linear, Random Forest (RF), and Support Vector Machine (SVM) Regression. The resulting SDB residuals have been analyzed in relation to Chlorophyll, Turbidity, and TSM in coastal seawater. The study found that most of the erroneous SDB residuals and extreme values were distributed in high or medium concentrated OAS regions. The result of the study may further enhance SDB estimation accuracy by considering the influence of the above three OAS in coastal waters.

Spatiotemporal variability in bio-optical characteristics of the southwestern tropical Indian Ocean during boreal summer: Biophysical influences

The bio-optical characteristics of the surface ocean play a pivotal role in radiative transfer and photosynthetic carbon fixation. To examine spatiotemporal variability of bio-optical properties in the tropical Indian Ocean, a 10-day time-series comparative study was made at one fixed and 3 variable stations in the southwestern tropical Indian Ocean (SWTIO), and 4-stations in the Equatorial Indian Ocean (EIO) during June 2014. A total of 24 hyperspectral radiometer profiles were collected along with physicochemical parameters and phytoplankton pigments analysis. A negative sea level anomaly (SLA), higher precipitation, a more stable mixed layer, and higher buoyancy frequency (N2) were observed in SWTIO compared to EIO. The deep chlorophyll maxima (DCM) in SWTIO were characterised by deep biomass maxima (DBM) whereas in the EIO it was due to deep photoacclimation maxima (DAM). The photosynthetically available radiation (PAR) profiles were obtained by integrating downwelling irradiance from 400 to 700 nm suggesting that the SWTIO had a greater abundance of optically active substances (OAS) than the EIO. Surface chlorophyll a (Chl-a) was found to have significant relationships with phytoplankton absorption coefficient (aph(443)). Chl-a-specific phytoplankton absorption coefficients (a*ph(443)), total absorption coefficient (at(443)), and particulate backscattering (bbp(555)) were also related to Chl-a implying that the phytoplankton was in growth/active phase. The time-series variability of the phytoplankton and changes in the photosynthetic pigments (PSP) to photoprotective pigments (PPP) ratio at SWTIO suggested phytoplankton community shift during 5th to 7th days. Microplankton percent-abundances showed distinct linear relationships with Chl-a and bbp(555) implying that carbon fixation per unit Chl-a was higher in microplankton than in nano and picoplankton at SWTIO. The in situ Chl-a, aph(443), adg(443), Kd(490) and C¯pd were used to validate the various empirical algorithms available as well as the satellite-retrieved data. The MODIS and VIIRS data correlated moderately. This study would help towards improving regional optimized algorithms for OAS and provide a baseline understanding of the scale of variability of primary production and carbon flux from space. In addition, this would contribute valuable bio-optical data from an under-sampled geographical area.

Understanding the changes of optically active substances (OACs) in Hulun Lake in the past 35 years and its indication to the degradation of aquatic ecology

Lake eutrophication and ecological degradation are among the most serious environmental problems in the world and identifying changes in key water quality parameters and associated drivers is critical for lake management. Optically active substances (OACs) are important water quality indicators that affect lake water ecology and evaluate lake eutrophication. Climate change and human activities are important factors driving the changes of OACs in lakes. Understanding how and why OACs varies in long-term trend in response to different influencing factors and its driving mechanism is needed to understand. In this study, based on the Landsat series satellite data, this study investigated the historical changes of OACs in Hulun Lake, quantitatively explained the response relationship of OACs to meteorological, hydrological and human activity elements, and discussed their indicative role in the degradation of water ecology. The results showed that during 1986–2020, OACs had obvious periodic changes; from 1986 to 1999, Chla, CDOM and TSM are relatively stable at lower values; during 2000–2009, the concentration of total suspended matter (TSM) and coloured dissolved organic matter (CDOM) increased, and the concentration of chlorophyll a (Chla) decreased; during 2010–2020 The annual TSM and CDOM concentration decreased, and the Chla concentration increased. The results showed that among the hydrometeorological and anthropogenic variables selected to explain changes in the OAC, gross domestic product (GDP) is the key factor influencing changes in the OACs, explaining 39.3%, 71.8% and 60.6% of the changes in Chla, CDOM and TSM, respectively. Air temperature (AT) can explain 24.0% of the variation in Chla, evapotranspiration (Eva) can explain 19.1% and 5.52% of the variation in CDOM and TSM, and precipitation (P) can explain 13.8% and 7.5% of the variation in Chla and TSM. Water level is an important pathway affecting OACs in Hulun Lake, explaining 50.7% of the variation in CDOM, 48.0% of the variation in TSM and 23.1% of the variation in Chla. Warming and drought in the Hulun Lake basin and human activities ultimately lead to changes in OACs. Finally, a pattern diagram to assess the degree of ecological degradation of lake was proposed. This study has substantial implications to monitor and assess the ecological degradation status of lake and maintain the sustainable development of lakes in cold and arid regions.

Underwater Use of a Hyperspectral Camera to Estimate Optically Active Substances in the Water Column of Freshwater Lakes

Freshwater lakes provide many important ecosystem functions and services to support biodiversity and human well-being. Proximal and remote sensing methods represent an efficient approach to derive water quality indicators such as optically active substances (OAS). Measurements of above-ground remote and in situ proximal sensors, however, are limited to observations of the uppermost water layer. We tested a hyperspectral imaging system, customized for underwater applications, with the aim to assess concentrations of chlorophyll a (CHLa) and colored dissolved organic matter (CDOM) in the water columns of four freshwater lakes with different trophic conditions in Central Germany. We established a measurement protocol that allowed consistent reflectance retrievals at multiple depths within the water column independent of ambient illumination conditions. Imaging information from the camera proved beneficial for an optimized extraction of spectral information since low signal areas in the sensor’s field of view, e.g., due to non-uniform illumination, and other interfering elements, could be removed from the measured reflectance signal for each layer. Predictive hyperspectral models, based on the 470 nm–850 nm reflectance signal, yielded estimates of both water quality parameters (R² = 0.94, RMSE = 8.9 µg L−1 for CHLa; R² = 0.75, RMSE = 0.22 m−1 for CDOM) that were more accurate than commonly applied waveband indices (R² = 0.83, RMSE = 13.2 µg L−1 for CHLa; R² = 0.66, RMSE = 0.25 m−1 for CDOM). Underwater hyperspectral imaging could thus facilitate future water monitoring efforts through the acquisition of consistent spectral reflectance measurements or derived water quality parameters along the water column, which has the potential to improve the link between above-surface proximal and remote sensing observations and in situ point-based water probe measurements for ground truthing or to resolve the vertical distribution of OAS.

Inter-annual variability in the inherent optical properties along the southeastern Arabian Sea from 2009 to 2015

A study was performed on the coastal waters of the southeastern Arabian Sea to understand the changes in the inherent properties of optically active substances (OAS). The objective of this study was to obtain insight into the temporal dynamics of OASs for various remote sensing applications. Both the absorption of light by phytoplankton (aph(440)) and detritus (ad(440)) varied the 2nd orders of magnitude, whereas the absorption of light by chromophoric dissolved organic matter (CDOM; aCDOM(440)) exhibited an increasing trend over the years. aCDOM(440) increased from 2009 to 2015 at a rate 0.012 m−1 year−1. The validation of the satellite data with in situ data indicated that ad(443) underestimated the in situ ad(443) when the in situ ad(443) was less than 0.3 m−1. The ternary analysis of ocean-colour-climate-change initiative (OC-CCI)–derived inherent optical properties (IOP) during the postmonsoon season indicated increased contribution by ad(443). Detritus was the primary light absorber in the coastal waters of Kochi followed by phytoplankton and CDOM. These long-term data sets and their seasonal variation will enable development of seasonal specific regional algorithms for chlorophyll-a, which are employed in potential fishing zone advisories and other applications.

Long-term chlorophyll-a dynamics in tropical coastal waters of the western Bay of Bengal.

The long-term distribution of in situ optically active substances (OAS), accuracy assessment of satellite retrieved chlorophyll-a (chl-a) and its long-term trend has been carried out at a coastal site of the north-western Bay of Bengal. The temporal distribution of chl-a, total suspended matter (TSM) and absorption due to coloured dissolved organic matter at 440nm (aCDOM440) discerned a common peak during southwest monsoon season (August-October). Chl-a also showed a prominent peak during pre-southwest monsoon period (March-April). The spatial variability of TSM and aCDOM440 was maximum during southwest monsoon, whereas in the case of chl-a, it was during pre-southwest monsoon. The accuracy assessment of chl-a retrieved from Moderate Resolution Imaging Spectroradiometer-Aqua (MODISA), Ocean Colour Monitor-2 (OCM-2) and Visible Infrared Imager Radiometer Suite (VIIRS) showed overestimation in nearshore waters. The error in satellite measurement of chl-a was within the range of 33 to 51%. The chl-a retrieved from MODISA was most accurate as indicated by statistical analysis. The long-term trend in satellite chl-a clearly indicated bi-modal distribution with a primary peak during pre-southwest monsoon attributed to recurrent phytoplankton bloom that was mostly confined to nearshore waters. Whereas, the secondary peak in chl-a, during the end of southwest monsoon, spreads far offshore.

Seasonal variation in optically active substances at a coastal site along western Bay of Bengal

Optically active substances (OAS) such as chlorophyll-a (chl-a), chromophoric dissolved organic matter (CDOM) and total suspended matter (TSM) play significant role in health assessment of aquatic ecosystem. Temporal variability in OAS plays important role in modulating coastal water ecology. This demand for continuous monitoring of OAS in coastal waters. The present study highlights on temporal variability of OAS off Gopalpur, a coastal site along the north-western Bay of Bengal. The OAS were found to be having strong seasonal influence apart from large variability in concentration. Chl-a concentration showed fourfold variability (0.03–12.29 mg m−3) with seasonal trend of pre-monsoon > post-monsoon > monsoon. Absorption due to CDOM at 440 nm (aCDOM440) varied between 0.02 and 4.48 m−1 following seasonal trend of pre-monsoon > monsoon > post-monsoon. TSM concentration was ranged within 0.1–28.21 mg l−1 showing maximum during monsoon and minimum during post-monsoon season. The higher concentration of chl-a and aCDOM440, during pre-monsoon season, was predominantly due to ecosystem disrupting red tide event of Noctiluca bloom during pre-monsoon season of 2014. The high load of TSM during monsoon was due to increased river influx attributed to upstream precipitation. There was no significant relation observed among the OAS indicating that multiple sources of OAS in optically complex waters of the north-western Bay of Bengal. The study provides understanding on long-term variations in OAS which is essential for development or tuning of bio-optical algorithms for accurate remote estimation of geophysical products.

Modeling photodegradation kinetics of organic micropollutants in water bodies: A case of the Yellow River estuary

Predicting photodegradation rate constants (k) of pollutants in water bodies is important for assessing their persistence and fate. This prediction used to be based on the k values determined under laboratory conditions that seldom consider underwater downward sunlight attenuation in the field. We studied a procedure to predict k taking the Yellow River estuary and two model chemicals (sulfamethoxazole and acyclovir) as a case. Models were developed for predicting underwater sunlight intensities from optically-active substances. Based on the predicted underwater sunlight intensities, hourly variation of k for the model compounds was predicted as a function of water depth, for a fresh water, an estuarine water and a seawater body in the estuary. Results show that photodegradation half-lives (t1/2) of the two compounds will be underestimated by dozens of times if underwater downward sunlight attenuation and intensity variation are not considered. Outdoor validation experiments show the maximum deviation between the predicted and measured k values is a factor of 2. The developed models can be employed to predict k of environmental chemicals in coastal water bodies once they are locally calibrated.

Performance of operational satellite bio-optical algorithms in different water types in the southeastern Arabian Sea

Summary The in situ remote sensing reflectance (Rrs) and optically active substances (OAS) measured using hyperspectral radiometer, were used for optical classification of coastal waters in the southeastern Arabian Sea. The spectral Rrs showed three distinct water types, that were associated with the variability in OAS such as chlorophyll-a (chl-a), chromophoric dissolved organic matter (CDOM) and volume scattering function at 650 nm (β650). The water types were classified as Type-I, Type-II and Type-III respectively for the three Rrs spectra. The Type-I waters showed the peak Rrs in the blue band (470 nm), whereas in the case of Type-II and III waters the peak Rrs was at 560 and 570 nm respectively. The shifting of the peak Rrs at the longer wavelength was due to an increase in concentration of OAS. Further, we evaluated six bio-optical algorithms (OC3C, OC4O, OC4, OC4E, OC3M and OC4O2) used operationally to retrieve chl-a from Coastal Zone Colour Scanner (CZCS), Ocean Colour Temperature Scanner (OCTS), Sea-viewing Wide Field-of-view Sensor (SeaWiFS), MEdium Resolution Imaging Spectrometer (MERIS), Moderate Resolution Imaging Spectroradiometer (MODIS) and Ocean Colour Monitor (OCM2). For chl-a concentration greater than 1.0 mg m−3, algorithms based on the reference band ratios 488/510/520 nm to 547/550/555/560/565 nm have to be considered. The assessment of algorithms showed better performance of OC3M and OC4. All the algorithms exhibited better performance in Type-I waters. However, the performance was poor in Type-II and Type-III waters which could be attributed to the significant co-variance of chl-a with CDOM.

Effect of optically active substances and atmospheric correction schemes on remote-sensing reflectance at a coastal site off Kochi

The present study focused on understanding the variability of optically active substances (OASs) and their effect on spectral remote-sensing reflectance (Rrs). Furthermore, the effect of atmospheric correction schemes on the retrieval of chlorophyll-a (chl-a) from satellite data was also analysed. The OASs considered here are chl-a, coloured dissolved organic matter (CDOM), and total suspended matter (TSM). Satellite data from the Moderate Resolution Imaging Spectroradiometer (MODIS) on the Aqua satellite was used for this study. The two atmospheric correction schemes considered were: multi-scattering with two-band model selection NIR correction (hereon referred as ‘A1’) and Management Unit of the North Sea Mathematical Models (MUMM) correction and MUMM NIR calculation (hereafter referred as ‘A2’). The default MODIS bio-optical algorithm (OC3M) was used for the retrieval of chl-a. Analysis of OASs showed that chl-a was the major light-absorbing component, with highly variable distribution (0.006–25.85 mg m–3). Absorption due to CDOM at 440 nm (aCDOM440) varied from 0.002 to 0.31 m–1 whereas TSM varied from 0.005 to 33.44 mg l–1. The highest concentration of chl-a was observed from August to November (i.e. end of the southwest monsoon and beginning of the northeast monsoon), which was attributed to coastal upwelling. The average value of aCDOM440 was found to be lower than the global mean. A significant negative relationship between aCDOM440 and salinity during the southwest monsoon indicated that much of the CDOM during this season was derived from river discharge. Spectral Rrs was found to be strongly linked to the variability in chl-a concentration, indicating that chl-a was the major light-absorbing component. Satellite-derived spectral Rrs was in good agreement with that in situ when chl-a concentration was lower than 5 mg m–3. The validation of chl-a, derived from in situ Rrs, showed moderate performance (correlation coefficient, R2 = 0.64; log10(RMSE) = 0.434; absolute percentage difference (APD) = 43.6% and relative percentage difference (RPD) = 42.33%). However the accuracy of the algorithm was still within acceptable limits. The statistical analysis for atmospheric correction schemes showed improved mean ratio of measured to estimated chl-a (‘r’ = 1.6), log10(RMSE) (0.49), APD (25.46%), and RPD (17.57%) in the case of A1 as compared with A2, whereas in the case of A2, R2 (0.56), slope (0.26), and intercept (0.27) were better as compared with A1. The two atmospheric correction schemes did not show any significant statistical difference. However the default atmospheric correction scheme (A1) was found to be performing comparatively better probably due to the fact that the concentration of TSM and CDOM was much lower to overcome the impact of chl-a.

Optical characterization of coastal lagoons in Tunisia: Ecological assessment to underpin conservation

Ghar El Melh is a shallow lagoon (average depth of 0.8m) that has undergone a eutrophication process due to growing human pressures. To obtain a global frame of the ecosystem functioning, an optical and an ecological classification were used in parallel. Downwelling and upwelling spectral irradiances were measured in situ in 22 sampling stations across the water body; then Apparent Optical Properties (AOPs), namely reflectance R(λ) and vertical attenuation coefficient Kd(λ) were calculated for each wavelength of visible spectrum, furnishing typical spectra from turbid waters, rich in dissolved and suspended matter. From water samples of the same stations the concentrations of OASs (Optically Active Substances), i.e. Chromophoric Dissolved Organic Matter (CDOM), Non-Algal Particulate (NAP) and Phytoplankton, were assessed. The use of an optical classification for water bodies rich in TSM and CDOM, integrating AOPs and OASs, highlighted a great spatial heterogeneity, well overlapping with hydrology and human impacts patterns. A modified version of the Ecological Evaluation Index (EEI), considering the macrophyte distribution (based on a visual assessment of macrophyte coverage, without quantitative sampling) was then used, highlighting an intermediate ecological condition, despite high water turbidities. The integrated use of both systems thus furnished a complete characterization, rapidly detecting the most impacted sectors and the possible primary causes. The method might be applied as a monitoring procedure in other Mediterranean coastal lagoons, with the aim to adopt a common conservation strategy for these important transitional water bodies.

Long-term light environment variability in Lake Biwa and Lake Kasumigaura, Japan: modeling approach

Light environment variability was investigated in the two Japanese lakes Biwa and Kasumigaura, which offer a broad range of optical conditions in the water bodies due to their diverse morphometries and limnological characteristics. To elucidate their light environments, Secchi depths (SDs) were related to long-term monitored datasets of concentrations of optically active substances (OASs) using two approaches based on statistical and mechanistic models. A good estimate for the nonphytoplanktonic suspended solids (NPSS) concentration gained using a monthly factor δ (which represents the phytoplanktonic portion in total suspended solids) from a long-term analysis was utilized to develop robust models. Using the mechanistic model, the contribution of each OAS to the SD can be understood and investigated in more detail than possible with a statistical approach, but the statistical model yields better results in terms of SD prediction. Based on the results of an analysis of the contribution of each OAS to the SD, it was clear that NPSS was the component that exerted the most influence on the light environments in the two lakes; in this respect, this study agrees with other studies that show the importance of suspended inorganic particles as the main contributor to the SD in inland waters. Using ANOVA, we analyzed how specific inherent optical properties may have changed spatially and temporally, and the results indicated that the temporal (monthly) effect was primarily responsible for the loss of accuracy in the models. In addition, the ANOVA analysis suggested that grouping the data improved the predictive performances of the statistical models. Finally, we concluded that combining the two models yields the most reliable results in terms of SD prediction and determining the contribution of each OAS to the SD at present.

Seasonal Variability of Optical Properties in a Highly Turbid Lake (Laguna Chascomús, Argentina)

We study the underwater light field seasonality in a turbid lake, Laguna Chascomús (Buenos Aires, Argentina). We report (1) relationships between optical properties (OPs) and optically active substances (OASs); (2) relationships between inherent (IOPs) and apparent (AOPs) optical properties; and (3) the seasonal variability in OASs and OPs. Light absorption was dominated by the particulate fraction. The contributions of phytoplankton pigments and unpigmented components were similar. The best predictors of total particulate absorption, unpigmented particulate absorption, turbidity and vertical attenuation coefficient were total suspended solids or their ash content. Many OASs and OPs varied seasonally. The concentrations of OASs were higher during spring and summer, resulting in lower transparency and higher turbidity. However, mass-specific absorption coefficients displayed lower values during spring and summer. Thus, the higher light attenuation observed during spring and summer resulted from higher concentrations of relatively less absorptive OASs. Collectively, these results suggest that: (1) light extinction is enhanced during spring and summer; (2) the enhanced light extinction is due to changes in the particulate fraction; (3) the enhanced light extinction is mostly due to an increase in the amount of particulate material; and (4) the increase of particulate matter also enhanced light extinction through increased scattering.

Dependence of spectral distribution of inherent optical properties of lake waters on the concentrations of different water constituents

The study focuses on the analysis of inherent optical properties of lake waters characterized by consisting of several optically active substances (OAS) (phytoplankton, suspended particles and dissolved organic matter). The results obtained can be useful for deriving the algorithms of optical remote sensing models which need the spectral data on inherent optical properties of the aquatic environment. The spectral absorption and attenuation coefficients were measured in situ using the instrument ac-9 for eight wavelengths in the range of 400–700 nm. The investigation sites were five Finnish and six Estonian lakes. The concentrations of chlorophyll a and total suspended particulate matter, as well as the absorption coefficient of coloured dissolved organic matter (at 380 nm) were determined in a laboratory from water samples taken concurrently with ac-9 measurements. There are three main objectives: (1) to calculate the values of absorption and specific absorption coefficients of tripton on the basis of ac-9 and laboratory data; (2) to estimate the contribution of pure water, chlorophyll a, dissolved organic matter and tripton to the total absorption and attenuation coefficients (results for ac-9 wavelengths and also for the PAR region, 400–700 nm); and (3) to determine the slope (and its variability) of the scattering coefficient spectrum for each measurement series and to estimate the reasons of this variability. At the wavelength of 532 nm the values of the specific absorption coefficient of tripton varied between 0.013 and 0.098 L mg−1 m−1, while the slope of its spectrum ranged from 0.0060 to 0.0109 nm−1. The contributions of different components of the aquatic medium to the total absorption/attenuation coefficients varied noticeably from lake to lake. The slope of the scattering coefficient spectrum was in the range of 0.32–2.5, the multiple regression between the slope and three main optically active substances jointly gave the determination coefficient R2(adjusted)=0.655.

Reconstruction of the Time Series of the Underwater Light Climate in a Shallow Turbid Lake

Lake Ulemiste is a shallow, eutrophic lake which has served the city of Tallinn as a water reservoir for many centuries. Its light climate was studied by combining a routinely measured data set with a modelling approach. For 26 years (1978–2004), data was collected on such optically active substances (OAS) and water parameters as water colour, turbidity and phytoplankton biomass. Simple modelling enabled the quantification of long-term time-series data and the subsequent calculation of the diffuse attenuation coefficient, euphotic depth and average light of the mixed layer. Several changes in the hydrological cycle have taken place during the period under study, among which are an increase in the water level of about 0.5 m and a decrease in the external water load from 108 million m3 year−1 to about 25 million m3 year−1. At the same time euphotic depth has shown a distinct trend towards increasing since the early 1990s. The euphotic depth also showed an increase (from 1.1 to 1.4 m) due to an improvement in underwater light conditions – mainly in the spring (April and May) and autumn (October and November) because of the lower amount of dissolved organic matter in the lake. The average light availability in the mixed layer has increased, but this has not affected the phytoplankton biomass as the latter is not light-limited during the summer period.

Variability of remote sensing reflectance and implications for optical remote sensing—A study along the eastern and northeastern waters of Arabian Sea

[1] Situated in the eastern and northeastern regions and subjected to similar oceanic processes, the inshore waters north of 20°N and east of 65°E are more optically complex than the offshore waters and waters south of 20°N and east of 65°E. This has been observed through the analysis of variations of the remote sensing reflectance (Rrs(λ)) in the optical spectrum of Electromagnetic radiation. The optical complexity has further been studied through the examination of optically active substances (OAS) such as chlorophyll_a, suspended sediment and coloured dissolved organic matter (CDOM). It is found that CDOM is the significant component in making the area optically non-linear. For the first time multi-spatial/temporal band-ratio algorithms are developed to map OAS from these waters through Ocean Colour Monitor flown on IRS – P4 satellite.

Analysis of estuarine colour components during non-monsoon period through Ocean Colour Monitor

Simultaneous acquisition of water samples, radiance and irradiance measurements were carried out from 40 stations in the Mandovi–Zuari estuaries during February to May 2002. From the samples collected, inherent and apparent optical properties (IOP and AOP) such as absorption coefficient ( a), upwelling diffuse attenuation coefficient ( k u) and subsurface reflectance ( R) were derived. Using these optical properties, radiative transfer at each water column is examined. On the basis of the radiative transfer outcome, band-ratio algorithms are derived for three optically active substances (OAS), viz, chlorophyll- a, suspended sediment and coloured dissolved organic matter (CDOM). The respective algorithms are 670/555, 490/670 and 412/670 nm for chlorophyll- a, suspended sediment and CDOM. These algorithms are applied to Ocean Colour Monitor (OCM), onboard Indian Remote Sensing Satellite (IRS)-Polar Satellite Launch Vehicle (P4), scenes (digital data), to synoptically analyze these OAS. The synoptic analysis of OAS revealed different hydrodynamic characteristics of the estuaries during non-monsoon seasons.