Optical models of mesotrophic and eutrophic water bodies

Abstract

Interpretation techniques have been developed to process the spectra of upwelling radiation from water in the visible. Spectral indices to retrieve the concentration of optically active components have been suggested. They make it possible to retrieve the chlorophyll-a concentration with a standard error of less than 2.6mgm^?3, mineral suspended matter concentration less than 4mgl^?l, and dissolved organic matter concentration less than O.5mgCl^?1. The instrumentation is described for measurments of the upwelling radiation as well as for operational determination of the concentrations of phytoplankton chlorophyll-a, mineral suspended matter and dissolved organic matter.     

Surface chlorophyll a estimation in the Arabian Sea using IRS-P4 Ocean Colour Monitor (OCM) satellite data

The IRS-P4 satellite launched on 26 May 1999 by Indian Space Research Organisation (ISRO) carried an Ocean Colour Monitor (OCM) payload, primarily designed to measure ocean colour, the spectral variations of waterleaving radiance that can be related to the concentration of phytoplankton pigments, coloured dissolved organic matter and suspended particulate matter. Remote sensing data obtained from IRS-P4 OCM sensor is processed for the removal of atmospheric effects of Rayleigh and aerosol scattering to derive normalized water-leaving radiance in visible channels of 412, 443, 490, 512 and 555 nm. Ocean chlorophyll-2 (OC2) global bio-optical algorithm was used to convert normalized water-leaving radiance data to chlorophyll a concentration. For quantitative accuracy assessment, OCM-derived chlorophyll a concentration values were compared to the in situ measurements of chlorophyll a, conducted during ORV Sagar Kanya SK-149c ship cruise during 15-27 November 1999 in Arabian Sea. In general OCM-derived chlorophyll a showed a good correlation ( r 2 =0.90, N = 8, RMS=0.125 mgm -3 ) with measured chlorophyll values.     

Determination of chlorophyll concentration changes in Lake Garda using an image-based radiative transfer code for Landsat TM images

The distribution of phytoplankton chlorophyll concentration in Lake Garda (Italy) was estimated using Landsat Thematic Mapper (TM) data acquired at two different times, February 1992 and March 1993. To investigate the waterleaving radiance adequately, the contribution of the atmospheric path radiance reaching the sensor should be removed. In this work a completely image-based atmospheric correction method was applied by means of an inversion technique based on a simplified radiative transfer code (RTC). A semi-empirical approach of relating atmospherically corrected TM spectral reflectances to in situ measurements through regression analysis was used. Limnological parameters were measured near to the TM images dates; some of the in situ measurements were used to define algorithms relating chlorophyll concentration measurements to water surface reflectance and the others too were used to validate the results of the predictive model. The models developed, which performed better (r² = 0.818) when concentrations were higher than > 3.0 mg m³, were used to map chlorophyll concentration throughout the lake. Spatial distribution maps of chlorophyll concentration and concentration changes were produced with contour intervals of 1 mg m³.     

Satellite-derived parameters for biological modelling in coastal waters: Illustration over the eastern continental shelf of the Bay of Biscay

In biological modelling of the coastal phytoplankton dynamics, the light attenuation coefficient is often expressed as a function of the concentrations of chlorophyll and mineral suspended particulate matter (SPM). In order to estimate the relationship between these parameters over the continental shelf of the northern Bay of Biscay, a set of in situ data has been gathered for the period 1998-2003 when SeaWiFS imagery is available. These data comprise surface measurements of the concentrations of total SPM, chlorophyll, and irradiance profiles from which is derived the attenuation coefficient of the photosynthetically available radiation, K_PAR. The performance of the IFREMER look-up table used to retrieve the chlorophyll concentration from the SeaWiFS radiance is evaluated on this new set of data. The quality of the estimated chlorophyll concentration is assessed from a comparison of the variograms of the in situ and satellite-derived chlorophyll concentrations. Once the chlorophyll concentration is determined, the non living SPM, which is defined as the SPM not related to the dead or alive endogenous phytoplankton, is estimated from the radiance at 555 nm by inverting a semi-analytic model. This method provides realistic estimations of concentrations of chlorophyll and SPM over the continental shelf all over the year. Finally, a relationship, based on non living SPM and chlorophyll, is proposed to estimate K_PAR on the continental shelf of the Bay of Biscay. The same formula is applied to non living SPM and chlorophyll concentrations, observed in situ or derived from SeaWiFS radiance.     

CASI data utilized for mapping suspended matter concentrations in sediment plumes and verification of 2-D hydrodynamic modelling

A combined tunnel and bridge between Denmark and Sweden, The Øresund Link, is presently under construction (1995-2001). The dredging spills and environmental effects are monitored continuously. The monitoring involves the simulation of suspended matter concentrations in sediment plumes from dredging operations using 2-D hydrodynamic modelling (MIKE 21 PA/MT). A Compact Airborne Spectrographic Imager (CASI) acquisition was used to map suspended matter concentrations in a plume from 10 June 1997. Suspended matter concentrations were in the range of 1-30mgl^-1 with background concentrations around 1-3mgl^-1, while concentrations in the plume were 3-30mgl^-1. A map of suspended matter concentration was derived from an empirical algorithm between in situ irradiance reflectance at 550 nm and suspended matter concentration determined from water samples taken both inside and outside the plume. The map was used to verify the results derived by the 2-D hydrodynamic model. A comparison of the concentration variations derived using the two methods presented here shows a reasonable agreement, in spite of the fact that the remote sensing data have far more spatial details than the MIKE 21 simulation.     

Feasibility of airborne imaging spectrometry for lake monitoring—a case study of spatial chlorophyll a distribution in two meso-eutrophic lakes

The spatial distribution of the sum of chlorophyll a and phaeophytin a concentrations (chl-a) under light wind (0–2 m s^?1) conditions was studied in two lakes with an AISA airborne imaging spectrometer. Chl-a was interpreted from AISA radiance data using an algorithm based on the near-infrared (700–710 nm) to red (660–665 nm) ratio. The results of Lake Lohjanjärvi demonstrate that the use of one monitoring station can result in over- or underestimation by 29–34% of the overall chl-a compared with an AISA-based estimation. In Lake Hiidenvesi, the AISA-based estimation for the mean chl-a with 95% confidence limits was 25.19±2.18 µg l^?1. The use of AISA data together with chl-a measured at 15 in situ sampling stations decreased the relative standard error of the mean chl-a estimation from 20.2% to 4.0% compared with the use of 15 discrete samples only. The relative standard error of the mean chl-a using concentrations at the three routine monitoring stations was 15.9 µg l^?1 (63.1%). The minimum and maximum chl-a in Lake Hiidenvesi were 2 and 101 µg l^?1, 6 and 70 µg l^?1 and 11 and 66 µmg l^?1, estimated using AISA data, data from 15 in situ stations and data from three routine in situ stations, respectively.     

应用Landsat TM影像估算渤海叶绿素a和总悬浮物浓度

利用23个实测样点的渤海叶绿素a和总悬浮物浓度数据及同步Landsat TM影像数据,分别分析了Landsat TM离水辐射亮度对渤海叶绿素a和总悬浮物浓度的敏感性,选择合适的波段,通过回归分析构建了基于Landsat TM离水辐射亮度的渤海叶绿素a和总悬浮物浓度反演模型。结果表明,TM1、TM2和TM3波段对叶绿素a的敏感性较高,以TM4/TM1和TM3/TM2的对数为自变量,以叶绿素a浓度的对数为因变量的线性估算模型可以有效反演渤海叶绿素a浓度,决定系数R2达到0.97;TM3波段对悬浮物的敏感性最高,以TM2、TM3和TM3/TM2为自变量,以总悬浮物浓度的以10为底的对数为因变量的多元线性模型获得的结果最佳,决定系数R2达到0.91。     

Suspended particulate matter in Lake Erie derived from MODIS aquatic colour imagery

This paper explores the use of Moderate Resolution Imaging Spectroradiometer (MODIS) wavebands in the red/near-infra-red for estimating concentrations of suspended particulate matter (SPM) in the moderately turbid, optically complex waters of Lake Erie. Observations show that at wavelengths shorter than 550 nm, more than 50% of the absorption signal is accounted for by dissolved organic matter and phytoplankton, confirming that algorithms incorporating these wavelengths may not be appropriate for these waters. Single band and band ratios at wavelengths greater than 667 nm are tested for their suitability for monitoring SPM concentrations in these waters. A simplified regional semi-analytical model is utilized which is independent of variations in dissolved organic matter and chlorophyll absorption, enabling estimates of SPM concentrations from MODIS water-leaving radiance at 748 nm with an average root mean square (RMS) error of 40%. Knowledge of the vertical distribution of particles enables estimates of total water column suspended loads which are then related to wind re-suspension events. The method is applied to MODIS water-leaving radiance at 748 nm to produce a time series of surface and total water column suspended loads in Lake Erie for the period 2003–2007.     

Optical properties of inorganic suspended solids and their influence on ocean colour remote sensing in highly turbid coastal waters

The influence of the optical properties of inorganic suspended solids (ISS) on in-water algorithms was evaluated using an optical model in highly turbid coastal water, whose ISS concentration reached several hundred grams per cubic metre. The measurements were conducted in the upper Gulf of Thailand. The backscattering coefficient of the ISS was calculated using the Lorenz–Mie scattering theory. On the basis of the measurement, the ISS size distribution was parameterized as a function of ISS concentration, and both the spherical and non-spherical particle shape models were evaluated. For ISS concentrations of 10 g m^?3, an estimate of the chlorophyll-a (chl-a) concentration within a factor of 2 on a logarithmic scale is possible in a [chl-a] range of 4–30 mg m^?3. The differential coefficient of remote sensing reflectance was calculated to evaluate its respective sensitivities for chl-a and ISS concentrations. The use of radiometric data at 670 nm (700–900 nm) is valid for in-water algorithms used to estimate chl-a (ISS) concentration in highly turbid coastal waters.     

The spectral responses of algal chlorophyll in water with varying levels of suspended sediment

The purpose of this paper is to investigate the spectral responses of algal chlorophyll and water, under natural sunlight with varying suspended sediment concentrations (SSC). Twenty levels of SSC with each of two sediment types were generated, ranging from 50 to 1000 mgl^?1, in 75101 of water containing chlorophyll-a concentrations of 718 μgl^?1 and 295 μgl^?1. Results indicate that suspended sediments do not eliminate the prominent spectral patterns of algal chlorophyll, even as SSC reached 1000 mgl^?1. Between 400 and 900 nm, the relation between reflectance and SSC satisfies the expression: d²R(λ)/dS²<0. The effects of varying SSC on the positions and magnitudes of pronounced chlorophyll features were investigated. The ratio between the NIR and red wavelengths was totally independent of SSC. Thus, our finding supports using it as an index for measuring chlorophyll in natural surface water containing suspended sediments.     

Linear Spectral Mixing Model for estimating optically active components in estuarine waters of Patos Lagoon,Brazil

Understanding about type and concentration of components in large continental waterbodies is of great value to environmental studies. The synoptic view of multispectral remote-sensing images has the potential to systematically estimate important parameters like suspended solids (SS) and chlorophyll (Chl). However, measures derived directly from radiometric image data usually retrieve inaccurate estimations, preventing discrimination of the spectra and amount of specific components. This work proposes the application of Linear Spectral Mixing Model to estimate concentrations of SS and Chl in the Patos Lagoon estuary, Brazil, through Landsat-8 Operational Land Imager sensor data. The linear mixing model was applied to produce fraction images of SS, sandy bottom, and dissolved organic matter. Simple and multiple linear regressions were performed in order to produce empirical models confronting parameters measured in situ with traditional radiometric data from spectral bands and with the resulting fraction images. The proposed empirical models estimates reached concentration of SS (coefficient of determination, R² = 0.84) and Chl (R² = 0.77) in comparison to the data collected in situ. These results showed great potential for using Spectral Mixing Models to indirectly estimate water components by remote-sensing data.     

Interpretation of the 685nm peak in water-leaving radiance spectra in terms of fluorescence,absorption and scattering,and its observation by MERIS

One of the major design goals of the Medium Resolution Imaging Spectrometer (MERIS) was the capability to use the signal from chlorophyll fluorescence stimulated by ambient sunlight to detect and map phytoplankton. This is considered tobe especially useful in coastal waters, where the determination of chlorophyll from water-leaving radiance spectra using the conventional blue/green ratio method is often complicated by high concentrations of gelbstoff and suspended matter. Based on a variety of studies, three spectral channels centred at 665, 681.25 and 705nm were included in the design of MERIS for retrieving the fluorescence signal. This paper presents observations with highresolution spectrometers which demonstrate the main factors affecting the observed signal in the red part of the spectrum. These factors are absorption by pure water, scattering by suspended particles, absorption and fluorescence of chlorophyll and the influence of submerged macrophyta. The influence of exceptional blooms such as 'red tides' on radiance spectra is also discussed. The paper shows how the combination of these effects can be understood using simple and easy-to-use radiative transfer models, and can be exploited by MERIS for improved mapping of phytoplankton, red tides and coastal, submerged and tidal flat vegetation.     

SeaWiFS potential for remote sensing of marine Trichodesmium at sub-bloom concentration

The SeaWiFS potential for identifying the marine cyanobacterium Trichodesmium, at low concentration in oligotrophic case I waters, has been investigated by numerical simulations using a three-component model of sea colour. The results obtained indicate that simple algorithms, which operate on data recorded by SeaWiFS bands 2, 3. and 5, should have the capability to discriminate pixels containing Trichodesmium at concentrations as low as 0·1–0·3 mgm^-3 chlorophyll a, as well as to yield an estimate of the concentration. The validity range of the procedure has been explored by a sensitivity analysis, assuming realistic variations in the input data to the optical model.     

Sun-stimulated chlorophyll fluorescence 2: Impact of atmospheric properties

Abstract

The Sun-stimulated chlorophyll fluorescence is a small but significant property of phytoplankton which can be detected using remote-sensing techniques. Besides the influence of oceanic properties, chlorophyll fluorescence is masked by atmospheric extinction. While an increase in chlorophyll concentration of 1 mg/m³ causes an increase in the upwelling radiances of about 0·03Wm^?2sr^?1 μm^?1 just above the water surface and due to the chlorophyll fluorescence, the upward radiances measured at λ_F = 685nm and at the top of the atmosphere ranges from 8 to 20Wm^?2sr^?1 μm^?1 for realistic atmospheric turbidity variations and a solar zenith distance of Θ_s = 50·7°. Additionally, the fluorescence, peaking at λ_F = 685nm with a half-width of about 10 nm, is reduced by the absorption of O₂ and H₂O. However, the fluorescence signal is nearly unaffected, when wavelengths λ≥686nm are exluded and a spectral interval of Δλ_F = 5nm is used for the radiance measurements.     

Remote sensing of phytoplankton An attempt from the Landsat Thematic Mapper

Abstract

An atmospheric correction scheme using Landsat Thematic Mapper (TM) bands 1, 2 and 4 was developed and the data corrected for atmospheric effects due to Rayleigh scattering and aerosols. Secondly phytoplankton pigment mapping was achieved through substitution of corrected radiances in TM bands 1 and 2 into a bio-optical algorithm developed for the study area off Azhikal in the Arabian Sea. A C map showing near-surface concentrations of phytoplankton pigments (mgm_?3)and a K map showing the pigment distribution (m?1) in one attenuation length were generated.     

Assessment of estuarine water-quality indicators using MODIS medium-resolution bands: Initial results from Tampa Bay, FL

Using Tampa Bay, FL as an example, we explored the potential for using MODIS medium-resolution bands (250- and 500-m data at 469-, 555-, and 645-nm) for estuarine monitoring. Field surveys during 21-22 October 2003 showed that Tampa Bay has Case-II waters, in that for the salinity range of 24-32 psu, (a) chlorophyll concentration (11 to 23 mg m⁻³), (b) colored dissolved organic matter (CDOM) absorption coefficient at 400 nm (0.9 to 2.5 m⁻¹), and (c) total suspended sediment concentration (TSS: 2 to 11 mg L⁻¹) often do not co-vary. CDOM is the only constituent that showed a linear, inverse relationship with surface salinity, although the slope of the relationship changed with location within the bay. The MODIS medium-resolution bands, although designed for land use, are 4-5 times more sensitive than Landsat-7/ETM+ data and are comparable to or higher than those of CZCS. Several approaches were used to derive synoptic maps of water constituents from concurrent MODIS medium-resolution data. We found that application of various atmospheric-correction algorithms yielded no significant differences, due primarily to uncertainties in the sensor radiometric calibration and other sensor artifacts. However, where each scene could be groundtruthed, simple regressions between in situ observations of constituents and at-sensor radiances provided reasonable synoptic maps. We address the need for improvements of sensor calibration/characterization, atmospheric correction, and bio-optical algorithms to make operational and quantitative use of these medium-resolution bands.     

On the potential of MODIS and MERIS for imaging chlorophyll fluorescence from space

The first images from MODIS (Moderate Resolution Imaging Spectroradiometer) are now being used to evaluate information on the in vivo fluorescence peak near 685?nm from chlorophyll-a, stimulated by sunlight. The Fluorescence Line Imager (FLI) airborne imaging spectrometer was used in the 1980s to demonstrate the mapping of this signal from an aircraft, showing that it gave significant rejection of confusing signals from atmospheric radiance. For imaging from space, a major limitation is sensor sensitivity, which tends to restrict imaging to relatively high concentrations under good solar illumination. Noise-equivalent chlorophyll concentrations for MODIS and MERIS (Medium Resolution Imaging Spectrometer) have been estimated as 0.07 and 0.1?mg?m^?3, respectively, under zenith Sun, clear sky conditions. Although MERIS has slightly poorer sensitivity, it has the advantage of flexible band placing and presence of a band at 709?nm in the baseline band-set that allows better definition of the continuum spectrum above which fluorescence is measured. This band should also allow detection of bright plankton blooms (red-tide events) through the peak radiance near 709?nm caused by a combination of in-water scattering and absorption.     

Bio-optical properties of east coast Malaysia waters in relation to remote sensing of chlorophyll

Surface water samples collected during the monsoon and inter-monsoon seasons of 2009 off the east coast of Peninsular Malaysia have been analysed for concentrations of total chlorophyll, suspended particulates and coloured dissolved organic matter (CDOM). Spectral absorption coefficients of dissolved and particulate materials have also been measured. Significant seasonal variabilities in concentrations and optical properties were reported with high concentrations of all parameters during the northeast monsoon (NEM) season and low during the southwest monsoon (SWM) and inter-monsoon seasons. Contrary to previous reports on the oligotrophic nature of the waters during the inter-monsoon season, relatively high concentrations of chlorophyll (>3 mg m^?3) were observed at offshore stations in the study area in the spring and fall inter-monsoon months. The chlorophyll-specific absorption spectrum changes with the seasons with the greatest absorption per unit chlorophyll during the SWM and the least during the inter-monsoon seasons, probably in response to seasonal changes in phytoplankton community and cell size structure. The water is classified as optical case 2. At the blue end of the spectrum (440 nm), light absorption by non-phytoplankton materials (CDOM and detritus) accounts for nearly 70% of the total non-water absorption regardless of the season. At the wavelength (676 nm) of the secondary chlorophyll absorption peak in the red part of the spectrum, light absorption by chlorophyll contributes 80–90% to total non-water absorption at most stations and this may provide the basis for remote sensing of phytoplankton chlorophyll in these waters.     

Validation of MERIS ocean-color products in the Bohai Sea: A case study for turbid coastal waters

An extensive in situ data set in the Bohai Sea of China was collected to assess radiometric properties and concentrations of ocean constituents derived from Medium Resolution Imaging Spectrometer (MERIS). The data collected include spectral normalized water-leaving radiance L_wn(λ) and concentrations of suspended particulate matter (SPM) and chlorophyll a (Chl-a). A strict spatio-temporal match-up method was adopted in view of the complexity and variability of the turbid coastal area, resulting in 13, 48 and 18 match-ups for MERIS L_wn(λ), SPM and Chl-a estimates, respectively. For MERIS L_wn(λ), the match-ups showed mean absolute percentage differences (APD) of 17%-20% in the 412, 443, 620 and 665 nm bands, whereas L_wn(λ) at bands from 490 and 560 nm had better APD of 15-16%. The band ratio of L_wn(490) to L_wn(560) of the satellite data was in good agreement with in situ observations with an APD of 4%. MERIS SPM and Chl-a products overestimated the in situ values, with the APD of approximately 50% and 60%, respectively. When match-up criteria were relaxed, the assessment results degraded systematically. Hence, in turbid coastal areas where temporal variability and spatial heterogeneity of bio-optical properties may be pronounced as the result of terrestrial influences and local dynamics, the strict spatio-temporal match-up is recommended.