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Distinguishing between chlorophyll-a and suspended solids in lake water using hyperspectral data
Abstract
Classifying surface water bodies according to trophic status by remote sensing techniques has had limited success in lakes with relatively high nonalgal turbidity levels. Since the trophic status of a lake is typically defined based on its chlorophyll-a concentration, and since relatively high suspended solids concentrations masks chlorophyll absorption and reflectance peaks, determining trophic status remotely is typically only partially successful. Hoer, we were interested in exploring hyperspectral data analysis for estimating trophic status. Hyperspectral data (10 nm resolution between 262 and 850 nm) of light attenuation were measured in Lake Texoma (USA) at the surface, 0.1, 0.5, 1.0 and 1.5 meters in depth, while simultaneously analyzing the water column for chlorophyll-a and suspended solids concentration. Data were collected at five sampling stations, each representative of a major zone in the 36,000 hectare lake, approximately monthly, during 1996/97 hydrologic year. Downwelling and upwelling vertical attenuation coefficients were calculated using Bouger-Lambert's law. First and second order derivatives, as well as higher order derivatives were applied to the spectral data. The results showed a clear correlation between first order derivatives and turbidity, while the second order derivatives were correlated to chlorophyll-a concentrations.
A numerical trophic state index for lakes has been developed that incorporates most lakes in a scale of 0 to 100. Each major division ( 10, 20, 30, etc. ) represents a doubling in algal biomass. The index number can bc calculated from any of several parameters, including Secchi disk transparency, chlorophyll, and total phosphorus. My purpose here is to present a new ap- proach to the trophic classification of lakes. This new approach was developed because of frustration in communicating to the pub- lic both the current nature or status of lakes and their future condition after restoration when the traditional trophic classification system is used. The system presented hcrc, termed a trophic state index (TSI), in- volves new methods both of defining trophic status and of determining that status in lakes. All trophic classification is based on the division of the trophic continuum, howcvcr this is defined, into a series of classes termed trophic states. Traditional systems divide the continuum into three classes: oligotrophic, mesotrophic, and cutrophic. There is often no clear delineation of these divisions. Determinations of trophic state are made from examination of several di- verse criteria, such as shape of the oxygen curve, species composition of the bottom fauna or of the phytoplankton, conccntra- tions of nutrients, and various measures of biomass or production. Although each changes from oligotrophy to eutrophy, the changes do not occur at sharply defined places, nor do they all occur at the same place or at the same rate. Some lakes may be considered oligotrophic by one criterion and eutrophic by another; this problem is
The relative contribution of different components to the attenuation of photosynthetically active radiation was determined in the Charlotte Harbor estuarine system based on laboratory and in situ measurements. Agreement between laboratory and in situ measurements of the attenuation coefficient (kt) was good (r2 = 0·92). For all in situ measurements (n = 100), suspended, non-chlorophyll matter accounted for an average of 72% of kt, dissolved matter accounted for 21%, suspended chlorophyll for 4%, and water for the remaining 3%.For individual determinations, suspended non-chlorophyll matter, dissolved matter, suspended chlorophyll, and water, each accounted for as much as 99%, 79%, 21%, and 18% of kt. Attenuation by suspended matter was greatest near the mouth of the northern tidal rivers and was variable over the rest of the estuarine system. Attenuation by dissolved matter was greatest in the brackish tidal rivers and decreased with increasing salinity. Attenuation due to dissolved matter was positively correlated with water color. The source of the color was basin runoff. Wavelength transmittance changed along the salinity gradient. Maximum transmittance shifted from 500 to 600 nm in gulf waters to 650 to 700 nm in colored, brackish waters. Dissolved matter was primarily responsible for the large attenuation at short wavelengths (400–500 nm).
The major objective of this study was to assess the technical feasibility of using TM data to evaluate, both qualitatively and quantitatively, the general water quality of southern Green Bay and central Lake Michigan. An empirical approach of relating TM data with simultaneously acquired 'sea truth' data through multiple linear regression analysis was employed. Subsequently, the regression models were used to prepare digital cartographic products depicting the water quality and thermal distributions over the entire study area.-from Author
Photosynthesis involves the capture of the Sun's energy into biochemical energy that sustains all life on the Earth. It is a massive process that has radically changed the composition of the atmosphere. Its scale can be judged from the fact that man's entire fossil fuel use from 1860 to 1988 was equivalent to only 2 years’ global photosynthesis. Although photosynthesis occurs in a wide range of organisms (bacteria, cyanobacteria, algae, and plants), and about a third of it occurs in the oceans, this discussion concentrates on plants.
Discrimination of the chlorophyll signal from those of suspended sediments and the water itself has proven to be a difficult problem in optical remote sensing of algal biomass. Our study uses numerical differentiation of high resolution spectral data collected at close range over experimental tanks to address this problem. Results indicate that pure water effects can be reduced by a first-order derivative curve and suspended sediment effects can be removed by a second-order transformation. Chlorophyll content is correlated with the difference between the second derivatives at 660 and 895 nanometers. This relationship holds even in the presence of background turbidity. Thus, it is an effective means of compensating for interference from suspended solids. Our findings are discussed as they relate to the use of remote sensing for lake and reservoir management.
The role of light interception by photosynthetic pigments on the productivity of phytoplankton was studied year‐round in mesotrophic Lake Constance. The phytoplankton showed wide seasonal fluctuations in both biomass (Chl a ) and photosynthesis.
Linear regression was used to determine pigment‐specific light attenuation and nonalgal light extinction. Fractional light absorption by photosynthetic pigments ranged from 4 to 70%. Pheopigments comprised 27.4 ± 8.2% of total pigments. Since pheopigments do not transfer energy to the photosystems, their light absorption must be added to the background light attenuation by nonphotosynthetic material. If pheopigments are considered, estimates of fractional light absorption decrease accordingly.
A plot of integral photosynthesis vs. maximum photosynthesis at the depth of the optimum light intensity gives a saturation curve, due to increasing fractional light absorbance by photosynthetic pigments at rising concentrations of biomass. Integral photosynthesis would reach a theoretical upper limit if all light were absorbed by active photosynthetic pigments. Since pheopigment concentrations usually rise with active Chl a , background light attenuation increases also, leading to diminished productivity per unit of lake surface area.
Concurrent in situ water quality and SPOT (Systeme Pour l'Observation de la Terre) satellite data were obtained for three reservoirs located in north Texas. In situ data included measurement of chlorophyll a, pheophytin a, total suspended solids, total dissolved solids, and turbidity. The SPOT data from locations of water samples were subset and digital data were examined in their raw states as well as numerous transformations. Low but significant correlations were observed between digital numbers and both turbidity and chlorophyll a. The degree of correlation was not as great as in previously reported studies using SPOT and Landsat data. To be useful for reservoir scale monitoring in north Texas, the techniques still need to be refined sufficiently to detect differences within the range of water quality typically found in the area under study.
Absorption and emission spectroscopy in the ultraviolet and visible regions, apart from being the earliest physical techniques of analysis, have great utility in solving a variety of structural and analytical problems [l-41. However, in many cases the quantitative determination of individual components in a mixture by UV-VIS spectroscopy becomes very difficult owing to the spectral similarities and overlapping of weak absorption bands of one component by strong absorption bands of other components. It was recently recognized that derivative spectrophotometry, introduced about three decades ago [5-81 and originally proposed by Rutherford [9], can be a very useful analytical tool for characterizing an analyte band that is overlapped by other absorption bands with different halfwidths. On differentiation, the intensity of the absorption band with a small halfwidth increases more than that of a band with a large halfwidth [10]. The central idea behind the development of the science of derivatized spectra rests upon the measurement of the first or the higher derivatives of the absorbance A or the luminescence intensity I with respect to the wavelength λ, e.g., dI/dλ, dA/dλ, dI/dλ, etc. These derivatives, when plotted or scanned against wavelength, yield an array of derivative spectra.
An important influence on gross photosynthetic rates per unit area is the success with which phytoplankton competes for the available light with non-algal suspended material and dissolved organic compounds. Using a range of Irish lakes, with euphotic zones varying between 0.7 and 20 m and with chlorophyll-α values between 1 and 860 mg m−3, the effect on gross rates of photosynthesis is analysed for changes in the balance between the factors contributing to light attenuation. Net values per unit area are also likely to be modified in well-mixed systems as the ratio of light to dark regions in the water column are altered with changes in light penetration. Depth gradients in dark respiration are reported for L. Neagh which vary according to previous light history and nutrient stress. Possible cases of restraint on phytoplankton growth are discussed for optically deep situations.