John F. Schalles

Estimation of chlorophyll a from time series measurements of high spectral resolution reflectance in an eutrophic lake

We acquired high spectral resolution reflectance data in Carter Lake, a eutrophic oxbow on the Iowa–Nebraska border, from April 1995 to April 1996. Chlorophyll a, total seston, sestonic organic matter, Secchi depth, and nephelometric turbidity were determined for each respective spectral measurement. Changes in algal taxonomic structure and abundance coincided with the development and senescence of a midsummer through autumn bloom of Anabaena. Taxonomic structure was more diverse in late winter and spring when Synedra sp. (diatom) and several chlorophytes and dinoflagellates were present. Overall, chlorophyll a varied from about 20 to 280 μg·L−1, Secchi transparency from 18 to 74 cm, and seston dry weight from 11 to 48 mg·L−1 in February and September, respectively. Particulate matter completely dominated lake water light attenuation. Dissolved organic matter had low optical activity. The most sensitive spectral feature to variation in chlorophyll a concentration was the magnitude of the scattering peak near 700 nm. The 700‐nm peak correlated to chlorophyll concentration through the relationships between algal pigment absorption near 670 nm and the cell biomass and surface‐related scattering signal in the near infrared. An algorithm relating the height of the 700‐nm reflectance peak above a reference baseline between 670 and 850 nm to chlorophyll a was accurate and robust despite large variations in optical constituents caused by both strong seasonality in the algal system and short‐term variations in seston from wind‐induced sediment resuspension. The present algorithms were successfully used in other systems with different seasonality and productivity patterns. The coefficients of the models relating chlorophyll a and spectral reflectance variables appeared to be ecosystem specific: both the intercept and slope for the models in this study were moderately lower than for several other recently published results. We validated our algorithm coefficients with a second, independent dataset. The standard error for chlorophyll a prediction was ±28 μg·L−1.

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

Abstract 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.

Fluorescence component in the reflectance spectra from coastal waters. Dependence on water composition

Based on HYDROLIGHT simulations of more than 2000 reflectance spectra from datasets typical of coastal waters with highly variable optically active constituents as well as on intercomparisons with field measurements, the magnitude of chlorophyll fluorescence was analyzed and parameterized as a function of phytoplankton, CDOM, and suspended inorganic matter concentrations. Using the parameterizations developed, we show that variations in the fluorescence component of water leaving radiance in coastal waters are due more to the variability of attenuation in the water than to the variability of the fluorescence quantum yield, which we estimate to be relatively stable at around 1%. Finally, the ranges of water conditions where fluorescence plays a significant role in the reflectance NIR peak and where it is effectively undetectable are also determined.

Comparative reflectance properties of algal cultures with manipulated densities

A comparative study was conducted with representatives of four ecologically important freshwater algal phyla (Chlorophyta, Cyanophyta, Bacillariophyta and Pyrrophyta) in order to (1) analyze the relationship between reflectance spectral features and algal density, and (2) to elucidate and characterize possible diagnostic spectral reflectance features for identification of dominant groups in algal bloom states. Algae (two chlorophytes, a cyanophyte and a bacillariophyte) were cultured in mesocosm tanks and then their densities were reduced through dilution with clear water in four experiments. In a fifth experiment, use was made of the pyrrophyte Peridinium gatunense, which bloomed in Lake Kinneret, Israel, practically forming a unialgal ‘culture’. The basic quantitative procedures for spectroradiometeric measurements and support data collection (pigment concentrations) were consistent among the experiments. Several spectral reflectance characteristics were found to be common to all the species examined in the study: a depression between 440 and 500 nm, a salient trough around 670 nm, and prominent peaks centered around 570 nm and 700 nm. The increase in magnitude of a near infra-red peak and shift of its position towards longer wavelengths with increased chlorophyll- a concentration was found to be a common trait for all the species studied. Other specific spectral reflectance features were also found and are discussed here. Quantitatively different relationships of magnitude and position of spectral reflectance features with chlorophyll- a concentration reflect different optical properties of algae (their scattering and absorption). This may be useful for taxonomic characterization by remote sensing.

Carolina bays of the Savannah River Plant

Much of the research to date on the Carolina bays of the Savannah River Plant and elsewhere has focused on certain species or on environmental features. Different levels of detail exist for different groups of organisms and reflect the diverse interests of previous investigators. This report summarizes aspects of research to date and presents data from numerous studies. 70 refs., 14 figs., 12 tabs.

The response of volume reflectance to manipulated algal concentrations above bright and dark bottoms at various depths in an experimental pool

Abstract We investigated changes in spectral reflectance associated with manipulations of algal densities and both bright and dark “bottoms”; at various depths in a 9500 L pool. Spectroradiometer data were collected at close range under natural sunlight. We found orderly patterns of either increasing or decreasing volume reflectance dependent on algal density, bottom color, and bottom depth. The color of the submerged bottom affected reflectance signals, especially the magnitude of the difference between the points of peak near‐infrared reflectance and chlorophyll‐dependent red absorption. Wavelength shifts in reflectance parks were detected as algal chlorophyll was reduced from 336 to 32 mg/m3. The wavelength of peak green reflectance increased from 552 to 567 nm and the near‐infrared peak from 695 to 717 nm.

A comparison between above-water surface and subsurface spectral reflectances collected over inland waters

The objective of the research was to undertake a quantitative comparison of spectral-reflectance measurements made slightly above the surface of water bodies with the measurements made slightly below the surface. The study is focused on three rivers; two in Georgia, USA and one in Japan. As expected, the differences in reflectance are not constant and vary with the wavelength. The contribution of surface-reflection effects to the surface reflectance measured slightly above the water is both pronounced and highly variable, but although they do alter the magnitude of the upwelling signal, they do not change the general shape of the spectral profiles. The correction of surface-reflection effects by assuming a proportionality factor (ρ) is not considered to be efficient for inland fresh water bodies. For in situ spectroscopy, the recommended approach is to measure upwelling radiance slightly below the waters surface as a means of minimizing extraneous noise. Researchers should be aware of the potential for diminishing the validity of findings because of measurement errors.

The influence of suspended clays on phytoplankton reflectance signatures and the remote estimation of chlorophyll

Wavelength specific scattering and absorption by phytoplankton produces distinctive reflectance spectra which are discernable with high spectral resolution sensors. These spectra are diagnostically useful for predicting and monitoring chlorophyll a in surface waters (OEKKER et al. 1991, GITELSON et al. 1993, R!CHARDSON 1996, SCHALLES et al. 1998). In many inland and coastal waters, inorganic and organic non-algal seston are present in concentrations which modif)r or even dominate the composite warer column optical processes and rhe upwelling radiation field detectable by sensors at or above the surface (CURRAN & Novo 1988, Novo et al. 1989, VERTUCCI and LIKENS 1989, BuKATA et al. 1991, QuiBELL 1991, RuNDQUIST et al. 1995). Optically, these diverse types of particles may scatter and absorb light differendy according to their physical, chemical, and/or biological characteristics (BUKATA et al. 1991, DEKKER et al. 1991). Clay an d other mineral particles cause strong light scattering and have a relatively fixed o r weakly varying relationship between their scattering and absorbing properties across visible and near infrared wavelengths (CURRAN & Novo 1988).

Ultraviolet-Visible and Fluorescence Spectral Evidence of Natural Organic Matter (NOM) Changes along an Estuarine Salinity Gradient

A transect of the St Marys River estuary from above the point of maximum salt wedge penetration to coastal salinities was conducted in July 1999. None of the parameters examined—dissolved organic carbon (DOC) content, UV light absorbance at 254 nm, and Total Luminescence spectra—follow the rule of conservative mixing. The characteristics of the different molecular size fractions of the St Marys River natural organic matter (NOM), as well as the results of a laboratory mixing experiment, provided evidence that loss of larger molecular size compounds from riverine NOM may occur by coagulation at salinities up to 10. An apparent gain of carbon in the lower estuary was attributed to exports from abundant coastal marshes in this area.The Total Luminescence spectra of the riverine NOM can be described by two peaks, centered respectively around 340/445 nm, and 230/430 nm Excitation/Emission Wavelength Pair (EEWP), which are characteristic of humic materials of aquatic origin. The samples from the high salinity stations exhibit peaks at lower emission wavelength EEWP 320/424 nm, which can be considered as marine humic-like material. The presence of amino acid-tryptophan like peaks were observed, with EEWP 300/350 nm in some of the high salinity samples. This peak was of high relative fluorescence intensity. It is hypothesized that the intense biological activity of the salt marsh and near coastal area is responsible for the carbon addition as well as the appearance of the highly fluorescence amino acid-protein like material.

Examination of Abiotic Drivers and Their Influence on Spartina alterniflora Biomass over a Twenty-Eight Year Period Using Landsat 5 TM Satellite Imagery of the Central Georgia Coast

We examined the influence of abiotic drivers on inter-annual and phenological patterns of aboveground biomass for Marsh Cordgrass, Spartina alterniflora, on the Central Georgia Coast. The linkages between drivers and plant response via soil edaphic factors are captured in our graphical conceptual model. We used geospatial techniques to scale up in situ measurements of aboveground S. alterniflora biomass to landscape level estimates using 294 Landsat 5 TM scenes acquired between 1984 and 2011. For each scene we extracted data from the same 63 sampling polygons, containing 1222 pixels covering about 1.1 million m2. Using univariate and multiple regression tests, we compared Landsat derived biomass estimates for three S. alterniflora size classes against a suite of abiotic drivers. River discharge, total precipitation, minimum temperature, and mean sea level had positive relationships with and best explained biomass for all dates. Additional results, using seasonally binned data, indicated biomass was responsive to changing combinations of variables across the seasons. Our 28-year analysis revealed aboveground biomass declines of 33%, 35%, and 39% for S. alterniflora tall, medium, and short size classes, respectively. This decline correlated with drought frequency and severity trends and coincided with marsh die-backs events and increased snail herbivory in the second half of the study period.