Robert P. Bukata

Trends in Water Clarity of the Lower Great Lakes from Remotely Sensed Aquatic Color

ABSTRACT Satellite observations of aquatic colour enable environmental monitoring of the Great Lakes at spatial and temporal scales not obtainable through ground-based monitoring. By merging data from the Coastal Zone Color Scanner (CZCS) and the Sea-viewing Wide Field-of-view Sensor (SeaWiFS), monthly binned images of water-leaving radiance over the Great Lakes have been produced for the periods 1979–1985 and 1998–2006. This time-series can be interpreted in terms of changes in water clarity, showing seasonal and inter-annual variability of bright-water episodes such as phytoplankton blooms, re-suspension of bottom sediments, and whiting events. Variations in Secchi disk depth over Lakes Erie and Ontario are predicted using empirical relationships from coincident measurements of water transparency and remotely-sensed water-leaving radiance. Satellite observations document the extent to which the water clarity of the lower Great Lakes has changed over the last three decades in response to significant events including the invasion of zebra mussels. Results confirm dramatic reductions in Lake Ontario turbidity in the years following mussel colonization, with a doubling of estimated Secchi depths. Evidence confirms a reduction in the frequency/intensity of whiting events in agreement with suggestions of the role of calcium uptake by mussels on lake water clarity. Increased spring-time water clarity in the eastern basin of Lake Erie also corroborates previous observations in the region. Despite historical reports of localised increases in transparency in the western basin immediately following the mussel invasion, image analysis shows a significant increase in turbidity between the two study periods, in agreement with more recent reports of longer term trends in water clarity. Through its capacity to provide regular and readily interpretable synoptic views of regions undergoing significant environmental change, this work illustrates the value of remotely sensing water colour to water clarity monitoring in the lower Great Lakes.

Use of chromaticity in remote measurements of water quality

Abstract Optical cross sections (absorption and scattering per unit concentration of chlorophyll a , suspended mineral, and dissolved organic carbon) are utilized to generate subsurface irradiance reflectance spectra and upwelling irradiance spectra for a variety of water masses. Standard CIE chromaticity analyses are applied to the upwelling irradiance spectra. The results of these analyses serve as a basis for a discussion of severe restrictions in the application of chromaticity to water quality prediction.

Estimation of Organic and Inorganic Matter in Inland Waters: Optical Cross Sections of Lakes Ontario and Ladoga

The high degree of optical complexity of inland water masses necessitates the development of models which consider the optical competitiveness of several co-existing aquatic components. A model is described which was developed to simultaneously estimate chlorophyll a, suspended mineral, and dissolved organic carbon concentrations from a single measurement of the subsurface volume reflectance spectrum in the optically complex waters of Lake Ontario. To estimate such aquatic concentrations requires a quantification, as a function of wavelength, of the amount of scattering and absorption that may be ascribed to a unit concentration of each aquatic component (i.e., the pertinent optical cross sections). Cross section spectra are presented for Lake Ontario and it is illustrated how such cross sections may be utilized in conjunction with directly-measured subsurface volume reflectance spectra and optimization analyses to extract the organic and inorganic components. Cross section spectra obtained in a similar manner for Lake Ladoga are also presented and compared to those of Lake Ontario. Similarities in absorption, but differences in backscattering cross section spectra for suspended inorganic matter were observed for the two lakes. Near-identical absorption and very similar backscattering cross section spectra suggest that Lakes Ontario and Ladoga are characterized by optically-comparable populations of chlorophyll-bearing biota.

Particulate concentrations in Lake St. Clair as recorded by a shipborne multispectral optical monitoring system

Abstract Methodology is developed to convert the upwelling multispectral radiance observed by a ship-mounted dual radiometer reflectance system into the subsurface volume reflectance normalized to a standard solar zenith angle and a standard downwelling light field. This normalized multispectral subsurface volume reflectance is then utilized to evaluate the capacity of effectively conducting, from a moving research vessel, a continuous monitor of the total seston and suspended sediment concentrations in the generally turbid, shallow waters of Lake St. Clair. Good agreement is obtained between remotely predicted and directly sampled concentrations of both seston and suspended sediments.

Satellite Monitoring of Optically-Active Components of Inland Waters: An Essential Input to Regional Climate Change Impact Studies

Abstract Consistent with the climate change objectives of the IGBP is the need to remotely monitor and map both global and regional biological productivity over lands, oceans, and inland waters. Models and algorithms are currently being developed to infer aquatic primary production from near-surface chlorophyll concentration values determined from satellite sensors. Data from Lake Ladoga are utilized to illustrate that the algorithms currently being used to monitor near-surface chlorophyll concentrations in oceanic waters are inadequate when applied to water masses optically complicated by their proximity to land masses. Methodologies originally developed for retrieving simultaneous concentrations of chlorophyll, suspended minerals, and dissolved organic carbon from volume reflectance measurements of Lake Ontario are shown to display success in Lake Ladoga that could not be duplicated by six different oceanic chlorophyll retrieval algorithms. The principal requirements for water quality satellite monitoring are the cross sections of the optically-active components of the water body being remotely monitored. It is argued that, despite the spatial and temporal variability of such cross sections, their determination for principal water bodies should comprise both global and regional climate change studies.

Spectral Attenuation and Irradiance in the Laurentian Great Lakes

Abstract Optical data collected between 1973 and 1979 are utilized to study spectral attenuation and irradiance in the midlake waters of four Laurentian Great Lakes. Particular attention is given to the photosynthetic available radiation (PAR), its incident spectral distribution, its spectral attenuation, and its qualitative change with depth. Curves are shown illustrating these spectral properties as well as the relationships between PAR and the photosynthetic usable radiation (PUR) for each of the lakes. These curves, along with the included mathematical relationships, enable quantitative estimates to be obtained of incident PAR, subsurface PAR, and subsurface PUR, and qualitative information to be obtained on subsurface irradiance levels for the Laurentian Great Lakes.

Tracking the Propagation of Solar Ultraviolet Radiation: Dispersal of Ultraviolet Photons in Inland Waters

Abstract Solar extraterrestrial ultraviolet radiation is tracked from its impingement upon the Earths atmosphere to its dispersal in natural inland waters. This is accomplished through the use of a solar spectral irradiance model, a water column optical model, directly measured (AES monitoring network) values of ground-level ultraviolet radiation and stratospheric ozone, optical properties of aquatic matter indigenous to Lake Ontario and Waskesiu Lake, Setlow DNA damage action spectrum, and phyto-plankton photosynthesis inhibition spectrum. The conclusions from such a combined theoretical/experimental study include a) the presence of dissolved organic matter (DOM) provides an effective UV-B shield for aquatic organisms despite the absorption spectra of dissolved organic carbon (DOC) indigenous to North American waters displaying substantial variations, b) UV absorption for a given DOC concentration varies from water body to water body, c) when UV irradiance is weighted according to the Set-low DNA damage action spectrum, the physical depth of the 1% of surface irradiance level decreases, i.e., attenuation ofSetlow-weighted irradiance is greater than attenuation of actual UV-B irradiance, d) a single mathematical relationship can relate the change in Setlow-weighted daily irradiation at the surface of the Earth to ozone layer depletion while another single relationship can relate the change in depth-averaged Setlow-weighted daily irradiation to ozone layer depletion, these single relationships being applicable throughout the year for all mid-to-high latitudes, e) the increase in Setlow-weighted irradiation due to ozone layer depletion decreases with depth in the water column, f) phytoplankton photosynthesis inhibition by UV-A significantly exceeds inhibition by UV-B, g) inhibition of phytoplankton photosynthesis in many inland waters will not be significantly impacted by ozone layer thinning, h) modeling ground-level and subsurface values of ultraviolet radiation provides a suitable surrogate for the direct measurement of the minuscule and spectrally steep UV radiation, i) the highly diffusive character of the ground-level UV-B irradiance results in the reflectance of UV-B from the water being essentially constant (5% to 8%) throughout the year and the transmission of UV-B across the air-water interface being virtually unaffected by surface waves, j) in the absence of suspended sediments, the contribution of chlorophyll-bearing biota to the spectral UV photon budget is in anti-phase with the contribution of DOM, while the contributions of water and surface reflectance are comparable, small, and relatively constant, k)for the water bodies considered, dissolved organic matter accounts for the largest component of the UV photon budget.

Relationships Among Secchi Disk Depth, Beam Attenuation Coefficient, and Irradiance Attenuation Coefficient for Great Lakes Waters☆

Abstract Optical data collected between 1973 and 1979 are utilized to discuss the relationships among the directly observed Secchi disk depths and the directly measured total attenuation coefficients and irradiance attenuation coefficients in Lakes Erie, Ontario, Superior, and Huron, as well as Georgian Bay. Tables and curves are presented depicting these mathematical relationships obtained by statistical regressions. These relationships are used to effect an intercomparison of the four Laurentian Great Lakes. In addition, subsurface, vertically downward sighting ranges are estimated and compared to the Secchi disk depths as determined from the mathematical regressions. Since there exists a vast amount of historical Secchi disk depth measurements, the need for such relationships certainly exists. It is intended that the cautious use of these regressions will assist the interpretation of such historical data bases wherein Secchi depths comprise the only available optical information.

Validation of a radiometric color model applicable to optically complex water bodies

Abstract A radiometric color model relating the color of optically complex (non-Case I) waters to the organic and inorganic color-producing agents (CPA) responsible for that color has been previously applied to the waters of Lake Ontario, Canada and Lake Ladoga, Russia. Additional underwater optical measurements and water quality data from Lakes Erie and Michigan and several boreal lakes in northern Ontario, as well as comparable data from the European lakes Krasnoye, Zug, and Lucerne are utilized to illustrate both the validity of the model, as well as the universality of its application. It was found that, due to the limited availability of specific spectral scattering and absorption properties (optical cross-section spectra) for CPA indigenous to natural water bodies on a global scale, the use of optical cross-section spectra appropriate to the CPA indigenous to Lake Ontario and those appropriate to the CPA indigenous to Lake Ladoga provided a more-than-adequate surrogate for the water bodies considered herein (inclusive of river systems in the British Columbia Canadian Cordillera for which results of a study relating river water color to hydrographic basin features are also revisited). Similarities among the optical cross-section spectra pertinent to freshwater biota and similarities among the optical cross-section spectra pertinent to freshwater dissolved organic matter would appear to allow such a liberal use of site-specific aquatic optical properties. However, greatest discrepancies in indigenous inland water CPA optical cross-section spectra are consequences of global geologic diversities. A compensation for geologic diversities is illustrated by the use of Lake Ladoga cross-section spectra with the European lakes and the Lake Ontario cross-section spectra with the North American lakes, also providing validation of both the radiometric model and its universality. Such geologic similarities could alleviate labor-, time-, and cost-intensive determinations of optical cross-section spectra for many inland water bodies.

Modelling the Radiometric Color of Inland Waters: Implications to a) Remote Sensing and b) Limnological Color Scales

Utilizing a bio-optical model previously developed for Lake Ontario, the responsiveness of chromaticity coordinates (X, Y, Z), dominant wavelength (λdom), and associated spectral purity (p) to the abundance of color-producing agents (CPA) residing within the Lake Ladoga water column was determined. CPA considered were phytoplankton (chl), suspended minerals (sm), and dissolved organic carbon (doc). Waters that contain simultaneously low concentrations of chl, sm, and doc are shown to appear blue to turquoise in color (472–500 nm). Highly turbid waters (i.e., waters containing high concentrations of chl and/or sm) with low concentrations of doc are shown to display colors ranging from green to brown (> 500 nm). Waters with large concentrations of doc, irrespective of turbidity, are shown to be invariably brownish (560–570 nm). With increasing CPA content, X, Y, and Z (and, consequently, λdom) asymptotically approach constant limit values. An “end-point” dominant wavelength at about 572 nm appears to be intrinsically characteristic of all natural waters. It is shown that when one or more CPA exceeds a critical concentration, the spectral purity p asymptotically approaches values in the range 0.35 to 0.45 for all waters (exceptive of those containing solely chl in the restricted concentration range < 0.5 μg/L). Optical distinctiveness, particularly with respect to indigenous doc, of natural waters, while impacting the spectral purity of the “end-point” radiometric color, does not produce a comparable impact on the “end-point” color itself. This work reinforces the restrictive application of chromaticity analyses to the remote sensing of binary aquatic systems comprised of water plus one CPA. It also illustrates that neither panchromatic nor two-channel ratio images can provide unambiguous inference of water quality parameters. Correspondence between radiometric water color descriptors (X, Y, Z, λdom, and p) and water color scales traditionally used in limnology is established, illustrating that the platinum-cobalt scale would be most appropriate for assessing waters that were radiometrically yellow, provided that the yellow hue were not invariably attributed to doc.