Expanding understanding of optical variability in Lake Superior with a 4-year dataset (original) (raw)
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Journal of Geophysical Research: Oceans, 2013
1] Satellite remote sensing offers one of the best spatial and temporal observational approaches. However, well-validated satellite imagery has remained elusive for Lake Superior. Lake Superior's optical properties are highly influenced by colored dissolved organic matter (CDOM), which has hindered the retrieval of chlorophyll concentration through band-ratio algorithms. This study evaluated seven existing inversion algorithms. The top-performing inversion algorithm was tuned to a Lake Superior optical data set and applied to satellite imagery. The retrieval of chlorophyll concentration via inversion algorithms was not possible due to errors in derived CDOM absorption being greater than phytoplankton absorption values and the very small contribution of phytoplankton absorption to the overall absorption budget. However, the retrieval of absorption due to CDOM from satellite imagery was encouraging. To ensure that the best satellite remotely sensed reflectance estimates were used in the retrieval of absorption due to CDOM, several atmospheric correction schemes were evaluated. The absorption due to CDOM was greatest in the western arm of Lake Superior and near river mouths and decreased with distance offshore. The absorption due to CDOM had a bimodal distribution over the annual cycle with the greatest peak in fall and a smaller peak in spring.
Optical characterization of Lake Champlain: Spatial heterogeneity and closure
Journal of Great Lakes Research, 2013
A robust optical characterization of the underwater and emergent light fields of Lake Champlain was conducted for sites (n = 11) throughout the lake in August 2011, based on in situ measurements with modern instrumentation and laboratory measurements of optically active constituents (OACs) and components (a x) of the absorption coefficient (a). Inherent optical property (IOP) measurements included a, a x , and the particulate scattering and backscattering coefficients. Metrics of apparent optical properties (AOPs) included Secchi depth, the diffuse attenuation coefficients for downwelling [K d (λ)] and scalar (K 0) irradiance and remote sensing reflectance [R rs (λ)]. The credibility of the measurements is demonstrated through: (1) consistency of relationships between OACs and IOPs and AOPs, (2) the approach toward equivalence of laboratory and field measurements, and (3) the extent of closure of predictions of K d (λ) and R rs (λ), based on IOP measurements and radiative transfer expressions, with paired observations of these AOPs (average differences of 9.4 and 19.3%). Wide spatial differences in OACs, and the resulting IOPs and AOPs, are documented throughout the bounds of the lake and are the result of its morphologic complexity and differing external loading. The lake is a complex case 2 system, with uncoupled variations in OACs and a x over the bounds of the lake. Both empirical and radiative transfer expressions are used to predict changes in AOPs in response to hypothetical changes in OACs.
Journal of Great Lakes Research, 2014
Thirteen years of SeaWiFS data (1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010) from the early spring isothermal period (March-April) were used to determine trends of water attenuation coefficient (K dPAR ), chlorophyll a (Chl a), Photosynthetic Available Radiation (PAR), and modeled primary production in southern Lake Michigan. Surface PAR values remained unchanged between 1998 and 2010, but there was an 18-22% drop in K dPAR during the March/April isothermal period as water clarity increased. This transparency increase was accompanied by a 41-53% decline in Chl a concentration (μg · L −1 ) and a 42-46% decline in modeled primary production (Great Lakes Primary Production Model). These changes were most pronounced in 2001-2003 which coincided with the period of initial colonization of the quagga mussels. Statistically significant spatial differences were noted in Chl a (μg · L −1 ) concentrations between mid-depth (z = 30-90 m deep), and offshore (z N 90 m deep) waters. Chl a concentrations in the mid-depth region (30-90 m) decreased at a higher rate compared to offshore waters (N90 m) likely as a result of filtration activities of quagga mussel.
Light-absorbing components in the Great Lakes
Journal of Great Lakes Research, 2013
Features of light absorption are critical in regulating the optical signal available for remote sensing. The magnitudes, spectral characteristics, spatial patterns, and, to a lesser extent, dynamics of light-absorbing components are documented for the Laurentian Great Lakes. This includes the open waters of each of the five lakes, and selected rivers, embayments and near-shore areas. The absorption coefficient, a(m −1), is partitioned according to the additive components (a x) of colored dissolved organic matter (a CDOM), non-algal particles (a NAP), phytoplankton (a φ), and water itself (a w ; known). Dependencies of a x on various metrics of optically active constituents (OACs), cross-sections, are evaluated. A wide range of magnitudes of a x and a, and contributions of a x to a are documented. For example, the magnitude of a at a wavelength of 440 nm was nearly 10-fold greater in the western basin of Lake Erie than in the open waters of Lake Huron. Rivers, embayments, and near-shore areas generally had higher levels than the open waters. The largest a x throughout the system was a CDOM , originating mostly from terrestrial sources. Most of a NAP was associated with clay mineral particles. The distribution of a φ was highly correlated to chlorophyll concentration. The collected data set is appropriate to support initiatives to develop and preliminarily test mechanistic retrieval algorithms for OACs in the Great Lakes.
Remote Sensing, 2016
The importance of lakes and reservoirs leads to the high need for monitoring lake water quality both at local and global scales. The aim of the study was to test suitability of Sentinel-2 Multispectral Imager's (MSI) data for mapping different lake water quality parameters. In situ data of chlorophyll a (Chl a), water color, colored dissolved organic matter (CDOM) and dissolved organic carbon (DOC) from nine small and two large lakes were compared with band ratio algorithms derived from Sentinel-2 Level-1C and atmospherically corrected (Sen2cor) Level-2A images. The height of the 705 nm peak was used for estimating Chl a. The suitability of the commonly used green to red band ratio was tested for estimating the CDOM, DOC and water color. Concurrent reflectance measurements were not available. Therefore, we were not able to validate the performance of Sen2cor atmospheric correction available in the Sentinel-2 Toolbox. The shape and magnitude of water reflectance were consistent with our field measurements from previous years. However, the atmospheric correction reduced the correlation between the band ratio algorithms and water quality parameters indicating the need in better atmospheric correction. We were able to show that there is good correlation between band ratio algorithms calculated from Sentinel-2 MSI data and lake water parameters like Chl a (R 2 = 0.83), CDOM (R 2 = 0.72) and DOC (R 2 = 0.92) concentrations as well as water color (R 2 = 0.52). The in situ dataset was limited in number, but covered a reasonably wide range of optical water properties. These preliminary results allow us to assume that Sentinel-2 will be a valuable tool for lake monitoring and research, especially taking into account that the data will be available routinely for many years, the imagery will be frequent, and free of charge.
Journal of Great Lakes Research, 2006
In this paper we utilize 7 years of SeaWiFS satellite data to obtain seasonal and interannual time histories of the major water color-producing agents (CPAs), phytoplankton chlorophyll (chl), dissolved organic carbon (doc), and suspended minerals (sm) for Lake Michigan. We first present validation of the Great Lakes specific algorithm followed by correlations of the CPAs with coincident environmental observations. Special attention is paid to the satellite observations of the extensive episodic event of sediment resuspension and calcium carbonate precipitation out of the water. We then compare the obtained time history of the CPA's spatial and temporal distributions throughout the lake to environmental observations such as air and water temperature, wind speed and direction, significant wave height, atmospheric precipitation, river runoff, and cloud and lake ice cover. Variability of the onset, duration, and spatial extent of both episodic events and seasonal phenomena are documented from the SeaWiFS time series data, and high correlations with relevant environmental driving factors are established. The relationships between the CPAs retrieved from satellite data and environmental observations are then used to speculate on the future of Lake Michigan under a set of climate change scenarios.
Journal of Great Lakes Research, 2013
Observations of remote sensing reflectance (R rs), the signal available to support remote sensing of optically active constituents (OACs) of water quality interest, are presented for multiple sites within each of the Laurentian Great Lakes based on in situ measurements made with a hyperspectral radiometer. R rs (λ) spectra are contrasted among these lakes and in time and space within selected systems. Qualitative analyses of spectra are provided that identify the inherent optical property (IOP) and coupled OAC conditions responsible for the differences in R rs (λ). The much higher R rs peaks observed in the green wavelengths for the lower Great Lakes (Erie and Ontario) are attributed to elevated backscattering levels caused by higher concentrations of minerogenic particles. The credibility of the R rs (λ) spectra is established through successful closure analyses that demonstrate good matches with IOP-based predictions and consistency of coefficient values for radiative transfer expressions with related literature and theory. A mechanistic forward model of R rs (λ) is developed that accommodates the effects of three OACs, including metrics of phytoplankton biomass, minerogenic particles and colored dissolved organic material. This includes the development of the critical cross-section relationships that quantify the couplings between the OACs and IOPs, and in turn the IOPs and the R rs (λ) signal. The model is demonstrated to perform well in matching observations in Lake Erie, and to be sensitive to the representation of the spectral dependency of backscattering and likely variations in the dependence of phytoplankton absorption on chlorophyll. The modeled predicted responses of Lake Erie to different OAC levels are presented.