Donna L Witter

Assessing the application of SeaWiFS ocean color algorithms to Lake Erie

ABSTRACT The feasibility of satellite-based monitoring of phytoplankton chlorophyll a concentrations in Lake Erie is assessed by applying globally calibrated, ocean-derived color algorithms to spatially and temporally collocated measurements of SeaWiFS remote sensing reflectance. Satellite-based chlorophyll a retrievals were compared with fluorescence-based measurements of chlorophyll a from 68 field samples collected across the lake between 1998 and 2002. Twelve ocean-derived color algorithms, one regional algorithm derived for the Baltic Seas Case 2 waters, and a set of regional algorithms developed for the western, central and eastern basins of Lake Erie were considered. While none of the ocean-derived algorithms performed adequately, the outlook for the success of regionally calibrated and validated algorithms, with forms similar to the ocean-derived algorithms, is promising over the eastern basin and possibly the central basin of the lake. In the western basin, each of the regional algorithms considered performed poorly, indicating that alternative approaches to algorithm development, or to satellite data screening and analysis procedures will be needed.

Multivariate approach to estimate colour producing agents in Case 2 waters using first-derivative spectrophotometer data

The complex composition and distribution of colour producing agents (CPAs) in turbid aquatic environments such as the Western Basin of Lake Erie (WBLE) presents a challenge to the application of remote sensing data for differentiating among in-water constituents and estimating their concentrations independently. In this study, multivariate procedures are applied to lab-based spectrophotometer data to estimate the concentration of chlorophyll-a and suspended matters in the WBLE. Principal Component Analysis of first-derivative transformed hyper-spectral data from the spectrophotometer extracted three significant spectral components for each cruise, explaining up to 88% of the spectral variability. Spectral matching using reference spectra indicated that two of the extracted patterns represent signatures of in-water constituents that govern the optical properties of the WBLE, namely, cyanobacteria and diatoms associated with green algae. The spectrophotometer data clearly revealed known spectral features associated with phytoplankton, such as the absorption minima near 550 and 700 nm, which can be attributed to the minimum of absorption and fluorescence of chlorophyll-a, respectively. The method also extracted the absorption peaks due to chlorophyll-a, near 670 nm, and due to phycocyanin, near 620 nm. Principal component regression of chlorophyll-a on the PC scores indicated that 63.4% of variation of chlorophyll-a in the WBLE can be explained by two components. Factors 2 and 3 explain 60% of the joint spatiotemporal variability of suspended matters in the WBLE. The results illustrate the potential of multivariate technique applied to remote sensing data in isolating the patterns that represent constituents in turbid Case 2 waters.

Evaluating multiple colour-producing agents in Case II waters from Lake Erie

Lake Erie is part of the Great Lakes systems in North America, which represent the largest continental lake systems in the world. Anthropogenic eutrophication in the Western Basin of Lake Erie, a Case II environment, has an adverse impact on the surrounding ecosystems and the regional economy. The optical complexity found in Lake Erie is a feature of many aquatic environments making it a challenging setting for remote-sensing applications. To assess the controls on these optical properties, we sampled 20 locations, encompassing a variety of optical environments in the Western Basin and Sandusky Bay during four research cruises. Strong correlations between light extinction and phycocyanin (correlation coefficient, r ≥ 0.95), suspended sediment (r = 0.90), and chlorophyll-a (r ≥ 0.86) indicate that surface conditions are representative down to at least the first optical depth. Application of varimax-rotated principal component analysis to lab-based, hyperspectral reflectance data identified three components related to a diatom/green algae community, and two blue-green algae communities, one of which was associated with suspended sediment. Phycocyanin and chlorophyll-a content inferred using a semi-analytic red/near-infrared algorithm correlated well with concentrations measured in situ using a multiparameter sonde. Chlorophyll-a retrievals from a regional, blue : green algorithm developed for the Western Basin of Lake Erie compared well with retrievals from the semi-analytic algorithm for all samples from the Western Basin and 25% of samples from Sandusky Bay. Chlorophyll-a retrieval errors using the blue : green algorithm occurred when high ratios of suspended sediment to phycocyanin biased samples from the extremely turbid waters of Sandusky Bay. The bias likely resulted when suspended sediment altered the blue : green ratio or when phycocyanin interfered with the chlorophyll-a absorption peaks. This approach can be applied to other Case II environments to provide insights during the design of experimental field studies and for spectral band selection with the next generation of visible near-infrared remote-sensing instruments.