Light detection and ranging (LiDAR) and multispectral studies of disturbed Lake Superior coastal environments

Due to its high spatial resolution and excellent water penetration, coastal light detection and ranging (LiDAR) coupled with multispectral imaging (MSS) has great promise for resolving shoreline features in the Great Lakes. Previous investigations in Lake Superior documented a metal-rich ‘‘halo’’ around the Keweenaw Peninsula, related to past copper mining practices. Grand Traverse Bay on the Keweenaw Peninsula provides an excellent Great Lakes example of global mine discharges into coastal environments. For more than a century, waste rock migrating from shoreline tailings piles has moved along extensive stretches of coast, damming stream outlets, intercepting wetlands and recreational beaches, suppressing benthic invertebrate communities, and threatening critical fish breeding grounds. In the bay, the magnitude of the discarded wastes literally ‘‘reset the shoreline’’ and provided an intriguing field experiment in coastal erosion and spreading environmental effects. Employing a combination of historic aerial photography and LiDAR, we estimate the time course and mass of tailings eroded into the bay and the amount of copper that contributed to the metal-rich halo. We also quantify underwater tailings spread across benthic substrates by using MSS imagery on spectral reflectance differences between tailings and natural sediment types, plus a depth-correction algorithm (Lyzenga Method). We show that the coastal detail from LiDAR and MSS opens up numerous applications for ecological, ecosystem, and geological investigations. Light detection and ranging (LiDAR) is an airborne laser-ranging technique that acquires high-resolution elevation and bathymetric data (Ackermann 1999). The data are collected with aircraft-mounted lasers capable of recording elevation measurements at a rate of 10–200kHz pulses s21 for above-water topographic stretches and 1–10-kHz for coastal bathymetric surveys, with a maximum vertical precision of 15 cm (Crow et al. 2007). In coastal surveys, the aircraft travels over a water stretch at about 60 m s21, pulsing two varying laser beams toward earth through an opening in the plane’s fuselage: a red wavelength (infrared) beam that is reflected by the water surface and a narrow, blue-green wavelength beam that penetrates the water surface and is reflected from the bottom surface. The LiDAR sensor records the time difference between the two signals to derive measurements of water depth. An infrared version of LiDAR is used in forest applications, principally for biomass surveys and profiling of canopies (Lefsky et al. 1999). More recently, attention has expanded to underwater marine and freshwater applications. Under ideal conditions in coastal waters, blue-green laser penetration allows detection of structures down to depths approximately three times greater than passive light reflection. LiDAR has penetrated to a recorded maximum of 35 m in oceanic environments (Guenther 2007). Applications of blue-green laser techniques to mapping underwater structures have recently expanded. Marine studies include mapping of coral reefs (Brock et al. 2004), characterization of Atlantic barrier islands (Nayegandhi et al. 2005), and studies of Gulf of Mexico estuaries (Brock et al. 2002). In freshwater applications, recent river channel-bed characterizations include erosional surveys along braided streams (Bowen and Waltermire 2002). Here we emphasize potential applications in the Great Lakes and marine coastal environments, particularly under circumstances where past and present mine tailings discharges or tailings pond failures are of concern. Multispectral sensors (MSS) are instruments that acquire passive reflectance images in many continuous spectral bands throughout the visible, near-infrared, mid-infrared, and thermal portion of the spectrum. These systems can discriminate aboveand below-water surface features that have diagnostic absorption and reflectance characteristics. In our case, keying off albedo and spectral differences, they were used jointly with LiDAR to characterize stamp sand dispersal along the shoreline and under the water. Studies were carried out along the southern shoreline of Lake Superior, on the Keweenaw Peninsula. Recent investigations in Lake Superior documented a metal-rich ‘‘halo’’ around the Keweenaw Peninsula associated with past copper mining practices (Fig. 1; Kerfoot et al. 2004, 2009; Gewurtz et al. 2008). In Grand Traverse Bay (Fig. 2), * Corresponding author: [email protected] Limnol. Oceanogr., 57(3), 2012, 749–771 E 2012, by the Association for the Sciences of Limnology and Oceanography, Inc. doi:10.4319/lo.2012.57.3.0749