A satellite-based multi-temporal assessment of the extent of nuisance Cladophora and related submerged aquatic vegetation for the Laurentian Great Lakes

In the Laurentian Great Lakes, the prolific growth of submerged aquatic vegetation (SAV, dominated by the filamentous green alga Cladophora) is negatively impacting human and wildlife health, fisheries, and aesthetic conditions. The distribution of Cladophora and similar SAV in the Great Lakes and how that has changed over recent decades has been unknown, and the magnitude of the current problem relative to historic growth is difficult to assess given the lack of field data and monitoring. Here, we present the first comprehensive map of SAV in the nearshore zone of the lower four Great Lakes, where 'nearshore' indicates the zone where the bottom reflectance is sufficient to characterize bottom substrate type, using primarily the visible bands of the Landsat TM sensor. Multiple spectral bands were combined to minimize the water-depth-dependent variance in lake bottom reflectance, and pixels were classified as dense vegetation, sparse vegetation or uncolonized substrate. Changes in vegetation cover and water clarity between the mid-1970s and present were also evaluated for several focus areas across the lakes utilizing historical Landsat TM and MSS imagery. The Landsat-based bottom classification achieved an overall accuracy of 83% based on comparison to ground truth data. Lake-wide SAV cover ranged from 15% (Lake Huron) to 40% (Lake Ontario) of the mappable lake bottom based on detection depth. The total mapped area of vegetation corresponds to a conservative biomass estimate of 129 kilotonnes dry weight. The observed changes in the distribution of SAV over time indicate that increasing water clarity in the Lakes is expanding Cladophora habitat.