Abstract
Although rivers are of immense practical, aesthetic, and recreational value, these aquatic habitats are particularly sensitive to environmental changes. Increasingly, changes in streamflow and water quality are resulting in blooms of bottom-attached (benthic) algae, also known as periphyton, which have become widespread in many water bodies of US national parks. Because these blooms degrade visitor experiences and threaten human and ecosystem health, improved methods of characterizing benthic algae are needed. This study evaluated the potential utility of remote sensing techniques for mapping variations in algal density in shallow, clear-flowing rivers. As part of an initial proof-of-concept investigation, field measurements of water depth and percent cover of benthic algae were collected from two reaches of the Buffalo National River along with aerial photographs and multispectral satellite images. Applying a band ratio algorithm to these data yielded reliable depth estimates, although a shallow bias and moderate level of precision were observed. Spectral distinctions among algal percent cover values ranging from 0 to 100% were subtle and became only slightly more pronounced when the data were aggregated to four ordinal levels. A bagged trees machine learning model trained using the original spectral bands and image-derived depth estimates as predictor variables was used to produce classified maps of algal density. The spatial and temporal patterns depicted in these maps were reasonable but overall classification accuracies were modest, up to 64.6%, due to a lack of spectral detail. To further advance remote sensing of benthic algae and other periphyton, future studies could adopt hyperspectral approaches and more quantitative, continuous metrics such as biomass.
Highlights
Rivers are iconic features of landscapes and waterways all over the world, including those managed by the United States (US) National Park Service (NPS)
The results of this study suggested that, for the conditions observed along the Buffalo River at the time the images were acquired, mapping benthic algae from readily available remotely sensed data using established depth retrieval and image classification methods was more difficult than anticipated
Blooms of benthic algae have become increasingly common in many of the parks managed by the NPS and, presumably, in rivers worldwide
Summary
Rivers are iconic features of landscapes and waterways all over the world, including those managed by the United States (US) National Park Service (NPS). Benthic algae are an integral part of the ecological base within lotic (running water) environments, forming the basis of many aquatic food webs and providing structural support, excessive blooms can compromise aesthetic quality, limit recreational opportunities, and degrade the experience of millions of park visitors each year. Such blooms can have a number of deleterious effects on an ecosystem.
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