A Multi-Prong Approach for Monitoring Hydrilla [Hydrilla verticillate (L. fil.) Royle] in Lakes and Reservoirs

Hydrilla verticillatata is an invasive aquatic macrophyte that has negatively impacted freshwater ecosystems in areas around the world. As a result, lake managers often seek ways to manage hydrilla and mitigate its spread. Having effective methods for assessing hydrilla abundance in a system is imperative, but traditional assessment methods are expensive, time consuming, and may be inaccurate. Contemporary remote sensing techniques have the potential to provide a faster and more effective means for obtaining hydrilla coverage estimates. Therefore, we set out to determine what methodologies provide the most efficient and effective approach for assessing hydrilla in a large reservoir. We used spectral indices on satellite imagery and unoccupied aerial vehicle (UAV) imagery to develop an image classification scheme for quantifying hydrilla occurrence. We also used down-scan sonar in conjunction with the BioBase software to generate hydrilla coverage and biovolume estimates. the normalized difference vegetation index derived from 3-m resolution multispectral satellite imagery proved effective for training an image classification, providing a reliable means at quantifying hydrilla colonization through time. The coloration index combined with UAV imagery yielded pixel values for hydrilla that were distinct from other aquatic plant species and can be used to verify results of satellite imagery. However, these methods were ineffective when hydrilla had not yet grown to maximum shoot length. In these conditions, down-scan sonar remained a valuable assessment tool to supplement aerial remote sensing techniques. These results equip lake managers with the knowledge to make more informed decisions, quickly assess hydrilla occurrence, and develop effective management strategies.