Monitoring Lake Okeechobee Harmful Algal Bloom populations and dynamics with a long-duration Autonomous Surface Vehicle

This article describes the main findings of a full year of continuous operation of a 2-meter Autonomous Sail and Solar Surface Drone, the Nav2 (Navocean Inc.), as part of a Harmful Algal Bloom (HAB) monitoring program in Lake Okeechobee. The Nav2 was equipped with a set of water quality and atmospheric sensors, that recorded high frequency measurements ({inverted exclamation} 1 min) and transmitted near real-time information to allow reporting through a web portal for assessment and operation responses. Major findings include detection of HABs early in the year through chlorophyll (chl-a) and phycocyanin (phyco) fluorometric measurements, as well as different spatial scales of variability in the algal patches. The 24/7 high resolution monitoring allowed detection of patch motion and discrimination between growth and motion along a transect. Furthermore, the platform can potentially fingerprint specific HAB species based on the relatively fine-scale spatial expression of the phyco to chl-a ratio, which essentially captures the bloom macrostructure (e.g. surface scums versus more uniform sub-surface waves over 0.1 - 1 km scale). Sensor outputs, when converted to concentrations based on calibrated with pure laboratory standards, did not accurately yield true chl-a or phyco values when compared to validation samples, likely due to the high turbidity of the lake. However, routine solid-state validations of fluorometric measurements proved useful for assessing consistency in optical sensors to check for sensor drift (e.g. to due biofouling), which was not significant. Overall this demonstration shows that the Nav2 can uniquely and reliably provide in situ HAB and environmental monitoring capabilities in a large, turbid, shallow lake. We envision that platform as an innovative technology for water resource managers by providing turn-key long-duration baseline environmental data (hands-off waypoint navigation), early warnings of HABs for protecting human health, and for HAB mitigation monitoring.