Tracing the transport of algal blooms from reservoir to estuary using satellite imagery and in situ hydrologic data

Abstract:

Algal blooms forming downstream of water control structures are often attributed to bloom export from upstream reservoirs, yet the role of these structures as drivers of bloom transport is not well understood. To investigate the relationship between water control structures and downstream algal blooms, our primary objective was to develop and apply a method for bloom transport estimation using satellite imagery. We studied Lake Okeechobee and the St. Lucie Estuary in South Florida, connected systems impacted by recurrent cyanobacterial blooms. Blooms in the St. Lucie are believed to be exported from water released from Lake Okeechobee throughout the year to prevent the lake from overtopping its banks. However, when analyzing flow from Lake Okeechobee into the St. Lucie and downstream chlorophyll-a concentrations, no correlation is apparent. The lack of correlation indicates export events are episodic rather than chronic, contingent on other factors within the system. Considering the spatial distribution of blooms in Lake Okeechobee during discharge events could better inform the role of water control structures in downstream bloom presence. We hypothesized bloom export from Lake Okeechobee into the St. Lucie would occur when blooms were concentrated near the canal outlet from Lake Okeechobee, and discharge rates exceeded 1,500 cubic feet per second. Bloom events were identified from in situ chlorophyll-a samples within Lake Okeechobee and the St. Lucie. The spatial distribution of cyanobacteria within Lake Okeechobee was assessed using Sentinel-3 Ocean and Land Colour Instrument images and the Cyanobacteria Index algorithm. Cyanobacterial cell concentrations were summed across different radii surrounding the canal outlet to identify an area where blooms should be evaluated to predict export. We defined a radius of influence across the lake by analyzing water release rate with respect to change in downstream chlorophyll. Many events were analyzed from 2016 to 2019 to assess the influence of water release from Lake Okeechobee on bloom presence in the St. Lucie. This study provided insight on how managers can harness satellite imagery to better inform decisions regarding water releases, limiting the amount of algae entering downstream waters.