Improvement of field fluorometry estimates of chlorophyll-a concentration in a cyanobacteria-rich eutrophic lake

Instrumented buoys are used to monitor water quality, yet there remains a need to evaluate whether in-vivo fluorometric measures of chlorophyll a (Chl-a) produces accurate estimates of phytoplankton abundance. Here six years (2014-2019) of in vitro measurements of Chl-a by spectrophotometry were compared with coeval estimates from buoy-based fluorescence measurements in eutrophic Buffalo Pound Lake, Saskatchewan, Canada. Analysis revealed that fluorometric and in vitro estimates of Chl-a differed both in terms of absolute concentration and patterns of relative change through time. Three models were developed to improve agreement between metrics of Chl-a concentration, including two based on Chl-a and phycocyanin (PC) fluorescence and one based on multiple linear regressions with measured environmental conditions. All models were examined in terms of two performance metrics; accuracy (lowest error) and reliability (\% fit within confidence intervals). The model that is based on PC fluorescence was most accurate (error = 35\%), whereas that using environmental factors was most reliable (89\% within 3σ of mean). Models were also evaluated on their ability to produce spatial maps of Chl-a using remotely-sensed imagery. Here newly-developed models significantly improved system performance with a 30\% decrease in Chl-a errors and a two-fold increase in the range of reconstructed Chl-a values. Superiority of the PC model likely reflected high cyanobacterial abundance, as well as the excitation-emission wavelength configuration of fluorometers. Our findings suggest that a PC fluorometer, used alone or in combination with environmental measurements, performs better than a single-excitation-band Chl-a fluorometer in estimating Chl-a content in highly eutrophic waters.