Abstract

Concentrations of the photosynthetic pigment chlorophyll a are commonly used as a proxy for phytoplankton biomass in aquatic systems. Traditional methods for extracting chlorophyll a in discrete samples limit measurement frequency, while in situ sensor technology provides high frequency chlorophyll a fluorescence measurements at a more appropriate time scale to study processes related to phytoplankton turnover and a more rapid response to harmful algal blooms. Caveats to in situ measurements include that temperature, fluorescent dissolved organic matter, and turbidity can affect accuracy and that chlorophyll a fluorescence does not always correlate well to extracted chlorophyll a. Therefore, practitioners need guidance to interpret fluorescence data considering potential errors and relationships to extracted chlorophyll a to inform management decisions. Before this project, the nature of those relationships had not been tested across a wide gradient in water properties. To address these gaps, a one-year study was conducted across 12 biogeochemically diverse sites to quantify possible fluorescence interferences and test the predictability of extracted chlorophyll a using uncorrected in situ chlorophyll a (in relative fluorescence units) combined with routinely measured water quality parameters. Overall, sensor fluorescence was positively correlated with extracted chlorophyll a, but the strength and drivers of the relationship varied by site. Temperature, turbidity, and fluorescent dissolved organic matter influenced sensor readings independently of phytoplankton biomass. Hence, considerations for in situ sensor implementation depend on monitoring goals and resources. Where feasible, we recommend the simultaneous use of in situ and extractive approaches to track short-term variability and long-term change in chlorophyll a.