Fluoro-Electrochemistry Based Phytoplankton Bloom Detection and Enumeration; Field Validation of a New Sensor for Ocean Monitoring

Phytoplankton are essential for the health of our oceans, yet existing in situ methods for monitoring phytoplankton abundance and community structure are limited, with relatively poor spatiotemporal coverage and taxonomic resolution, particularly among the nanoplankton size range. Here, we build on previous work and present field testing of a novel reagent-free fluoro-electrochemical technique for monitoring changes in nanoplankton abundance and community structure in natural seawater samples. This was achieved through the construction of a prototype sensor, which was then tested over a 3-month Spring–Summer period in 2023 with samples collected from the L4 station (Western English Channel). The measurements made by our sensor were successfully validated alongside microscope-based taxonomic enumerations and analytical flow cytometry. Of the phytoplankton functional groups of interest, our results demonstrate particularly strong correlations between the sensor and both microscope-based taxonomy and flow cytometry for enumerating small coccolithophorids (i.e., calcifying Isochrysidales, of the Gephyrocapsa genus) and between the prototype and microscope-based taxonomy for enumerating diatoms. We demonstrate that the inclusion of traditionally hard to identify nanoflagellates in our classifications has minimal effect on our ability to monitor overall shifts in community structure and bloom detection. Taking things forward, the potential for in situ deployment is discussed.