Fluorometer optical path interference via zooplankton phototaxis: Implications for high‐frequency data collection

Abstract

To measure chlorophyll a (Chl a) fluorescence (Fchl), fluorometers use an excitation wavelength that is within the visible spectrum of most zooplankton, and as a result has the potential to cause a phototactic response in zooplankton. The transparent bodies of herbivorous zooplankton may allow viable chlorophyll a within an individual's digestive tract to fluoresce in response to sensor excitation light, resulting in measurement bias. To test for this bias, a fully factorial (± zooplankton and ± light) experiment was conducted in an oligotrophic lake. Excitation light from fluorometers triggered a positive phototactic response during nighttime hours, resulting in swarms of zooplankton congregating beneath the sensor. The maximum hourly mean Fchl from nighttime/open treatments was higher and more variable than nighttime/zooplankton exclusion treatments, with the greatest single hour difference of 7.34 relative fluorescence units (RFU) vs. 0.26 RFU. In open treatments, sustained periods of Fchl exceeded 31x the values of exclusion treatments. A second series of experiments pulsed excitation lights in alternating periods in order to characterize zooplankton response times. Sensor bias was detected in as little as 20 s after initial illumination. Collectively, these results suggest that swarms of phototactic zooplankton can cause substantial bias in Fchl measurements at night. To correct for this bias, post‐processing methods using time series decomposition were demonstrated to remove the majority of Fchl bias.

Understanding and mitigating global change with aquatic sensors: current challenges and future prospects

“…A great deal of lake chemistry and biology changes above, below, and within the thermocline (Lind et al, 2022) By using a vertical profiler that uses a computer-controlled winch to lower a suite of sensors from the lake surface to near the lake bottom, the depth of the thermocline can be determined (Li et al, 2022). On the subsequent lowering of the sensors, adaptive sampling algorithms can be used to increase the frequency of sampling while moving through the thermocline and decreased through the epilimnion and hypolimnion, enabling the sensors to optimally sample at the depths where the most interesting dynamics are occurring (Moriarty et al, 2021).…”