Biased measurements by stationary turbidity‐fluorescence instruments due to phototactic zooplankton behavior

Abstract

Submersible fluorescence and turbidity sensors are widely used in studies of oceans and lakes. To reduce the instrument size, an overlapping interrogation volume is commonly used for the two sensors. Fluorescence sensors emit blue light for excitation and measure the red light emitted by excited chlorophyll pigments. However, during the night, many phototactic zooplankters are attracted to the blue light. If the instrument is fixed in place (e.g., on a mooring), the aggregation of the attracted animals may bias both the fluorescence and turbidity readings. To examine this potential bias, we carried out experiments with natural assemblages of zooplankton and Artemia nauplii in tanks equipped with a commercially available fluorescence‐turbidity sensor. Our findings indicate that zooplankters were attracted by the blue light emitted from a fluorometer during the dark, biasing the reading of both sensors. The bias in fluorescence was likely due to phytoplankton in the guts of the aggregated zooplankton. The induction of turbulence in the tank greatly reduced the bias, likely due to the inability of the zooplankton to counteract the resulting flow and swim toward the light. Field observations carried out with a similar instrument in a coastal station off Japan were consistent with the laboratory experiments. Our findings indicate a need to redesign coupled fluorescence‐turbidity sensors and to reevaluate the results of past studies where they had been used with stationary observing systems.

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

“…Using manipulative lab experimentation, similar swarms observed within the optical path of the fluorometer were tightly correlated with increased sensor bias. Both laboratory and field results align with the findings of Tanaka et al (2019), confirming the potential for FIZ bias as a problem in both marine and freshwater environments.…”

“…While phototaxis may be a primary behavior driving FIZ bias, this response should not be taken out of the broader context of DVM and its relation to zooplankton grazing. Tanaka et al (2019) linked sensor bias with the fluorescence of ingested phytoplankton, and given previous work using fluorometers to detect fluorescence through the carapace of zooplankton (Karaköylü et al 2009), it can be assumed that the quality, quantity, and residency times of florescent compounds within the digestive tract of a zooplankton will influence the magnitude of FIZ bias. Gut residency times of the freshwater cladoceran Daphina pulicaria were found to range from 4 to 106 min (Murtaugh 1985).…”

Using machine learning to correct for nonphotochemical quenching in high‐frequency, in vivo fluorometer data

“…This indicates that there were several larger than expected decreases in fluorescence between night and day profiles that are unexplained by the features of the model. These larger changes are potentially due to movement in the DCM (perhaps due to internal seiche waves), changes in sensor calibration, or even optical path interference by zooplankton (Tanaka et al 2019; V. W. Moriarty et al pers. comm.…”

Detecting Mesopelagic Organisms Using Biogeochemical‐Argo Floats

“…Previous studies have suggested that periods of spikes in optical profiles were associated with the periodic interaction of zooplankton with transmissometers (Bishop et al, 1999;Bishop & Wood, 2008). Zooplankton are also associated with backscattering "spikes", defined here as a sharp and large increase in the optical signal (similar to Briggs et al, 2011, and detailed in the supporting information), in measurements collected by sensors mounted on a surface underway system and on an underwater stationary observatory (Burt & Tortell, 2018;Tanaka et al, 2019). These observations suggest that the presence of a subset of mesopelagic organisms could be estimated using BGC-Argo optical profiles and that the presence of spikes in optical instruments should not be interpreted a priori as an indication of sinking particles.…”