Returned signal strength intensity measurements from an acoustic Doppler current profiler deployed at an open-water site in Lake Superior from May-July 2019 were used to estimate the scattering intensity over the top 50 m of the water column. It displayed a strong diel signal, with higher scattering intensity at night, consistent with the normal diel vertical migration of planktonic organisms. However, the intensity of the nocturnal scattering signal decreased abruptly on 10 June, coincident with the end of winter stratification (cold water on top of warmer water) and the beginning of the neutrally stratified period. We hypothesize that the sudden change in zooplankton behavior is due the availability of phytoplankton deep in the water column after the transition, made possible by radiatively driven convection mixing the entire water column on time scales of hours.
Bibliographical noteFunding Information:
Funding for the fieldwork was provided by NSF OCE-1829895. The authors would like to thank Samuel Kelly for the use of the Nortek Signature 500 ADCP. JA collected the data, performed the analysis and initial interpretation, and wrote the bulk of the manuscript. TH contributed to concepts, interpretation of data and sections of writing. DB contributed to interpretation of data, writing, and editing. The authors would like to thank two anonymous reviewers for constructive criticism of our first draft. All data and code necessary to recreate results in this manuscript are available at the Data Repository at the University of Minnesota (DRUM) at https://doi.org/10.13020/eqra-hp74
Funding for the fieldwork was provided by NSF OCE-1829895. The authors would like to thank Samuel Kelly for the use of the Nortek Signature 500 ADCP.
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- Thermal structure