The Imprint of Primary Production on High-Frequency Profiles of Lake Optical Properties

Abstract

Water inherent optical properties (IOPs) contain integrative information on the optical constituents of surface waters. In lakes, IOP measurements have not been traditionally collected. This study describes how high-frequency IOP profiles can be used to document short-term physical and biogeochemical processes that ultimately influence the long-term trajectory of lake ecosystems. Between October 2018 and May 2020, we collected 1373 high-resolution hyperspectral IOP profiles in the uppermost 50 m of the large mesotrophic Lake Geneva (Switzerland−France), using an autonomous profiler. A data set of this size and content does not exist for any other lake. Results showed seasonal variations in the IOPs, following the expected dynamic of phytoplankton. We found systematic diel patterns in the IOPs. Phases of these diel cycles were consistent year-round, and amplitudes correlated to the diurnal variations of dissolved oxygen, clarifying the link between IOPs and phytoplankton metabolism. Diel amplitudes were largest in spring and summer under low wind condition. Wind-driven changes in thermal stratification impacted the dynamic of the IOPs, illustrating the potential of high-frequency profiles of water optical properties to increase our understanding of carbon cycling in lake ecosystems.

LéXPLORE: A floating laboratory on Lake Geneva offering unique lake research opportunities

“…LéXPLORE operation in open water settings allows deployment of the automated Thetis profiler (Minaudo et al, 2021) adjacent to the platform (Figure 3). This profiler acquires bulk IOP measurements at high frequencies and high vertical resolution (Appendix S1).…”

“…These exemplary profiles show a warming period in April 2020 interrupted by 2 days of wind‐driven mixing/upwelling (black arrow in c) and subsequent chlorophyll‐a and oxygen accumulation in the epilimnion. Reprinted with permission from Minaudo et al (2021)…”

“…This profiler acquires bulk IOP measurements at high frequencies and high vertical resolution (Appendix S1). The tool can capture relatively poorly understood diel and vertical variations in IOPs that influence a wide variety of aquatic processes including primary production (Figure 3), whiting, grazing, and intrusion of riverine suspended particles (Minaudo et al, 2021). Using these novel measurements as inputs to radiative transfer models will yield simulated AOP results showing water‐leaving or top‐of‐atmosphere reflectance.…”

“…Primary production depends on both vertical distribution and vertical flux of nutrients and algal communities, as well as on carbon cycling and light regimes. As documented by the first LéXPLORE publications, teams working simultaneously and using diverse approaches can build effective lines of evidence to address these questions (Fernández Castro et al, 2021; Minaudo et al, 2021; Perolo et al, 2021).…”

Effects of natural surfactants on the spatial variability of surface water temperature under intermittent light winds on Lake Geneva

“…As a result, its attenuation coefficient depends on the transparency of nearsurface water. This data was not available for our field campaign; instead, hyperspectral underwater irradiance measurements made with an autonomous profiler (Minaudo et al, 2021) the following year during the same period at the LéXPLORE platform were used to estimate kd visible . In the second half of April 2020, the weighted average of spectral attenuation coefficients was calculated as ͞ k d = 0.45 ± 0.05 m -1 , and was implemented in the calculations (Fig.…”