Alleviation of hypoxia by biologically generated mixing in a stratified water column

Abstract

Daily vertical migrations of zooplankton have been shown to affect nutrient distributions and dissolved gas concentrations in lakes and oceans via active internal transport and metabolic consumption. Additionally, mixing generated by these migrations has been shown to have the capacity to alter the physical structure of a water column, with potential further implications for its biogeochemical structure. In this work, we use laboratory experiments to investigate the importance of biologically generated mixing relative to other processes in determining the biogeochemical structure of a water column inhabited by migrating zooplankton. Specifically, we consider oxygen, a highly ecologically relevant scalar, and the competition between metabolic consumption and biogenic mixing in a stably stratified water column with a hypoxic layer. Using laboratory experiments and a one-dimensional model informed by those measurements, we illustrate the potential for migrating animals to alleviate hypoxia, introducing complex feedbacks between the presence of animals and the biogeochemical state of their surroundings. Furthermore, we demonstrate the feasibility of oxygen as a potential indicator of biogenic mixing for future in situ investigations given its low diffusivity and higher signal-to-noise ratio. Daily vertical migrations of zooplankton are found throughout lakes and oceans, with centimeter-scale animals traversing tens of meters in lakes and hundreds of meters in the open ocean (Ringelberg 2014), often in large aggregations. These movements, the world’s largest migration of biomass, are possiblymotivated by predator avoidance, allowing feeding at the surface during the night and a return to depth during the day to reduce the risk of visual predation (Ringelberg 2009; Sato et al. 2013). The depth of migration is controlled by a variety of factors, with animals often seeking cover from predation below the euphotic zone as well as within difficult to inhabit regions, such as oxygen-limited waters (Hembre and Megard 2003; Vanderploeg et al. 2009; Bianchi et al. 2013). Because many zooplankton are better able to withstand periods of hypoxia relative to their larger predators, oxygen-limited waters can serve as a daytime refuge (Vanderploeg et al. 2009), and observations of daytime zooplankton location show correlation with oxygen-depleted regions in both lakes (Hembre and Megard 2003) and oceans (Bianchi et al. 2013). Specifically, for an oxygen-stratified lake, zooplankton concentration was significantly enhanced in the “refuge zone” containing 94–156 μmol O2 L , an intermediate level of oxygen where the zooplankton were not physiologically stressed by the reduced oxygen but their larger predators (i.e., trout) could not survive. By feeding at the surface and subsequently returning to depth when satiated, migrating zooplankton contribute significantly to the active vertical flux of organic matter (Longhurst and Glen Harrison 1988; Zhang and Dam 1997; Manno et al. 2015; Butterfield 2018). Additionally, the active selection of oxygen-limited regions by vertical migrators can intensify the oxygen minimum zones that are simultaneously driving the depth of migration (Bianchi et al. 2013; Butterfield 2018). Beyond internal carbon transport and metabolic consumption of oxygen, physical mixing generated by animal propulsion has been hypothesized as potentially relevant to the physical structure (e.g., density stratification) of the water column (Huntley and Zhou 2004; Dewar et al. 2006; Katija and Dabiri 2009; Noss and Lorke 2014;Wilhelmus andDabiri 2014;Wang andArdekani 2015; Simoncelli et al. 2018). However, non-negligible physical mixing of a density stratification necessitates fluid motion at scales comparable to the length scales of stratification (Osborn 1980), often orders of magnitude larger than an individual zooplankton (Visser 2007). Despite small individual animal size, the behavioral tendency of many zooplankton species to form large, dense aggregations *Correspondence: [email protected] This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.