Environmental drivers of mixotrophs in boreal lakes

Abstract

Mixotrophy is increasingly recognized as an important trophic pathway among phytoplankton, yet its underlying drivers remain largely unknown and unexplored. Here, we present a study utilizing 69 lake samplings in boreal Quebec, Canada, identifying variables driving the success of phytoplankton that have a capacity for mixotrophy and pointing to the underlying mechanisms. We found that the success of mixotrophs (% of total biomass) was positively influenced by both colored dissolved organic matter (cDOM) and dissolved CO 2 concentration but limited by the abundance of crustacean zooplankton. The effect of cDOM manifested as a consequence of limited autotrophic phytoplankton biomass in lakes with reduced light penetration. We observed a nonlinear (u-shaped) relation between CO 2 and mixotrophs, with biomass favored at both low and high CO 2 concentrations. A reduced fitness of mixotrophs at near-atmospheric CO 2 concentrations is likely owing to the costs of rapidly switching between or maintaining multiple trophic strategies. The abundance of zooplankton had a negative effect on mixotroph biomass but a positive effect on autotrophic phytoplankton. We also found that while the community composition of potentially mixotrophic phytoplankton was to some degree likely influenced by zooplankton biomass, composition was unaffected by the CO 2 and cDOM gradients. Overall, this study highlights mixotrophy in boreal lake systems as a strategy of persistence, with the maintenance of a moderate but constant presence across a changing gradient of light and trophic conditions. The results of our study support the hypothesis that phytoplankton with a capacity for mixotrophy provide a superior and stable stoichiometric food source for zooplankton, implicating mixotrophs as a vital component of boreal lake food webs.

Relative importance of top‐down vs. bottom‐up control of lake phytoplankton vertical distributions varies among fluorescence‐based spectral groups

“…We found that brown algae and cryptophytes were most sensitive to zooplankton as a driver of their vertical distribution, in support of our hypothesis H2 that highly edible taxa would be most sensitive to potential top-down control by zooplankton grazing (Holm et al 1983;Brett et al 2000Brett et al , 2006Hansson et al 2019). The brown algae spectral group, likely comprised of predominantly chrysophytes in our study lakes (Fig.…”

Do inferences about freshwater phytoplankton communities change when based on microscopy or high‐throughput sequencing data?

“…The lower biodiversity of Chrysophyta with microscopy can be related to preservation and magnification issues because these algal cells preserve poorly in Lugol and accurate taxonomic assignment depends on details only visible with scanning electron microscopy (Wujek & O’Kelly, 1992). In many oligotrophic and cold freshwater ecosystems where the mixotrophic nature of Chrysophyta (Rothhaupt 1996) is an advantage (Hansson, Grossart, del Giorgio, St‐Gelais, & Beisner, 2019) a high diversity of Chrysophyta‐related OTUs has been observed along with a high total abundance of sequences (e.g. 25–50% Charvet et al, 2012; 25–60% Lara et al, 2015; 37% Ortiz‐Álvarez, Triadó‐Margarit, Camarero, Casamayor, & Catalan, 2018; 33% Llorens‐Marès et al 2020).…”

Freshwater zooplankton metapopulations and metacommunities respond differently to environmental and spatial variation

“…Environmental variation between sites was categorized using 16 variables known to impact zooplankton community composition (Hansson et al 2019). Twelve environmental variables were measured at each site: pH, dissolved oxygen, conductivity, temperature, maximum depth, water transparency, chlorophyll a, total phosphorus, total nitrogen, dissolved organic carbon, dissolved inorganic carbon, and colored dissolved organic matter (detailed sampling methods can be found in Appendix S1).…”