Environmental drivers of mixotrophs in boreal lakes

• Published in: Limnology and oceanography 2019-07, Vol.64 (4), p.1688-1705
• Main Authors: Hansson, Truls Hveem, Grossart, Hans-Peter, del Giorgio, Paul A., St-Gelais, Nicolas F., Beisner, Beatrix E.
• Format: Article
• Language: English
• ISSN: 0024-3590; 1939-5590
• DOI: 10.1002/lno.11144

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 CO₂ 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 CO₂ and mixotrophs, with biomass favored at both low and high CO₂ concentrations. A reduced fitness of mixotrophs at near-atmospheric CO₂ 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 CO₂ 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.