Under-ice mesocosms reveal the primacy of light but the importance of zooplankton in winter phytoplankton dynamics

Abstract

Factors that regulate planktonic communities under lake ice may be vastly different than the open-water season. However, under-ice food webs in temperate lakes are poorly understood, despite expected changes in light availability, ice cover, and snowfall associated with climate change. We hypothesized that light limitation (bottom-up control) outweighs zooplankton grazing (top-down control) on phytoplankton biovolume and community structure under ice in a north temperate lake. Using in situ under-ice mesocosms, we found that light had stronger effects on phytoplankton abundance than zooplankton, as expected. Specifically, low light limited growth of diatoms, cryptophytes, chrysophytes, and chlorophytes. Zooplankton, however, also significantly affected phytoplankton by decreasing diatoms and cryptophytes, in contrast to the common assumption that zooplankton grazing has negligible effects under ice. Ammonia and soluble reactive phosphorus decreased in high light treatments presumably through uptake by phytoplankton, whereas ammonia and soluble reactive phosphorus increased in high zooplankton treatments, likely through excretion. In situ experimental studies are commonly applied to understand food web dynamics in open-water conditions, but are extremely rare under ice. Our results suggest that changes in the light environment under ice have significant, rapid effects on phytoplankton growth and community structure and that zooplankton may play a more active role in winter food webs than previously thought. Changes in snow and ice dynamics associated with climate change may alter the light environment in ice-covered systems and significantly influence community structure.

Photoperiod influences the shape and scaling of freshwater phytoplankton responses to light and temperature

“…Given that the greatest difference between predicted and observed growth rates occurred in short photoperiod regimes, researchers should be particularly cautious when making predictions for high latitude environments and/or winter months when daily irradiance is low. In fact, our review indicates that the recent emphasis on exploring the wintertime dynamics of freshwater environments (Bertilsson et al 2013, Hrycik and Stockwell 2021) is unfolding in the backdrop of photoperiod regimes that have been typically understudied in laboratory environments (Fig. 1).…”

Integrating Perspectives to Understand Lake Ice Dynamics in a Changing World

“…While in situ mesocosm experiments are powerful in their natural context, implementation during winter poses technological challenges (Block et al, 2019) and only recently such in situ experiments have been conducted (Hrycik & Stockwell, 2019). Advances in technology and new infrastructural developments such as ice‐resistant mesocosm systems have created opportunities for experimental studies on lake ice dynamics.…”

Warmer Winters Increase the Biomass of Phytoplankton in a Large Floodplain River

“…This can foster productivity of algae adapted to low temperatures (Jewson et al., 2009) and that respond quickly to light availability (Twiss et al., 2014; Uehlinger et al., 2000). In addition, grazing by zooplankton and other planktivores may be lower in winter than other seasons as a result of lower temperatures, thereby releasing phytoplankton from top‐down pressures (Hyrcik & Stockwell, 2020). Thus, winter phytoplankton productivity in rivers may exceed current expectations.…”