Under Ice and Early Summer Phytoplankton Dynamics in Two Arctic Lakes with Differing DOC

We investigated Arctic lake phytoplankton response along vertical gradients in the water column during seasonal succession from ice-covered to open-water conditions. Two oligotrophic lakes in West Greenland with different dissolved organic carbon (DOC) concentrations were selected. We assessed which factors: (1) promote under-ice growth of phytoplankton, and (2) trigger shifts in the community structure. Our results suggest that DOC is an important driver of the seasonal distribution of phytoplankton biomass—high DOC exacerbates light limitation under ice resulting in low phytoplankton biomass, but supports phytoplankton growth during the open-water period when photolytic and biological degradation of organic matter contributes to the pool of available nutrients. Under-ice phytoplankton biomass in a clear, low DOC lake was as high as during the open-water period, suggesting the importance of under-ice processes for lake metabolic balance, also evidenced by persistent oxic conditions in the hypolimnion. Species composition was dynamic, reflecting rapidly changing conditions over the course of the season. Our data suggest that phytoplankton under ice provides seed populations for early spring blooms and that the largest shifts in species composition occur later during the summer, likely as a result of nutrient depletion and intensified grazing pressure. Early spring temperatures in West Greenland have risen rapidly in recent decades, triggering shifts in the timing of lake ice-out and the onset of stratification; therefore, it is important to understand phytoplankton seasonal dynamics to disentangle the effects of climate-driven shifts on aquatic biota.