From minute to day: Ecophysiological response of phytoplankton to fluctuating light exposure during vertical mixing

Underwater light is a highly dynamic resource for phytoplankton. Fluctuating light influences photosynthesis, respiration, biosynthesis, and growth at different timescales, but the interplay of these processes is not well-understood. Subsamples of a phytoplankton community from the turbid, well-mixed lake TaiHu (China) were either vertically moved through different mixing depths or incubated at fixed depths of matching daily light supply. Growth, photosynthetic electron transport rates, net oxygen production, and night respiration were measured for 9 days. Production was investigated during cyclic mixing, throughout the day, and among days of different integral light intensities. Electron transport rates were higher during vertical mixing and increased with mixing depth, therefore mixing phytoplankton better exploited short periods of surface illumination. On the other hand, light-stimulation of oxygen consumption and photoperiod shortening increased the compensation light intensities for daily production and growth. The communities kept at the surface and sub-surface were photoinhibited; their net oxygen production declined and compensation light intensity increased in the afternoon. There was no sign of afternoon depression of net oxygen production in mixing phytoplankton which metabolized photosynthetic products in periodic phases of low light in deeper layers. Mixing phytoplankton produced more oxygen during increasing light intensities and respired more at decreasing light intensities, decreasing net oxygen production during the day. This cyclic post-illumination enhancement of respiration seemed to be hardwired to fluctuating light, regardless of mixing depth and uncoupled from night respiration rates. Photosynthesis and respiration seemed more tightly connected under fluctuating than constant light.