Phytoplankton growth depends not only on mean intensity but also on the dynamics of the light supply. In surface mixed layers, phytoplankton may rapidly move between strong light and almost darkness. The nonlinear light‐dependency of growth may differ between constant and fluctuating light because of the different frequency distribution of light and/or acclimation processes. The present study compares for the first time light‐dependency of photosynthesis and growth of phytoplankton communities in situ under defined mixing conditions and at fixed depths. Maximum growth rates per day were not significantly different, but the growth efficiency was much higher under constant light than under fluctuating light of sub‐saturating daily irradiance. Phytoplankton incubated under fluctuating light needed about three times higher mean daily irradiances to balance photosynthesis and losses than under constant light. The difference in growth efficiency was mostly caused by the different frequency distribution of underwater light, as was estimated by a photosynthesis model of sufficient temporal resolution. The present study indicates a considerable overestimation of phytoplankton growth at sub‐saturating light in well‐mixed water layers by the common growth measurements under constant light. This implies an underestimation of the compensation light intensities and respective overestimations of the critical mixing depths.
Resource distribution heterogeneity offers niche opportunities for species with different functional traits to develop and potentially coexist. Available light (photosynthetically active radiation or PAR) for suspended algae (phytoplankton) may fluctuate greatly over time and space. Species‐specific light acquisition traits capture important aspects of the ecophysiology of phytoplankton and characterize species growth at either limiting or saturating daily PAR supply. Efforts have been made to explain phytoplankton coexistence using species‐specific light acquisition traits under constant light conditions, but not under fluctuating light regimes that should facilitate non‐equilibrium coexistence. In the well‐mixed, hypertrophic Lake TaiHu (China), we incubated the phytoplankton community in bottles placed either at fixed depths or moved vertically through the water column to mimic vertical mixing. Incubations at constant depths received only the diurnal changes in light, while the moving bottles received rapidly fluctuating light. Species‐specific light acquisition traits of dominant cyanobacteria (Anabaena flos‐aquae, Microcystis spp.) and diatom (Aulacoseira granulata, Cyclotella pseudostelligera) species were characterized from their growth–light relationships that could explain relative biomasses along the daily PAR gradient under both constant and fluctuating light. Our study demonstrates the importance of interspecific differences in affinities to limiting and saturating light for the coexistence of phytoplankton species in spatially heterogeneous light conditions. Furthermore, we observed strong intraspecific differences in light acquisition traits between incubation under constant and fluctuating light – leading to the reversal of light utilization strategies of species. This increased the niche space for acclimated species, precluding competitive exclusion. These observations could enhance our understanding of the mechanisms behind the Paradox of the Plankton.
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 wellunderstood. 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.
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