Abstract
AbstractEffects of prolonged darkness on Arctic phytoplankton composition were investigated with lab experiments and a pigment time series in Kongsfjorden, Spitsbergen (78°55′N). Chlorophyll a (Chl a), pigment composition, particulate organic carbon, cell numbers, and photosynthetic characteristics were studied in Arctic diatoms (Thalassiosira antarctica, Thalassiosira nordenskioeldii) and flagellates (Rhodomonas sp., Micromonas sp.) during 8 weeks of darkness and subsequent recovery in irradiance. Loss of photosynthetic functionality after 2 weeks of darkness was reversible in all species when returned to irradiance. Diatoms were more resistant to prolonged darkness (> 2 weeks) compared to the flagellates, with lower decline rates of Chl a and maximum quantum yield of PSII. T. nordenskioeldii showed rapid growth during recovery throughout 8 weeks of dark incubation, whereas recovery of flagellates diminished within 4 weeks. Ratios of taxonomic marker pigments relative to Chl a of all species showed limited variation during 8 weeks of dark incubation. The experimentally observed enhanced dark survival of diatoms was in agreement with pigment observations during four polar nights (2013–2017) in Kongsfjorden, which showed increased relative diatom abundance during declining biomass (down to 0.02 mg Chl a m−3). Therefore, a period of prolonged darkness gives Arctic diatoms a head start during the early stages of the spring bloom. The taxon‐specific survival traits can influence the geographical distribution of diatoms and flagellates within the polar oceans and their phenology. Furthermore, the persistence of Chl a of nonviable phytoplankton during darkness might influence biomass estimates during the polar night.
Highlights
particulate organic carbon (POC), and cell numbers during darkness Chlorophyll a (Chl a) concentration of Micromonas sp. cultures increased during the first week of darkness and declined afterward, whereas Rhodomonas sp
These differences might be due to differences in the allocation of energy reserves, with diatoms utilizing a more conservative approach as compared to flagellates, resulting in prolonged survival of long periods of darkness, while maintaining the ability to photosynthesize when returned to irradiance (Walter et al 2017)
The taxon-specific survival traits observed during the dark incubation experiments suggest that periods of prolonged darkness such as the polar night can influence phytoplankton composition, that is, the geographic distribution and phenology of flagellates and diatoms
Summary
Vegetative stages of temperate benthic diatoms survived 1 yr of darkness, which coincided with an exponential decline in photosynthetic pigments (Veuger and van Oevelen 2011). Field observations during the Arctic polar night have so far not provided a clear picture on how the absence of irradiance affects phytoplankton species composition and physiology. Phytoplankton assemblages (≥ 20 μm) obtained from Kongsfjorden during the polar night showed rapid initiation of carbon fixation when exposed to irradiance (Kvernvik et al 2018). We tested the hypothesis that diatoms survive longer periods of darkness than flagellates by investigating the photophysiological responses of two Arctic diatoms and two flagellates to 8 weeks of darkness. We analyzed 4 yr of phytoplankton pigment observations in a high Arctic fjord (Kongsfjorden, Spitsbergen) to assess the abundance of diatom and flagellates during the polar night. We discuss the potential of these observations as a driver for high latitude phytoplankton spring bloom composition
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