Phytoplankton photosynthetic activity dynamics in a temperate macrotidal ecosystem (the Strait of Dover, eastern English Channel): Time scales of variability and environmental control

The temporal variability in phytoplankton photosynthetic activity was studied in the coastal waters of the Strait of Dover and related to environmental conditions. Phytoplankton photosynthetic activity data collected from October 2008 and August 2010 at different time scales (hourly to interannual) using a nested sampling design are presented. Photosynthetic parameters including the maximum quantum yield (Fv/Fm), maximal light utilization efficiency (α), maximum electron transport rate (ETRm) and light saturation coefficient (Ek) were measured using Pulse Amplitude Modulation (PAM) fluorometry. Information on the taxonomic composition of phytoplankton assemblages and physicochemical parameters were also collected. Marked changes in photosynthetic parameters were observed at the different time scales investigated. The variability of photosynthetic parameters at sub-seasonal scale (hourly to monthly) could be of the same order of magnitude as at seasonal scale. At short time scale (hour to week), there was no consistent pattern of diel periodicity in photosynthetic parameters. There was a decline of ETRm and α during the day as often as there was a peak at the maximum of irradiance. Light conditions were the main influencing factor on photosynthetic parameters at these time scales. The optimal use of incident irradiance seemed focused on between-day rather than within-day variability. At longer time scale (seasonal to interannual), close interplays between shifts in community taxonomic composition and changes in the physicochemical characteristics of the environment controlled the variability in photosynthetic parameters. Whatever the time scale and period of year considered, variability in photosynthetic parameters was “Ek-independent”. In well-mixed macrotidal ecosystems, such as the Strait of Dover, a nested sampling design allowing to characterize the short- (intraday and daily) as well as long-term variability (within months, monthly, seasonal, annual and interannual) in photosynthetic parameters is required to get a suitable picture of phytoplankton photosynthesis and avoid potential problems of aliasing.