Study of dynamics of phytoplankton and photosynthetic parameters using opportunity ships in the western English Channel

• Characterisation of three areas, based on the hydrological properties of the water mass.
• Dynamics of phytoplankton in these areas, importance of DIP spot.
• Estimation of productivity dynamics by using PAM from ships of opportunity.
• Regulation of photosynthetic parameters by nutrients and light.
• Multivariate analyses coupling physical, chemical and biological factors.

The dynamics of physicochemical, phytoplankton communities and photosynthetic parameters were monitored monthly along a transect in the western English Channel from a ship of opportunity. The transect was divided into three areas, based on the hydrological properties of the water mass: the French area (area 1), a station in the centre of the Channel (area 2), and the English area (area 3). In area 1, a bloom occurred in summer/autumn. Due to the gradual changes of physicochemical parameters, area 1 can be described as an ecocline where species adapted to a wide range of physicochemical properties were observed. The area was characterised by the co-dominance of diatoms and dinoflagellates. In area 2, a spot with a high concentration of dissolved inorganic phosphate (DIP) was sampled before and after a bloom of Karenia mikimotoï, suggesting that DIP availability may explain the occurrence of this dinoflagellate species in this area. Area 3 was characterised by two phytoplankton peaks, one in spring and one in late summer/early autumn. Production was controlled by light in area 1, by nutrients in area 2, and by light in spring and by nutrients in summer/autumn in area 3. A link between the chlorophyll a (Chla) biomass and primary production was not systematically observed, suggesting that Chla is not a good proxy for production. On the other hand, a strong link was observed between the relative maximum electron transport rate (rETRmax) and the maximum electron transport rate per chlorophyll unit (Chl_ETRmax). rETRmax consequently appears to be a good indicator of the dynamics of productivity in this area, making it possible to monitor it using PAM (pulse amplitude modulated) from ships of opportunity.