Vertical Variability and Diel Dynamics of Picophytoplankton in the Strait of Sicily (Mediterranean Sea) in Summer

Phytoplankton pigment diversity and photoacclimation during the natural day-night cycle was investigated at a fixed location in the Strait of Sicily in July 1997, using HPLC pigment analysis on fractionated samples (<3 micrometers and >3 micrometers) and flow cytometry. Picophytoplankton dominated phytoplankton biomass in terms of chla with an average value of 57%, and was mainly represented by prokaryotes, prymnesiophytes and pelagophytes. Prochlorococcus and picoeukaryotes equally contributed to the picophytoplankton in terms of chla, but Prochlorococcus were numerically more abundant and were represented by two ecotypes, one replacing the other with depth. Larger phytoplankton was dominated by prymnesiophytes and diatoms. Photoacclimation was evident from changes in pigment content and strongly increased with depth. The deep chlorophyll maximum (DCM), present between 75 and 90 m, showed a diverse and rich phytoplankton community with the two size classes almost equally represented. Growth rates of Prochlorococcus and Synechococcus, estimated from cell cycle measurements, were 0.67 and 0.41 d-1, respectively at 75 m. Only Prochlorococcus was able to sustain a good growth rate of 0.43 d-1 at the base of the DCM (90 m), where only 0.5% of incident light was available. Light-shift experiments using on-board incubated natural seawater showed much faster kinetic coefficients for acclimation in picophytoplankton than in larger algae. In general, the data present evidence about the dynamics of picophytoplankton and its light adaptation through the water column and in the DCM, and can be considered representative of stable summer conditions in the Mediterranean Sea.

BRUNET Christophe;  CASOTTI Raffaella;  VANTREPOTTE Vincent;  CONVERSANO F.; 

2007-11-14

INTER-RESEARCH

JRC37630

0171-8630,   

https://publications.jrc.ec.europa.eu/repository/handle/JRC37630,   

10.3354/meps07017,