Vertical variability and diel dynamics of picophytoplankton in the Strait of Sicily, Mediterranean Sea, in summer

Abstract

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 µm) and flow cytometry. Picophytoplankton dominated phytoplankton biomass in terms of chl a with an average value of 57% and was mainly represented by prokaryotes, prymnesiophytes and pelagophytes. Prochlorococcus and picoeukaryotes con- tributed equally to the picophytoplankton in terms of chl a, but Prochlorococcus were numerically more abundant and were represented by 2 ecotypes, one replacing the other according to depth. Larger phytoplankton were dominated by prymnesiophytes and diatoms. Photoacclimation was evi- dent from changes in pigment content and strongly increased with depth. The deep chlorophyll max- imum (DCM), present between 75 and 90 m, showed a diverse and rich phytoplankton community with the 2 size classes almost equally represented. Growth rates of Prochlorococcus and Synechococ- cus, 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 onboard incubated natural seawater showed much faster kinetic coefficients for acclimation in picophytoplankton than in larger algae. In general, the data describe the dynamics of picophytoplankton and its light adapta- tion through the water column and in the DCM, and can be considered representative of stable sum- mer conditions in the Mediterranean Sea.