Relationship of yearly changes of phytoplanktonic fluorescence to upwelling in the Straits of Messina.

Large gradients of tidal displacements as well as topographic constrictions determine upwelling of deeper waters to the surface layer in the Straits of Messina. This work describes the seasonal variability of surface distribution of phytoplankton biomass depending on upwelling phenomena. Temperature, salinity, nitrates and phytoplankton fluorescence were measured in 1994 and 1995 by continuous underway surface real-time measurements on board dedicated research boats. Each survey was performed following the dynamic phases of flooding and ebbing tides. Large spatial gradients of physical-chemical and biological parameters were mainly found at low water slack, while at high water slack a diffused phytoplankton fluorescence was observed only in autumn, concurrent with the seasonal thermocline. Generally, during winter, the upwelling distribution was limited to narrow zones, while in summer it involved the middle of the Straits and southern zones. During spring in the southern zone of the Straits, maximum of chlorophyll-a fluorescence was detected (0.32 gl?1 Chl-a) in summer, when back and forth tidal movements become intense between the Tyrrhenian and the Ionian seas, values was everywhere lower. The Straits system can be compared to an "intermittent pump" which, during the different seasons, before enriched itself and then it provides nutrients to the surrounding basins.