Seasonal variability of phytoplankton fluorescence in relation to the Straits of Messina (Sicily) tidal upwelling

In the Straits of Messina the large gradients of tidal displacements as well as the topographic constrictions determine the upwelling of deeper waters in the surface layer. 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. Tidal currents are essentially southward during the high tide and northward during the low tide. During the low water slack, large spatial gradients of physical-chemical and biological parameters were mainly found, while in the high water slack a diffused phytoplankton fluorescence was observed only in autumn, in coincidence of a seasonal thermocline. Salinity, nitrate and chlorophyll-a fluorescence data revealed a significant positive intercorrelation, whereas they are inversely correlated with temperature. 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 (May 1995, 0.32 µg-Chla l-1); in summer, when back and forth tidal movements become intense between the Tyrrhenian and the Ionian seas, values was everywhere lower. The data set from continuous and repeatable acquisition has allowed the study of different time-space scales in the Straits of Messina, a very strong dynamic environment. 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.