Hydrographic variability, nutrient distribution and water mass dynamics in Strymonikos Gulf (Northern Greece)

Abstract

Four seasonal hydrographic sampling cruises, covering a grid of 36 stations, were organized in the area of Strymonikos and Ierissos Gulfs, in Northern Greece, during 1997–1998. The aim of the study was to provide insight into the spatial and seasonal variability of physical and chemical oceanographic parameters, to define the baroclinic circulation, and to describe the dynamics of Strymon River plume area and the related freshwater and nutrient transport processes. Results, in all cruises, indicated the relatively limited contribution of river inputs, the strong influence of Black Sea Water (BSW) in the gulf's hydrography, and the summer intrusion of Levantine Intermediate Water in the outer Strymonikos Gulf. The baroclinic circulation (5/40 dbar) revealed the presence of mesoscale and smaller-scale features (meanders, frontal zones and eddies), associated with Strymon and Richios River discharges. Freshwater input covered the surface nearshore zone of Strymonikos Gulf, classifying the area as a ‘wide shelf’ region. The total freshwater content in the gulf was calculated to vary from 12.6×10 6 to 65.8×10 6 m 3 in the summer and winter period, respectively. This variability leads to a freshwater residence time ranging between 5 and 48 days, according to the Strymon River flow conditions. A distinct nutrient gradient was observed between freshwater inflow, the mixed plume zone and the remaining Strymonikos Gulf. Dissolved inorganic nitrogen (DIN) and phosphorus (DIP) concentrations followed opposite temporal variability patterns, with DIN being exhausted in the summer and increased in the winter. DIN:DIP ratios illustrate that the system appears shifting from a winter phosphorus limitation to a summer nitrogen limitation behavior.