Assessing Chlorophyll-a in the Southwestern Coastal Waters of the Caspian Sea

Caspian Sea with an average depth of 27m is the largest enclosed water body in the world. Despite its enormity and valuable biotic and economic resources, investigations on the biota and seawater properties are mosaic at best. In previous studies, the monitoring of the chlorophyll-a concentrations in the Southern Caspian Sea was organized based on satellite data sets; however, vertical distribution of chlorophyll-a concentrations and its variations in deeper layers of the southern Caspian seawater are not well known. The aim of this research was to study the variations of seasonal distributions of chlorophyll-a near Anzali Port in northern Iran. Data collection was performed at 23 stations, 2 km apart, along four survey lines perpendicular to the coastline and two transects parallel to the coast every season using a research vessel. Field measurements in the study area were carried out. A portable CTD probe was applied for profiling from sea surface to bottom. Results of seasonal field measurements showed variations of the seawater properties (e.g. temperature, salinity, density and chlorophyll-a). Maximum depth of the profiling stations was more than 470 m and vertical structure of temperature in the southern Caspian Sea waters with a significant seasonal thermocline between 20-50 m depths in summer with a vertical difference of 16oC were characterized. Seasonal average of the salinity was in the range of 12.27-12.37 PSU for the period of measurements. The highest Chlorophyll-a content in the range of 0.2-3.4 mg m-3, was found below that of the sea surface in summer. Variations of the chlorophyll-a concentration in the study area can be attributed to the effect of changes in seawater characteristics in various seasons, stratification and heating of the sea surface layer in the warm seasons and discharge of lagoon and rivers in the study area. The range of the concentrations at the sea surface in August and November were higher than the measured values in April in the study area.