Phytoplankton biovolume and chlorophyll-a  concentrations are of major concern in primary productivity (PP) estimatiors in aquatic ecosystems. The goals of the paper are: study of chlorophyll-a concentration and its relations to variations of some phytoplankton community structure parameters and abiotic factors (environmental and nutrients matters) in costal waters of Caspian Sea- Goharbaran region in 2013-2014. Monthly water samples were collected from different layers (surface, 5 and 10m) in three depths (5, 10 and 15 m). Result showed increasing trend of phytoplankton biovolume from spring into winter (except in summer). The minimum and maximum mean (±SE) values of chlorophyll-a recorded in spring (0.39± 0.05) and autumn (4.56± 0.23) mg/m³, respectively. The Change of seasonal taxonomic phytoplankton pattern showed important role in relationship between chlorophyll-a cells content and abiotic factors. The effect of temperature (in summer and autumn), phosphorous (in spring), nitrogen (in summer) and silicon (in spring and winter) on decoupling between chlorophyll-a and biovolume were found by determining their correlation with the residuals of the regression between the two parameters (chlorophyll-a and biovolume). As conclusion, the decoupling observed between the phytoplankton biovolume and chlorophyll-a concentration the need to suitable select and caution of the two PP estimators and algal blooms prediction particularly in the cage and pen culture sites. Since the rapid changes in the chlorophyll-biovolume relationship could lead to significant errors in the explanation of results and unexpected field observations.