Exploring the Caspian Sea Basic Ecosystem Dynamics by Means of the SeaWiFS Historical Record (1997-2009)

Abstract

Patterns of algal blooming, described by variations in the concentration of chlorophyll-like pigments (Chl), are considered to be effective indicators of ecological dynamics in coastal and marine environments. A time series of Chl statistical maps, derived from data collected by the Sea-viewing Wide Field-of-View Sensor (SeaWiFS), from September 1997 to July 2009, were used to explore the space and time heterogeneity of algal blooming in the Caspian Sea. The imagery details the diverse characteristics of the northern region, dominated by the impact of Volga river runoff, and of the southern region, where intense events occur along the coast and on prominent shelf breaks. Between these, a central transitional region presents more variable environmental conditions. The observed seasonal pattern is essentially bimodal, with a late-summer, early-fall period of extended blooming, and a winter-spring period of less intense events. The northern region has a dominant role in determining this general pattern, while the middle and the southern regions do present the same pattern , but with some summer variations. The inter-annual variability is not very pronounced, with annual Chl anomalies showing limited oscillations around zero over most of the basin. In the north, large anomalies are related to the Volga river plume variability, while in the south negative the anomalies of the first years are followed by prevailing positive anomalies, after the exceptional 2001 summer event. A rather peculiar, unusual patchiness, composed of large, circular, intense spots of high concentration, appears to be typical of the coldest months.