Caspian Sea is eutrophying: the alarming message of satellite data

Anahita Modabberi,Bjørn Kløve,Ali Danandeh Mehr,Roohollah Noori,Farhad Hooshyaripor,Amir Houshang Ehsani,Kaveh Madani

doi:10.1088/1748-9326/abc6d3

Abstract

The competition over extracting the energy resources of the Caspian Sea together with the major anthropogenic changes in the coastal zones have resulted in increased pollution and environmental degradation of the sea. We provide the first evaluation of the spatiotemporal variation of chlorophyll-a (Chl-a) across the Caspian Sea. Using remotely sensed data from 2003 to 2017, we found that the Caspian Sea has suffered from a growing increase in Chl-a, especially in warmer months. The shallow parts of the sea, near Russia and Kazakhstan, especially where the Volga and Terek rivers discharge large nutrient loads (nitrogen- and phosphorus-rich compounds) into the sea, have experienced the highest variations in Chl-a. The Carlson’s trophic state index showed that during the study period, on average, about 12%, 26%, and 62% of the Caspian Sea’s area was eutrophic, mesotrophic, and oligotrophic, respectively. The identified trends reflect an increasing rate of environmental degradation in the Caspian Sea, which has been the subject of conflict among its littoral states that since the collapse of the Soviet Union have remained unable to agree on a legal regime for governing the sea and its resources.

Highlights

The nitrogen and phosphorus released from household wastewaters and caused by the substantial use of agricultural fertilizers in the Caspian Sea basin are transported into the sea through rivers and drainage systems
This is due to the high concentrations of detergents and nutrients in the rivers that flow into the north Caspian Sea
This study investigated the variations of Chl-a, as the most important indicator of eutrophication, over the Caspian Sea, using traditional statistical indices and the proper orthogonal decomposition (POD) model based on satellite data

Summary

Introduction

Caspian Sea is the largest lake in the world by area with no regular connection to open seas except through the Volga–Don canal that connects this aquatic environment to the Sea of Azov and Black Sea [1] It has a rich biodiversity, provides about 90% of the world’s caviar supply [2, 3], and forms resting regions for ten million migratory birds [4, 5]. High concentrations of detergents and ammonium have been previously reported in north of the sea [12] (see table S1 (available online at stacks.iop.org/ERL/15/124047/mmedia)) This is due to the high concentrations of detergents and nutrients in the rivers that flow into the north Caspian Sea (table S2). According to the United Nations Environment Program [13], respectively, about 886 100 and 104 280 tons of total nitrogen (TN) and total phosphorus (TP), were annually discharged in the sea from rivers, industries, and municipalities within the Caspian Sea basin. This situation can potentially result in phytoplankton population increase, a good indicator of the nutrients’ enrichment, evidenced by high chlorophyll-a (Chl-a) concentrations across the sea [16]

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