Accordingly, this study aimed to: (a) investigate the spatiotemporal variation of Chl- a in the Caspian Sea using the proper orthogonal decomposition (POD) method, a method with limited previous applications in oceanic studies. Jamshidi and Bin Abu Bakar investigated the Chl- a distribution in Bandar-e Anzali, a port in the southwest of the sea.īridging the gap in our understanding of the spatiotemporal variation of Chl- a over time and space across the Caspian Sea was the major motivation of this study. Bagheri et al studied phytoplankton distribution in southwest of the sea during 2001–2002. Nasrollahzadeh et al conducted a study on the trophic status in the southern part of Caspian Sea. Most of the previous studies have limited coverage in terms of time and space. įew studies have evaluated the spatiotemporal variation of Chl- a in the Caspian Sea. 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. The resulting nutrient concentrations can potentially reach a level that converts more areas of the sea from oligotrophic (TN and TP concentration less than 0.661 and 0.008 mg l −1, respectively) to eutrophic (TN and TP concentration greater than 1.39 and 0.084 mg l −1, respectively) and accelerates eutrophication. In the case of discharged TP loads, the shares of Russia, Iran, Azerbaijan, Turkmenistan, and Kazakhstan were, respectively, about 85.70%, 5.43%, 4.31%, 3.81%, and 0.75%. The contributions of Russia, Azerbaijan, Iran, Kazakhstan, and Turkmenistan to the discharged TN loads were, respectively, about 91.40%, 3.76%, 3.24%, 1.54%, and 0.06%. According to the United Nations Environment Program, 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 is due to the high concentrations of detergents and nutrients in the rivers that flow into the north Caspian Sea (table S2). High concentrations of detergents and ammonium have been previously reported in north of the sea (see table S1 (available online at /ERL/15/124047/mmedia)). The Volga, Ural, and Terek rivers carry a wide variety of nutrients that end up in the Caspian Sea. 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. Nitrogen- and phosphorus-rich compounds (hereafter referred to as nutrients) are among the concerning Caspian Sea pollutants. In addition, the rapid urbanization, industrial and agricultural developments in the coastal areas, as well as political and economic competition in using the Caspian Sea resources have further exacerbated the environmental conditions in the sea. Annually, about one million tons of oil is leaking into the sea. As a major oil production site, the Caspian Sea region has been exposed to large pollution loads originated from oil and gas industries. Since the sea is landlocked, the pollutants discharged into the sea remain in place and accumulate. It has a rich biodiversity, provides about 90% of the world's caviar supply, and forms resting regions for ten million migratory birds. 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.Ĭaspian 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. 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 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. 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. We provide the first evaluation of the spatiotemporal variation of chlorophyll- a (Chl- a) across the Caspian Sea. 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.
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