Study of the variability of the monthly average chlorophyll a in the Caspian Sea in the period 2010-2022 based on some physical parameters

Document Type : Research Article

Authors

Department of Space Physics, Institute of Geophysics, University of Tehran, Tehran, Iran.

Abstract

Some of the algae bloom phenomena in marine environments are harmful and under increase due to climate change and marine degradation in most coastal areas of the world. Caspian Sea also is experiencing these phenomena, which can really harm this fragile ecosystem. In this paper, this phenomenon is studied for the Caspian Sea for the last decade. Data of sea surface color (as the concentration of chlorophyll), surface temperature, surface currents and surface winds data acquired from two sites, namely NASA and AVISO, including some data of surface color and temperature of MODIS satellite data have been used. Their spatial resolution is about half a degree and the time resolution is weekly for chlorophyll and sea surface temperature and surface currents data and monthly for wind data. Time series and spectra analyses were used to consider time and periodic variations of cholorophyll concenterations as well as surface currents in the midlle and southern Caspian Sea. Some Hovmöller diagrams for the concentrations of the cholorophyll and currents are used to consider the maps of changes for the whole period of study.
The results show that these phenomena occur often in the last months of summer and some time in winter. It occurs almost extensively every one to two year and mainly starting in the northern basin of the Caspian Sea. The upwelling due to wind, especially in the eastern coastal areas of the Caspian Sea seems to be very important in this event. The surface circulation can transport and redistribute the chlorophyll produced be the events of algae bloom. It also appears that the number of blooms has slightly decreased in recent decade but its intensity seems to have increased. This is particularly so for the events between 2015 and 2020, in which September 2017 and 2018 has experienced strong outbreak of algae blooms that have spread into the southern Caspian Sea as well. In some cases, as in September 2017, it might have reached the southern coast of the Caspian Sea that could have harmed the coastal facilities and fisheries. These are well shown on the Hovmöller diagrams for chlorophyll concentrations and surface currents for the whole period of study in the Caspian Sea.
There are 4 yearly, yearly and seasonal variability in the occurrence of algae blooms and long cyclic changes of the variability may be due to the large scale and long period oscillation, as Enso in the atmosphere-ocean system. The spectra of Nini 3.4 also shows that some 4 to 5 years’ cyclic variation exists in Enso signal. Although the Enso singal shows that following an El Nino event, there might be an algae bloom event in the Caspian Sea (as the event of September 2017). This is expected in face of present climate change and sea surface temperature increase in recent years.

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References

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Journal of the Earth and Space Physics
Volume 51, Issue 2
online publication date: 20 September 2025
September 2025
Pages 431-451

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