https://anwaar.squ.edu.om/en/2018/06/25/dead-zones-of-the-western-arabian-sea/

Oxygen minimum zones (also called as the dead zones or hypoxic zones), have been reported for over 400 geographical regions of the World Ocean and regional seas. The phenomenon is a key stressor of open ocean and coastal ecosystems worldwide. In this regard, hypoxic zones became a serious environmental challenge to fishery management. As one of the most prominent oxygen depletions of the ocean, the Arabian Sea oxygen minimum zone extends from the central part of the basin to the coasts of Oman, Iran, Pakistan and India and makes fisheries vulnerable to the shelf hypoxia. Shelf oxygen depletion causes numerous fish kill incidents along the Omani coast. However, the scenario of long-term changes of the shelf oxygen minimum zones and its impact on fish catches are still poorly investigated.

Dr. Sergey Piontkovski from the Department of Marine Sciences & Fisheries at Sultan Qaboos University acted as the Principal Investigator in a number of research projects, which tackled the issue of physical-biological interactions in the western Arabian Sea including the Omani shelf. Recently, Dr. Piontkovski with the team of technicians from the department and his colleague Dr. Bastien Queste from the University of East Anglia (UK), introduced a cutting edge technology in studies of the Omani shelf – the Sea gliders.

The Sea gliders, which are autonomous robotic systems equipped with numerous sensors allowing recording of physical, chemical, and biological parameters of the water column. These records could be carried out every 6 hours throughout the year. On the way to the surface (from the depth of several hundred meters and back), the sea glider transmits recorded data via satellites and gets a new command- on what to do and where to sail. Moreover, the research team has complemented the sea glider data on vertical distribution of temperature, salinity, water density, dissolved oxygen concentration, and phytoplankton biomass (based on fluorescence measurements), by sampling on board SQU research vessel “Al-Jamiah” coming from time to time to visit the glider. Series of sea glider deployment allowed the team to understand important details is seasonal changes of the water column structure over the Omani shelf, in the Muscat region.

Also, Drs. Sergey Piontkovski and Bastien Queste have accomplished the analysis of historical data from 53 International oceanographic expeditions carried out in the western Arabian Sea during the past 50 years. Data were published in a number of high-ranked international journals. The analysis was based on about 30,000 vertical profiles of temperature, salinity, water density and over 2,000 vertical profiles of dissolved oxygen recorded in the depth range from the surface to 300m. Scientists have found out that the thermal stratification of the water column increased and the oxygen minimum zone shoaled from about 150m in the 1960s to 80m in the 2000s. The discovered phenomenon is a warning sign pointing at the fact that the habitat of pelagic fishes is becoming thinner and pushed up to the surface. Perhaps, these oxygen changes were underlain by and related to long-term changes in the intensity of monsoonal winds and thermal structure of the water column.

For example, the summer temperature increase in the western Arabian Sea over the past 50 years exceeded the mean rate reported for tropical latitudes of the world ocean. In being land locked, the Arabian Sea beats world records. The most pronounced changes have been taking place in the upper 30m layer. This means that the thermal stratification of the water column has increased. This could have affected the shoaling of the oxygen minimum. Plus, warming of the water column decreases the solubility of the oxygen coming from the atmosphere.

Large pelagic fishes are active swimmers which require a lot of oxygen for their metabolism, especially in warm tropical waters. The dissolved oxygen concentration below 2 milliliters per liter induces symptoms of stress for many tropical pelagic fishes; therefore, this concentration is believed to be the hypoxic threshold. If this threshold slowly moves up in the water column (over years), fish populations become compressed in the upper layer, which makes them more exposed to fishery.

A vulnerable question to be asked is what will happen to the pelagic ecosystem of Omani shelf in the nearest future, if the current depth of critical oxygen concentration is about 80m and the oxygen minimum zone is still moving up to the surface? The “uncomfortable shelves” are used to be abandoned, by large pelagic fishes. In the case of Omani shelf this means that declining landings of large pelagic fishes (catches per unit of effort) could have an environmental origin, along with over-fishing.

As far as the upward motion of the critical oxygen concentration is concerned, the trend seems to be dangerous indeed, but it is quite possible that shoaling of the critical concentration depth would be switched to and replaced by a subsequent deepening. In other words, let’s hope that we are dealing with a long-term fluctuation of water mass properties; the event which is quite common in oceanography. Long-term fluctuations of physical, chemical, and biological characteristics of the ocean are climate driven phenomena. In the Arabian Sea, they are mediated by a basin scale and global scale atmospheric anomalies, like the Siberian High atmospheric pressure system, the Indian Ocean Dipole, and the El-Ninõ Southern Oscillation.

In turn, the cyclicity of atmospheric processes (like monsoonal winds) could be mediated by extra-terrestrial forces; for instance, lunar tidal actions which have the periodicity of 56, 95, 125 and 1470 years. These cycles were detected within the thickness of sediment core layers extracted from the Oxygen Minimum Zone of the Pakistan shelf. The clarification of this and the other physical-biological coupling needs funding.

In order to provide further insights into the issue of oxygen minimum zones (which pronouncement is affected by algal blooms), Dr. Sergey Piontkovski and his colleagues from UK and USA came up with the proposal “An Innovative Approach to National Action Plan on Monitoring Algal Blooms”, which was submitted to the Research Council (Oman). The proposed project team consists of specialists from Sultan Qaboos University (SQU), the University of East Anglia (UEA, UK), the National Oceanic and Atmospheric Administration (NOAA, USA).

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