تغییرات مکانی و زمانی زیستگاه های ساحلی خلیج گرگان تحت تاثیر نوسانات دریای خزر

گرمایش زمین و افزایش سطح تراز آب اقیانوس ‌ها تهدید جدی برای محیط زیست تالاب ‌های ساحلی می ‌باشد.  عکس ‌العمل محیط زیستی بزرگترین دریاچه کره زمین در مقابل عوامل اقلیمی طی هفتاد سال اخیر به گونه ‌ای رقم خورده است که کاهش سطح تراز آب دریای خزر موجب خشک شدن سطح وسیعی از تالاب ‌های ساحلی شده است.  هدف اصلی این پژوهش، ارزیابی تغییرات بوم شناختی زیستگاه ‌های ساحلی مهم خلیج گرگان طی سال ‌های 1995 تا 2019 میلادی می‌باشد.  با آنالیز تصاویر ماهواره ‌ای لندست و پردازش نتایج داده ‌ها در محیط سامانه اطلاعات جغرافیایی نرخ تغییرات خطوط ساحلی، وضعیت عمق بستر، تبدیل پوشش زمین و تغییر زیستگاه ‌های ساحلی خلیج گرگان محاسبه و طبقه بندی گردید و نتایج به دست آمده با شواهد میدانی صحت سنجی شد.  نتایج مطالعات نشان داد که با کاهش  150 سانتی متری سطح تراز آب دریای خزر طی سال ‌های 1995  تا 2019  میلادی، وسعت خلیج گرگان به میزان  176 کیلومتر مربع کاهش یافته است.  نقشه عمق نسبی حاصل از مطالعات شبیه سازی در سنجش از دور نشان می ‌دهد که عمق بستر خلیج در راستای شرق به غرب به شدت کاهش یافته است.  مهم ‌ترین عوارض زمین و زیستگاه ‌های ساحلی خلیج گرگان و درصد فراوانی پوشش آن ‌ها در سال  2019  میلادی شامل خاکریز ساحلی (2 %) مارش شور (7 %)، مارش لب شور (14 %)، تالاب کم عمق (15 %)، پهنه‌های گلی (7 %)، پوشش گیاهی جنگلی (10 %) و آبگیر خلیج گرگان و تالاب میانکاله  0.45 %) بود.  همچنین مساحت زیستگاه ‌های تالابی، پوشش گیاهی جنگلی و خاکریز ماسه‌ ای با افت سطح تراز آب دریای خزر دچار کاهش شده بود در عوض به وسعت زیستگاه ‌های مارش لب شور، شور، پهنه‌های گلی و پیت ‌زارها افزوده شده بود.  این موضوع به خوبی جایگزینی بوم سازگان ‌های خشک‌ زی و نیمه آبزی را در قلمرو بوم سازگان آبزی تایید می ‌کند که نوسانات سطح تراز آب دریای خزر تاثیر مستقیم بر روی تغییرات محیط زیستی تالاب ‌های ساحلی داشته است.

Spatial and temporal variations of Coastal habitats in Gorgan Bay resulting from fluctuations in the Caspian Sea

Extended Abstract: IntroductionIncreased density of Co2 in the atmosphere during the Anthropocene epoch has resulted in pervasive concerns for the global environment. Global warming has resulted in sea level rise and coastal flooding. Forecasting has indicated that a vast area of coastal countries and their economic and social infrastructureswill be damaged due to 200 cm sea level rise by2100. Sea level rise in oceans has caused coastal erosion and flooding. Thus, it is considered as a real threat to coastal environment. The Caspian Sea environment has reacted differently to climate changesduring the last 70 years and vast areas of its coastal lagoons have dried. Therefore, the present study primarily seeks toinvestigate ecological variations of coastal habitats in the Gorgan Bay during the period of 1995 to 2019. Materials and methodsGorgan Bay and Miankaleh Lagoon are considered to be among global biosphere reservesand the most important protected areasalong the southern coasts of the Caspian Sea. The present study has evaluated coastal variations, such as shoreline displacement, changes in the depth of sea bed, land cover and coastal habitats using satellite images and GIS processing. Shorelines of Gorgan Bay are determined usingremote sensing software Envi 5.3, while land cover and coastal habitats are evaluated through GIS processing in Arc – Map 10. 5. The shoreline is determined through the calculationsperformedonthe proportion of green and blue bands in reflected electromagnetic waves and histogram thresholding of near infrared (NIR) spectrum in Envi 5.3. The total area of Gorgan Bay was determinedusingthe Normalized Distance Water Index (NDWI).The most important land covers and coastal habitats are classified using Support Vector Machine (SVM). Finally,variations of coastal habitats are calculated using Change Detection Workflow index and the final maps areproduced in Arc Map 10.5. Results and discussionResults indicate that due to about 150 cm decrease in the Caspian Sea level from 1995 to 2019, the total area ofGorgan Bay has faced about 176 km decrease. Bathymetric maps shows that the depth of Gorgan Bay has decreased dramatically along the East to west side. The depth of the Ashouradeh and Chopoghlei inlets have also decreased and vast areas of these water bodies haveturned into arid islands. The Gorgan Bay is connected to the Caspian Sea through some narrow channels. The most important land covers and coastal habitats of the Gorgan Bay in 2019 include sandy beach (2%), salt marsh (7%), brackish marsh (14%), wetland (15%), mudflat (7%), coastal forest (10%) and coastal lagoons (45%). The total area of coastal lagoons, vegetation covering and sandy beaches have decreased from 1995 to 2019 and the area of the brackish marsh, salt marsh, mud flat and pit wetlands have increased at the same time. The total area of sandy beaches have decreased about 52 Km2 since 1995.Instead,the area covered by salt marshes and brackish marshes have increased by about 87 and 60 Km2 during the same period.62 Km2 of mud flat have been created during the same time,and thus, the area of Miankaleh Lagoon and Gorgan Bay have decreased by about 176 Km2. The environment of Gorgan Bay and Miankaleh Lagoon is directly related to the fluctuations in theCaspian Sea level. Survival of these coastal lagoons depends on permanent water exchange between the Caspian Sea and Gorgan Bay. Rapid fluctuations of the Caspian Sea level and high levelof deposition are considered to be among the most important factorsof coastal habitats destruction and ecosystems displacement. These natural phenomena happened twice during the Anthropocene period (1945 1978 and 19952019). ConclusionResults have confirmed that arid ecosystems have replaced aquatic ecosystems in study area. The main results of the study have confirmed that the fluctuation in the Caspian Sea level has direct impact on coastal habitats of the study area and decreasing sea level could change marginal ecosystems. Due to the decrease in water exchange volume rate between the Caspian Sea and Gorgan Bay during the 1995 2019 period, a 32 percent decrease has happened in the area ofGorgan Bay and salt marshes have dominated along the Gorgan Bay coastal area. Unfortunately, the continual decrease inthe Caspian Sea level can destroy biodiversity and coastal habitats in the future. Therefore,integrated coastal zone management (ICZM (is influential insaving and preserving of the Gorgan Bay.