Geographical planning of space quarterly journal

Geographical planning of space quarterly journal

The application of satellite images Sentinel 2 and Landsat 8 and comparison of their capabilities in estimating chlorophyll-a concentration in the Gorgan Bay

Document Type : Research Paper

Authors
Fatemeh Rahmani Khalili 1
Sarah Haghparast 1
KAMRAN NASIRAHMADI 2
Vahid Kheirabadi 1
1 Fisheires Department, Faculty of Animal Sciences and Fisheries, Sari University of Agricultural Sciences and Natural Resources, Sari, Iran
2 Civil Engineering Department, Faculty of Chemical, Industrial and Civil Engineering, Mazandaran University of Science and Technology, Behshahr, Iran
10.30488/gps.2025.460663.3750
Abstract
A B S T R A C T
Regarding the changes in the water quality of Gorgan Bay, this study aimed to investigate the capabilities of satellite images Sentinel 2 and Landsat 8 and compare their efficacy in estimating chlorophyll-a concentration. In this research, satellite images of the bay were obtained from Sentinel-2 and Landsat-8 from April 2021 to August 2022. The preparation of 29 images including atmospheric correction, separation of the water zone, and image processing were performed. Then, chlorophyll-a concentration maps (mg/m3) using the models of NDCI, D05, M09, and T07 were obtained from ENVI software and compared with field data in the stations of interest. To estimate chlorophyll-a concentration, the results showed that NDCI and M09 models were well-correlated with field data. Based on Sentinel-2 satellite images and in comparison to other models, the highest correlation coefficient (R) with the  field data was found in the NDCI model (0.633) which showed the lowest root mean square error (8.8 mg/m3). According to the Landsat 8 images and compared to other models, the M09 model had the highest correlation coefficient with the field data (0.743) and showed the lowest root mean square error (2.33 mg/m3). Results of the present work indicated that for Sentinel-2 satellite images, model NDCI, and based on Landsat 8 satellite images, the M09 model had a reasonable efficacy in the Gorgan Gulf
Extended Abstract
Introduction
Gorgan Gulf, one of the largest Gulfs of the Caspian Sea, is located in the southeast corner of this sea. From ecological and economic viewpoints, the Gorgan Gulf and Miankale Wetland are significant. The Caspian Sea level, nearby rivers, and the Miankale Peninsula influence the Gorgan Gulf. It plays a vital role in the growth and reproduction of aquatic animals and bony and cartilaginous fish and attracts winter migratory birds.
The concentration of chlorophyll-a plays a vital role as a quality indicator of water bodies, and the presence of phytoplankton is used to check the water quality and biochemical status. Common measurements of water quality parameters, such as estimating the amount of chlorophyll-a and water-soluble substances, require field sampling, analysis, and laboratory measurements, which are very costly and time-consuming. Remote sensing techniques provide an overview of large areas in real-time. Remote sensing algorithms that rely more on the detection of specific spectral features using the blue, green, yellow, red, or near-infrared range are well used to distinguish and detect algal blooms from other natural phenomena.
Despite the high importance of the Gorgan Gulf ecosystem, few studies have been done regarding the estimation of chlorophyll-a using remote sensing data in this area. In this study, an attempt was made to estimate the chlorophyll data in the Gorgan Gulf using ENVI software and satellite images, and the results obtained through field sampling and previous studies were validated in this region to provide an index for the correct estimation of chlorophyll-a data in the Gorgan Gulf. Finally, among the models applied to determine the concentration of chlorophyll-a, the best model suitable for the studied area and correlated well with the field data was selected to prepare Chlorophyll-a concentration maps.
 
Methodology
Field sampling and measurement of chlorophyll-a concentration were performed monthly at three sites (Bandar Gaz, Nowkandeh, and Bandar Torkman) using the Algatorch device with an accuracy of 0.1 from April 2021 until August 2022 at surface depth. Algatorch device is one of the most famous sensors for measuring chlorophyll-a and cyanobacteria, and it can measure portable and the profile of chlorophyll accumulation at different depths. Satellite data images were received simultaneously with field data collection to analyze and estimate the required parameters. Landsat-8 and Sentinel-2 multispectral satellite images were used to estimate the concentration of chlorophyll a. Landsat-8 multispectral images have a spatial resolution of 30 m and a temporal resolution of 16 days. The number of 29 images related to 2021 to 2022 of Collection-2, Level-1 data type was downloaded from the US Geology website (https://earthexplorer.usgs.gov). Twelve images of Sentinel-2 data on the dates collected in the field were downloaded from the website (https://apps.sentinel-hub.com). To extract the concentration of chlorophyll-a, NDCI, D05, M09, and T07 models were used in ENVI software and then compared with the field data in the desired stations. Statistical parameters were applied to evaluate the efficacy of different algorithms: root mean squared error (RMSE) and R-squared relationship coefficient (R2).
 
Results and discussion
The results of field measurement of chlorophyll-a concentration in terms of mg/m3 in the studied area showed that the concentration of chlorophyll-a varies in the measured stations in different months of the year. The chlorophyll-a concentration results from Sentinel-2 and Landsat-8 satellite images showed that based on the analysis of Sentinel-2 satellite images, the correlation coefficient (R) between the relationship between the NDCI model and field data was equal to 0.633. The root mean square error (RMSE index) was the lowest error for the NDCI model at the rate of 8.8 mg/m3, with the highest correlation and the lowest error compared to other models. Also, based on the analysis of Landsat-8 satellite images, the correlation coefficient (R) between the M09 model and the field data was equal to 0.743, and the root mean square error (RMSE index), the lowest error for the M09 model was 2.33 mg/m3. NDCI and M09 models estimating chlorophyll-a concentration correlated well with field data. The result obtained by the NDCI method in Landsat-8 is more suitable than Sentinel-2.
 
Conclusion
Based on the results of the present study, the changes in the chlorophyll-a concentration during 2021-2022 were very significant due to the decrease in the water level of the gulf, and its maximum value was observed in winter 2021. Using the NDCI model on Landsat-8 satellite images and the M09 model on Sentinel-2 satellite images, a good estimate of the chlorophyll-a concentration in the turbid waters of the Gorgan Gulf could be provided. Through these models, biological studies can be done at a low cost and a higher speed.
 
Funding
There is no funding support.
 
Authors’ Contribution
Fatemeh Rahmani Khalili: satellite data collection, software implementation, analysis of results and writing. Sara Haqparast: methodology, validation and final writing. Kamran Nasir Ahmadi: data collection and editing. Vahid Khairabadi: data collection and calibration..
 
Conflict of Interest
Authors declared no conflict of interest.
 
Acknowledgments
The author thanks and appreciates the cooperation of the experts of the General Department of Environmental Protection of Golestan and Mazandaran provinces who helped us in conducting this research, especially those who did the qualitative evaluation of the article. Also, the current research has been done in the form of a master's thesis at Sari University of Agricultural Sciences and Natural Resources..
Keywords
Satellite images
Gorgan Gulf
Sentinel-2
Chlorophyll A
Landsat 8

Subjects

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Geographical planning of space quarterly journal
Volume 14, Issue 4
Autumn 2024
Pages 117-131