Abstract

Given the great importance of lakes in Earth’s environment and human life, continuous water quality (WQ) monitoring within the frame of the Water Framework Directive (WFD) is the most crucial aspect for lake management. In this study, Earth Observation (EO) data from Landsat 7 Enhanced Thematic Mapper Plus (ETM+) and Landsat 8 Operational Land Imager (OLI) sensors have been combined with co-orbital in situ measurements from 50 lakes located in Greece with the main objective of delivering robust WQ assessment models. Correlation analysis among in situ co-orbital WQ data (Chlorophylla, Secchi depths, Total phosphorus-TP-) contributed to distinguishing their inter-relationships and improving the WQ models’ accuracy. Subsequently, stepwise multiple regression analysis (MLR) of the available TP and Secchi depth datasets was implemented to explore the potential to establish optimal quantitative models regardless of lake characteristics. Then, further MLR analysis concerning whether the lakes are natural or artificial was conducted with the basic aim of generating different remote sensing derived models for different types of lakes, while their combination was further utilized to assess their trophic status. Correlation matrix results showed a high and positive relationship between TP and Chlorophyll-a (0.85), whereas high negative relationships were found between Secchi depth with TP (−0.84) and Chlorophyll-a (−0.83). MLRs among Landsat data and Secchi depths resulted in 3 optimal models concerning the assessment of Secchi depth of all lakes (Secchigeneral; R = 0.78; RMSE = 0.24 m), natural (Secchinatural; R = 0.95; RMSE = 0.14 m) and artificial (Secchiartificial; R = 0.62; RMSE = 0.1 m), with reliable accuracy. Study findings showed that TP-related MLR analyses failed to deliver a statistically acceptable model for the reservoirs; nevertheless, they delivered a robust TPgeneral (R = 0.71; RMSE = 1.41 mg/L) and TPnatural model (R = 0.93; RMSE = 1.43 mg/L). Subsequently, trophic status classification was conducted herein, calculating Carlson’s Trophic State Index (TSI) initially throughout all lakes and then oriented toward natural-only and artificial-only lakes. Those three types of TSI (general, natural, artificial) were calculated based on previously published satellite-derived Chlorophyll-a (Chl-a) assessment models and the hereby specially designed WQ models (Secchi depth, TP). The higher deviation of satellite-derived TSI values in relation to in situ ones was detected in reservoirs and shallower lakes (mean depth < 5 m), indicating noticeable divergences among natural and artificial lakes. All in all, the study findings provide important support toward the perpetual WQ monitoring and trophic status prediction of Greek lakes and, by extension, their sustainable management, particularly in cases when ground truth data is limited.

Highlights

  • Surface freshwater is one of the most essential resources for the terrestrial ecosystem and the predominant source of drinking water on Earth [1]

  • In the framework of the effort of [65] to develop water quality (WQ) empirical algorithms across certain Spanish lakes and ponds, they evaluated three different atmospheric correction methods (DOS; ATCOR3; MODTRAN5). Those methods were applied to Landsat 7 ETM+ bands, and the results indicated that the DOS method performed better than the others, reporting the lowest errors

  • Spearman r values that resulted from the correlation analysis among all available band transformations and Secchi depth values, log, ln, and SQRT Secchi depth values ranged from −0.56 to +0.56

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Summary

Introduction

Surface freshwater is one of the most essential resources for the terrestrial ecosystem and the predominant source of drinking water on Earth [1]. Over the past few decades, climate change and human activities have deteriorated water quality (WQ) [2]. Some factors responsible for it include rapid development, as well as changes in land use/land cover (LULC) patterns, industrialization, and urbanization [3]. The close proximity of water reservoirs to settlements may reduce the price of water to consumers. It may prevent the sustainable management of water resources against deteriorating activities and inappropriate disposal of urban sewage generated within drainage basins [4]. Deterioration of lake systems’ WQ has resulted in many lake eutrophication problems; environmental scientists have tried to monitor, manage, and limit it for more than two decades [5]. WQ monitoring is the most crucial aspect for lake management [6]

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