Χρήση γεωχωρικών τεχνολογιών και υδροακουστικής για την παρακολούθηση υδάτινων οικοσυστημάτων

Inland waters play a major role in many important environmental processes, among the most fundamental of which are the provision of habitats for a variety of species, the participation in the hydrological cycle, as well as the nutrient and carbon cycles. Furthermore, their contribution to social well-being, health and human activities has begun to receive increasing attention within the broader frame of ecosystem services. Nevertheless, pressure from human activities, climate change and land use alteration have led to a global degradation of inland waters, negatively impacting their role and functions. In light of this, a number of national and international legislative directives have been put forth, with the collective aim of highlighting the problem and enforcing the protection and improvement of inland water ecological status. To provide an integral understanding of the way in which aquatic ecosystems respond to various forms of external pressure, as well as to clarify the factors that influence their manifestations, large volumes of geographic data are needed. Αquatic ecosystems exhibit a significantly large diversity with respect to their hydrological and morphological characteristics. As a result, the study of the species that they host, as well as their physico-chemical composition, the management, monitoring and inference with regards to ecosystem behavior cannot be secured exclusively based on point-wise field measurements and observations. To this end, Remote Sensing provides an important modern geospatial technology, which has been widely used for the analysis of information related to the status of aquatic ecosystems. Using passive (e.g. multi-spectral or hyper-spectral), as well as active (e.g. radar) satellite systems, it is possible to monitor the condition of aquatic ecosystems, as well as their drainage catchments. At the same time, recent years have seen an increase in use of aerial remote sensing, owing, in part, to the increasing popularity and accessibility to unmanned aerial vehicles (UAVs), which offer the acquisition of data with high spatial resolution at an increased temporal resolution and flexibility. Despite the fact that satellite-, as well as aerial remote sensing, have been used and contributed to the identification, classification and detection of variations in aquatic ecosystem status, they cannot be used to collect reliable information for water depths greater than those of the proximal surface layer (or up to several meters under favourable conditions). To tackle these limitations, hydroacoustics presents a standardised, widely used method for the observation and monitoring of biological and hydromorphological characteristics of aquatic ecosystems. This thesis comprises an effort to demonstrate the capabilities offered by modern geospatial technologies and hydroacoustics, with the goal of achieving the holistic monitoring of aquatic ecosystems. In specific, a number of potential applications of geospatial technologies and hydroacoustics for the observation and monitoring of morphometric, physico-chemical and biological parameters of aquatic ecosystems, are presented through targeted case studies.Τα εσωτερικά ύδατα συμμετέχουν σε πολύ σημαντικές περιβαλλοντικές λειτουργίες και διαδικασίες, μεταξύ των οποίων η παροχή ενδιαιτημάτων για πληθώρα ειδών και η συμμετοχή τους στον υδρολογικό κύκλο, καθώς και στους κύκλους θρεπτικών αλάτων και άνθρακα. Επίσης, η συμβολή τους στην κοινωνική ευημερία, την υγεία και τις ανθρώπινες δραστηριότητες έχει αρχίσει να γίνεται ευρέως αντιληπτή στο πλαίσιο των υπηρεσιών των οικοσυστημάτων. Παρόλα αυτά, οι ανθρωπογενείς πιέσεις, η κλιματική αλλαγή και οι μεταβολές των χρήσεων γης έχουν οδηγήσει στην υποβάθμιση των εσωτερικών υδάτων σε παγκόσμια κλίμακα και έχουν αλλοιώσει τις λειτουργίες αυτών των οικοσυστημάτων. Στο πλαίσιο αυτό, διάφορες εθνικές και διεθνείς οδηγίες έχουν θεσμοθετηθεί, με σκοπό την ανάδειξη του προβλήματος και την προστασία-βελτίωση της οικολογικής τους κατάστασης. Με σκοπό την ολοκληρωμένη κατανόηση του τρόπου, με τον οποίον ανταποκρίνονται τα υδάτινα οικοσυστήματα στις διάφορες πιέσεις, καθώς και τον προσδιορισμό των αιτιών της απόκρισής τους αυτής, απαιτείται μεγάλος όγκος γεωγραφικών δεδομένων και παρατηρήσεων. Τα υδάτινα οικοσυστήματα εμφανίζουν μεγάλη ποικιλομορφία ως προς τα υδρολογικά και μορφολογικά χαρακτηριστικά τους, ως προς τα είδη που φιλοξενούν, καθώς και ως προς τη φυσικοχημική τους κατάσταση. Ως εκ τούτου, η διαχείριση, η παρακολούθηση και η γενίκευση της συμπεριφοράς των οικοσυστημάτων, βασιζόμενη αποκλειστικά σε σημειακές παρατηρήσεις πεδίου, είναι συχνά επισφαλής, οικονομικά ασύμφορη ή/και επιχειρησιακά αδύνατη. Η Τηλεπισκόπηση αποτελεί μία σημαντική σύγχρονη γεωχωρική επιστήμη και τεχνολογία, η οποία έχει χρησιμοποιηθεί ευρέως για την ανάλυση πληροφοριών σχετικά με την κατάσταση των υδάτινων οικοσυστημάτων. Τόσο από παθητικά (για παράδειγμα, πολυφασματικά ή υπερφασματικά), όσο και από ενεργητικά (για παράδειγμα ραντάρ) δορυφορικά συστήματα, είναι δυνατή η παρακολούθηση της κατάστασης των υδάτινων οικοσυστημάτων και της λεκάνης απορροής τους. Παράλληλα, τα τελευταία χρόνια, έχει αυξηθεί η χρήση της εναέριας τηλεπισκόπησης, μέσω και των μη-επανδρωμένων εναέριων οχημάτων (UAVs), τα οποία παρέχουν τη δυνατότητα συλλογής δεδομένων με μεγαλύτερη χωρική ανάλυση και μεγαλύτερη χρονική ευελιξία. Παρότι τόσο η δορυφορική, όσο και η εναέρια τηλεπισκόπηση, έχουν χρησιμοποιηθεί και έχουν συνεισφέρει στην αναγνώριση, ταξινόμηση και ανίχνευση των μεταβολών των υδάτινων οικοσυστημάτων, δεν μπορούν να χρησιμοποιηθούν για την ανάκτηση δεδομένων από μεγαλύτερα βάθη, πέραν του εγγύς επιφανειακού υδάτινου στρώματος (ή σε βάθος μερικών μέτρων, υπό προϋποθέσεις). Για την αντιμετώπιση των περιορισμών αυτών, η υδροακουστική είναι μία πρότυπη μέθοδος τηλεπισκόπησης που έχει χρησιμοποιηθεί ευρέως για την παρακολούθηση βιολογικών και υδρομορφολογικών χαρακτηριστικών των υδάτινων οικοσυστημάτων. Στο πλαίσιο της διατριβής αυτής γίνεται προσπάθεια εφαρμογής και ανάδειξης των δυνατοτήτων που προσφέρουν οι σύγχρονες γεωχωρικές τεχνολογίες και η υδροακουστική, με σκοπό την ολιστική παρακολούθηση υδάτινων οικοσυστημάτων. Συγκεκριμένα, μέσα από στοχευμένες μελέτες περίπτωσης, παρουσιάζονται χαρακτηριστικές εφαρμογές των γεωχωρικών τεχνολογιών και της υδροακουστικής στην παρακολούθηση μορφομετρικών, φυσικοχημικών και βιολογικών παραμέτρων υδάτινων οικοσυστημάτων

Assessing the Fish Stock Status in Lake Trichonis: A Hydroacoustic Approach

Fish stock monitoring is an important element for the sustainable management of inland water resources. A scarcity of data and the lack of systematic monitoring for Lake Trichonis precludes an up-to-date assessment. To assess the current status of pelagic fish stock, a hydroacousting survey was conducted for the first time in Lake Trichonis, Greece. In October 2019, the lake was acoustically surveyed with two, horizontally and vertically mounted, 120 kHz transducers during day and night. A decreasing gradient in pelagic fish density from the western to the eastern shores of the lake was observed. Fish density was significantly higher in the intermediate layers of the water column, in the eastern region, compared to the western region. The lake appears to host primarily communities of small-sized fish (TL: 0–5 cm), whereas larger fish (TL: 5–50 cm) are a small minority of the total fish stock. The overall average estimated fish length was approximately 2.4 cm. The adoption of routine inland fish stock monitoring through hydroacoustic methods could be a promising step in the effort to improve the understanding of unique inland water ecosystems with minimum impact on endemic species, as well as to mitigate human impact and achieve long-term sustainable management

Monitoring water quality parameters of lake Koronia by means of long time-series multispectral satellite images

In this study, a comprehensive 30-year (1984-2016) water quality parameter database for Lake Koronia - one of the most important Ramsar wetlands of Greece - was compiled from Landsat imagery. The reliability of the data was evaluated by comparing water Quality Element (QE) values computed from Landsat data against in situ data. Water quality algorithms developed from previous studies, specifically for the determination of Water Temperature and pH, were applied to Landsat images. In addition, Water Depth, as along with the distribution of floating vegetation and cyanobacterial blooms, were mapped. The performed comprehensive analysis posed certain questions regarding the applicability of single empirical models across multi-temporal, multi-sensor datasets, towards the accurate prediction of key water quality indicators for shallow inland systems. Overall, this assessment demonstrates that despite some limitations, satellite imagery can provide an accurate means of obtaining comprehensive spatial and temporal coverage of key water quality characteristics.17618

Monitoring water quality parameters of Lake Koronia by means of long time-series multispectral satellite images

In this study, a comprehensive 30-year (1984–2016) water quality parameter database for Lake Koronia – one of the most important Ramsar wetlands of Greece – was compiled from Landsat imagery. The reliability of the data was evaluated by comparing water Quality Element (QE) values computed from Landsat data against in situ data. Water quality algorithms developed from previous studies, specifically for the determination of Water Temperature and pH, were applied to Landsat images. In addition, Water Depth, as along with the distribution of floating vegetation and cyanobacterial blooms, were mapped. The performed comprehensive analysis posed certain questions regarding the applicability of single empirical models across multi-temporal, multi-sensor datasets, towards the accurate prediction of key water quality indicators for shallow inland systems. Overall, this assessment demonstrates that despite some limitations, satellite imagery can provide an accurate means of obtaining comprehensive spatial and temporal coverage of key water quality characteristics

Estimating Environmental Preferences of Freshwater Pelagic Fish Using Hydroacoustics and Satellite Remote Sensing

In this study, a remote sensing-based method of mapping and predicting fish spatial distribution in inland waters is developed. A combination of Earth Observation data, in-situ measurements, and hydroacoustics is used to relate fish biomass distribution and water-quality parameters along the longitudinal transect of the Římov Reservoir (Czech Republic) using statistical and machine learning techniques. Parameter variations and biomass distribution are estimated and validated, and apparent trends are explored and discussed, together with potential limitations and weaknesses. Water-quality parameters exhibit longitudinal gradients along the reservoir, while calculations reveal a distinct fish assemblage pattern observed as a patchy overall biomass distribution. Although the proposed methodology has a great potential for sustainable water management, careful planning is needed to ensure the simultaneous acquisition of remote sensing and in-situ data to maximize calibration accuracy

Production, Validation and Morphometric Analysis of a Digital Terrain Model for Lake Trichonis Using Geospatial Technologies and Hydroacoustics

Covering an area of approximately 97 km2 and with a maximum depth of 58 m, Lake Trichonis is the largest and one of the deepest natural lakes in Greece. As such, it constitutes an important ecosystem and freshwater reserve at the regional scale, whose qualitative and quantitative properties ought to be monitored. Depth is a crucial parameter, as it is involved in both qualitative and quantitative monitoring aspects. Thus, the availability of a bathymetric model and a reliable DTM (Digital Terrain Model) of such an inland water body is imperative for almost any systematic observation scenario or ad hoc measurement endeavor. In this context, the purpose of this study is to produce a DTM from the only official cartographic source of relevant information available (dating back approximately 70 years) and evaluate its performance against new, independent, high-accuracy hydroacoustic recordings. The validation procedure involves the use of echosoundings coupled with GPS, and is followed by the production of a bathymetric model for the assessment of the discrepancies between the DTM and the measurements, along with the relevant morphometric analysis. Both the production and validation of the DTM are conducted in a GIS environment. The results indicate substantial discrepancies between the old DTM and contemporary acoustic data. A significant overall deviation of 3.39 ± 5.26 m in absolute bottom elevation differences and 0.00 ± 7.26 m in relative difference residuals (0.00 ± 2.11 m after 2nd polynomial model corrector surface fit) of the 2019 bathymetric dataset with respect to the ~1950 lake DTM and overall morphometry appear to be associated with a combination of tectonics, subsidence and karstic phenomena in the area. These observations could prove useful for the tectonics, geodynamics and seismicity with respect to the broader Corinth Rift region, as well as for environmental management and technical interventions in and around the lake. This dictates the necessity for new, extensive bathymetric measurements in order to produce an updated DTM of Lake Trichonis, reflecting current conditions and tailored to contemporary accuracy standards and state-of-the-art research in various disciplines in and around the lake