Geomorphological and environmental characterization of three estuaries on the spanish coast

Resumen

The main aim of this PhD thesis is to develop and validate a methodology that allows to understand the integral functioning of the estuaries, and to characterize the estuarine subsystems and their connectivity. To achieve this, three study zones have been selected: (1) San Vicente de la Barquera estuary (N Spain), (2) Guadiana River estuary (SW border between Spain and Portugal) and (3) Ebro River Delta estuary (NE Spain), which cover most of the environmental range and oceanographic conditions that occurs on the Iberian Peninsula coasts. Different specific objectives have been proposed, and a particular methodology has been developed to achieve each of them. The first one consists in a historical observation and analysis of the state of the three study sites to understand which processes have occurred on them in the last years. To do that, a hierarchical classification representing estuarine eco-geomorphological features relevant to estuarine dynamics and functioning has been proposed to unify and standardize the terms when mapping these types of environments. After that, a 30-year time series of high resolution maps has been developed, analysing the changes in feature surfaces by means of Geographic Information System (GIS). The results explain the changes with time on each of the study sites at a long-term scale. Besides, a common general behaviour to all transitional systems, as well as the relationships among main subsystems (namely shoreline sandy environments, dunes, tidal flats and drainage network) and processes inherent to each one have been described. Through this methodology, it is demonstrated how beaches and dunes serve as protective barriers for the tidal flats, acting as sediment buffer for the entire system. Besides, the subsystems are connected by the drainage network, responsible for the exchange of matter and energy in between them. These results add an important perspective towards a general understanding of the dependence of the main estuarine features on intrinsic and boundary conditions. Based on results from the previous specific objective, the second specific objective is to evaluate the most relevant natural processes and the anthropogenic influence on their functioning to understand what is happening within the estuary. For that, San Vicente de la Barquera estuary has been selected as test site, due to the quality and amount of available information for this site. Specifically, saltmarshes in San Vicente de la Barquera estuary have been studied in detail as they have been previously identified as the most changed features within the estuary. To achieve this specific objective, saltmarsh evolution has been studied in a spatial context by means of detailed maps of change in vegetation cover combined with topographic data, to identify patterns of change. The results support the evidences of saltmarsh disappearing in this estuary. Despite no clear pattern of vegetation loss/gain in relation to elevation has been identified over the entire study period, indications of physical stress within the saltmarsh linked to self-organized process are described. In addition, regional and local factors have been studied to identify possible causes of saltmarsh deterioration. With this methodology it is demonstrated the importance of both natural forcing factors and human interventions when describing saltmarshes evolution and future trends. The last two specific objectives proposed are to define a set of variables to characterize all the above described processes in a simple manner and suitable to other estuarine zones, and to design and validate with them an environmental characterization index for estuaries (EstuarIndex). This final part of the PhD thesis pretends to answer the question of what is going to happen in each of the study zones as well as what needs to be done to develop a comprehensive methodology for estuarine management in Spain. For that purpose, different variables have been proposed to evaluate the environmental status of the main estuarine subsystems based on previous index-based studies. Three field campaigns were carried out in each study area to collect information in situ, two of them at the end of summer time and one at the end of winter time, to obtain information on seasonal peaks. Besides, an extensive bibliographical review has been made to obtain information about those variables that did not require measurements. The results obtained by means of the index provide a good approach to the environmental status of the estuaries. All of them show medium-high index values. Besides, in the three study cases, results obtained are consistent with what was observed in the field during the field campaigns. For this reason, this index could be considered as a first valid approximation to characterize the main dominant processes within estuaries, which are responsible for their long-term evolution. Despite an “static view” of the system is represented by a unique final value, the developed index provides a final view based on processes with a wide time scale. These results can be used by coastal managers to make adequate plans regarding estuarine management, as they are simple and reliable tools. Furthermore, the methodology here presented has been designed to be applied to any estuary of mid-latitudes.