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Haïfa University (2013)

Intrusion of coastal waters into the pelagic Eastern Mediterranean : in situ and satellite-based characterization

Efrati Shai

Titre : Intrusion of coastal waters into the pelagic Eastern Mediterranean : in situ and satellite-based characterization

Auteur : Efrati Shai

Etablissement de soutenance : Haïfa University

Grade : Master 2013

Résumé
In the highly populated ultra-oligotrophic eastern Mediterranean sea, a scenario of pollution from anthropogenic sources that influences the open sea is very likely to happen. When observing satellite images of this area one can notice chlorophyll-a patches stretching from the eastern coasts into the open sea. These patches are very common and appear in different locations and time periods. By integrating datasets of near real time multi satellite and in situ measurements from a high-resolution survey, this study aims to characterize physicalbiogeochemical properties of such patches in the eastern Mediterranean basin. For over two decades, satellites have provided continuous spatial data of physical and biological properties. Combination of data from different independent sources, such as ocean color and altimetry satellites, allows an understanding of the relationship between different properties. It is a major tool in studying the dynamics of ocean flow, as they control substances spread and aid in the development of monitoring tools. Satellite data is derived using a variety of algorithms for different applications, such as chlorophyll-a from ocean color and geostrophic currents from altimetry. Satellite derived maps of geostrophic currents allow observation and study of coherent features of water flow. Analysis of the dynamics of water flow can be expanded using mathematical methods separated into Eulerian and Lagrangian analysis. In this work, by using an advanced Lagrangian analysis in the notion of Lagrangian coherent structures, sequences of geostrophic currents datasets are interpreted and described as transport barriers. Using a near-real-time satellite based monitoring system constructed especially for this study, the eastern Mediterranean was monitored with the intention of finding an event of a chlorophyll-a patch stretching from the eastern coasts to the open sea. When such an event was detected, its physical and biogeochemical properties were sampled during a dedicated scientific cruise. 19 depth profiles were analyzed using a CTD and 13 stations were sampled IN and OUT of the patch for laboratory biogeochemical analysis. Spatial analysis of the combined dataset from satellites and comparison with insitu measurements indicates that the studied patch was a semi-enclosed system, bounded within the mixed layer and separated from ambient waters by transport barriers induced by horizontal stirring. As such, the patch was characterized by physical-biogeochemical properties that significantly differed from those of the waters surrounding it, with lower salinity, higher temperatures, higher concentrations of silicic acid and chlorophyll a, and higher abundance of Synechococcus and Picoeukaryotes cells. Based on estimates of patch dimensions (40 km width and 25m depth) and propagation speed (0.09ms−1), the volume flux associated with the patch was found to be in the order of 0.1 Sv. These observations suggest that horizontal stirring by surface currents is likely to have an important impact on the ultraoligotrophic Levantine basin ecosystem, through (1) transport of nutrients and coastal derived material, and (2) formation of local, dynamically isolated, niches. In addition, this study provides a satellite-based framework for planning and executing high resolution sampling strategies in the interface between coast and the open sea.

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