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EARSeL: 2nd Workshop on Remote Sensing of the Coastal Zone Porto, Portugal, 9-11 June 2005 |
SESSION OCEAN COLOUR |
Vittorio Barale, Frédéric Mélin
Joint Research Centre of the EC, Ispra, 21020, Italy
vittorio.barale@jrc.it, frederic.melin@jrc.it
Antonella Lugliè, Nicola Sechi
Dipartimento di Botanica ed Ecologia Vegetale, Università di Sassari, Italy
luglie@uniss.it, sechi@uniss.it
Maria Grazia Giacobbe
Istituto per l’Ambiente Marino Costiero, CNR, Messina, 98122, Italy
mariagrazia.giacobbe@iamc.cnr.it
Mercedes Masó, Magda Vila
Institut de Ciències del Mar, Barcelona, 08003, Spain
meme@icm.csic.es, magda@icm.csic.es
Remote sensing from Earth orbit can be used to assess phytoplankton growth patterns in the coastal and marine environment, at large (basin) scales and over long (annual) periods. Optical observations, in particular, allow the determination of blooming indicators, such as the concentration of chlorophyll-like pigments (chl). In near-coastal waters, significant uncertainties can arise in the computation of chl absolute values and statistics, due to the presence of other optically active materials (i.e. dissolved organic matter and suspended inorganic particles). Nevertheless, the analysis of historical satellite data can provide information on anomalous algal blooms, and related environmental boundary conditions, with potential applications for the monitoring and management of water quality in coastal regions. In the present case, data collected by the Sea-viewing Wide Field-of-View Sensor (SeaWiFS), in the period 1998-2003, were used to explore the large-scale planktonic field of the Mediterranean Sea, in correspondence to blooming episodes of the genus Alexandrium, the dinoflagellates which cause most (harmful) algal blooms in the basin. Various species of this genus have been spreading in coastal areas of the western basin, in recent years, while in the eastern basin the genus has been observed only in Alexandria, Egypt. The comparison of in situ records with the SeaWiFS imagery, suggest that the eastern basin being much more oligotrophic than the western one, intense phytoplankton blooms in that basin are more difficult to occur. Furthermore, the data collected by the SeaWiFS – albeit with a spatial resolution too poor to distinguish individual near-coastal blooms – highlight the planktonic field characteristics in a series of known hot spots for Alexandrium blooming, located along the Spanish and Italian coastlines (Catalunia and Balearic Islands, Sardinia and Sicily). The investigation considered blooms ranging from those of noxious species (e.g. A. taylori) that generate elevated biomasses, causing a deterioration of coastal water quality along popular beaches, to those of toxic species (e.g. A. minutum and A. catenella), that exhibited a rapid geographical spread during the past decade, mostly in harbours and shellfish farms, causing substantial economic losses. In most cases, recurrent blooms have been described as localised phenomena, linked to increased nutrient availability and low water renewal, due to human presence/impact on site (e.g. crowded beaches or marinas), as well as to specific geographical and meteorological conditions (e.g. enclosed bays during summer, when hydrodynamic forcing is low). A preliminary analysis of quasi-concurrent satellite images also indicates that this blooming might be linked to local (near-coastal) factors, and only occasionally coincidental with the impact of larger (mesoscale) dynamical features such as offshore eddies or river plumes. Finally, it is interesting to note that the appearance and increasing recurrence of Alexandrium blooms in the Mediterranean Sea seems to correspond to a general decreasing trend of the chl indicator, estimated of the order of 10% of the annual mean value for the entire basin, in the period of SeaWiFS coverage. Considering the indications emerging from the present comparison between in situ and satellite data, it appears that remote sensing of bio-optical indicators can provide a novel insight into the Mediterranean biological cycles and into the basin’s potential for coastal eutrophication phenomena, contributing to the assessment of its main ecological provinces.
Last Update: 2005-03-16