Identification of similar environmental areas in Tagus estuary by using multivariate analysis

An observational programme consisting of physical, chemical and biological measurements has been carried out along Tagus estuary covering 13–18 stations. The multiparameter dataset generated in two distinct winter (March 2001, February 2004) and summer (July 2001 and July 2003) periods are analysed in this work. Principal component analysis (PCA) and cluster analysis were applied to data matrices including 10 variables (salinity, temperature, dissolved oxygen, nitrate + nitrite, ammonia, phosphate, silicate, suspended particulate matter, chlorophyll a and phaeopigments) in order to assess the seasonality of the respective patterns and to highlight areas of similar variables influence. In both winter and summer seasons, PCA clearly separates the upper estuary stations from the lower estuary ones, on the basis of nutrients, productivity, and salinity. The application of cluster analysis further supports and clarifies seasonal and spatial patterns, giving a better insight into other features. In summer, an increment of primary productivity is evident in mid-estuary region, while under extreme flow conditions the accumulation of particulate material, nutrients (as silicate and ammonia) and phytoplankton organisms seems to be favoured nearby extensive salt marshes.