Efecte del gradient de salinitat en la comunitat del biofilm de la Riera Salada

Abstract

The Riera Salada is a unique river ecosystem, as it presents a very high salinity of natural origin, being the highest peak (at the start of the river) of 43.53 conductivity mS/cm approximately three times more than the conductivity of the sea. The high salinity is caused by evaporites of Keuper, sediments that are found in different sites of the river. One of the curious facts is the strong gradient of salinity which results as a consequence of the successive entrance of water of the tributaries. At the end point of the sampling it falls down to 0.60 mS/cm, the conductivity of a freshwater river. The main objective is to see and identify which species inhabit these conditions (halophilic conditions) and how they may vary along the salinity gradient, as to identify if there are significant differences according to the different substrates of the river bed (sediments, pebbles and leaves). In this study they are especially observed the primary producers, algae and diatoms. As they were the most representative group as a percentage of abundance, a specific protocol for their observation was realised. It consisted to burn all the remains of organic matter and leave only the frustules of the diatoms for their observation under the microscope. From the various data have been calculated diversity index. It has been found that the salinity is not the determining factor, but it is the presence or absence of nutrients, it seems that the absence of nutrients affects more to the diversity and abundance than salinity does. It is also noticed that the substrate with more chlorophyll and diversity are the leaves, and then the boulders. It appears that the most representative organisms are diatoms followed by Bangia atropurpurea and Enteromorpha, halophiles species present in most salted sites. Within the group of diatoms the most abundant genus is Acnanthes. The results suggest significant changes in the composition of the biofilm between the sampling points and the type of sediment. Finally we noted that the substrate most affected by salinity is sediments, although the diversity index are equal, the community changes a lot of species and genera