Assessment of eutrophication effects on charophytes in Mediterranean ponds (North-Western Spain)

The distribution of charophyte vegetation in 57 ponds located in the basin of the River Duero in North-Western Spain was studied in relation to eutrophication. A total of 11 charophyte taxa were recorded in 40 ponds. However, only Chara fragilis Desv., Chara connivens Salzm. ex A. Braun and Nitella translucens (Pers.) C. Agardh were found in more than 17 % of the ponds where charophytes occurred. Signifi cant differences between ponds with and without charophytes were found for the following parameters: volatile suspended solids (VSS), total suspended solids (TSS), turbidity, chlorophyll-a, nitrate and total phosphorus (TP). These variables were significantly lower in ponds where charophytes were present. Charophytes were found over a wide range of TP concentrations (20-6884 mu g L-1). However, taxa showed differing sensitivities to phosphorus concentrations. Thus, Chara connivens and Chara fragilis showed the highest optimum values for TP concentration (1875 and 551 mu g L-1 TP, respectively) while Nitella translucens registered the lowest optimum value for TP (69 mu g L-1). The charophyte cover showed a signifi cant negative correlation with VSS and chlorophyll-a. As for charophyte taxa, Chara connivens cover was positive and significantly correlated with orthophosphate (SRP), while Nitella translucens cover showed a signifi cant negative correlation with TP and TSS. The ordination of charophyte taxa showed a principal gradient of mineralization and ecological condition and a secondary gradient of eutrophication.