SUMMER PHYTOPLANKTON ASSEMBLAGES ACROSS TROPHIC GRADIENTS IN HARD-WATER RESERVOIRS

Summer phytoplankton assemblages are described and characterised accor ding to their prevalence in a series of hardwater reservoirs of easter n Spain that had been classified in trophic categories on OECD criteri a. Distribution patterns of phytoplankton species were ordinated stati stically by principal components analysis (PCA). The first component w as strongly related to trophic gradient and it particularly discrimina ted the eutrophic and hypertrophic reservoirs. The second component se gregated life-forms, so that (1), on the oligo-mesotrophic side, large dinoflagellates were separated from small centric diatoms, unicellula r chrysophytes and filamentous ullotrichales and, on the eu-hypertroph ic side (2), colonial greens and large desmids were separated from uni cellular volvocales and small centric diatoms. The large differences b etween eutrophic and hypertrophic reservoirs were also clearly identif ied in a second PCA, in which physical and chemical factors were used with the principal components solved from the phytoplankton data. From these results, a new trophic category was discerned, for which we pro pose the name 'holotrophic'. This category applies to water bodies hav ing the following main features: (1) concentrations of chorophyll, tot al P and total N in the range of the hypertrophic systems, but with mu ch higher concentrations of dissolved phosphorus and ammonia and (2) p hytoplankton predominantly composed by unicellular green flagellates ( Pteromonas, Chlamydomonas) and chlorococcales (Scenedesmus), without c yanobacterial blooms.