Summer phytoplankton assemblages across trophic gradients in hard-water reservoirs

Summer phytoplankton assemblages are described and characterised according to their prevalence in a series of hardwater reservoirs of eastern Spain that had been classified in trophic categories on OECD criteria. Distribution patterns of phytoplankton species were ordinated statistically by principal components analysis (PCA). The first component was strongly related to trophic gradient and it particularly discriminated the eutrophic and hypertrophic reservoirs. The second component segregated life-forms, so that (1), on the oligo-mesotrophic side, large dinoflagellates were separated from small centric diatoms, unicellular chrysophytes and filamentous ullotrichales and, on the eu-hypertrophic side (2), colonial greens and large desmids were separated from unicellular volvocales and small centric diatoms. The large differences between eutrophic and hypertrophic reservoirs were also clearly identified 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 propose the name 'holotrophic'. This category applies to water bodies having the following main features: (1) concentrations of chorophyll, total P and total N in the range of the hypertrophic systems, but with much higher concentrations of dissolved phosphorus and ammonia and (2) phytoplankton predominantly composed by unicellular green flagellates (Pteromonas, Chlamydomonas) and chlorococcales (Scenedesmus), without cyanobacterial blooms.