SEASONAL INFLUENCE OF NITROGEN AND PHOSPHORUS ENRICHMENT ON THE FLORISTIC COMPOSITION OF THE ALGAL PERIPHYTIC COMMUNITY IN A SHALLOW TROPICAL, MESOTROPHIC RESERVOIR (SÃO PAULO, BRAZIL)

Abstract

Effects of N and/or P experimental addition on species richness and floristic composition of  periphytic algal community in a shallow, mesotrophic reservoir was evaluated four times during the year (spring, summer, fall and winter). Four treatments were designed using nutrient diffusing substrates (polystyrene vials filled up with agar solution and nutrients -- control: no nutrient addition, N+: 0.75 M, P+: 0.05 M and NP+: combined addition of N and P, molar N:P ratio = 15). Vial mouth was covered with a 20 µm mesh cloth for periphyton colonization. Samplings were performed on the 15th, 20th, 25th and 30th days of colonization. Two hundred and three taxa were identified; Chlorophyceae was the dominant group. Species richness per sample varied from 33 to 66 and was greater during summer and fall, mostly influenced by the time of the year than by nutrient treatment. Community similarity was mainly determined by the kind of treatment, grouping algal associations of (1) control, (2) P+ and NP+ treatments and (3) N+ treatment.  TWINSPAN analysis indicated that hierarchic classification of species was defined by P availability. Species and classes richness were not sensitive to changes due to nutrient enrichment, nitrogen amendments markedly contributing to the total species numbers, whereas species associations were clearly influenced by P availability. Present results indicated that the reservoir, due to its shallowness and prevalence of littoral biota, may have profound changes in its native associations with P inputs.