| Benthic algal communities are often the dominant primary producers and important energy source o higher trophic levels in rivers and streams. The composition and abundance of benthic algae and relationships with physicochemical parameters were studied during 2006 in five streams minimally impacted by human activity of central mountainous region (Bosque Olimpia, San Viron and La Mina) and eastern portion of Puerto Rico (Sonadora and Puente Roto-Mameyes). Monthly composite samples were obtained for enumeration and identification of species, and biomass by ash free dry weight and chlorophyll a. These biological parameters were used to estimate the autotrophic index. In addition, total phosphorus, dissolved nitrate, turbidity, pH, dissolved oxygen, conductivity, dissolved organic carbon, and temperature were determined. Altogether, 120 species were identified; diatoms were most abundant (94 species) followed by green algae (16 species), cyanobacteria (9 species) and euglenophyta (1 species). The diatoms Achnanthes rupestoides, Adlafia muscora, Gomphonema clavatum, G. dubravicense, G. truncatum, Gomphonema pumilum, Navicula constans, Navicula tripunctata, Navicula cf. recens, Nitzschia dissipata and Pinnularia subgibba are new records for Puerto Rico. The stream with the highest diversity index was La Mina (H'= 2.91 with 92 species) while Sonadora (H'= 2.03 with 57 species) registered the lowest species diversity. A correlation existed between concentration of chlorophyll a and nutrients, with La Mina having with the maximum value of chlorophyll a (10.03 mg/m2) and Sonadora with the minimum value (2.09 mg/m2). An autotrophic index superior to 100 in temperate zones indicates poor water quality. In this study however, the observed autotrophic index was high in all streams but associated to significant contribution of heterotrophic biomass in the epilithic community instead of local anthropogenic impacts. Streams located on the central region exhibited a higher nutritional status than sites located at the eastern portion of the island. Total phosphorus and nitrate explained 55% and 49% of the variation in the algal density, respectively. According to the concentrations of nutrients and chlorophyll a, the streams of the interior of the island (Bosque Olimpia, San Viron, and La Mina) are classified as mesotrophic while Mameyes and Sonadora are considered oligotrophic. The nutrient values and the evaluation of the epilithic community indicate that Sonadora could be used as a reference site in Puerto Rico with the least anthropogenic impact. |
