experimental evaluation of the herbivory interaction between physa sp. and periphyton

the influence of the herbivorous snail (physa sp.) on the algal periphyton community from thegamasiyab river, kermanshah province in western iran was examined by establishing a gradient of physa sp.density in artificial streams. the impact of physa grazing was evaluated by measuring dry mass, ash-free drymass (afdm) and chlorophyll a in the periphyton community. snails significantly depressed periphytonbiomass. following twelve days of grazing of the grazer densities of 0, 0.015, 0.03 and 0.06 individuals/cm2the dry masses of the periphyton communities were 3.04, 2.9, 2.66 and 1.07 mg/cm2 respectively. snails alsodecreased ash-free dry mass by 2.04, 1.78, 1.63 and 0.71 mg/cm2 and chlorophyll a by 8.6, 8.06, 5.25 and 4.6mg/m2. during this experiment the survival percentage for 0.015, 0.03 and 0.06 (individuals/cm2) treatmentswas 77.7%, 58.3% and 30% respectively. during the 12 days of the grazing period in the artificial stream thesnail growth also showed a density dependent pattern. diatom biovolumes calculated in this study are57.1×106, 36×106×31×106, and 5.8×106 μ m3/cm2 for snail densities of 0, 0.015, 0.03 and 0.06individuals/cm2 respectively. a group of species which are found primarily in the grazed treatments includeachnanthes lanceolata, cocconeis pediculus, and ulothrix sp. in contrast, ungrazed assemblage had highrelative abundance of non adnate diatoms such as nitzcshia linaris, gomphonema sp. and non filamentouschlorophytes such as scenedesmus sp