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 the Gamasiyab 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 dry mass (AFDM) and chlorophyll a in the periphyton community. Snails significantly depressed periphyton biomass. Following twelve days of grazing of the grazer densities of 0, 0.015, 0.03 and 0.06 individuals/cm2 the dry masses of the periphyton communities were 3.04, 2.9, 2.66 and 1.07 mg/cm2 respectively. Snails also decreased 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 4.6 mg/m2. During this experiment the survival percentage for 0.015, 0.03 and 0.06 (individuals/cm2) treatments was 77.7%, 58.3% and 30% respectively. During the 12 days of the grazing period in the artificial stream the snail growth also showed a density dependent pattern. Diatom biovolumes calculated in this study are 57.1×106, 36×106×31×106, and 5.8×106 μ m3/cm2 for snail densities of 0, 0.015, 0.03 and 0.06 individuals/cm2 respectively. A group of species which are found primarily in the grazed treatments include Achnanthes lanceolata, Cocconeis pediculus, and Ulothrix sp. In contrast, ungrazed assemblage had high relative abundance of non adnate diatoms such as Nitzcshia linaris, Gomphonema sp. and non filamentous chlorophytes such as Scenedesmus sp.