Esparra-Escalera, Héctor J.

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    Nutrient release and grazing effects by two exotic Thiaridae snails on periphyton biomass under artificial conditions
    (2018-05) Esparra-Escalera, Héctor J.; Santos-Flores, Carlos J.; College of Arts and Sciences - Sciences; Sotomayor-Ramírez, David R.; Rodríguez-Minguela, Carlos M.; Vélez-Díaz, Ana; Department of Biology; Fernández-Van Cleve, John
    Thiara (Tarebia) granifera and Melanoides tuberculata are invasive parthenogenetic snails that impact food webs in tropical freshwater systems. Due to their high reproductive rate, both snails dominate the community of herbivores during conditions of higher nutrient availability and when the benthic biomass is high. These species should impact the aquatic trophic status by differential grazing and should affect the diversity and abundance of primary producers. Previous research shows knowledge gaps related to how these two snails may independently (or jointly could) influence the periphyton biomass and diversity, as well as niche-related environmental conditions. In this thesis, I examined the nutrient release of both snail species and how this is related to the periphyton biomass as judged by measurements of chlorophyll-a, total nitrogen and total phosphorus. Four treatments consisting of a control, T. (T.) granifera, M. tuberculata, and both species interacting, were set in artificial systems (mesocosms) and replicated four times. Each experiment unit (except control) consisted of 14 adult snails. Nutrient release assays, physico-chemical parameters measured in-situ (dissolved oxygen, temperature, pH and turbidity) and standing crop of the periphyton (via chlorophyll-a determinations and taxonomic assessment) were evaluated at five different times during 28 to 30 days after the introduction of the snails. Both species were measured (size, length of opercular aperture and spire) at the beginning and at the end of the sampling periods. Born juveniles were quantified at the end of the sampling periods. TN and NO3-N released by snails varied by treatments and periods of sampling; however, in the interaction of both snails, the concentrations were similar. Among the parameters measured in-situ, temperature varied significantly by periods of sampling. DO and pH showed a lineal correlation in presence of T. (T.) granifera alone. Also, T. (T.) granifera, either alone or interacting with M. tuberculata, released more juveniles. M. tuberculata showed a significant difference (p < 0.05) in rate of change of the shell length in competition with T. (T.) granifera. Both snails showed correlations in external morphological traits (opercular aperture, shell length and spire), especially when each species was kept apart. T. (T.) granifera seemed to reduce the periphyton standing crop (lower chl-a), probably due to grazing effects; the presence of this snail seemed was related to the dominance by cyanobacteria, such as Chroococcus, Anabaena, and species of Microcystaceae. During periods of sampling, the treatment with M. tuberculata showed the largest periphyton biomass and the highest proliferation of green algae (Chlorophyta) (as a possible food source), suggesting that it exerts a different grazing pressure on periphyton, when compared to T. (T.) granifera. Thus, both Thiarid species seemed to exert different effects on the nutrient concentrations and on the abundance and diversity of the algal species.