Nutrients effect and top-down control on macroalgae by invertebrate grazers in a rocky intertidal at São Sebastião channel (São Paulo, Brazil).

BAZTERRICA, M.CIELO; HIDALGO, FERNANDO JOSÉ; FLORES, AUGUSTO

Mar del Plata

Encuentro; IX Encuentro de Biólogos en Red; 2014

Facultad de Ciencias Exactas y Naturales, UNMdP

In marine environments, consumers and nutrients change macroalgae assemblages, affecting the structure of natural communities. In sub-tropical intertidals, ephemeral macroalgae are absent from wave protected areas, probably because the reduced splash that increment the desiccation stress. Recently, it was proposed that macroalgae is also limited by the low nutrient delivery resulting for the reduced wave action. In this context, we hypothesized that increased nutrient supply in protected areas, would increase the growth of macroalgae, similarly to exposed sites, where spray is higher. Under this hypothesis, our objective was to assess nutrient effects, considering the top-down control on macroalgae by intertidal grazers (mainly limpets and periwinkles), in a rocky intertidal at São Sebastião Channel (São Paulo, Brazil; 23°41´S, 45°19´W). We used an experimental approach, including and excluding grazers, under natural (no-manipulation) and increased (added manually) nutrient concentrations, in the middle intertidal levels from low and high wave energy sites (protected and exposed). Results showed that at exposed sites, the biofilm and macroalgae grew towards the end of the experiment (ca. 45 days) with a marginally higher mean percent cover in absence of grazers, and with higher variability under nutrient addition. In protected sites, no nutrient effects were observed, macroalgae were almost absent and only the biofilm grew without grazers. Results suggest that grazers limit the biofilm and macroalgae in exposed and protected sites, and that consumer effects is affected by nutrient concentration in exposed, but not in protected sites. These results provide evidence about processes structuring these communities, allowing the development of alternative hypotheses, for example, about the ability of macroalgae to intake nutrients in different environmental conditions (i.e., more or less drying), where a possible explanation is that the effectiveness of macroalgae in capture nutrients is driven by a balance between the availability of nutrients and stress conditions.