Periphyton and grazer stoichiometry along a light gradient in a mountain stream

DÍAZ VILLANUEVA, VERÓNICA; E. BALSEIRO

Conferencia; Gordon Research Conference: Metabolic Basis of Ecology; 2012

Headwater forest streams are light limited, so autotrophy is very low. Downstream, as the channel widens and canopy covers decrease, autotrophy increase as a consequence of more light availability. This would also lead to an increase in periphitic C:nutrient ratios. If stream grazers were strict homeostatic, changes in periphyton stoichiometry would lead to metabolic mechanisms to compensate the excess of C. However, some evidence shows that grazers can track changes in nutrient content of food sources. To determine if larvae of the species Meridialaris chiloeensis (Ephemeroptera) are homeostatic with regards changes in periphyton nutrient content along a light gradient of a stream continuous, we performed periphyton and larvae sampling at four sites along a mountain catchment with a gradient from upstream forest sites (S1 and S2), to downstream open canopy sites (S3 and S4). We sampled water, periphyton from stones, and larvae to measure carbon and nutrient (N and P) contents. Sampling was carried out in May (autumn). We found a downstream gradient in water from higher to lower total dissolved P but not in total dissolved N. Periphyton C to chlorophyll ratio was higher downstream. However, against our predictions, C:N did not significantly vary among reaches while C:P and N:P ratio were lowest downstream. On the contrary, all larvae ratios varied along the gradient. While larval C:N was positively related to periphytic C:N, larval C:P and N:P increased downstream, because of extremely low %P in larvae (0.24%). These results may indicate that not all periphyton is an available food. The presence of Nostoc colonies in the downstream open sites could be a not available food for larvae, which become highly P-limited. This sampling would be carried out again in spring, where changes in periphyton biomass and taxonomy could lead to changes in elemental composition of larvae and reduce P-limitations. Experiments with larvae from the different sites fed on periphyton from different origins will also be necessary to elucidate the metabolisms of this abundant grazer in Andean Patagonian streams.