Effect of riparian invasion by the crack willow (Salix fragilis) on ecosystem metabolism in North Patagonian streams

In Argentine Patagonia, the crack willow (Salix fragilis) is a woody invader, which develops in riparian zones of streams. Information about the effect of these trees on the stream functions and whether they can modify the streams' metabolism (i.e. production and respiration) is scarce. We compared the metabolism of streams invaded and non-invaded by S. fragilis. To this end, we used the open-channel two-station method to estimate whole ecosystem production and respiration in selected stream reaches and the in-stream metabolism-chamber method to estimate the production and respiration of single stream communities. We also determined geomorphologic, hydraulic, and biological characteristics at invaded and non-invaded riparian reaches. Stream reaches had low gross production (0.027- 0.398 g O-2 m(-2) day-1) and respiration values (0.020 - 0.409 g O-2 m(-2) day-1). The presence of S. fragilis could not exclusively explain differences in whole stream metabolism. However, willows promoted intense habitat alteration such as a reduction of macrophyte patches and abundant accumulations of woody debris. The latter led, in part, to a reduction in primary production and an increase in community respiration, associated with live and dead willow components (i.e. in-stream exposed roots, retained logs, twigs and leaf litter). In non invaded reaches, the macrophyte was the community compartment with the highest production and respiration. In contrast, in S. fragilis-invaded reaches where macrophytes were highly reduced, epilithic and sand biofilms drove both ecosystem processes.