Organic matter dynamics in headwater forest streams of Northern Patagonia, Argentina

ALBARIÑO, R.J.; BURIA, L.M.; DÍAZ VILLANUEVA, V.

Valdivia, Chile

Workshop; Final international workshop in the framework of 10 years bilateral cooperation between Ghent University (Belgium) and Universidad Austral de Chile (Chile); 2008

Universidad Austral, Valdivia Chile

Streams are open and unidirectional systems where matter exchange with the surrounding landscape and exportation processes are important. Therefore, any mechanism delaying transportation rates will enhance biotic processes increasing in situ matter mineralization. Organic matter (OM) balances are defined by matter inputs and outputs within a system reach. Additionally, biotic components of the system are responsible for a major fraction of OM breakdown and nutrient cycling determining the transference of matter and energy up in the food web. The headwaters of large Patagonian rivers in Argentina discharge chemically diluted waters from forested Nothofagus catchments (pH circumneutral, conductivity ~50 µS cm-1, SRP ~2 µg L-1, NO3 ~15 µg L-1). Those small streams run through deciduous or mixed primary forests. The bulk of leaf litter enters in late summer-autumn (60-120 g m-2 mo-1) and is highly characterized by species with intermediate to slow decaying rates (k<0.01, t50%= 90-180 days). Spates occurring in late autumn (rainy season) serve to redistribute leaf litter along headwater extensions (annual OM standing stock in headwaters: 9 g m-2, min-max: 1-40 g m-2) but they also remove and transport a substantial amount of OM subsidizing the biota and ecological processes along downstream reaches. As in many other headwaters of the world, benthic invertebrates represent the dominant fraction of consumers. Aquatic insects, mostly belonging to dipteran, plecopteran, ephemeropteran and trichopteran species, dominate the assemblage. Many representatives are endemic at species to family level. Functional feeding groups are dominated by detritivores, mainly surface-collectors in terms of abundance, but co-dominating with shredders, in terms of biomass. Although primary producers are constrained to shade-tolerant species, they may support a substantial fraction of grazers. A representative area of the major basins of the Atlantic slopes in Patagonia is under the protection of the Administration of National Parks, Argentina. In this regard, water and catchment issues that may threat biodiversity and ecological integrity of those fluvial systems are still sparse and mostly associated to human settlements (i.e. urbanization, forestry, water subtraction, crops and cattle raising, tourism). Perhaps the most widespread threat is the ongoing dispersion of the invasive brown and rainbow trout. This conservation issue conflicts with the importance of such fishery having recreational and economical benefits. Waterfalls acting as natural physical barriers prevent the upstream settlement of trout and, in doing so, allow pristine functional and structural conditions to occur as in pre-trout introduction times.