Quantifying seasonal variations in water source and nutrient concentrations: a catchment comparison in Valles Caldera National Preserve, NM, USA

Abstract

The purpose of this study is to examine the interactions between physical and biological processes, and their influence on nutrient cycling and export in two semiarid, montane, headwater catchments. We measured stream chemistry in two neighboring catchments within the Valles Caldera National Preserve, New Mexico from February through August 2005 to identify (1) how variable water sources and flowpaths affect carbon and nutrient concentrations, and (2) how these solutes were modified as they were transported out of the catchments. Both catchments were characterized by a large snowmelt flush of carbon and nutrients in spring and a smaller flush of carbon and nutrients during the monsoon season. Although similar in elevation, soil, vegetation, and climate, the catchments exhibited significantly differences in stream water C, N, and P concentrations during the spring flush. End member mixing using conservative solutes identified the cause of this variability was due primarily to differences in hydro logic residence time and streamflow generation between catchments. These mixing models for each catchment indicated that variability in carbon and nutrients was explained by physical transport during the spring snowmelt and the first flushing events of the monsoon season. In contrast, conservative mixing did a poor job of predicting carbon and nutrient chemistry during other season suggesting biological modification during transport was a major control on streamwater chemistry. After correcting for variability in water sources, both catchments exhibited higher than expected N concentrations during winter and snowmelt, switching to higher than expected P concentrations during the summer monsoon season suggesting a seasonal switch in limiting nutrients. These data demonstrate how simple, quantitative evaluation of hydrologic flowpaths and residence time can be used to separate physical and biological controls on catchment-scale stream water chemistry.