Stoichiometric impact of calcium carbonate deposition on nitrogen and phosphorus supplies in three montane streams

The absolute concentrations of nitrogen (N) and phosphorus (P) and their relative availabilities (N:P stoichiometry) can influence numerous ecological processes. In streams, N:P stoichiometry is influenced by different hydrologic and biogeochemical processes that also affect the downstream transport of these nutrients to receiving waters. Calcium carbonate (CaCO3) deposition, a widespread geochemical process in alkaline streams and other aquatic ecosystems, can lower phosphate concentrations and, potentially, decrease P availability relative to N availability. We evaluated the effects of CaCO3 deposition on stream water stoichiometry using a 3-year dataset of stream physicochemistry and several metrics of CaCO3 deposition across three streams in the Huachuca Mountains of southern Arizona, USA. CaCO3 deposition rates varied across and within streams, with benthic coverage of travertine as high as 70 % and deposition rates up to 8.3 μg Ca2+ L−1 m−1. Redundancy analysis revealed a strong, negative correlation between stream water phosphate concentrations and CaCO3 deposition rates, a relationship that also extended to total P concentrations, and a strong, positive correlation between inorganic N concentrations and CaCO3 deposition rates. Furthermore, we found a significant positive relationship between CaCO3 deposition rates and N:P ratios. These results support the role of coprecipitation of phosphate with CaCO3 deposition in reducing P supply. They also suggest that reduced concentrations of P in the water column may reduce biological N uptake, amplifying the stoichiometric signal of CaCO3 deposition.