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.