Calcium carbonate deposition drives nutrient cycling in a calcareous headwater stream

Calcium carbonate (Ca CO 3 ) deposition is common in aquatic ecosystems and may reduce phosphorus availability via coprecipitation of phosphate, an impact with broad implications for ecosystem processes. To determine if Ca CO 3 deposition in streams increases phosphorus (P) retention in minerals while reducing P availability to organisms, we studied paired streams (with and without active Ca CO 3 deposition) subjected to experimental shading and monitored changes in ecosystem attributes (e.g., periphyton biomass content, nutrient spiraling, periphyton nutrient limitation, and leaf litter decomposition). Shading reduced rates of Ca CO 3 deposition by over 50%, suggesting that a substantial proportion of Ca CO 3 deposition is supported by photosynthetically induced changes in alkalinity. Shading‐induced reductions in Ca CO 3 deposition led to increases in epilithon biomass P content ( P  <   0.05) and periphyton growth ( F 2,12  = 5.79, P  <   0.05). Reductions in Ca CO 3 deposition also relieved P limitation of periphyton growth ( F 3,16  = 59.32, P  <   0.001), extended P uptake lengths at least an order of magnitude, and reduced both P mass transfer velocity and areal uptake rates by over 80% ( F 2,3  = 22.62, P  <   0.05 and F 2,3  = 13.19, P  <   0.05, respectively). Finally, while shading caused reductions in leaf litter decomposition in the non‐Ca CO 3 depositing stream ( F 5,7  = 22.45, P  <   0.001), shading had no effect on leaf litter decomposition in the stream with active Ca CO 3 deposition. These results indicate that Ca CO 3 deposition can regulate P bioavailability and retention in streams and may drive streams to be P limited, as has been suggested in lake and wetland ecosystems.