Calcium carbonate deposition drives nutrient cycling in a calcareous headwater stream

Calcium carbonate (CaCO3) 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 CaCO3 deposition in streams increases phosphorus (P) retention in minerals while reducing P availability to organisms, we studied paired streams (with and without active CaCO3 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 CaCO3 deposition by over 50%, suggesting that a substantial proportion of CaCO3 deposition is supported by photosynthetically induced changes in alkalinity. Shading-induced reductions in CaCO3 deposition led to increases in epilithon biomass P content (P < 0.05) and periphyton growth (F-2,F-12 = 5.79, P < 0.05). Reductions in CaCO3 deposition also relieved P limitation of periphyton growth (F-3,F-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,F-3 = 22.62, P < 0.05 and F-2,F-3 = 13.19, P < 0.05, respectively). Finally, while shading caused reductions in leaf litter decomposition in the non-CaCO3 depositing stream (F-5,F-7 = 22.45, P < 0.001), shading had no effect on leaf litter decomposition in the stream with active CaCO3 deposition. These results indicate that CaCO3 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.