A major component of the phosphorus cycle in inland waters is its interaction with calcium carbonate (CaCO3). Yet, climate change, hydrology, eutrophication, and even varying terminology forCaCO3-producing ecosystems (e.g., chalk, karst, travertine) highlight the challenges in understanding this part of the phosphorus cycle. In this talk, I explore howCaCO3 formation and dissolution differs across lakes, rivers, streams, and wetlands, and the implications for ecosystem processes. For example, both biotic and abiotic processes influenceCaCO3 formation or dissolution. So, if aerobic photosynthesis promotesCaCO3 precipitation, which in turn sequesters phosphorus, does this present a challenge or opportunity for those primary producers? This review explores this question and others to better understand the dynamic role ofCaCO3 in the phosphorus cycle and stimulate further research into this coupled biogeochemical cycle.