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Since the start-up of the St. Lawrence Mesocosm (SLM) at the Montreal Biodome in 1992, phosphorus has accumulated slowly, reaching about 18 mg P l −1 in 2000. It was decided that this concentration should be lowered to about 2 mg P l −1 to maintain a safe nitrogen : phosporus (N : P) ratio of about 10. Before deciding what type of treatment to use for the removal of phosphorus, a P budget was estimated for 1998 in order to evaluate the different pathways of phosphorus in the mesocosm. The resulting budget had only a 1% difference between the inputs ( CV =12.9%) and the sum of the outputs and changes in P pools ( CV =12.5%). P inputs amounted to 40.5 kg for 1998: food for fish and invertebrates contributed 76% of the inputs while seabird guano contributed 20%. Filtration and general cleaning removed 51% of the inputs while water losses removed 22%. The slight but constant difference between total phosphorus and dissolved reactive phosphorus (DRP) in 1998 and previous years led us to believe that only DRP (mostly orthophosphate) accumulated in the system. The accumulation of DRP was 10% of the inputs in 1998. The budget showed that the importance of water losses is relative and depends on the DRP concentration in the SLM. Furthermore, it was possible to compare this P budget with an N budget of the SLM prepared in 1995. The comparison helped us understand why nitrate in closed-circuit mesocosms are characterized by a high, never-ending accumulation while DRP is characterized by a net increase in the first few years after start-up followed by a very small increase in the following years. Considering its low CV , this P budget was considered a useful water quality management tool in designing a P removal unit for the SLM. This budget may also serve as a guideline for managers of closed-circuit systems such as marine aquariums and aquacultures as well as for designers of P removal units.