N budget as water quality management tool in closed aquatic mesocosms

Following a high rise in nitrate levels in St Lawrence mesocosm of the Mont real Biodome in 1995, inputs, outputs and changes in N pools were independe ntly measured to estimate the N budget of the system. The resulting budget had only an 8% difference between the inputs (CV = 11.1%) and the sum of th e outputs and changes in N pools (CV = 9.6%). N inputs amounted to 342 kg f or 1995; 58% was contributed by the food for fish and invertebrates and 35% by the seabird guano. Filtration and general cleaning removed 20% of the i nputs. The pool vacuum cleaner designed by the Biodome staff proved to be a n important addition to the system since it increased by 30% the efficacy o f the life support systems (LSS) at removing N, and removed detritus glued to surfaces or trapped in sediments that could not be removed by the LSS; 6 5% of N accumulated in mesocosm mainly as NO3 (92%), but also as fish and i nvertebrate biomass (8%). Assuming 21 mM as the maximum NO3 concentration f or survival of the animals on display, the N budget showed that, without in tegrated dissimilation or assimilation processes, marine aquaria similar in design and operation to the SLM must be opened after 5-7 years of operatio n unless N inputs can be reduced or N outputs increased. As suggested by th e budget, the removal of one seabird species (gannets) and an increase in p artial water changes (from 9 to 16% per year) in 1996 have, since then, sta bilized the level of NO3 to 12-13 mM. The N budget was effective in identif ying the role of each component in the system, suggesting changes in the op erating practices, and predicting the impact of these changes. It is a usef ul water quality management tool for artificial environments, such as close d aquatic mesocosms. (C) 2000 Elsevier Science Ltd. All rights reserved.