N budget as water quality management tool in closed aquatic mesocosms

Following a high rise in nitrate levels in St Lawrence mesocosm of the Montreal Biodome in 1995, inputs, outputs and changes in N pools were independently 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 the outputs and changes in N pools (CV=9.6%). N inputs amounted to 342 kg for 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 inputs. The pool vacuum cleaner designed by the Biodome staff proved to be an important addition to the system since it increased by 30% the efficacy of 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; 65% of N accumulated in mesocosm mainly as NO3 (92%), but also as fish and invertebrate biomass (8%). Assuming 21 mM as the maximum NO3 concentration for survival of the animals on display, the N budget showed that, without integrated dissimilation or assimilation processes, marine aquaria similar in design and operation to the SLM must be opened after 5–7 years of operation unless N inputs can be reduced or N outputs increased. As suggested by the budget, the removal of one seabird species (gannets) and an increase in partial water changes (from 9 to 16% per year) in 1996 have, since then, stabilized the level of NO3 to 12–13 mM. The N budget was effective in identifying the role of each component in the system, suggesting changes in the operating practices, and predicting the impact of these changes. It is a useful water quality management tool for artificial environments, such as closed aquatic mesocosms.