Nutrient dynamics in the Biosphere 2 coral reef mesocosm:: water velocity controls NH4 and PO4 uptake

The Biosphere 2 coral reef biome is a large, fully enclosed, self-sustaining mesocosm. Water is moved throughout the mesocosm by waves. Inorganic and organic nutrients were monitored weekly from 1995 to 2000. Eight nutrient-uptake experiments were conducted to measure uptake-rate constants (S, m s(-1)) for NH4, PO4, and NO3. Nutrient concentrations were low, except for DON, and typical of coral-reef ecosystems (means: NH4 = 0.63, NO3 = 0.62, PO4 = 0.05, SiO3 = 9.5, DON = 41.2, DOP = 0.26 mmol m(-3)). Nutrient uptake-rate constants varied in the range 54-126 x 10(-6) m s(-1) (4.6-11 m day(-1)) and correlated with water velocity. These rates, however, are 2-3-fold higher than rates for equivalent water velocities in steady, non-wave flows. Nutrients are recycled within the biome at rates sufficient to support gross production and, even in this recycling system, nutrient-uptake rates are mass transfer-limited.