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

Abstract

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 54–126×10^–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.