Effects of unidirectional and oscillatory water flow on nitrogen fixation (acetylene reduction) in coral reef algal turfs, Kaneohe Bay, Hawaii

Abstract

Rates of acetylene reduction (nitrogenase activity) by algal turf communities from Kaneohe Bay, Oahu, Hawaii, were measured as a function of increasing water flow speeds under unidirectional and oscillatory flow regimes in an enclosed incubation chamber. Water flow speeds, shear stress, and turbulence intensities were measured with thermistor probes in the chamber and over the turfs in the field. The thickness of the boundary layer varied significantly and linearly with bulk water flow speeds in the field. Although the boundary layer in the chamber also decreased with increasing flow speeds, turbulence intensity and flow speeds in the chamber were mostly lower than those typically measured in the field. Rates of acetylene reduction were positively related to water flow speed. Oscillatory water flow, which increased turbulence intensity five times, resulted in a significant increase in acetylene reduction compared to unidirectional flow. Even at the lowest mean flow speeds measured in the field (<0.1 m s −1), mean rates of acetylene reduction (27 nmol ethylene cm −2 h −1±11 SD) were high under oscillatory flow. Equivalent high rates under unidirectional flow were not achieved until flow speed was more than doubled. The slope of log–log linear regressions of acetylene reduction versus flow speed was 0.5 for both oscillatory and unidirectional regimes. This result suggests that acetylene reduction rates in the chamber were controlled by mass transfer of a rate-controlling solute, such as acetylene or oxygen (inhibitory to nitrogenase), through a laminar diffusion boundary layer. Because coral reefs exist in areas of very low nitrogen availability, nitrogen fixation is fundamentally important for coral reef primary production and biogeochemistry. Yet, current understanding of nitrogen fixation on coral reefs has been derived primarily from measurements made under unnatural conditions of no or low water flow. This study lends support to the importance of water flow as a major control of the metabolism of organisms occupying coral reefs.