Effects of nonlocal turbulence on the mass transfer of dissolved species to reef corals

Falter, James L., Marlin J. Atkinson, Ryan J. Lowe, Stephen G. Monismith, Jeffrey R. Koseff

Limnol. Oceanogr., 52(1), 2007, 274-285 | DOI: 10.4319/lo.2007.52.1.0274

ABSTRACT: We examined the importance of nonlocal turbulence to the mass transfer of dissolved species from both cylinders and models of reef coral. Solid gypsum cylinders and gypsum-coated model coral were dissolved in an outdoor flume at mean flow speeds of 4.5 and 10 cm s-1. Three model corals were used: two branched forms of the genera Acropora and Pocillopora, and one lobate form of the genus Platygyra. Turbulence intensities (Tu) were measured as the ratio of the standard deviation of the time-variant flow to the mean. Tu of between 5% and 55% were created independently of the mean flow by pumping water through two vertical arrays of jets. Mass transfer coefficients increased by 10-70% with turbulence intensity for all forms at both mean flow speeds considered (p < 0.05); however, mass transfer coefficients for the branched coral were not as sensitive to changes in turbulence intensity as for the lobate coral. Results for the cylinders were consistent with the engineering literature and indicate that the effects of turbulence intensity on mass transfer become weaker when branch size or flow speed (or both) become very small. Turbulence intensities measured around experimental coral communities and above naturally occurring reef communities typically vary between 15% and 35%. Assuming an average turbulence intensity of around 25%, then ignoring the effects of nonlocal turbulence altogether would result in uncertainties in mass transfer coefficients of at most ±10% for the lobate coral form and ±5% for the branched coral forms. Measuring external turbulence in bulk flows is not necessary for most evaluations of biogeochemical rates in benthic reef systems.

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