Mass transfer limitation of photosynthesis of coral reef algal turfs |
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Authors: | Robert C Carpenter Susan L Williams |
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Institution: | (1) Department of Biology, California State University, Northridge, CA 91330-8303, USA;(2) Department of Biology, San Diego State University, San Diego, CA 92182-0057, USA;(3) Present address: Bodega Marine Laboratory, P.O. Box 247, Bodega Bay, CA 94923-0247, USA |
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Abstract: | Algal turfs are the major primary producing component on many coral reefs and this production supports higher levels in the
complex reef trophic web. Rates of metabolism of algal turfs are related positively to water motion, consistent with limitation
by the diffusion of a substance through a boundary layer. Based on engineering mass transfer theory, we hypothesized that
photosynthesis of algal turfs is controlled by rates of mass transfer and responses of photosynthesis to increasing flow speed
should be predicted by engineering correlations. This hypothesis was tested in ten experiments where photosynthesis was estimated
in a flume/respirometer from changes in dissolved oxygen at eight flow speeds between 0.08 and 0.52 m/s. Flow in the flume
and over the reef at Kaneohe Bay, Oahu, Hawaii was estimated using hot-film thermistor and electromagnetic current meters.
Rates of photosynthesis were related positively to flow in all experiments and plots of the log of the average Sherwood number
(Sh
meas) versus log Reynolds number (Re
D) for each experiment are lower than predicted for mass transfer through a turbulent boundary layer. Algal turf-covered plates
are characterized as hydrodynamically transitional to fully rough surfaces and the lower than predicted slopes suggest that
roughness reduces rates of mass transfer. A negative correlation between algal turf biomass and slopes of the log Sh
meas−log Re
D plots suggests that mass transfer to algal turfs is affected significantly by the physical structure of the algal community.
Patterns of photosynthesis based on changes in dissolved oxygen and dissolved inorganic carbon concentrations (DIC) indicate
that the flow speed effect is not the result of increased flux of oxygen from the algal turfs, and combined with the short
response time to flow speed, suggest that DIC may limit rates of photosynthesis. Although there are differences between flow
in the flume and flow over algal turfs on the reef, these results suggest that photosynthesis is controlled, at least in part,
by mass transfer. The chemical engineering approach provides a framework to pose further testable hypotheses about how algal
canopy height, flow oscillation, turbulence, and substratum roughness may modulate rates of metabolism of coral reef algal
turfs. |
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Keywords: | |
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