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We provide evidence from field experiments that within-coral colony variation in light- limited electron transport rates through Photosystem II is positively correlated with water flow over the coral surface. We made in situ paired measurements of water flow and photosynthetic electron transport rates (rETR) over the upstream and downstream faces of several coral colonies at Conch Reef, Key Largo, Florida, USA. Using pulse amplitude modulated (PAM) fluorometry, we estimated rETR at increasing levels of photosynthetically active radiation (PAR). The rETR versus PAR relation- ship was fitted to a negative exponential curve parameterized by the initial slope (α) and the asymp- totic maximum (rETRmax). We showed that flow speeds measured at 3 mm above the upstream faces of multiple branching colonies of Porites porites and hemispherical colonies of Siderastrea siderea were more than double those over the downstream faces. In both P. porites and S. siderea, these higher upstream flow speeds were associated with α values that were 4 and 12% higher, respec- tively, than those over the downstream faces. In contrast, there were no differences in flow speed, or in α between the upstream and downstream faces of plate-like colonies of Montastrea annularis. Ancillary measurements of oxygen concentrations over intact coral colonies in the field demonstrated that oxygen concentrations within 2 mm of the coral surface were between 2 to 110% higher than ambient (measured >1 m from the coral). Moreover, paired measurements over a single colony of S. siderea showed steeper velocity gradients over the upstream face relative to the downstream face, accompanied by lower oxygen concentrations, a thinner diffusion boundary layer, and higher α. Our results support the hypothesis that within-colony variation in photosynthesis by corals is related to water flow, and suggests that oxygen flux from the coral may be important.

Intracolony variability in photosynthesis by corals is affected by water flow: role of oxygen flux