Enduring physiological and reproductive benefits of enhanced flow for a stony coral

We experimentally examined the in situ, long-term effects of flow on the coral Pocillopora verrucosa over a 2-yr period. Eighteen coral nubbins were positioned on stages protected from the ambient flow with transparent walls. Six of the corals were exposed to ‘‘enhanced flow’’ with underwater pumps (15–20 cm s21), while another other six were grown under ‘‘reduced-flow’’ conditions (, 1 cm s21), and a further six nubbins were set up as a control treatment exposed to ambient natural flow conditions on stages without walls. Mortality was 50% in the reduced- flow experiment. Compared with the reduced-flow and the control treatments, colonies growing in enhanced-flow conditions developed a more compact morphology and had significantly higher chlorophyll and protein concentrations, a higher density of zooxanthellae, and, most importantly, higher reproductive output. However, corals grown in reduced-flow conditions had lower skeleton density compared with those grown under ambient flow or enhanced flow. Flow rate had no measurable influence on average pH at the site of calcification as monitored by skeletal boron isotopic composition (d11B). Hence, the skeletal d11B–pH proxy in this coral appears to be independent of flow and the biological attributes that are modulated by flow, such as zooxanthellae density, chlorophyll concentrations, and rates of photosynthesis, respiration, and calcification. Calcification site pH in all colonies was raised by +0.3 relative to ambient seawater pH. Together with light, flow should be considered a key abiotic factor determining core biological characteristics of P. verrucosa.