Utechnical Comments

Hajime Kayanne et al. (1) investigate diurnal changes in CO2 partial pressure (PC02) and community metabolism on Shiraho reef (Ishigaki Island, Japan) and conclude that "reefs might serve as a sink, not a source, for atmospheric CO2" because of high net organic production relative to net calcification (Table 1). This conclusion is at odds with recent reports (2, 3, 4) and data (5) suggesting that reefs are net sources of atmospheric C02, albeit of limited importance in the current carbon budget. Furthermore, it is well established that the value of gross primary production (Pg) is close to that of respiration (R) in most coral reef flats, and that the net production (Pn) is around zero (6). One would expect some reefs to depart from this general trend, but consideration of currents and community distributions suggests that the data in the report by Kayanne et al. (1) represent neither the whole Shiraho reef nor all coral reefs. The reef flat at Shiraho comprises five different benthic communities and is under strong tidal influence (1, 7). Because Kayanne et al. measured PCO2at a single station over several days, the water masses at hand had different origins and crossed different communities depending on the direction of the current. Therefore, the observed changes in PCO2 likely did not result from the metabolic activity of the entire reef flat but, rather, from a variable combination of the communities located around the study site. Also, Kayanne et al. calculated mean daytime and nighttime reef water Pco2 with the use of a predictive regression equation