Rates of net community carbon production (mmol C m(-2) h(-1)) were measured continuously in an algal-dominated reef flat community on the Kaneohe Bay barrier reef, Hawaii, for 12 days at the end of October 2006. The weather became increasingly cloudy during the last 5 days of measurements, resulting in a sevenfold decline in daily incident light (28-4 Ein m(-2) d(-1)). In response, gross primary production (P) for the reef flat community also decreased sevenfold, varying linearly with light (r (2) = 0.92, n = 12). Community respiration (R) decreased fivefold over this same period and was highly correlated with changes in P (r (2) = 0.84, n = 12). We reason that this short-term coherence between P and R indicates that most of the carbon fixed during this period was rapidly metabolized via plant respiration. We further conclude that the dominance of autotrophic respiration under general conditions of nutrient-limited growth can explain much of the balance between P and R that is commonly observed in shallow reef communities.