[1] An investigation was conducted to determine the effects of elevated pCO 2 on the net production and calcification of an assemblage of corals maintained under near-natural conditions of temperature, light, nutrient, and flow. Experiments were performed in summer and winter to explore possible interactions between seasonal change in temperature and irradiance and the effect of elevated pCO 2 . Particular attention was paid to interactions between net production and calcification because these two processes are thought to compete for the same internal supply of dissolved inorganic carbon (DIC). A nutrient enrichment experiment was performed because it has been shown to induce a competitive interaction between photosynthesis and calcification that may serve as an analog to the effect of elevated pCO 2 . Net carbon production, NP C , increased with increased pCO 2 at the rate of 3 ± 2% (mmol CO 2 aq kg À1 ) À1 . Seasonal change of the slope NP C -[CO 2 aq] relationship was not significant. Calcification (G) was strongly related to the aragonite saturation state W a . Seasonal change of the G-W a relationship was not significant. The first-order saturation state model gave a good fit to the pooled summer and winter data: G = (8 ± 1 mmol CaCO 3 m À2 h À1 )(W a À 1), r 2 = 0.87, P = 0.0001. Both nutrient and CO 2 enrichment resulted in an increase in NP C and a decrease in G, giving support to the hypothesis that the cellular mechanism underlying the decrease in calcification in response to increased pCO 2 could be competition between photosynthesis and calcification for a limited supply of DIC.Citation: Langdon, C., and M. J. Atkinson (2005), Effect of elevated pCO 2 on photosynthesis and calcification of corals and interactions with seasonal change in temperature/irradiance and nutrient enrichment,

The median C:N:P atomic ratio of benthic marine macroalgae and seagrasses is about 550:30:1. Benthic plants are much more depleted in P and less in N, relative to C, than phytoplankton. The amount of nutrients required to support a particular level of net production is much lower for benthic marine plants than it is for phytoplankton.

Abstract. The concentration of CO2 in the atmosphere is projected to reach twice the preindustrial level by the middle of the 21 st century. This increase will reduce the 2 concentration of CO3-of the surface ocean by 30% relative to the preindustrial level and will reduce the calcium carbonate saturation state of the surface ocean by an equal percentage.Using the large 2650 m 3 coral reef mesocosm at the BIOSPHERE-2 facility near Tucson, Arizona, we investigated the effect of the projected changes in seawater carbonate chemistry on the calcificafion of coral reef organisms at the commtmity scale. Our experimental design was to obtain a long (3.8 years) time'series of the net calcificafion of the complete system and all relevant physical and chemical variables (tem•rature, salinity, light, nutrients, Ca 2+, pCO2, TCO2, and total alkalinity). Periodic additions of NaHCOz, Na2CO•, and/or CaC12 were made to change the calcium carbonate saturation state of the water. We found that there were consistent and reproducible changes in the rate of calcificafion in response to our manipulations of the saturation state. We show that the net community calcificafion rate suggests that saturation state or a closely related quantity is a primary environmental factor that influences calcffication on coral reefs at the ecosystem level. We compare the sensitivity of cal½ification to short-term (days) and long-term (months to years) changes in saturation state and found that the response was not significantly different. This indicates that coral reef organisms do not seem to be able to acclimate to changing saturation state. The predicted decrease in coral reef calcification •een the years 1880 and 2065 A.D. based on our longterm results is 40%. Previous small-scale, short-term organismal studies predicted a calcification reduction of 14-30%. This much longer, community-scale study suggests that the impact on coral reefs may be greater than previously suspected. in the next century coral reefs will be less able to cope with rising sea level and other anthropogenic stresses.

A 2 week field experiment was conducted to measure surface wave dissipation on a barrier reef at Kaneohe Bay, Oahu, Hawaii. Wave heights and velocities were measured at several locations on the fore reef and the reef flat, which were used to estimate rates of dissipation by wave breaking and bottom friction. Dissipation on the reef flat was found to be dominated by friction at rates that are significantly larger than those typically observed at sandy beach sites. This is attributed to the rough surface generated by the reef organisms, which makes the reef highly efficient at dissipating energy by bottom friction. Results were compared to a spectral wave friction model, which showed that the variation in frictional dissipation among the different frequency components could be described using a single hydraulic roughness length scale. Surveys of the bottom roughness conducted on the reef flat showed that this hydraulic roughness length was comparable to the physical roughness measured at this site. On the fore reef, dissipation was due to the combined effect of frictional dissipation and wave breaking. However, in this region the magnitude of dissipation by bottom friction was comparable to wave breaking, despite the existence of a well‐defined surf zone there. Under typical wave conditions the bulk of the total wave energy incident on Kaneohe Bay is dissipated by bottom friction, not wave breaking, as is often assumed for sandy beach sites and other coral reefs.

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