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.

This is the second paper of a series that reports on our investigation of the clustering properties of AGNs in the ROSAT All-Sky Survey (RASS) through cross-correlation functions (CCFs) with Sloan Digital Sky Survey (SDSS) galaxies. In this paper, we apply the Halo Occupation Distribution (HOD) model to the CCFs between the RASS Broad-line AGNs with SDSS Luminous Red Galaxies (LRGs) in the redshift range 0.16 < z < 0.36 that was calculated in paper I. In our HOD modeling approach, we use the known HOD of LRGs and constrain the HOD of the AGNs by a model fit to the CCF. For the first time, we are able to go beyond quoting merely a 'typical' AGN host halo mass, M h , and model the full distribution function of AGN host dark matter halos. In addition, we are able to determine the large-scale bias and the mean M h more accurately. We explore the behavior of three simple HOD models. Our first model (Model A) is a truncated power-law HOD model in which all AGNs are satellites. With this model, we find an upper limit to the slope (α) of the AGN HOD that is far below unity. The other two models have a central component, which has a step function form, where the HOD is constant above a minimum mass, without (Model B) or with (Model C) an upper mass cutoff, in addition to the truncated power-law satellite component, similar to the HOD that is found for galaxies. In these two models we find that the upper limits on α are still below unity, with α 0.95 and α 0.84 for Model B and C respectively. Our analysis suggests that the satellite AGN occupation increases slower than, or may even decrease with, M h in contrast to the satellite HODs of luminosity-threshold samples of galaxies, which, in contrast, grow approximately as N s ∝ M α h with α ≈ 1. These results are consistent with observations that the AGN fraction in groups and clusters decreases with richness.

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This is the fourth paper in a series that reports on our investigation of the clustering properties of active galactic nuclei (AGNs) identified in the ROSAT All-Sky Survey and Sloan Digital Sky Survey (SDSS). In this paper we investigate the cause of the X-ray luminosity dependence of the clustering of broad-line, luminous AGNs at z 0.16 0.36. <