Worldwide, coral reef ecosystems are experiencing increasing pressure from a variety of anthropogenic perturbations including ocean warming and acidification, increased sedimentation, eutrophication, and overfishing, which could shift reefs to a condition of net calcium carbonate (CaCO3) dissolution and erosion. Herein, we determine the net calcification potential and the relative balance of net organic carbon metabolism (net community production; NCP) and net inorganic carbon metabolism (net community calcification; NCC) within 23 coral reef locations across the globe. In light of these results, we consider the suitability of using these two metrics developed from total alkalinity (TA) and dissolved inorganic carbon (DIC) measurements collected on different spatiotemporal scales to monitor coral reef biogeochemistry under anthropogenic change. All reefs in this study were net calcifying for the majority of observations as inferred from alkalinity depletion relative to offshore, although occasional observations of net dissolution occurred at most locations. However, reefs with lower net calcification potential (i.e., lower TA depletion) could shift towards net dissolution sooner than reefs with a higher potential. The percent influence of organic carbon fluxes on total changes in dissolved inorganic carbon (DIC) (i.e., NCP compared to the sum of NCP and NCC) ranged from 32% to 88% and reflected inherent biogeochemical differences between reefs. Reefs with the largest relative percentage of NCP experienced the largest variability in seawater pH for a given change in DIC, which is directly related to the reefs ability to elevate or suppress local pH relative to the open ocean. This work highlights the value of measuring coral reef carbonate chemistry when evaluating their susceptibility to ongoing global environmental change and offers a baseline from which to guide future conservation efforts aimed at preserving these valuable ecosystems.

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Two coral reef sponges were examined in situ off Puerto Rico for fluxes of dissolved inorganic nitrogen. Chondrilla nucula, a ubiquitous Caribbean sponge with cyanobacterial symbionts, releases large amounts of nitrate [600 nmol N g I (dry wt) h-l; 4,000 Mmol N me2 h-l]. Since C. nucula covers a mean of 12% of the substratum, it potentially contributes between 50 and 120% of the nitrogen required to sustain reef productivity. Anthosigrnella varians, a common sponge with zooxanthellae symbionts releases lesser amounts of nitrate [ 19 nmol N g-' (dry wt) h--l; 20 pmol N m-2 h -I]. Alth ough A. varians may cover between 1.2 and 11% of the reef substratum, the potential contribution is < 1% of the nitrogen requiredi For reef productivity. These release rates for C. nucula are comparable to those reported for nitrogen fixation in reef environments and in excess of rates reported for nitrate production by other reef substrata. Large populations of nonphotosynthetic symbiotic bacteria are considered to nitrify amino nitrogen to nitrate and the differences are attributed to the size of bacterial populations and the nature of photosynthetic symbionts.

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