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Agua Hedionda Lagoon (AHL), a tidal estuary located on the southern California coast, supports a diverse ecosystem while serving numerous recreation activities, a marine fish hatchery, a shellfish hatchery, and the largest desalination plant in the western hemisphere. In this work, a 1-year time series of carbon dioxide data is used to establish baseline average dissolved inorganic carbon conditions in AHL. Based on a mass balance model of the outer basin of the lagoon, we propose that AHL is a source of inorganic carbon to the adjacent ocean, through advective export, at a rate of 5.9 × 106 mol C year−1, and a source of CO2 to the atmosphere of 0.21 × 106 mol C year−1 (1 mol C m−2 year−1), implying a net heterotrophic system on the order of 6.0 × 106 mol C year−1 (30 mol C m−2 year−1). Although variable with a range throughout the year of 80% about the mean, the ecosystem remained persistently heterotrophic, reaching peak rates during the summer season. Using results from the mass balance, the annual cycle of selected properties of the aqueous CO2 system (pH, pCO2, and CaCO3 saturation state) were mathematically decomposed in order to examine the relative contribution of drivers including advection, ecosystem metabolism, and temperature that act to balance their observed annual cycle. Important findings of this study include the identification of advection as a prime driver of biogeochemical variability and the establishment of a data-based estimate of mean flushing time for AHL. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Understanding bacterioplankton community dynamics in coastal hypoxic environments is relevant to global biogeochemistry because coastal hypoxia is increasing worldwide. The temporal dynamics of bacterioplankton communities were analysed throughout the illuminated water column of Devil's Hole, Bermuda during the 6-week annual transition from a strongly stratified water column with suboxic and high- pCO2 bottom waters to a fully mixed and ventilated state during 2008. A suite of culture-independent methods provided a quantitative spatiotemporal characterization of bacterioplankton community changes, including both direct counts and rRNA gene sequencing. During stratification, the surface waters were dominated by the SAR11 clade of A lphaproteobacteria and the cyanobacterium S ynechococcus. In the suboxic bottom waters, cells from the order C hlorobiales prevailed, with gene sequences indicating members of the genera C hlorobium and P rosthecochloris - anoxygenic photoautotrophs that utilize sulfide as a source of electrons for photosynthesis. Transitional zones of hypoxia also exhibited elevated levels of methane- and sulfur-oxidizing bacteria relative to the overlying waters. The abundance of both T haumarcheota and E uryarcheota were elevated in the suboxic bottom waters (> 109 cells l−1). Following convective mixing, the entire water column returned to a community typical of oxygenated waters, with E uryarcheota only averaging 5% of cells, and C hlorobiales and T haumarcheota absent. [ABSTRACT FROM AUTHOR]

Concerns have been raised about how coral reefs will be affected by ocean acidification, but projections of future seawater CO2 chemistry have focused solely on changes in the pH and aragonite saturation state (Ωa) of open-ocean surface seawater conditions surrounding coral reefs rather than the reef systems themselves. The seawater CO2 chemistry within heterogeneous reef systems can be significantly different from that of the open ocean depending on the residence time, community composition and the main biogeochemical processes occurring on the reef, that is, net ecosystem production (NEP = gross primary production − autotrophic and heterotrophic respiration) and net ecosystem calcification (NEC = gross calcification − gross CaCO3 dissolution), which combined act to modify seawater chemistry. On the basis of observations from the Bermuda coral reef, we show that a range of projected biogeochemical responses of coral reef communities to ocean acidification by the end of this century could partially offset changes in seawater pH and Ωa by an average of 12-24% and 15-31%, respectively. [ABSTRACT FROM AUTHOR]