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Pickett, M, Andersson AJ.  2015.  Dissolution rates of biogenic carbonates in natural seawater at different pCO2 conditions: a laboratory study. Aquatic Geochemistry.   10.1007/s10498-015-9261-3   Abstract

The bulk dissolution rates of six biogenic carbonates (goose barnacle, benthic foraminifera, bryozoan, sea urchin, and two types of coralline algae) and a sample of mixed sediment from the Bermuda carbonate platform were measured in natural seawater at pCO2 values ranging from approximately 3000 to 5500 μatm. This range of pCO2 values encompassed values regularly observed in porewaters at a depth of a few cm in carbonate sediments at shallow water depths (<15 m) on the Bermuda carbonate platform. The biogenic carbonates included calcites of varying Mg content (2–17 mol%) and a range of specific surface areas (0.01–2.7 m2 g−1) as determined by BET gas adsorption. Measured rates of dissolution increased with increasing pCO2 treatment for all substrates and ranged from 2.5 to 18 μmol g−1 h−1. The highest rates of dissolution were observed for the bryozoans and the lowest rates for the goose barnacles. The relative ranking in dissolution rates between different substrates was consistent at all pCO2 levels, indicating that substrates dissolve sequentially and that some substrates will be more vulnerable than others to rising CO2 and ocean acidification. Furthermore, dissolution rates were found to increase with increasing Mg content, though the relative dissolution rates were observed to be a function of both Mg content and microstructure (surface area).

Paulsen, ML, Andersson AJ, Aluwihare L, Cyronak T, D'Angelo S, Davidson C, Elwany H, Giddings SN, Page HN, Porrachia M, Schroeter S.  2018.  Temporal changes in seawater carbonate chemistry and carbon export from a Southern California estuary. Estuaries and Coasts. 41:1050-1068.   10.1007/s12237-017-0345-8   AbstractWebsite

Estuaries are important subcomponents of the coastal ocean, but knowledge about the temporal and spatial variability of their carbonate chemistry, as well as their contribution to coastal and global carbon fluxes, are limited. In the present study, we measured the temporal and spatial variability of biogeochemical parameters in a saltmarsh estuary in Southern California, the San Dieguito Lagoon (SDL). We also estimated the flux of dissolved inorganic carbon (DIC) and total organic carbon (TOC) to the adjacent coastal ocean over diel and seasonal timescales. The combined net flux of DIC and TOC (FDIC + TOC) to the ocean during outgoing tides ranged from - 1.8 +/- 0.5 x 10(3) to 9.5 +/- 0.7 x 10(3) mol C h(-1) during baseline conditions. Based on these fluxes, a rough estimate of the net annual export of DIC and TOC totaled 10 +/- 4 x 10(6) mol C year(-1). Following a major rain event (36 mm rain in 3 days), FDIC + TOC increased and reached values as high as 29.0 +/- 0.7 x 10(3) mol C h(-1). Assuming a hypothetical scenario of three similar storm events in a year, our annual net flux estimate more than doubled to 25 +/- 4 x 10(6) mol C year(-1). These findings highlight the importance of assessing coastal carbon fluxes on different timescales and incorporating event scale variations in these assessments. Furthermore, for most of the observations elevated levels of total alkalinity (TA) and pH were observed at the estuary mouth relative to the coastal ocean. This suggests that SDL partly buffers against acidification of adjacent coastal surface waters, although the spatial extent of this buffering is likely small.

Parson, RJ, Nelson CA, Carlson CA, Denman CC, Andersson AJ, Kledzik AL, Vergin KL, McNally SP, Treusch AH, Giovannoni SJ.  2014.  Marine bacterioplankton community turnover within seasonally hypoxic waters of a subtropical sound: Devil’s Hole, Bermuda. Environmental Microbiology.   10.1111/1462-2920.12445   Abstract

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 Alphaproteobacteria and the cyanobacterium Synechococcus. In the suboxic bottom waters, cells from the order Chlorobiales prevailed, with gene sequences indicating members of the genera Chlorobium and Prosthecochloris – 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 Thaumarcheota and Euryarcheota 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 Euryarcheota only averaging 5% of cells, and Chlorobiales and Thaumarcheota absent.

Page, HN, Courtney TA, DeCarlo EH, Howins NM, Koester I, Andersson AJ.  2019.  Spatiotemporal variability in seawater carbon chemistry for a coral reef flat in Kane'ohe Bay, Hawai'i. Limnology and Oceanography. 64:913-934.   10.1002/lno.11084   AbstractWebsite

Coral reef community composition and ecosystem function may change in response to anthropogenic ocean acidification. However, the magnitude of acidification on reefs will be modified by natural spatial and temporal variability in seawater CO2 chemistry. Consequently, it is necessary to quantify the ecological, biogeochemical, and physical drivers of this natural variability before making robust predictions of future acidification on reefs. In this study, we measured temporal and spatial physiochemical variability on a reef flat in Kane'ohe Bay, O'ahu, Hawai'i, using autonomous sensors at sites with contrasting benthic communities and by sampling surface seawater CO2 chemistry across the reef flat at different times of the day during June and November. Mean and diurnal temporal variability of seawater CO2 chemistry was more strongly influenced by depth gradients (0.5-10 m) on the reef rather than benthic community composition. Spatial CO2 chemistry gradients across the reef flat reflected the cumulative influence from benthic metabolism, bathymetry, and hydrodynamics. Based on graphical assessment of total alkalinity-dissolved inorganic carbon data, reef metabolism in November was dominated by organic carbon cycling over inorganic carbon cycling, while these processes were closely balanced in June. Overall, this study highlights the strong influence of depth on reef seawater CO2 chemistry variability through its effects on benthic biomass to seawater volume ratio, seawater flow rates, and residence time. Thus, the natural complexity of ecosystems where a combination of ecological and physical factors influence reef chemistry must be considered when predicting ecosystem biogeochemical responses to future anthropogenic changes in seawater CO2 chemistry.

Page, HN, Andersson AJ, Jokiel PL, Rodgers K’uleiS, Lebrato M, Yeakel K, Davidson C, D’Angelo S, Bahr KD.  2016.  Differential modification of seawater carbonate chemistry by major coral reef benthic communities. Coral Reefs. :1-15.   10.1007/s00338-016-1490-4   AbstractWebsite

Ocean acidification (OA) resulting from uptake of anthropogenic CO2 may negatively affect coral reefs by causing decreased rates of biogenic calcification and increased rates of CaCO3 dissolution and bioerosion. However, in addition to the gradual decrease in seawater pH and Ω a resulting from anthropogenic activities, seawater carbonate chemistry in these coastal ecosystems is also strongly influenced by the benthic metabolism which can either exacerbate or alleviate OA through net community calcification (NCC = calcification – CaCO3 dissolution) and net community organic carbon production (NCP = primary production − respiration). Therefore, to project OA on coral reefs, it is necessary to understand how different benthic communities modify the reef seawater carbonate chemistry. In this study, we used flow-through mesocosms to investigate the modification of seawater carbonate chemistry by benthic metabolism of five distinct reef communities [carbonate sand, crustose coralline algae (CCA), corals, fleshy algae, and a mixed community] under ambient and acidified conditions during summer and winter. The results showed that different communities had distinct influences on carbonate chemistry related to the relative importance of NCC and NCP. Sand, CCA, and corals exerted relatively small influences on seawater pH and Ω a over diel cycles due to closely balanced NCC and NCP rates, whereas fleshy algae and mixed communities strongly elevated daytime pH and Ω a due to high NCP rates. Interestingly, the influence on seawater pH at night was relatively small and quite similar across communities. NCC and NCP rates were not significantly affected by short-term acidification, but larger diel variability in pH was observed due to decreased seawater buffering capacity. Except for corals, increased net dissolution was observed at night for all communities under OA, partially buffering against nighttime acidification. Thus, algal-dominated areas of coral reefs and increased net CaCO3 dissolution may partially counteract reductions in seawater pH associated with anthropogenic OA at the local scale.