No abstract

A coral reef represents the net accumulation of calcium carbonate (CaCO3) produced by corals and other calcifying organisms. If calcification declines, then reef-building capacity also declines. Coral reef calcification depends on the saturation state of the carbonate mineral aragonite of surface waters. By the middle of the next century, an increased concentration of carbon dioxide will decrease the aragonite saturation state in the tropics by 30 percent and biogenic aragonite precipitation by 14 to 30 percent. Coral reefs are particularly threatened, because reef-building organisms secrete metastable forms of CaCO3, but the biogeochemical consequences on other calcifying marine ecosystems may be equally severe.

[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,

Ocean acidification describes the progressive, global reduction in seawater pH that is currently underway because of the accelerating oceanic uptake of atmospheric CO 2. Acidification is expected to reduce coral reef calcification and increase reef dissolution. Inorganic cementation in reefs describes the precipitation of CaCO3 that acts to bind framework components and occlude porosity. Little is known about the effects of ocean acidification on reef cementation and whether changes in cementation rates will affect reef resistance to erosion. Coral reefs of the eastern tropical Pacific (ETP) are poorly developed and subject to rapid bioerosion. Upwelling processes mix cool, subthermocline waters with elevated pCO 2 (the partial pressure of CO 2) and nutrients into the surface layers throughout the ETP. Concerns about ocean acidification have led to the suggestion that this region of naturally low pH waters may serve as a model of coral reef development in a high-CO 2 world. We analyzed seawater chemistry and reef framework samples from multiple reef sites in the ETP and found that a low carbonate saturation state (⍀) and trace abundances of cement are characteristic of these reefs. These low cement abundances may be a factor in the high bioerosion rates previously reported for ETP reefs, although elevated nutrients in upwelled waters may also be limiting cementation and/or stimulating bioerosion. ETP reefs represent a real-world example of coral reef growth in low-⍀ waters that provide insights into how the biological-geological interface of coral reef ecosystems will change in a high-CO 2 world. coral reef persistence ͉ inorganic cementation ͉ ocean acidification ͉ climate change A tmospheric CO 2 is increasing exponentially because of the unregulated combustion of fossil fuels (1). Approximately one-third of all of the CO 2 released into the atmosphere since the industrial revolution has been absorbed by the oceans (2). This ongoing uptake of atmospheric CO 2 is causing a drop in seawater pH at the global scale, causing an acidification of the oceans (3-5).Ocean acidification results in a decrease in seawater [CO 3 2Ϫ ] and, consequently, a decrease in the saturation state (⍀) of carbonate minerals {⍀ ϭ [Ca 2ϩ ][CO 3 2Ϫ ]/KЈ sp , where KЈ sp is the apparent solubility product of a carbonate mineral (e.g., aragonite, calcite)}. Acidification is expected to reduce coral reef calcification and increase reef dissolution, and the relative rates of change will likely be related to the partial pressure of CO 2 (pCO 2 ) in surface seawater,

[1] Acidified waters are impacting commercial oyster production in the U.S. Pacific Northwest, and favorable carbonate chemistry conditions are predicted to become less frequent. Within 48 h of fertilization, unshelled Pacific oyster (Crassostrea gigas) larvae precipitate roughly 90% of their body weight as calcium carbonate. We measured stable carbon isotopes in larval shell and tissue and in algal food and seawater dissolved inorganic carbon in a longitudinal study of larval development and growth. Using these data and measured biochemical composition of larvae, we show that sensitivity of initial shell formation to ocean acidification results from diminished ability to isolate calcifying fluid from surrounding seawater, a limited energy budget and a strong kinetic demand for calcium carbonate precipitation. Our results highlight an important link between organism physiology and mineral kinetics in larval bivalves and suggest the consideration of mineral kinetics may improve understanding winners and losers in a high CO 2 world.

Eight‐month‐old blocks of the coral Porites lobata colonized by natural Hawaiian euendolithic and epilithic communities were experimentally exposed to two different aqueous pCO2 treatments, 400 ppmv and 750 ppmv, for 3 months. The chlorophyte Ostreobium quekettii dominated communities at the start and at the end of the experiment (65–90%). There were no significant differences in the relative abundance of euendolithic species, nor were there any differences in bioeroded area at the surface of blocks (27%) between pCO2 treatments. The depth of penetration of filaments of O. quekettii was, however, significantly higher under 750 ppmv (1.4 mm) than under 400 ppmv (1 mm). Consequently, rates of carbonate dissolution measured under elevated pCO2 were 48% higher than under ambient pCO2 (0.46 kg CaCO3 dissolved m−2 a−1 versus 0.31 kg m−2 a−1). Thus, biogenic dissolution of carbonates by euendoliths in coral reefs may be a dominant mechanism of carbonate dissolution in a more acidic ocean.

Ocean acidification (OA) refers to the ongoing decline in oceanic pH resulting from the uptake of atmospheric CO 2 . Mounting experimental evidence suggests that OA will have negative consequences for a variety of marine organisms. Whereas the effect of OA on the calcification of adult reef corals is increasingly well documented, effects on early life history stages are largely unknown. Coral recruitment, which necessitates successful fertilization, larval settlement, and postsettlement growth and survivorship, is critical to the persistence and resilience of coral reefs. To determine whether OA threatens successful sexual recruitment of reef-building corals, we tested fertilization, settlement, and postsettlement growth of Acropora palmata at pCO 2 levels that represent average ambient conditions during coral spawning (∼400 μatm) and the range of pCO 2 increases that are expected to occur in this century [∼560 μatm (mid-CO 2 ) and ∼800 μatm (high-CO 2 )]. Fertilization, settlement, and growth were all negatively impacted by increasing pCO 2 , and impairment of fertilization was exacerbated at lower sperm concentrations. The cumulative impact of OA on fertilization and settlement success is an estimated 52% and 73% reduction in the number of larval settlers on the reef under pCO 2 conditions projected for the middle and the end of this century, respectively. Additional declines of 39% (mid-CO 2 ) and 50% (high-CO 2 ) were observed in postsettlement linear extension rates relative to controls. These results suggest that OA has the potential to impact multiple, sequential early life history stages, thereby severely compromising sexual recruitment and the ability of coral reefs to recover from disturbance.T he susceptibility of reef-building corals to increasing CO 2 levels has been a central issue in the context of global climate change. Present-day atmospheric CO 2 (pCO 2 ) levels are estimated at 387 ppm, 30% higher than the natural range over the last 650,000 y (1). pCO 2 is increasing at an annual rate of 0.5% (2), 200 times faster than any changes that occurred during the last eight glacial cycles (1 Mounting experimental evidence suggests that ocean acidification (OA) will have negative consequences for numerous marine organisms, primarily oceanic calcifiers that rely on the delicate balance of dissolved inorganic carbon species for the formation of their shells and skeletons (4-7). Although recent research efforts have aimed to constrain the mechanisms and effects (both physiological and ecological) of elevated pCO 2 on adult corals, comparatively little attention has been given to the effect of OA on earlier life history stages.The majority of reef-building corals rely on external fertilization and the development, survival, and settlement of lecithotrophic (i.e., energy-limited) planula larvae (8). Coral larvae spend hours to days developing in the water column before they are capable of settling on the reef. Larval settlement requires the recognition of water-soluble and substrate-bound chemical cues, ph...

[1] This paper uses the extended multiple linear regression (eMLR) technique to investigate changes over the last decade in dissolved inorganic carbon (DIC) inventories on a meridional line (P16 along 152°W) up the central Pacific and on a zonal line (P02 along 30°N) across the North Pacific. Maximum changes in the total DIC concentrations along P02 are 15-20 mmol kg À1 over 10 years, somewhat higher than the $1 mmol kg À1 a À1 increase in DIC expected based on the rate of atmospheric CO 2 increase. The maximum changes of 15-20 mmol kg À1 along the P16 line over the 14/15-year time frame fit with the expected magnitude of the anthropogenic signal, but there is a deeper than expected penetration of the signal in the North Pacific compared to the South Pacific. The effect of varying circulation on the total DIC change, based on decadal alterations of the apparent oxygen utilization rate, is estimated to be greater than 10 mmol kg À1 in the North Pacific, accounting for as much as 80% of the total DIC change in that region.