Joan A. Kleypas

Ocean acidification: the other CO2 problem.

Rising atmospheric carbon dioxide (CO2), primarily from human fossil fuel combustion, reduces ocean pH and causes wholesale shifts in seawater carbonate chemistry. The process of ocean acidification is well documented in field data, and the rate will accelerate over this century unless future CO2 emissions are curbed dramatically. Acidification alters seawater chemical speciation and biogeochemical cycles of many elements and compounds. One well-known effect is the lowering of calcium carbonate saturation states, which impacts shell-forming marine organisms from plankton to benthic molluscs, echinoderms, and corals. Many calcifying species exhibit reduced calcification and growth rates in laboratory experiments under high-CO2 conditions. Ocean acidification also causes an increase in carbon fixation rates in some photosynthetic organisms (both calcifying and noncalcifying). The potential for marine organisms to adapt to increasing CO2 and broader implications for ocean ecosystems are not well known; both are high priorities for future research. Although ocean pH has varied in the geological past, paleo-events may be only imperfect analogs to current conditions.

Representing key phytoplankton functional groups in ocean carbon cycle models: Coccolithophorids

Carbonates are the largest reservoirs of carbon on Earth. From mid-Mesozoic time, the biologically catalyzed precipitation of calcium carbonates by pelagic phytoplankton has been primarily due to the production of calcite by coccolithophorids. In this paper we address the physical and chemical processes that select for coccolithophorid blooms detected in Sea-viewing Wide Field-of-view Sensor (SeaWiFS) ocean color imagery. Our primary goal is to develop both diagnostic and prognostic models that represent the spatial and temporal dynamics of coccolithophorid blooms in order to improve our knowledge of the role of these organisms in mediating fluxes of carbon between the ocean, the atmosphere, and the lithosphere. On the basis of monthly composite images of classified coccolithophorid blooms and global climatological maps of physical variables and nutrient fields, we developed a probability density function that accounts for the physical chemical variables that predict the spatiotemporal distribution of coccolithophorids in the world oceans. Our analysis revealed that areas with sea surface temperatures (SST) between 3° and 15°C, a critical irradiance between 25 and 150 µmol quanta m-2 s-1, and decreasing nitrate concentrations (N/t < 0) are selective for upper ocean large-scale coccolithophorid blooms. While these conditions favor both Northern and Southern Hemisphere blooms of the most abundant coccolithophorid in the modern oceans, Emiliania huxleyi, the Northern and Southern Hemisphere populations of this organism are genetically distinct. Applying amplified fragment length polymorphism as a marker of genetic diversity, we identified two major taxonomic clades of E. huxleyi; one is associated with the Northern Hemisphere blooms, while the other is found in the Southern Hemisphere. We suggest a rule of “universal distribution and local selection”: that is, coccolithophorids can be considered cosmopolitan taxa, but their genetic plasticity provides physiological accommodation to local environmental selection pressure. Sea surface temperature, critical irradiance, and N/t were predicted for the years 2060–2070 using the NCAR Community Climate System Model to generate future monthly probability distributions of coccolithophorids based upon the relationships observed between the environmental variables and coccolithophorid blooms in modern oceans. Our projected probability distribution analysis suggests that in the North Atlantic, the largest habitat for coccolithophorids on Earth, the areal extent of blooms will decrease by up to 50% by the middle of this century. We discuss how the magnitude of carbon fluxes may be affected by the evolutionary success of coccolithophorids in future climate scenarios.

Poorly cemented coral reefs of the eastern tropical Pacific: Possible insights into reef development in a high-CO2 world

Ocean acidification describes the progressive, global reduction in seawater pH that is currently underway because of the accelerating oceanic uptake of atmospheric CO2. 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 pCO2 (the partial pressure of CO2) 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-CO2 world. We analyzed seawater chemistry and reef framework samples from multiple reef sites in the ETP and found that a low carbonate saturation state (Ω) 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-Ω waters that provide insights into how the biological–geological interface of coral reef ecosystems will change in a high-CO2 world.

Future coral reef habitat marginality: temporal and spatial effects of climate change in the Pacific basin

Marginal reef habitats are regarded as regions where coral reefs and coral communities reflect the effects of steady-state or long-term average environmental limitations. We used classifications based on this concept with predicted time-variant conditions of future climate to develop a scenario for the evolution of future marginality. Model results based on a conservative scenario of atmospheric CO2 increase were used to examine changes in sea surface temperature and aragonite saturation state over the Pacific Ocean basin until 2069. Results of the projections indicated that essentially all reef locations are likely to become marginal with respect to aragonite saturation state. Significant areas, including some with the highest biodiversity, are expected to experience high-temperature regimes that may be marginal, and additional areas will enter the borderline high temperature range that have experienced significant ENSO-related bleaching in the recent past. The positive effects of warming in areas that are presently marginal in terms of low temperature were limited. Conditions of the late 21st century do not lie outside the ranges in which present-day marginal reef systems occur. Adaptive and acclimative capabilities of organisms and communities will be critical in determining the future of coral reef ecosystems.

Modeled estimates of global reef habitat and carbonate production since the Last Glacial Maximum

Estimated changes in reef area and CaCO3 production since the last glacial maximum (LGM) are presented for the first time, based on a model (ReefHab) which uses measured environmental data to predict global distribution of reef habitat. Suitable reef habitat is defined by temperature, salinity, nutrients, and the depth-attenuated level of photosynthetically available radiation (PAR). CaCO3 production is calculated as a function of PAR. When minimum PAR levels were chosen to restrict reef growth to 30 m depth and less, modern reef area totaled 584–746 × 10³ km². Global carbonate production, which takes into account topographic relief as a control on carbonate accumulation, was 1.00 Gt yr−1. These values are close to the most widely accepted estimates of reef area and carbonate production and demonstrate that basic environmental data can be used to define reef habitat and calcification. To simulate reef habitat changes since the LGM, the model was run at 1-kyr intervals, using appropriate sea level and temperature values. These runs show that at the LGM, reef area was restricted to 20% of that today and carbonate production to 27%, due primarily to a reduction in available space at the lower sea level and secondarily to lower sea surface temperatures. Nonetheless, these values suggest that reef growth prior to shelf flooding was more extensive than previously thought. A crude estimate of reef-released CO2 to the atmosphere since the LGM is of the same order of magnitude as the atmospheric CO2 change recorded in the Vostok ice core, which emphasizes the role of neritic carbonates within the global carbon cycle. This model currently addresses only the major physical and chemical controls on reef carbonate production, but it provides a template for estimating shallow tropical carbonate production both in the present and in the past. As such, the model highlights several long-standing issues regarding reef carbonates, particularly in terms of better defining the roles of light, temperature, aragonite saturation state, and topography on reef calcification.

Coral reef development under naturally turbid conditions : fringing reefs near Broad Sound, Australia

Reef coring and NOAA/AVHRR imagery were used to examine differences in reef colonisation and accumulation across a gradient of increasing tidal range and turbidity. AVHRR channel-1 reflectance, which was strongly correlated with suspended sediment concentration (SSC), demonstrated that SSC is due to tidal resuspension of sediments, and increases with increasing tidal range. Underwater surveys and reef coring revealed that reef development diminishes with increasing SSC toward Broad Sound. Few reefs near Broad Sound have formed reef flats; those that have are thinner and accumulated more slowly during the Holocene. The many submerged reefs in this area represent a mixture of reef “turn-ons” and “turn-offs”. Some are probably incipient reefs in the early stages of reef growth. Others appear to be coral communities growing as thin veneers on exposed rock surfaces, rather than coral reef communities with capacity for reef-building. Still others developed reef flats earlier in the Holocene, and have since turned-off.

Coral Reefs and Changing Seawater Carbonate Chemistry

Seawater carbonate chemistry of the mixed layer of the oceans is changing rapidly in response to increases in atmospheric CO2. The formation and dissolution of calcium carbonate is now known to be strongly affected by these changes, but many questions remain about other controls on biocalcification and inorganic cementation that confound our attempts to make accurate predictions about the effects on both coral reef organisms and reefs themselves. This chapter overviews the current knowledge of the relationship between seawater carbonate chemistry and coral reef calcification, identifies the hurdles in our understanding of the two, and presents a strategy for overcoming those hurdles.

Progress made in study of ocean's calcium carbonate budget

Many of the uncertainties in diagnostic and prognostic marine carbon cycle models arise from an imperfect understanding of the processes that control the formation and dissolution of calcium carbonate (CaCO3). On the production side of the equation, the factors that control the abundances of calcifying phytoplankton or zooplankton are largely unknown. On the dissolution side, changes in the depth of CaCO3 saturation horizons for both calcite and aragonite may produce large-scale changes in dissolution of shelf and slope sediments and reefs, with potentially significant implications for atmospheric carbon dioxide concentration and climate change, as well as for coralline organisms themselves. In recent years, concern about the long-term fate of anthropogenic CO2 in the oceans has re-ignited scientific interest in the fundamental abiotic and biotic processes that control the marine CaCO3 budget, since biological CaCO3 production and export are important mechanisms by which carbon is exported from the oceans surface to its abyss. CaCO3 precipitation releases CO2 to solution, while CaCO3 dissolution takes up CO2 from solution.

Comment on “Coral reef calcification and climate change: The effect of ocean warming”

McNeil et al. [2004] attempt to address an important question about the interactions of temperature and carbonate chemistry on calcification, but their projected values of reef calcification are based on assumptions that ignore critical observational and experimental literature. Certainly, more research is needed to better understand how changing temperatures and carbonate chemistry will affect not only coral reef calcification, but coral survival. As discussed above, the McNeil et al. [2004] analysis is based on assumptions that exclude potentially important factors and therefore needs to be viewed with caution. Copyright 2005 by the American Geophysical Union.

Present and future changes in seawater chemistry due to ocean acidification

The oceanic uptake of anthropogenic CO 2 changes the seawater chemistry and potentially can alter biological systems in the upper oceans. Estimates of future atmospheric and oceanic CO 2 concentrations, based on the Intergovernmental Panel on Climate Change (IPCC) emission scenarios, indicate that atmospheric CO 2 levels could approach 800 ppm by the end of the century. Corresponding models for the oceans indicate that surface water pH would decrease by approximately 0.4 pH units, and the carbonate ion concentration would decrease by as much as 48% by the end of the century. The surface ocean pH would be lower than it has been for more than 20 million years. Such changes would significantly lower the oceans buffering capacity, which would reduce its ability to accept more CO 2 from the atmosphere. Recent field and laboratory studies reveal that the carbonate chemistry of seawater has a profound impact on the calcification rates of individual species and communities in both planktonic and benthic habitats. The calcification rates of nearly all calcifying organisms studied to date decrease in response to decreased carbonate ion concentration. In general, when pCO 2 was increased to twice preindustrial levels, a decrease in the calcification rate ranging from about ―5% to ―60% was observed. Unless calcifying organisms can adapt to projected changes in seawater chemistry, there will likely be profound changes in the structure of pelagic and benthic marine ecosystems.