Impact of Ocean Acidification on Coral Reefs and Other Calcifiers
source : UCAR St Petersburg Workshop 18 - 20 April 2005
RESEARCH FINDINGS of the past decade have led to mounting concern that rising atmospheric carbon dioxide (C02) concentrations will cause changes in the ocean's carbonate chemistry system, and that those changes will affect some of the most fundamental biological and geochemical processes of the sea. Thanks to the efforts of large-scale physical and biogeochemical ocean programs such as WOCE, JGOFS, and OACES, ocean-wide changes in the carbonate system are now well documented. Since 1980 ocean uptake of the excess C02 released by anthropogenic activities is significant; about a third has been stored in the oceans. The rate of atmospheric C02 increase, however, far exceeds the rate at which natural feedbacks can restore the system to normal conditions. Oceanic uptake of C02 drives the carbonate system to lower pH and lower saturation states of the carbonate minerals calcite, aragonite, and high-magnesium calcite, the materials used to form supporting skeletal structures in many major groups of marine organisms.
A variety of evidence indicates that calcification rates will decrease, and carbonate dissolution rates increase, as CaC03 saturation state decreases. This evidence comes from principles of thermodynamics, the geologic record, and the evolutionary pathways of CaC03 secreting organisms. Further evidence, from controlled experiments of biocalcification under in¬creased C02 conditions, confirms that calcification rates of many organisms decrease with decreasing CaC03 saturation state. Extrapolation of these results to the real world suggests that calcification rates will decrease up to 60% within the 21st century. We know that such extrapolations are oversimplified and do not fully consider other environmental and biological effects (e.g., rising water temperature, biological adaptation); nor do they address effects on organism fitness, community structure, and ecosystem functioning. Any of these factors could increase or decrease the laboratory-based estimates, but it is certain that net production of CaC03 will decrease in the future.
The St. Petersburg Workshop, sponsored by NSF, NOAA, and the USGS, and held at the USGS Center for Coastal and Watershed Studies on 18-20 April 2005, was designed to take the next step toward unerstanding the response of marine calcification to
increasing atmospheric C02 concentration. The aims of the workshop were to summarize existing knowledge on the topic, reach a consensus on what the most pressing scientific issues are, and identify future research strategies for addressing these issues. Although workshop participants were drawn from a wide range of scientific disciplines, there was a clear convergence on the major scientific issues that should be pursued over the next 5-10 years. These include:
• Determine the calcification response to elevated C02 in benthic calcifiers such as corals (including cold-water corals), coralline algae, foraminifera, molluscs, and echinoderms; and in planktonic calcifiers such as coccolithophores, foraminifera, and shelled pteropods;
• Discriminate the various mechanisms of calcification within calcifying groups, through physiological experiments, to better understand the cross-taxa range of responses to changing seawater chemistry;
• Determine the interactive effects of multiple variables that affect calcification and dissolution in organisms (saturation state, light, temperature, nutrients) through continued experimen¬tal studies on an expanded suite of calcifying groups;
• Establish clear links between laboratory experiments and the natural environment, by combining laboratory experiments with field studies;
• Characterize the diurnal and seasonal cycles of the carbonate system on coral reefs, including commitment to long-term monitoring of the system response to continued increases in CO2;
• In concert with above, monitor in situ calcification and dissolution in planktonic and benthic organisms, with better characterization of the key environmental controls on calcification;
• Incorporate ecological questions into observations and experiments; e.g., How does a change in calcification rate affect the ecology and survivorship of an organism? How will ecosystem functions differ between communities with and without calcifying species?
shelf export of CaC03:
• Quantify and parameterize the mechanisms that contribute to the carbonate system, through bio¬geochemical and ecological modeling, and apply such modeling to guide future sampling and experimental efforts;
• Develop protocols for the various methodologies used in seawater chemistry and calcification measurements.
Some of these research objectives require technological development, but others can be addressed immediately. While much work remains toward answering the fundamental question: "How will marine calcification rates respond to increasing atmospheric C02 concentrations," we need to begin investigations that look forward to answering the question: "What are the consequences of reduced calcification in both planktonic and benthic calcifying communities and ecosystems?" We should not wait until we answer the former question before tackling the latter.
This report is intended as a guide to program managers and researchers toward designing research projects that address these important questions. It is written with the detail and references needed to serve as a resource for researchers, including graduate students, who wish to tackle projects within the sometimes confusing topic of marine carbonate chemistry and calcification.
7. CONCLUSIONS AND RECOMMENDATIONS
7.1 Impacts of Anthropogenic CO2 in the Oceans
THE UPTAKE OF ANTHROPOGENIC CO2 by the ocean changes the seawater chemistry and "will significantly impact biological systems in the upper oceans. Estimates of future atmospheric and oceanic C02 concentrations, based on the Intergovernmental Panel on Climate Change OPCC) emission scenarios and general circulation models indicate that atmospheric C02 levels could exceed 500 ppmv by the middle of the 21st century, and 800 ppmv by 2100. Corresponding models for the oceans indicate that by 2100, surface water pH will decrease by approximately 0.4 pH units relative to the preindustrial value, lower than it has been for more than 20 My. The carbonate ion concentration will also decrease by almost 50% relative to preindustrial levels. Such changes will significantly lower the ocean's buffering capacity and, therefore, reduce its ability to accept more CO2 from the atmosphere.
Recent field and laboratory studies reveal that the carbonate chemistry of seawater has a significant effect on the calcification rates of individual species and communities in both planktonic and benthic habitats. The calcification rates of most calcifying organisms studied to date decrease in response to decreased carbonate ion concentration. This response has been observed in multiple taxonomic groups - - from building corals to single-celled protists. Experimental evidence points to a 5-50% reduction in calcification rate under a CO2 level twice that of the preindustrial. The decreased carbonate ion concentration significantly reduces the ability of reef¬building corals to produce their CaC03 skeletons, affecting growth of individual corals and the ability of the larger reef to maintain a positive balance between reef building and reef erosion. Several groups of calcifying plankton-coccolithophorids (single-celled algae), forams, and pteropods (planktonic molluscs)¬also exhibit a reduction in their calcium carbonate structures. Many of these organisms are important components of the marine food web.
The effects of reduced calcification on individual organisms and on ecosystems have not been investigated, however, and have only been inferred from
knowledge about the role of calcification in organism and ecosystem functioning. This knowledge is limited because calcification rates have only recently been considered vulnerable to increased atmospheric CO2. Because calcification provides some advantage (or multiple advantages) to calcifying organisms, decreased calcification is likely to compromise the fitness or success of these organisms and could shift the competitive advantage toward non-calcifiers. There is also little information regarding the capacity of calcifying organisms to adapt to changing seawater chemistry. Coral reef organisms have not demonstrated an ability to adapt to decreasing carbonate saturation state, but experiments so far have been relatively short-term (hours to months). Some planktonic organisms, particularly those with rapid generation times, may be able to adapt to lowered saturation state via natural selection. Planktonic calcifiers that cannot adapt to future changes in seawater chem¬istry are likely to experience reductions in their geographic ranges, or latitudinal shifts. Decreased calcification in marine organisms is likely to impact marine food webs and, combined with other climatic changes in temperature, salinity, and nutrients, could substantially alter the biodiversity and productivity of the ocean.
Seawater pH is a master variable that impacts the speciation of the carbonate system, nutrients, and other major and trace element species in the oceans. It is largely unknown if, or how, various organisms will adapt to the large-scale pH changes that are anticipated over the next two to three centuries. At present, it is not possible to determine how the community structure will change or how these ecosystem changes might influence future climate feed¬back mechanisms. It is therefore important to develop new research strategies to better understand the long-term vulnerabilities of sensitive marine organisms to these changes. We are just beginning to understand the complex interactions between large¬scale changes in ocean chemistry and marine ecological processes. Clearly, seawater carbonate chemistry is changing over decadal and longer timescales and these changes will impact marine biota.
7.2 Research Needs
Data from across the scientific disciplines support the hypothesis that marine calcification and dissolution are largely controlled by carbonate chemistry, elevating the concern that increasing C02 poses a considerable threat to the health of our oceans. But these data are sparse, and extrapolating results from controlled experiments to the natural environment is risky. Several workshops and reports have addressed the overall scientific issues of marine calcification under elevated atmospheric C02. The St. Petersburg workshop attempted to summarize these issues, identify the most important gaps in our understanding, and provide guidance toward designing research to address them.
Understanding the biological consequences of ocean acidification and placing these changes in a historical context are in the early stages. Now is the time to coordinate scientific research strategies to maximize scientific findings. This is a complex scientific undertaking, and it is essential that new research is well informed by experimentalists and observationalists in marine chemistry, biology/ecology, and geology; and experts in ocean monitoring and technology, paleoreconstructions, and modelers. It is also essential to entrain young scientists into this field, and to provide them with materials that can help guide their research.
Given the broad array of research needs, participants of the St. Petersburg Workshop recommended a research design that could be logically phased based on criteria such as: (a) the most compelling research needs; (b) research that could be done now versus that which requires longer-term planning; (c) research that requires significant technological development; and (d) research that can take advantage of ongoing field activities. Table 7.1 lists only the most compelling research needs and should not be considered a complete list of necessary research; nonetheless it offers a framework for coordinating an overall research plan to tackle the issue of marine calcification under increasing atmospheric C02. Phase I represents high-priority research needs that can be initiated immediately. Phase II represents research that requires additional long-term planning and coordination, and Phase III represents research that requires some additional technological developments for success.
The St. Petersburg participants agreed on several parallel courses of research for the next 5-10 years. First, sustained observations of changes in the ocean carbon system should be continued. Second, additional field and laboratory investigations into the biological and ecological responses of calcifiers to increasing CO2 should be conducted. Among these,
Kleypas et al.-Impacts of Ocean Acidiflcatian - - - -
long-term field manipulation experiments present the most compelling and challenging research needs. Third, these observations and experiments should be founded on a strong set of proven standards for chemical and biological measurements, and should be augmented with paleo-records and proxies that can shed light on the natural response of the system over different timescales. Fourth, simultaneous development of ecosystem models is essential if we are to translate future changes in ocean chemistry and calcification/ dissolution rates to ecosystem response.
Many researchers have paved the way along these four courses toward tackling the important questions about calcification and dissolution response to increased ocean acidification. We can build on their efforts to understand the capacity for organisms and ecosystems to adapt to carbonate chemistry changes, and to predict the future of marine calcification and its feedback to the marine carbon cycle and global climate.
7.3 Research Collaborations
Collaborative research on the impacts of enhanced atmospheric CO2 on ocean chemistry and biology needs to be accelerated at the national and international levels. Emphasis should be placed on developing a better understanding of how changes in the metabolic processes at the cellular level will be manifested within the ecosystem or community structure, and how they will influence the climate feedbacks of the future. A fully integrated system of laboratory, mesocosm, field monitoring, and modeling approaches is required to provide policymakers with informed management strategies that address how humans might best mitigate or adapt to these long-term changes.
Such efforts should complement ongoing research programs in marine biogeochemistry and ecology (e.g., OCCC, Ocean Carbon and Climate Change; SO¬LAS, Surface Ocean-Lower Atmosphere Study; IM¬BER, Integrated Marine Biogeochemistry and Ecosystem Research; SCOR, Scientific Committee for Ocean Research; etc.). Many of these programs are international. Indeed, the St. Petersburg workshop and the production of this report included substantial in¬put from our non-U.S. partners and we strongly recommend strengthening these partnerships. Advances in carbon system and calcification measurements, in designing experimental mesocosms, in molecular studies, and in modeling, are among expertise seated across a suite of international labs. Most of the important questions outlined in the report are based on international research efforts, and should
Table 7.1: Key research activities, with a general indication of how they could be coordinated within a phased research plan.
x x x
x x x
x x x
Research Area Carbonate system monitoring
Physiology of calcification
Calcification response and organism response
Identify key areas for monitoring Standardize measurements, reporting
Coordinate carbonate system monitoring with existing observational systems
Increase monitoring, particularly in regions with high variability Develop technology: autonomous sensors for carbonate system and PIC; remote sensing applications
Conduct experiments on dissolution and its response to increased C02 (including better understanding of thermodynamic constants for high-Mg calcite)
Conduct experiments to determine the various mechanisms of calcification and the photosynthesis! calcification relationship in autotrophs and in heterotrophs with photosynthetic symbionts Develop and standardize methods for measuring calcification rates Investigate calcification response across multiple taxa: coccolithophorids; planktonic and benthic forams; pteropods;
reef-building and deep-sea corals; Halimeda; coralline algae; echinoderms; bryozoans; molluscs
Investigate effects of multiple controls on calcification (e.g., pC02, x x
T, light, nutrients)
Investigate potential for organisms to adapt x x
Investigate multiple life-stages of organisms x x
Develop field-based experiments to more realistically simulate x
pC02 effects on calcification
Develop skeletal proxies for paleo-calcification analysis x x
Engage benthic and planktonic ecologists and modelers to identify x key needs and design research to address ecosystem response
Develop and begin long-term monitoring and!or long-term x
experiments on ecological communities; coordinate with existing ecological monitoring
Develop appropriate ecosystem models for planktonic and benthic x x
Open ocean-investigate ecosystem shifts and feedbacks on x x
calcification, sedimentation, carbon cycle
Quantify "reef-building" and CaC03 budgets of other benthic x x
Develop technology such as remote-sensing applications x
be approached with a commitment to nurture these partnerships. In addition, many of the key regions for future research are in international waters, and many interdisciplinary efforts demand that expertise be drawn from beyond U.S. borders. Above all, the urgency of understanding the potential consequences of ocean acidification on marine calcifying ecosystems demands that we design future research on this issue as efficiently as possible, which requires ignoring traditional boundaries so that efforts are complementary rather than duplicated.
Editorial Comment: I think that this workshop introduction andummary of conclusions reflect the SCCT concerns over the future for both tropical and cold-water coral reef formation in both coastl and dee-water marine environments and the potential impact on
coastal and deep-water benthic (sea-bed dwelling) shell development and covering. We would also like to learn more about fish skeleton formation (is the ocean acidification contributing to skeletal deformation ??)