OS21A-1153 INVITED
Controls over Carbonate Saturation in Temperate Estuaries: Insights from Historical Records
Increasing atmospheric pCO2 has a direct impact on dissolved pCO2 and pH in the surface ocean, which is likely to have far-reaching effects on ecosystems based on the marine organisms that build carbonate structures. However, in estuaries and in the coastal ocean, the greatest impact of atmospheric pCO2 on carbonate saturation state may be indirect, occurring via its influence over the hydrologic cycle and ocean circulation. We analyzed published historical river flow and carbon system time-series data, combined with new time- series data collected from the same stations, to examine changes in pH, alkalinity, and aragonite saturation in Tomales Bay, northern California since 1987. In addition to strong seasonal variation in surface-water Ωarag due to fluctuations in both pH and alkalinity, a longer-term decline in Ωarag was observed between 1987-1995. Toward the end of the record, Ωarag approached the limit below which calcifying organisms have difficulty forming shells (<1). This record also shows that most of the decline in Ωarag occurred during an interval of increased terrestrial runoff from 1991-1995. We suggest that in estuaries such as Tomales Bay, Ωarag will be more sensitive to the impacts of salinity variations arising from changing rainfall patterns along the coast than to the direct pH effect from dissolved anthropogenic CO2. The effects of precipitation changes are difficult to predict and could include increased input of terrestrially-derived pCO2, dilution of alkalinity, and promotion of alkalinity production via sulfate reduction. Local calibrations and core processing are both underway to apply geochemical and faunal proxies in sediment cores for reconstructing salinity, temperature, pH, and carbonate saturation beyond the reach of historical records. We expect these records to provide insights into the geochemical and ecological responses to local human activities within the last 150-200 years.
OS21A-1154
The Effect of River Alkalinity on Coastal Aragonite Saturation
As atmospheric carbon dioxide levels increase, concern about ocean acidification is rising. Although a great deal of recent research has been focused on this topic, little has been done to explore how land and ocean interactions are affecting coastal acidification. This study addresses the regional variability of total alkalinity (TA) in rivers and its effect on the aragonite saturation state (Ω) at river mouths. The TA concentration of river water is its ability to buffer changes in pH, while Ω is an index that expresses the availability of calcium and carbonate ions (e.g. for shell formation). Local river sampling in New England (USA) and New Brunswick (CAN) showed regional differences in TA concentrations (203-2155 μmol/L), which are likely related to bedrock and land use patterns. USGS alkalinity data for rivers entering the East and Gulf Coasts (USA) ranged from 181-3598 μmol/L and showed a regional pattern of low TA in northern rivers and higher TA in the southern and Gulf rivers. Estimated aragonite saturation states (0.0- 5.6) revealed that most rivers were under-saturated (Ω < 1) and only a few rivers (mainly entering the Gulf) were super-saturated (Ω > 1). This widespread under-saturation near the coast may have detrimental effects for calcifying organisms that rely on aragonite to build their shells. Further study is needed to determine the spatial extent of river influence on acidification of coastal waters and to elucidate the ways in which anticipated climate change will alter the chemistry of river water entering the ocean.
OS21A-1155
Influence of terrestrial weathering on future ocean acidification.
Ocean uptake of anthropogenic CO2 causes a decline in seawater pH. This process is known as ocean acidification and is expected to have adverse effects for a variety of marine organisms. Future increase in pCO2 and consequential global warming, however, should enhance weathering of carbonate and silicate rocks. Weathering processes sequester atmospheric CO2 followed by carbonate burial in sediments. This negative feedback mechanism played an important role in regulating pCO2 over geologic timescales. Here we investigate whether continental weathering can enhance drawdown of CO2 and mitigate future ocean acidification using a carbon-cycle reservoir model coupled to a sediment model under a suite of carbon emission scenarios ranging from 600 to 5,000 Pg C total emissions released over 200 to 1,000 years. Our preliminary results indicate that enhanced weathering generally has a small effect on future pCO2 and ocean acidification. But on longer timescales, different weathering parameterizations introduce large uncertainties regarding the time when ocean pH will return to moderate levels in the future.
OS21A-1156
Economic Vulnerability Assessment of U.S. Fishery Revenues to Ocean Acidification
Ocean acidification, a predictable consequence of rising anthropogenic CO2 emissions, is poised to change marine ecosystems profoundly by decreasing average ocean pH and the carbonate mineral saturation state worldwide. These conditions slow or reverse marine plant and animal calcium carbonate shell growth, thereby harming economically valuable species. In 2006, shellfish and crustaceans provided 50% of the $4 billion U.S. domestic commercial harvest value; value added to commercial fishery products contributed $35 billion to the gross national product that year. Laboratory studies have shown that ocean acidification decreases shellfish calcification; ocean acidification--driven declines in commercial shellfish and crustacean harvests between now and 2060 could decrease nationwide time-integrated primary commercial revenues by $860 million to $14 billion (net present value, 2006 dollars), depending on CO2 emissions, discount rates, biological responses, and fishery structure. This estimate excludes losses from coral reef damage and possible fishery collapses if ocean acidification pushes ecosystems past ecological tipping points. Expanding job losses and indirect economic costs will follow harvest decreases as ocean acidification broadly damages marine habitats and alters marine resource availability. Losses will harm many regions already possessing little economic resilience. The only true solution to ocean acidification is reducing atmospheric CO2 emissions, but implementing regional adaptive responses now from an ecosystem-wide, fisheries perspective will help better preserve sustainable ecosystem function and economic yields. Comprehensive management strategies must include monitoring critical fisheries, explicitly accounting for ocean acidification in management models, reducing fishing pressure and environmental stresses, and supporting regional economies most sensitive to acidification's impacts.
OS21A-1157
Changing noise levels in a high CO2/lower pH ocean
We show that ocean acidification from fossil fuel CO2 invasion and from increased respiration/reduced ventilation, has significantly reduced ocean sound absorption and thus increased ocean noise levels in the kHz frequency range. Below 10 kHz, sound absorption occurs due to well known chemical relaxations in the B(OH)3/B(OH)4- and HCO3-/CO32- systems. The pH dependence of these chemical relaxations results in decreased sound absorption (α = dB/km) as the ocean becomes more acidic from increased CO2 levels. The scale of surface ocean pH change today from the +105 ppmv change in atmospheric CO2 is about - 0.12 pH, resulting in frequency dependent decreases in sound absorption that now exceed 12% over pre- industrial. Under reasonable projections of future fossil fuel CO2 emissions and other sources a pH change of 0.3 units or more can be anticipated by mid-century, resulting in a decrease in α by almost 40%. Increases in water temperature have a smaller effect but also contribute to decreased sound absorption. Combining a lowering of 0.3 pH units with an increase of 3°C, α will decrease further to almost 45%. Ambient noise levels in the ocean within the auditory range critical for environmental, military, and economic interests are set to increase significantly due to the combined effects of decreased absorption and increasing sources from mankind's activities. Incorporation of sound absorption in modeling future ocean scenarios (R. Zeebe, personal communication) and long-term monitoring possibly with the aid of modern cabled observatories can give insights in how ocean noise will continue to change and its effect on groups such as marine mammals which communicate in the affected frequency range.
OS21A-1158
Shallow water carbonate dissolution and ocean acidification: impacts and feedbacks
Ocean acidification is likely to have a significant impact on biogeochemical processes in shallow water carbonate sediments. Many of these sediments are heavily populated by seagrasses and based on our past results we have shown that sediment carbonate dissolution rates increase linearly with increasing seagrass density. We have also observed that this dissolution is actually net dissolution, i.e., the balance between gross dissolution and reprecipitation, and that reprecipitation increases linearly with net dissolution. Pore water and sediment incubation studies further suggest that this dissolution/reprecipiptation process involves Ostwald ripening. Reprecipitation results in the growth of larger carbonate grains at the expense of the preferential dissolution of smaller grains, driven by a decrease in surface free energy as the larger particles grow. Based on our recent estimates of the global significance of shallow water sediment carbonate dissolution the process appears to play a small role in the oceanic carbon cycle. However, a number of factors suggest that the dynamics of shallow water sediment carbonate dissolution will change dramatically in response to rising CO2. These changes could also potentially increase the global significance of this process. Ocean acidification and rising atmospheric CO2 will exert a positive feedback on seagrass growth, increasing sediment carbonate dissolution rates, and will therefore exert an overall negative feedback on rising CO2. At the same time, this increase in net dissolution will result in the coarsening of the remaining carbonate grains (by reprecipitation and Ostwald ripening). Because larger grains are likely to be less reactive than smaller grains (due to lower surface area), this process could act to diminish the strength of the above-discussed negative feedback on rising CO2. The net effect of these competing factors, as well as their overall effect on global and oceanic carbon cycles remains to be determined. However, these results suggest that seagrass-mediated shallow water carbonate dissolution could be of some importance in the response of oceanic ecosystems to ocean acidification.
OS21A-1159
Anthropogenic Ocean Acidification and its Effects on Calcifying Phytoplankton: The response of eight coccolithophore strains to changing pH
Rising partial pressures of CO2 in the atmosphere since the Industrial Revolution have caused a drop in ocean pH of 0.1 units. Ocean acidification is generally anticipated to result in a decrease in calcification by coccolithophores and other calcifying marine organisms. Coccolithophores are especially important in carbon cycling and ocean-atmosphere CO2 exchange since they not only fix organic carbon, but also calcify using inorganic carbon, resulting in the release of CO2 and the precipitation and transfer to the deep ocean of CaCO3. Recent studies have challenged early findings that coccolithophores reduce calcification in response to acidification (Iglesias-Rodriguez 2008). The present study investigates the response of a variety of coccolithophore genera from around the world to low seawater pH. Calcification (as inorganic C content) and organic carbon fixation (as organic C content) were measured for eight coccolithophore strains grown at low (7.6) and average present day seawater (8.2) pH. The results point to a varied response in both calcification and carbon fixation of different species and strains to low pH. Carbon and nitrogen isotopic composition of coccolithophore organic matter also indicate that the coccolithophores represent a diverse group of phytoplankton with varied strategies and responses to acidification. Interspecific as well as intraspecific differences in responses to acidification indicate that species as well as population biodiversity may be impacted by ocean acidification. This work indicates that coccolithophores are diverse in their responses to changing pH and highlights the need for further research on more species representative of this diversity if an accurate assessment is to be made of the effects of ocean acidification on carbon cycling by coccolithophores.
OS21A-1160 [WITHDRAWN]
The Effect Of Elevated Carbon Dioxide Concentration And Temperature On A Natural Phytoplankton Assemblage In A Controlled Mesocosm Experiment
Over the past two centuries, the accumulation of fossil-fuel CO2 in the upper ocean has increased the gaseous CO2 concentration and a climate warming has increased the surface temperature. In this study, we investigated the effects of elevated pCO2 concentration and temperature on phytoplankton physiology using a well-controlled mesocosm facility. The facility consists of a floating raft, nine impermeable cylindrical enclosures, pCO2 regulation units, bubble-mediated seawater mixers, and temperature regulation units. Each enclosure is two-thirds filled with seawater and is capped with a transparent dome that transmits incoming radiation. A performance of our mesocosm facility was thoroughly evaluated over a range of conditions and recommended a set of optimal operational settings. Such recommendations include attainment of target pCO2 concentration in the headspace and enclosure seawater, target seawater temperature, and the mixing efficiency of the seawater mixer. (Kim et al. 2008). This state-of-art facility will be used in our month-long experiment (September - October 2008) under three distinct conditions: the controlled (pCO2 = 380 ppm), the elevated CO2 (pCO2 = 950 ppm), and greenhouse (pCO2 = 950 ppm, temperature = 3°C elevation) treatments. This experiment will allow us to examine the synergistic response of a natural plankton community to rising pCO2 concentration and temperature.
OS21A-1161
Alteration of Oceanic Nitrification Under Elevated Carbon Dioxide Concentrations
Atmospheric carbon dioxide (CO2) concentrations are increasing exponentially and expected to double by the year 2100. Dissolution of excess CO2 in the upper ocean reduces pH, alters carbonate chemistry, and also represents a potential resource for autotrophic organisms that convert inorganic carbon into biomass--including a broad spectrum of marine microbes. These bacteria and archaea drive global biogeochemical cycles of carbon and nitrogen and constitute the vast majority of biomass in the sea, yet their responses to reduced pH and increased pCO2 remain largely undocumented. Here we show that elevated pCO2 may sharply reduce nitrification rates and populations of nitrifying microorganisms in the ocean. Multiple experiments were performed in the Sargasso Sea and the Southern California Bight under glacial maximum (193 ppm), present day (390 ppm), and projected (750 ppm) pCO2 concentrations, over time scales from hours to multiple days, and at depths of 45 m to 240 m. Measurement of nitrification rates using isotopically-labeled nitrogen showed 2-5 fold reduction under elevated pCO2--as well as an increase under glacial maximum pCO2. Marine Crenarchaeota are likely involved in nitrification as ammonia-oxidizing archaea (AOA) and are among the most abundant microbial groups in the ocean, yet this group decreased by 40-80% under increased pCO2, based on quantification of both 16S rRNA and ammonia monooxygenase (amoA) gene copies. Crenarchaeota also steadily declined over the course of multiple days under elevated pCO2, whereas ammonia-oxidizing (AOB) and nitrite-oxidizing bacteria (NOB) were more variable in their responses or were not detected. These findings suggest that projected increases in pCO2 and subsequent decreases in pH may strongly influence marine biogeochemistry and microbial community structure in the sea.
OS21A-1162
Ocean Acidification Impacts Larval and Juvenile Growth in the Native Oyster Ostrea lurida
The impacts of ocean acidification have only recently been recognized as a human-induced stressor on marine ecosystems. Ocean acidification can disrupt calcification in organisms that precipitate calcareous structures, including many ecologically and economically important species. We examined how decreased levels of carbonate saturation affected larval and juvenile growth and settlement in the native oyster Ostrea lurida. Larvae were cultured at three carbonate saturation levels that represent present day CO2 concentrations (380 ppm) and two future projected pCO2 scenarios (540 and 970 ppm). These treatments were maintained for 20 days throughout larval duration until settlement occurred. Larval and juvenile growth were determined by calculating change in shell area. Larvae exposed to 970 ppm grew 12% less than larvae held under control conditions (380 ppm). In addition, growth varied among larvae produced by different parents, suggesting that impacts of ocean acidification might vary intraspecifically. Juvenile growth (i.e., new shell added following settlement) was significantly different among CO2 treatments, and juveniles exposed to 970 ppm grew 24% less than juveniles held under control conditions (380 ppm). Carry-over effects from the larval stage influence juvenile growth, and because post-settlement mortality is often high for marine invertebrates, ocean acidification may negatively impact the size of native oyster populations.
OS21A-1163
Ecosystem-scale effects of aragonite saturation, temperature, and nutrients on coral-reef calcification
We investigated the relations between net calcification of an entire coral reef in the northern Red Sea and annual changes in aragonite saturation, temperature, and nutrient loading over a two year period, and augmented this study with similar observations in the Great Barrier Reef, Australia. In the Red Sea, average calcification rates varied between 60 ± 20 and 30 ± 20 mmol m-2 d-1 in the summer and winter, respectively. These changes primarily reflected seasonal differences in aragonite saturation and temperature. Data for the Great Barrier Reef are still being processed. Calcification rates at the ecosystem scale correlated remarkably well with the kinetics observed in inorganic aragonite precipitation experiments. This is a remarkable finding considering that calcification in coral reefs is primarily a biogenic process; this finding is yet to be explained mechanistically. These relationships are also in agreement with most laboratory studies on individual stony corals and coral mesocosms. The consistency of these responses at levels ranging from inorganic kinetics through the ecosystem scale suggests that these relationships are sufficiently robust to make predictions of coral reef response to ocean acidification and global warming. Finally, in a number of nighttime measurements, we observed net aragonite dissolution despite the supersaturated state of overlying reef water. This apparent dissolution is most likely biologically mediated. Aragonite dissolution, combined with decreasing aragonite precipitation, suggests that at least some reefs will be undergoing net dissolution by the time atmospheric carbon dioxide concentration doubles over its pre- industrial value.
OS21A-1164
U/Ca ratios in deep-sea corals: Testing the relationship with carbon dioxide in seawater
The oxygen minimum zone (OMZ), which has a vertical profile from depths of 200-800 m, increases as sea surface waters warm and become less soluble or as ocean circulation changes. As the [O2] decreases with depth, ΣCO2 increases and thereby lowering pH and dissolving CaCO3. Developing a proxy for past ocean ΣCO2 is crucial to understanding how the ocean will respond to future ΣCO2 changes. The calcite internodes of bamboo corals contain annual growth bands and have the potential to provide high-resolution paleo-oceanographic records. We examined the response of U/Ca incorporated into modern bamboo coral internodes collected from intermediate water depths (800-2000 m) in the eastern Pacific Ocean, to ΣCO2 in ambient seawater. The locations of the six bamboo coral samples span a wide range of ΣCO2, from 2330 to 2390 μmol/kg using data from the World Ocean Circulation Experiment (WOCE). Three samples perpendicular to the growth axis were collected from each of the six study corals from the exterior, middle, and interior of the specimen. Producing 1.00 microgram of powdered coral material required drilling an approximately 1.50 millimeter wide sample, integrating ~15 years of coral growth based on previously published studies of growth rates. Coral samples were analyzed for U/Ca using an inductively coupled plasma-mass spectrometer (ICP MS). U/Ca ratios varied both between corals and along the growth axis within individual corals. The U/Ca ratios of measured samples ranged from 0.01 to 0.03 μmol/mol. In addition, U/Ca varied from .012 to .021 μmol/mol within individual corals, suggesting a considerable amount of variation through time. The exterior coral sample U/Ca ratios measured on the ICP-MS were compared to WOCE data collected along line T17 (135°W), and found to positively correlate with an exponential trend (y = 4 × 10- 25*e0.022x, R2 of 0.88). Based upon this relationship, variability in U/Ca ratios for the past several hundred years can be used to understand changes in ΣCO2 and pH in the past and present oceans.
OS21A-1165
Investigating B/Ca of coccoliths using SIMS ion probe analysis
B/Ca ratios are proposed as a paleo-carbonate ion or paleo-pH proxy due to the preferential incorporation of borate ion into the calcite lattice, relative to boric acid which is the dominant species of B at lower pH. The relative importance of cellular regulation vs external pH on the carbonate B/Ca remains to be characterized for most organisms. Here we describe initial results of B/Ca analyses of coccoliths produced in laboratory culture under variable carbonate ion concentrations. Due to the impossibility of physically separating the micron-sized coccoliths from non-coccolith sediment material in quantities large enough for TIMS or ICP-MS analysis of B/Ca, eventual analysis of coccolith B/Ca from the fossil record will need to be conducted on individually picked coccoliths on the ion probe as is currently done for other trace element (eg. Sr/Ca) ratios. Hence, we employ the CAMECA IMS 1280 ion probe at the Northeast National Ion Microprobe Facility at Woods Hole Oceanographic Institution to measure B/Ca in coccoliths from cultures. We evaluated cleaning methods using a synthetic cleaning target (crushed marble) contaminated with noncalcifying algae. Cleaning is crucial for obtaining accurate B/Ca ratios and precluding sample charging. B/Ca ratios of different genera of modern coccoliths range from 5 to 25 umol/mol, 3 to 10fold lower than planktic foraminifera or abiogenic calcite precipitated in seawater in the same pH range. These low ratios suggest much more restricted uptake of B into the algae cell in the vesicle calcification used by coccolithophores, compared with the seawater vacuole calcification typical of foraminifera. Different coccolith species grown at the same pH exhibit different B/Ca ratios. A single species, Coccolithus pelagicus, cultured at a range of pH conditions from 7.7 to 8.4, exhibits no significant change in B/Ca ratios across the range of pH. One explanation is pH homeostasis at the calcification site. In possible support of pH homeostasis, the degree of calcification of this species was insensitive to variable CO2 and pH conditions in published experiments.