Chris Langdon

Effect of calcium carbonate saturation state on the calcification rate of an experimental coral reef

The concentration of CO2 in the atmosphere is projected to reach twice the preindustrial level by the middle of the 21st century. This increase will reduce the concentration of CO32− of the surface ocean by 30% relative to the preindustrial level and will reduce the calcium carbonate saturation state of the surface ocean by an equal percentage. Using the large 2650 m3 coral reef mesocosm at the BIOSPHERE-2 facility near Tucson, Arizona, we investigated the effect of the projected changes in seawater carbonate chemistry on the calcification of coral reef organisms at the community scale. Our experimental design was to obtain a long (3.8 years) time series of the net calcification of the complete system and all relevant physical and chemical variables (temperature, salinity, light, nutrients, Ca2+,pCO2, TCO2, and total alkalinity). Periodic additions of NaHCO3, Na2CO3, and/or CaCl2 were made to change the calcium carbonate saturation state of the water. We found that there were consistent and reproducible changes in the rate of calcification in response to our manipulations of the saturation state. We show that the net community calcification rate responds to manipulations in the concentrations of both Ca2+ and CO32− and that the rate is well described as a linear function of the ion concentration product, [Ca2+]0.69[CO32−]. This suggests that saturation state or a closely related quantity is a primary environmental factor that influences calcification on coral reefs at the ecosystem level. We compare the sensitivity of calcification to short-term (days) and long-term (months to years) changes in saturation state and found that the response was not significantly different. This indicates that coral reef organisms do not seem to be able to acclimate to changing saturation state. The predicted decrease in coral reef calcification between the years 1880 and 2065 A.D. based on our long-term results is 40%. Previous small-scale, short-term organismal studies predicted a calcification reduction of 14-30%. This much longer, community-scale study suggests that the impact on coral reefs may be greater than previously suspected. In the next century coral reefs will be less able to cope with rising sea level and other anthropogenic stresses.

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.

Ocean acidification compromises recruitment success of the threatened Caribbean coral Acropora palmata

Ocean acidification (OA) refers to the ongoing decline in oceanic pH resulting from the uptake of atmospheric CO2. 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 pCO2 levels that represent average ambient conditions during coral spawning (∼400 μatm) and the range of pCO2 increases that are expected to occur in this century [∼560 μatm (mid-CO2) and ∼800 μatm (high-CO2)]. Fertilization, settlement, and growth were all negatively impacted by increasing pCO2, 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 pCO2 conditions projected for the middle and the end of this century, respectively. Additional declines of 39% (mid-CO2) and 50% (high-CO2) 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.

Gas transfer experiment on Georges Bank using two volatile deliberate tracers

A gas exchange experiment was performed on Georges Bank using deliberate tracers sulfur hexafluoride (SF6) and helium 3 (3He). The concentrations of the tracers were measured in the water column over a period of 10 days. During this time the patch grew from an 8-km-long injection streak to an area of about 500 km2. The gas transfer velocity was determined from the change in the ratio of the tracers over time scaled to the ratio of their Schmidt numbers. A near-linear relationship between gas exchange and wind speed was observed based on four experimental points covering a wind speed range from 3 to 11 m/s. The results fall in the upper part of the range of gas transfer-wind speed relationships developed to date. Wind speeds during the experiment obtained from anemometers on the ship, on a free floating drifter, and on a fixed mooring showed significant differences. With the ability to measure gas transfer velocities over the ocean on timescales of several days, accurate wind speed/stress measurements are imperative to obtain a robust relationship between gas transfer and wind speed.

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.

Concurrent high resolution bio‐optical and physical time series observations in the Sargasso Sea during the spring of 1987

The evolution of bio-optical and physical properties of the upper layer of the open ocean has been examined at time scales from a few minutes to several months using recently developed multi-variable moored systems (MVMS). Concurrent, colocated time series measurements of horizontal currents, temperature, photosynthetically available radiation, transmission of a beam of collimated light (660 nm), stimulated chlorophyll fluorescence, and dissolved oxygen concentration were made. The systems were located at eight depths in the upper 160 m of the Sargasso Sea (34°N, 70°W) and were deployed three times for a total of 9 months in 1987. The first deployment data presented here show considerably more variability than those of the latter two deployments because of the dynamic springtime shoaling of the mixed layer and the accompanying phytoplankton bloom and more mesoscale variability associated with cold core rings and warm outbreak waters associated with the Gulf Stream. These data are used to demonstrate the utility of the MVMS and indicate the importance of high-frequency, long-term sampling of bio-optical and physical variables of the upper ocean for understanding and modeling dynamical changes in bio-optical properties, primary production, and carbon fluxes of the upper ocean on time scales ranging from minutes to seasons to decades. Some phenomena observed with the systems include (1) diurnal variations in bio-optical properties, (2) springtime stratification and rapid (∼2 days and less) episodic changes in the beam attenuation coefficient and in situ chlorophyll fluorescence, and (3) advective episodes associated with warm outbreaks of Gulf Stream waters and cold core Gulf Stream rings in the vicinity of the mooring.

Rates of respiration in the light measured in marine phytoplankton using an 18O isotope-labelling technique

Abstract For seven marine phytoplankton species, rates of net and gross O2 production (measured by the ΔO2 and 18O techniques) and 14C assimilation were determined in laboratory incubations. The 14C experiments were conducted in culture medium that was either N2-purged, air-purged or unpurged. In all species, the rates of respiration in the light are observed to be similar to, or greater than, the rates in the dark. For several species, respiration rates increased in the light but there was no Warburg effect (enhancement of 14C productivities in N2-purged cultures), indicating that photoenhancement of respiration was not due to photorespiration. In a few species, significant rates of photorespiration were detected with the 14C method, corresponding to 20 to > 100% of the rates of respiration in the light. The ratio of light respiration to production showed variable changes when phytoplankton were exposed to greater light intensities. These results suggest that phytoplankton do not necessarily increase rates of respiration when placed in an environment with increased irradiance.

Seasonal variability of bio-optical and physical properties in the Sargasso Sea

The seasonal variability of bio-optical and physical properties within the upper ocean at a site in the Sargasso Sea (34°N, 70°W) has been observed using multivariable moored systems (MVMS) during a 9-month period (March through November 1987). In addition, complementary meteorological data, sea surface height (Geosat) and sea surface temperature maps, and expendable bathythermograph (XBT) and shipboard profile data (physical and bio-optical) have been utilized for interpretation. The observations during March are characteristic of late wintertime conditions of a deep isothermal layer (∼18–19°C), but with intervening periods of warming due to the advection of warm outbreak waters associated with Gulf Stream meanders. The mixed layer depth shoals from greater than 160 m to about 25 m in late March (spring transition). Phytoplankton blooms follow the mixed layer shoaling. A succession of phytoplankton populations occurs during this transitional interval. Mesoscale variability associated with cold core rings and warm outbreak waters associated with the Gulf Stream are evident at various times. The mixed layer remains near 25 m for the summer and deepens in mid-September. A relatively intense subsurface maximum in chlorophyll develops at ∼75 m following the spring transition. The maximum persists, but weakens in mid-summer. The present study clearly indicates that important processes associated with and contributing to the seasonal cycle occur on short time and space scales and that integrated data sets obtained from moorings, ships, and satellites can be used to effectively study bio-optical and physical phenomena on time scales from minutes to seasons.

Dissolved oxygen monitoring system using a pulsed electrode: design, performance, and evoluation

An instrument for measuring dissolved oxygen in seawater is described. The device uses a conventional polarographic electrode in connection with chronoamperometry to overcome many of the problems limiting the performance of oxygen electrodes. The polarizing potential is applied as a pulse and the resulting current transient is sampled after a 1.5- to 3-s delay. Current during the first few seconds is 4 to 8 times the steady-state current and is unaffected by flow rate past the membrane provided a recovery time of 3 min or more is allowed between pulses. Secondary benefits include reproducibility typically better than ±0.8 μM and essentially drift-free operation for several weeks. At a pulsing rate of 120 h−1 analysis time is 2 to 4 min per bottle. A microcomputer controls all phases of the measuring process: pulse generation, data acquisition, reduction, and storage. Software is used to correct sensor output for temperature dependence exactly using the activation energy of permeation and the Arrhenius equation. An activity coefficient, a function of temperature and salinity, is computed to correct for salinity. Results of an intercomparison with Winkler determinations are presented and an example is given of the instruments use to measure the respiration of copepods in an unstirred 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.