Measurement of community metabolism and significance in the coral reef CO2 source-sink debate

Two methods are commonly used to measure the community metabolism (primary production, respiration, and calcification) of shallow-water marine communities and infer air–sea CO2 fluxes: the pH-total alkalinity and pH-O2 techniques. The underlying assumptions of each technique are examined to assess the recent claim that the most widely used technique in coral reefs (pH-total alkalinity), may have provided spurious results in the past because of high rates of nitrification and release of phosphoric acid in the water column [Chisholm, J. R. M. & Barnes, D. J. (1998) Proc. Natl. Acad. Sci. USA 95, 6566–6569]. At least three lines of evidence suggest that this claim is not founded. First, the rate of nitrification required to explain the discrepancy between the two methods recently reported is not realistic as it is much higher than the rates measured in another reef system and greater than the highest rate measured in a marine environment. Second, fluxes of ammonium, nitrate, and phosphorus are not consistent with high rates of nitrification and release of phosphoric acid. Third, the consistency of the metabolic parameters obtained by using the two techniques is in good agreement in two sites recently investigated. The pH-total alkalinity technique therefore appears to be applicable in most coral reef systems. Consequently, the conclusion that most coral reef flats are sources of CO2 to the atmosphere does not need revision. Furthermore, we provide geochemical evidence that calcification in coral reefs, as well as in other calcifying ecosystems, is a long-term source of CO2 for the atmosphere.

Global air-sea flux of CO2: An estimate based on measurements of sea–air pCO2 difference

Approximately 250,000 measurements made for the pCO2 difference between surface water and the marine atmosphere, ΔpCO2, have been assembled for the global oceans. Observations made in the equatorial Pacific during El Nino events have been excluded from the data set. These observations are mapped on the global 4° × 5° grid for a single virtual calendar year (chosen arbitrarily to be 1990) representing a non-El Nino year. Monthly global distributions of ΔpCO2 have been constructed using an interpolation method based on a lateral advection–diffusion transport equation. The net flux of CO2 across the sea surface has been computed using ΔpCO2 distributions and CO2 gas transfer coefficients across sea surface. The annual net uptake flux of CO2 by the global oceans thus estimated ranges from 0.60 to 1.34 Gt-C⋅yr−1 depending on different formulations used for wind speed dependence on the gas transfer coefficient. These estimates are subject to an error of up to 75% resulting from the numerical interpolation method used to estimate the distribution of ΔpCO2 over the global oceans. Temperate and polar oceans of the both hemispheres are the major sinks for atmospheric CO2, whereas the equatorial oceans are the major sources for CO2. The Atlantic Ocean is the most important CO2 sink, providing about 60% of the global ocean uptake, while the Pacific Ocean is neutral because of its equatorial source flux being balanced by the sink flux of the temperate oceans. The Indian and Southern Oceans take up about 20% each.

Mapping Intercellular CO2 Mole Fraction (Ci) in Rosa rubiginosa Leaves Fed with Abscisic Acid by Using Chlorophyll Fluorescence Imaging1 : Significance of Ci Estimated from Leaf Gas Exchange

Imaging of photochemical yield of photosystem II (PSII) computed from leaf chlorophyll fluorescence images and gas-exchange measurements were performed on Rosa rubiginosa leaflets during abscisic acid (ABA) addition. In air ABA induced a decrease of both the net CO2 assimilation (An) and the stomatal water vapor conductance (gs). After ABA treatment, imaging in transient nonphotorespiratory conditions (0.1% O2) revealed a heterogeneous decrease of PSII photochemical yield. This decline was fully reversed by a transient high CO2 concentration (7400 μmol mol−1) in the leaf atmosphere. It was concluded that ABA primarily affected An by decreasing the CO2 supply at ribulose-1,5-bisphosphate carboxylase/oxygenase. Therefore, the An versus intercellular mole fraction (Ci) relationship was assumed not to be affected by ABA, and images of Ci and gs were constructed from images of PSII photochemical yield under nonphotorespiratory conditions. The distribution of gs remained unimodal following ABA treatment. A comparison of calculations of Ci from images and gas exchange in ABA-treated leaves showed that the overestimation of Ci estimated from gas exchange was only partly due to heterogeneity. This overestimation was also attributed to the cuticular transpiration, which largely affects the calculation of the leaf conductance to CO2, when leaf conductance to water is low.

Microevolutionary responses in experimental populations of plants to CO2-enriched environments: parallel results from two model systems.

Despite the critical role that terrestrial vegetation plays in the Earth's carbon cycle, very little is known about the potential evolutionary responses of plants to anthropogenically induced increases in concentrations of atmospheric CO2. We present experimental evidence that rising CO2 concentration may have a direct impact on the genetic composition and diversity of plant populations but is unlikely to result in selection favoring genotypes that exhibit increased productivity in a CO2-enriched atmosphere. Experimental populations of an annual plant (Abutilon theophrasti, velvetleaf) and a temperate forest tree (Betula alleghaniensis, yellow birch) displayed responses to increased CO2 that were both strongly density-dependent and genotype-specific. In competitive stands, a higher concentration of CO2 resulted in pronounced shifts in genetic composition, even though overall CO2-induced productivity enhancements were small. For the annual species, quantitative estimates of response to selection under competition were 3 times higher at the elevated CO2 level. However, genotypes that displayed the highest growth responses to CO2 when grown in the absence of competition did not have the highest fitness in competitive stands. We suggest that increased CO2 intensified interplant competition and that selection favored genotypes with a greater ability to compete for resources other than CO2. Thus, while increased CO2 may enhance rates of selection in populations of competing plants, it is unlikely to result in the evolution of increased CO2 responsiveness or to operate as an important feedback in the global carbon cycle. However, the increased intensity of selection and drift driven by rising CO2 levels may have an impact on the genetic diversity in plant populations.