How does rising atmospheric CO2 affect marine organisms?

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Marsh Plants (Wetlands) -- Summary
Prior to the launching of CO2 Science, it had already been documented that atmospheric CO2 enrichment enhances vegetative productivity in wetland ecosystems (Jacob et al., 1995; Drake et al., 1996a). It had also been shown that elevated levels of atmospheric CO2 reduce insect and fungal damage to wetland plants (Drake et al., 1996b). We here review some additional studies that reinforce these findings and reveal still other positive wetland impacts of the ongoing rise in the air's CO2 content.

Rasse et al. (2003) developed a model for calculating net ecosystem exchange (NEE) of CO2 between C3 wetland sedge (Scirpus olneyi Gray) communities and the atmosphere, based on published ecophysiological data and measurements of various photosynthetic parameters made at the Chesapeake Bay CO2-enrichment study described by Curtis et al. (1989a,b). This model indicated that the S. olneyi community responded favorably to a near-doubled atmospheric CO2 concentration by increasing its NEE by 35-40%, which Rasse et al. compare to the mean net photosynthetic increase of 60% reported by Norby et al. (1999) in an extensive review that included several tree species and ecosystems. Rasse et al. then went on to say that "because Scirpus-dominated ecosystems are extremely productive (Drake and Leadley, 1991), a 35-40% productivity increase might represent a larger additional amount of carbon fixed as compared to a 60% increase in less productive forest ecosystems." In addition, because there are about six million square kilometers of wetlands worldwide, with approximately 15% of that area located in temperate regions (Mitsch et al., 1994), Rasse et al. concluded that "temperate C3 wetlands have a huge potential for increased plant productivity [and, therefore, carbon sequestration] during the 21st century."

Dakora and Drake (2000) exposed plant communities composed of Scirpus olneyi and the C4 grass Spartina patens to atmospheric CO2 concentrations of 360 and 660 ppm in open-top chambers in order to study the effects of elevated CO2 on nitrogenase activity and nitrogen fixation in these plants and in the non-symbiotic nitrogen-fixing microbes that inhabit the sediments in which the plants grow. They found that the extra CO2 increased nitrogenase activity by 35 and 13% in S. olneyi and S. patens, respectively, and that these stimulations led to increases in nitrogen incorporation of 73 and 23%, respectively, in the same plants. These responses were said by Dakora and Drake to be "in rough proportion to the relative effect of elevated CO2 on canopy photosynthesis measured throughout the day." They also found that the elevated CO2 significantly stimulated nitrogenase activity in the non-symbiotic nitrogen-fixing microbes that lived in the soil sediments, suggesting that increases in the air's CO2 content produce "an increase in the N2-fixing activity of free-living [microbes] in the marsh ecosystem."

These several findings suggest that ever more nitrogen will likely be made available to earth's terrestrial plants by the nitrogen-fixing components of the planet's biosphere as the air's CO2 content continues to climb, as discussed in our Editorial of 10 Dec 2003. The new evidence also contradicts the claim of Hungate et al. (2003) that "we should not count on carbon storage by land ecosystems to make a massive contribution to slowing climate change." It would appear that not only can we count on earth's forests, shrublands and grasslands to capture and sequester a massive amount of carbon, we can also count on the planet's coastal wetlands to do the same. In our Carbon Sequestration Commentary of 10 July 2002, for example, we indicate how marsh plants appear fully capable of readily obtaining the extra nitrogen they need to enable them to sequester the vast and rapidly-increasing amounts of carbon locked within them, further refuting the contention of Hungate et al. that there will not be enough nitrogen to accomplish this task in a future world of higher atmospheric CO2 concentration. In fact, we report that as the air's CO2 content rose by some 30% over the past century and a half, the total nitrogen sequestered in two Chesapeake Bay tidal marshes concurrently rose by a factor of three over the baseline value that had prevailed for the prior two millennia, noting that on the basis of this evidence, Hussein and Rabenhorst (2002) predict even greater nitrogen capture rates for the coming century.

Continuing in much the same vein, Hussein et al. (2004) measured carbon sequestration along two transects across submerging coastal landscapes (Hell Hook and Cedar Creek) of the Chesapeake Bay in Dorchester County, Maryland, USA, from which data they developed a model of carbon sequestration by coastal marshes. This work demonstrated that "coastal marsh soils are accreting vertically and migrating laterally over the [adjacent] low-lying forest soils to keep pace with sea-level rise," and that during the last 150 years, the rate of carbon sequestration by the marsh soils averaged 83.5 23 g m-2 yr-1, whereas prior to that period it had averaged 29.2 5.35 g m-2 yr-1.

These sequestration rates are much greater than those of either local forest or agricultural soils. What is more, the three scientists report that "carbon sequestration in mineral soils of agro- and upland-forest ecosystems is generally of limited capacity and tends to reach [a] steady-state condition within relatively short time," but that "in coastal marsh soils, carbon sequestration will continue to occur with time by accumulation in the organic horizons, and with increasing storage capacity." Based on a model they developed from their data, for example, Hussein et al. project that, driven by sea-level rise, carbon sequestration by coastal marsh ecosystems over the next 100 years will average 400 162 g m-2 yr-1. Their final conclusion, therefore, is that "coastal marsh ecosystems tend to sequester carbon continuously with increasing storage capacity as marsh age progresses," and that "carbon sequestration in coastal marsh ecosystems under positive accretionary balance acts as a negative feedback mechanism to global warming."

Returning to the Chesapeake Bay wetland study at the 17-year point of its progression, Rasse et al. (2005) evaluated the long-term effects of atmospheric CO2 enrichment on the net CO2 exchange, shoot density and shoot biomass of the wetland sedge, Scirpus olneyi, as well as how these effects have been influenced by salinity, which is one of the main environmental stressors of the wetland. In every year of the past 17 years, they found that the net CO2 exchange rate and shoot biomass and density of the plants growing in the CO2-enriched (ambient +340 ppm) air were all greater than they were among the plants growing in ambient air. In the case of the net CO2 exchange rate, the extra CO2 boosted it by 80% in the first year of the study, but the enhancement declined to about 35% by the end of the third year and remained relatively constant at that value over the following 15 years. Shoot biomass and density also increased, but whereas the CO2-induced stimulation of the net CO2 exchange rate remained essentially constant over the past 15 years, the CO2-induced stimulations of shoot biomass and density increased over time. After 5 years of a nearly constant stimulation of 16%, for example, shoot density increased in near linear fashion to a value 128% above the ambient-air value at the end of year 17. The response of shoot biomass to CO2 enrichment was also nearly linear, reaching a value approximately 70% above ambient at year 17. What is more, the trends in shoot density and biomass do not appear to be leveling off, leading one to wonder just how high the CO2-induced stimulations will ultimately rise.

Salinity was closely correlated with net CO2 exchange, shoot density and shoot biomass, such that the higher the salinity, the more detrimental were its effects on these variables. Nevertheless, even at the highest levels of salinity reported, atmospheric CO2 enrichment was able to produce a positive, albeit reduced, stimulatory effect on net CO2 exchange. For shoot biomass and density, the responses were better still. Not only did atmospheric CO2 enrichment essentially eradicate the detrimental effects of salinity, there was, in the words of Rasse et al., "circumstantial evidence suggesting that salinity stress increased the stimulation of shoot density by elevated atmospheric CO2 concentration."

Several important things are demonstrated by this experiment. First, as the researchers state, their results "leave no doubt as to the sustained response of the salt march sedge to elevated atmospheric CO2 concentration." Second, given the fact that the initial responses of the three growth variables declined or remained low during the first few years of the study, but leveled out or increased thereafter, it is clear that much more long-term research needs to be carried out if we are to ascertain the full and correct impacts of atmospheric CO2 enrichment on plants (see also, in this regard, our Editorial of 5 Mar 2003). In the case of the wetland sedge of this study, for example, it took about ten growing seasons before an increasing trend in the shoot density could clearly be recognized. Last of all, there is the researchers' "most important finding," i.e., "that a species response to elevated atmospheric CO2 concentration can continually increase when [it] is under stress and declining in its natural environment." This result is highly significant and once again bears witness to the fact that earth's rising atmospheric CO2 concentration is not a catastrophic disaster, as climate alarmists would have one believe, but actually a boon to the biosphere for which we will all someday be extremely grateful.

After one additional year of studying the Chesapeake Bay wetland, Erickson et al. (2007) presented data "on 18 years of measurement of above and belowground biomass, tissue N concentration and total standing crop of N for a Scirpus olneyi-dominated (C3 sedge) community, a Spartina patens-dominated (C4 grass) community and a C3-C4-mixed species community exposed to ambient and elevated (ambient + 340 ppm) atmospheric CO2 concentration [via open-top chamber technology] in natural salinity and sea level conditions of a Chesapeake Bay wetland." This report indicated that "elevated atmospheric CO2 enhancement of C3 biomass was sustained through time in the S. olneyi-dominated community, averaging about 40% for shoots and 26% for roots, whereas elevated CO2 had no significant overall effect on biomass production in the C4 grass community." In addition, they determined that "the greatest amount of carbon was added to the S. olneyi-dominated community during years when shoot N concentration was reduced the most, suggesting that the availability of N was not the most or even the main limitation to elevated CO2 stimulation of carbon accumulation in this ecosystem." These findings, according to the four researchers, "demonstrate that elevated CO2 effects on biomass production can be sustained through time," even when N availability is at the lowest of levels typically encountered in the wetland, and they note that similar CO2-induced "sustained enhancement of growth has been found in a scrub oak ecosystem (Dijkstra et al., 2002; Hymus et al., 2002), a tallgrass prairie (Owensby et al., 1999) and several forested ecosystems (Norby et al., 2005), indicating that increased productivity of many ecosystems will follow global increases in atmospheric CO2 concentration."

Working with Phragmites australis -- a wetland plant found in every U.S. state, as well as in numerous places throughout the world -- in a study with very different implications, Scholefield et al. (2004) measured isoprene emissions from plants growing at different distances from a natural CO2 spring located in central Italy, where atmospheric CO2 concentrations of approximately 350, 400, 550 and 800 ppm had likely prevailed for the entire lifetimes of the plants. This research indicated that as long-term atmospheric CO2 concentrations rose ever higher, plant isoprene emissions dropped ever lower: over the first 50-ppm increase in the air's CO2 concentration, they were reduced to approximately 65% of what they were at ambient CO2, while for CO2 increases of 200 and 450 ppm, they were respectively reduced to only about 30% and 7% of what they were in ambient-CO2 air, as best we can determine from the bar graph of the authors' relevant data.

What are the implications of these huge CO2-induced reductions in plant isoprene emissions?

To properly answer this question, one must understand that isoprene is a highly reactive non-methane hydrocarbon (NMHC) that is emitted by vegetation in copious quantities at current atmospheric CO2 concentrations, and that it is responsible for the production of vast amounts of plant- and animal-harming ozone (Chameides et al., 1988; Harley et al., 1999). It has been calculated by Poisson et al. (2000), for example, that current concentrations of NMHC emissions (the vast majority of which are isoprene) increase surface ozone concentrations by 50-60% over land and by as much as 40% over the world's oceans. In addition, biogenic NMHCs (with isoprene being the most important) play a major role in the global tropospheric chemistry of methane, which is one of the atmosphere's most powerful greenhouse gases, boosting methane's atmospheric lifetime by approximately 14% above what it would be in the absence of isoprene (Poisson et al., 2000). This being the case, if other plants behave similarly, and much evidence suggests that they do (Monson and Fall, 1989; Loreto and Sharkey, 1990; Sharkey et al., 1991; Loreto et al., 2001; Rosenstiel et al., 2003), we can expect the ongoing rise in the air's CO2 content to (1) enhance plant productivity, (2) mitigate the deleterious consequences of one of earth's worst air pollutants (ozone), and (3) reduce the atmospheric lifetime of one of the planet's most powerful greenhouse gases (methane).

And this is the substance whose rising concentration Al Gore and James Hansen consider to constitute the greatest threat ever to face the biosphere??? Give us a break! Carbon dioxide's accumulation in the atmosphere is one of the best things ever to happen to the planet and its many life-forms. We need more -- not less -- of this amazing elixir of life.

References
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Curtis, P.S., Drake, B.G., Leadly, P.W., Arp, W.J. and Whigham, D.F. 1989a. Growth and senescence in plant communities exposed to elevated CO2 concentrations on an estuarine marsh. Oecologia 78: 20-26.

Curtis, P.S., Drake, B.G. and Whigham, D.F. 1989b. Nitrogen and carbon dynamics in C3 and C4 estuarine marsh plants grown under elevated CO2 in situ. Oecologia 78: 297-301.

Dakora, F.D. and Drake, B.G. 2000. Elevated CO2 stimulates associative N2 fixation in a C3 plant of the Chesapeake Bay wetland. Plant, Cell and Environment 23: 943-953.

Dijkstra, P., Hymus, G.J., Colavito, D. et al. 2002. Elevated atmospheric CO2 stimulates shoot growth in a Florida scrub oak ecosystem. Global Change Biology 8: 90-103.

Drake, B.G. and Leadley, P.W. 1991. Canopy photosynthesis of crops and native plant communities exposed to long-term elevated CO2. Plant, Cell and Environment 14: 853-860.

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Last updated 24 October 2007