How does rising atmospheric CO2 affect marine organisms?

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Nectar -- Summary
Lake and Hughes (1999) grew nasturtiums (Tropaeolum majus) from seed through flowering and senescence (77 days) in growth chambers maintained at atmospheric CO2 concentrations of either 380 or 760 ppm in order to determine the effects of elevated CO2 on vegetative and reproductive growth, as well as flower characteristics, including numbers, longevity and nectar quantity and quality. This effort revealed that the doubled CO2 concentration of their study increased total plant biomass by 35% and root biomass by 78%. However, it did not affect reproductive biomass, nor did it alter individual flower dry weights. In addition, atmospheric CO2 enrichment did not affect time to flowering, flower longevity or number of flowers produced. Neither did it delay the onset of senescence. However, elevated CO2 positively impacted nectar volume in CO2-enriched flowers, increasing it by 2.4-fold over that produced in ambient-air flowers; and it did so without lowering the sugar and amino acid characteristics of the nectar.

Dag and Eisikowitch (2000) divided a 0.5-acre greenhouse located in the center of the Arava Valley in the southern part of Israel into two parts, one of which was exposed to ambient air and one of which was exposed to air that had a CO2 concentration of 1000 ppm throughout the morning, 400 ppm between 1300 and 1500 hours, and 600 ppm until the next morning. Under these conditions they grew melons (Cucumis melo); and in the early flowering stage they collected and measured the volume of nectar produced per flower between 0900 and 1530 hours along with the sugar concentration of the nectar. In doing so, they found that average nectar volumes per flower were significantly higher in the CO2-enriched sector of the greenhouse than in the control sector, sometimes by as much as 100%; and since the sugar concentration of the nectar was found to be the same in both treatments, sugar production per flower was stimulated by an identical amount (as much as 100%) in the CO2-enriched air. As a result, and noting that the only pollinator used in greenhouse production of melons in Israel is the honey bee, the two researchers concluded that "improvement in nectar reward can increase the attractiveness of the flowers to the bees, increase pollination activity and consequently increase the fruit set and the yield." Consequently, as ever more greenhouse managers begin to implement atmospheric CO2 enrichment techniques, we may expect to see huge benefits routinely accrue to them.

Erhardt et al. (2005) grew well-watered Epilobium angustifolium L. plants (perennial temperate clonal herbs that colonize nutrient-rich open habitats) from the seeds of five different genotypes in pots containing 12 liters of loamy soil maintained at high and low levels of nutrients by weekly supplying them with 25 ml of either 1.0 N (high level) or 0.5 N (low level) Hoagland's solution. The experiment lasted from April 1995 to July 1996 (two full growing seasons) and was conducted in naturally-lit controlled environment chambers housed within a greenhouse, half of which chambers were supplied with ambient air having a CO2 concentration of approximately 350 ppm and half of which were supplied with CO2-enriched air having a concentration of about 650 ppm. Under these conditions, and in the second year of the study when most of the plants were flowering, nectar was extracted from the flowers and its volume and sugar concentration were determined, along with its amino acid concentration and the total amino acid content per flower. This protocol revealed, in the words of the three researchers, that "elevated CO2 significantly increased nectar production per day (+51%, p < 0.01), total sugar per flower (+41%, p < 0.05), amino acid concentration (+65%, p < 0.05) and total amino acids per flower (+192%, p < 0.001)," and these responses occurred across the board with all genotypes.

What are the implications of these several findings? Erhardt et al. say that Galen and Plowright (1985) found that "increased nectar rewards led to longer bumblebee tenure on flowers and greater pollen receipt in E. angustifolium, and that bees visited more flowers per plant on plants with more nectar." In addition, they report that "in other plant species higher nectar rewards also usually led to increases in components of plant fitness (e.g., Thomson, 1986; Mitchell and Waser, 1992; Mitchell, 1993; Hodges, 1995; Irwin and Brody, 1999)."

As the CO2 content of the air continues to increase, therefore, it is likely that in addition to the increases in biomass we would expect to see produced by the aerial fertilization effect of the rising atmospheric CO2 concentration, plant fitness and flower pollination should also be benefited, all of which consequences should tend to increase the fruit, grain and vegetable yields of agricultural crops, as well as the analogous production of the world's natural vegetation, leading to a win-win situation for both components of the terrestrial biosphere.

Dag, A. and Eisikowitch, D. 2000. The effect of carbon dioxide enrichment on nectar production in melons under greenhouse conditions. Journal of Apicultural Research 39: 88-89.

Erhardt, A., Rusterholz, H.-P. and Stocklin, J. 2005. Elevated carbon dioxide increases nectar production in Epilobium angustifolium L. Oecologia 146: 311-317.

Galen, C. and Plowright, R.C. 1985. The effects of nectar level and flower development on pollen carry-over in inflorescences of fireweed (Epilobium augustifolium) (Onagraceae). Canadian Journal of Botany 63: 488-491.

Hodges, S.A. 1995. The influence of nectar production on hawkmoth behavior, self pollination, and seed production in Mirabilis multiflora (Nyctaginaceae). American Journal of Botany 82: 197-204.

Irwin, R.E. and Brody, A.K. 1999. Nectar-robbing bumble bees reduce the fitness of Ipomopsis aggregata (Polemoniaceae). Ecology 80: 1703-1712.

Lake, J.C. and Hughes, L. 1999. Nectar production and floral characteristics of Tropaeolum majus L. grown in ambient and elevated carbon dioxide. Annals of Botany 84: 535-541.

Mitchell, R.J. 1993. Adaptive significance of Ipomopsis aggregata nectar production: observation and experiment in the field. Evolution 47: 25-35.

Mitchell, R.J. and Waser, N.M. 1992. Adaptive significance of Ipomopsis aggregata nectar production: pollination success of single flowers. Ecology 73: 633-638.

Thomson, J.D. 1986. Pollen transport and deposition by bumble bees in Erythronium: influences of floral nectar and bee grooming. Journal of Ecology 74: 329-341.

Last updated 5 March 2008