For several multi-week periods, Idso (1997) grew specimens of corkscrew vallisneria (Vallisneria tortifolia) in several 10- and 29-gallon glass tanks (containing 10-cm bottom-layers of common aquarium gravel) that were filled with tap water maintained within 0.5°C of either 18.2°C or 24.5°C, while the semi-sealed air spaces above these "Poor Man's Biospheres," as he christened them, were maintained at a number of different CO2 concentrations. With the harvesting of plants at the end of the study, this protocol revealed that the CO2-induced growth enhancement of the plants was linear (in contrast to the gradually-declining CO2-induced growth enhancements typically exhibited by most terrestrial plants as the air's CO2 content climbs ever higher), and that the linear relationship extended to the highest atmospheric CO2 concentration studied: 2100 ppm. In addition, he found that the CO2-induced growth increase experienced by the plants in the higher of the two water temperature treatments (a 128% increase in going from an atmospheric CO2 concentration of 365 ppm to one of 2100 ppm) was 3.5 times greater than that of the plants in the lower water temperature treatment. Although this response may seem rather dramatic, it is not unique; for Idso reports that Titus et al. (1990), who studied the closely related Vallisneria americana, "observed that the biomass of their experimental plants also rose linearly with the CO2 content of the air above the water within which they grew, and that [it] did so from the value of the [then] current global mean (365 ppm) to a concentration fully ten times larger [our italics]."

In another study of a closely allied species, Yan et al. (2006) collected turions of Vallisneria spinulosa from Liangzi Lake, Hubei Province, China, and planted them in tanks containing 15-cm-deep layers of fertile lake sediments, topped with 40 cm of lake water, that were placed in two glasshouses - one maintained at the ambient atmospheric CO2 concentration of 390 ppm and the other maintained at an elevated concentration of 1000 ppm - where the plants grew for a period of 120 days, after which they were harvested and the dry weights of their various organs determined. As they describe it, this work indicated that the "total biomass accumulation of plants grown in the elevated CO2 was 2.3 times that of plants grown in ambient CO2, with biomass of leaves, roots and rhizomes increasing by 106%, 183% and 67%, respectively." Most spectacularly of all, they report that "turion biomass increased 4.5-fold," because "the mean turion numbers per ramet and mean biomass per turion in elevated CO2 were 1.7-4.3 and 1.9-3.4 times those in ambient CO2."

Over in Denmark, in a study of small slow-growing evergreen perennials called isoetids that live submersed along the shores of numerous freshwater lakes, Andersen et al. (2006) grew specimens of Littorella uniflora in sediment cores removed from Lake Hampen in 75-liter tanks with 10-cm overburdens of filtered lake water for a period of 53 days, while measuring various plant, water and sediment properties, after which they destructively harvested the plants and measured their biomass. Throughout this period, half of the tanks had ambient air bubbled through their waters, while the other half were similarly exposed to a mixture of ambient air and pure CO2 that produced a 10-fold increase in the air's CO2 concentration. This ultra-CO2-enrichment led to a 30% increase in plant biomass, as well as "higher O2 release to the sediment which is important for the cycling and retention of nutrients in sediments of oligotrophic softwater lakes." And when the ultra-CO2-enrichment was maintained for an entire growing season (May-November), Andersen and Andersen (2006) report that the ten-fold increase in aquatic CO2 concentration enhanced the biomass production of Littorella uniflora by a much larger 78%.

In a study of an "in-between" type of plant that has submersed roots and rhizomes that are anchored in water-body sediments, but which has floating leaves on the surface of the water and emergent flowers that protrude above the water surface, Idso et al. (1990) grew water lilies (Nymphaea marliac) for two consecutive years in sunken metal stock tanks located out-of-doors at Phoenix, Arizona (USA) and enclosed within clear-plastic-wall open-top chambers through which air of either 350 or 650 ppm CO2 was continuously circulated. This work revealed that in addition to the leaves of the plants being larger in the CO2-enriched treatment, there were 75% more of them than there were in the ambient-air tanks at the conclusion of the initial five-month-long growing season. Each of the plants in the high-CO2 tanks also produced twice as many flowers as the plants growing in ambient air; and the flowers that blossomed in the CO2-enriched air were more substantial than those that bloomed in the air of ambient CO2 concentration: they had more petals, the petals were longer, and they had a greater percent dry matter content, such that each flower consequently weighed about 50% more than each flower in the ambient-air treatment. In addition, the stems that supported the flowers were slightly longer in the CO2-enriched tanks; and the percent dry matter contents of both the flower and leaf stems were greater, so that the total dry matter in the flower and leaf stems in the CO2-enriched tanks exceeded that of the flower and leaf stems in the ambient-air tanks by approximately 60%.

Just above the surface of the soil that covered the bottoms of the tanks, there were also noticeable differences. Plants in the CO2-enriched tanks had more and bigger basal rosette leaves, which were attached to longer stems of greater percent dry matter content, which led to the total biomass of these portions of the plants being 2.9 times greater than the total biomass of the corresponding portions of the plants in the ambient-air tanks. In addition, plants in the CO2-enriched tanks had more than twice as many unopened basal rosette leaves.

The greatest differences of all, however, were hidden within the soil that covered the bottoms of the stock tanks. When half of the plants were harvested at the conclusion of the first growing season, for example, the number of new rhizomes produced over that period was discovered to be 2.4 times greater in the CO2-enriched tanks than it was in the ambient-air tanks; while the number of major roots produced there was found to be 3.2 times greater. And as with all other plant parts, the percent dry matter contents of the new roots and rhizomes were also greater in the CO2-enriched tanks. Overall, therefore, the total dry matter production within the submerged soils of the water lily ecosystems was 4.3 times greater in the CO2-enriched tanks than it was in the ambient-air tanks; while the total dry matter production of all plant parts - those in the submerged soil, those in the free water, and those in the air above - was 3.7 times greater in the high-CO2 enclosures.

Over the second growing season, the growth enhancement in the high-CO2 tanks was somewhat less; but the plants in those tanks were so far ahead of the plants in the ambient-air tanks that in their first five months of growth, they produced what it took the plants in the ambient-air tanks fully 21 months to produce.

Moving on to plants that are exclusively floating freshwater macrophytes, Idso (1997) grew many batches of the common water fern (Azolla pinnata) over a wide range of atmospheric CO2 concentrations at two different water temperatures (18.2°C and 24.5°C) in Poor Man's Biospheres for periods of several weeks. This work revealed that a 900-ppm increase in the CO2 concentration of the air above the tanks led to only a 19% increase in the biomass production of the plants floating in the cooler water, but that it led to a 66% biomass increase in the plants floating in the warmer water.

In an earlier study of Azolla pinnata, Idso et al. (1989) conducted three separate two- to three-month experiments wherein they grew batches of the floating fern out-of-doors in adequately-fertilized water contained in sunken metal stock tanks located within clear-plastic-wall open-top chambers that were continuously maintained at atmospheric CO2 concentrations of either 340 or 640 ppm, during which time the plants were briefly removed from the water and weighed at weekly intervals, while their photosynthetic rates were measured at hourly intervals from dawn to dusk on selected cloudless days. As a result of this protocol, they found that the photosynthetic and growth rates of the plants growing in ambient air "first decreased, then stagnated, and finally became negative when mean air temperature rose above 30°C." In the high CO2 treatment, on the other hand, they found that "the debilitating effects of high temperatures were reduced: in one case to a much less severe negative growth rate, in another case to merely a short period of zero growth rate, and in a third case to no discernible ill effects whatsoever - in spite of the fact that the ambient treatment plants in this instance all died."

Last of all, in a study of an emergent freshwater macrophyte, Ojala et al. (2002) grew water horsetail (Equisetum fluviatile) plants at ambient and double-ambient atmospheric CO2 concentrations and ambient and ambient + 3°C air temperatures for three years, although the plants were only subjected to the double-ambient CO2 condition for approximately five months of each year. This work revealed that the increase in air temperature boosted maximum shoot biomass by 60%, but that the elevated CO2 had no effect on this aspect of plant growth. However, elevated CO2 and temperature - both singly and in combination - positively impacted root growth, which was enhanced by 10, 15 and 25% by elevated air temperature, CO2, and the two factors together, respectively.

In light of the several experimental findings discussed above, we conclude that the ongoing rise in the air's CO2 content will likely have significant positive impacts on most freshwater macrophytes, including submersed, floating and emergent species.

References
Andersen, T. and Andersen, F.O. 2006. Effects of CO2 concentration on growth of filamentous algae and Littorella uniflora in a Danish softwater lake. Aquatic Botany 84: 267-271.

Andersen, T., Andersen, F.O. and Pedersen, O. 2006. Increased CO2 in the water around Littorella uniflora raises the sediment O2 concentration. Aquatic Botany 84: 294-300.

Idso, S.B. 1997. The Poor Man's Biosphere, including simple techniques for conducting CO2 enrichment and depletion experiments on aquatic and terrestrial plants. Environmental and Experimental Botany 38: 15-38.

Idso, S.B., Allen, S.G., Anderson, M.G. and Kimball, B.A. 1989. Atmospheric CO2 enrichment enhances survival of Azolla at high temperatures. Environmental and Experimental Botany 29: 337-341.

Idso, S.B., Allen, S.G. and Kimball, B.A. 1990. Growth response of water lily to atmospheric CO2 enrichment. Aquatic Botany 37: 87-92.

Ojala, A., Kankaala, P. and Tulonen, T. 2002. Growth response of Equisetum fluviatile to elevated CO2 and temperature. Environmental and Experimental Botany 47: 157-171.

Titus, J.E., Feldman, R.S. and Grise, D. 1990. Submersed macrophyte growth at low pH. I. CO2 enrichment effects with fertile sediment. Oecologia 84: 307-313.

Yan, X., Yu, D. and Li, Y.-K. 2006. The effects of elevated CO2 on clonal growth and nutrient content of submerged plant Vallisneria spinulosa. Chemosphere 62: 595-601.