Photosynthesis and Growth Rates in Salvinia molesta and Eichhornia crassipes

(1) Net carbon exchange was measured in communities of the floating aquatic macrophytes Salvinia molesta and Eichhornia crassipes. Maximum rates of carbon dioxide uptake were 20-23 mg dm-2(water surface) h-1and 30-35 mg dm-2h-1respectively. These occurred in well-developed communities in mid-summer, when irradiance and temperatures were at their highest. These rates are poor to mediocre compared with those of crop plants under similar conditions. When the leaf laminae were removed from a canopy of Eichhornia, leaving only the bulbous green petioles, the total carbon dioxide uptake dropped by 88%, although the surface area of the petioles was almost equal to the single surface area of the laminae. (2) Both net carbon uptake in the day and efflux at night were fairly constant when related to unit water surface area covered by the plant community, while the plant biomass per unit area had only a small effect on exchange rates. (3) Measured changes in dissolved inorganic carbon in the water accounted for only a small part of the total carbon dioxide exchange of the community, especially for Salvinia. (4) Both species had high relative growth rates and short doubling times with respect to both dry weight and leaf area when they were in uncrowded conditions. When plants became competitive with their neighbours these growth rates dropped rapidly, biomass per unit water surface area increased, and the plants proceeded to further developmental phases. However, all the while the plants could float into open water they remained in a vigorous colonizing phase with a high rate of new leaf production and many active meristems. Throughout this time they maintained high and constant relative growth rates and a low biomass per unit water surface area. (5) These two emergent macrophytes are often regarded as noxious weeds because of their ability to cover large water surfaces very rapidly. It is concluded that this ability depends not on any intrinsic photosynthetic advantage compared to other plants, but on their free-floating habit and many-branched growth pattern which enables them to remain in an active vegetative form until the water surface is covered.