Light-and C02-saturated photosynthetic rates of the submersed aquatic plants Hydrilla verticillata, Ceratophyllum demersum, and Myriophyllum spicatum were 50 to 60 nLmol OJmg Chl hr at 30 C. At ar levels of C02, the rates were less than 5% of those achieved by terrestrial C3 plants. The The majority of terrestrial plants can be classified as C3 or C. plants, based on specific characteristics associated with their photosynthetic pathways (4). C4 plants typically have higher photosynthetic rates and greater productivity than C3 plants (4). The photosynthetic mechanisms of submersed macrophytes, although basic to their productivity, have received limited attention. For Hydrilla and Ceratophyllum, the photosynthetic pathways and most of the associated characteristics are unknown. Myriophyllum apparently exhibits characteristics of both C3 and C4 plants. The initial product of CO2 fixation is 3-P-glycerate (29), as in C3 plants; but it reportedly also has a high optimum temperature for photosynthesis and a low CO2 compensation point (29), which are characteristics usually associated with C4 plants. In contrast, the submersed macrophytes Egeria densa and Lagarosiphon major, which belong to the same family as Hydrilla, possess CO2 compensation points similar to those of C3 plants, and their photosynthesis is inhibited by 02 (9). Thus, aquatic macrophytes appear to exhibit some diversity in regard to their photosynthetic mechanism.In an aquatic environment, the inorganic carbon can exist in several forms: free C02, H2CO3, HC03-, or C032-, depending on the pH. For both aquatic and terrestrial plants, free CO2 is the form most readily utilized for photosynthesis (25). A number of submersed plants, including Myriophyllum, reportedly can use HCO3-ions in addition to free CO2 for photosynthesis (30). Recent work, however, suggests that several aquatic species are unable to use HC03-ions (9). It has been argued that the ability to use HC03-ions would provide an aquatic plant with a competitive advantage in alkaline waters (21). A further factor that may influence the competitive success of an aquatic plant is its photosynthetic response to light. Egeria, for example, reportedly replaces both Elodea and Lagarosiphon because of its lower light requirement for photosynthesis (9). In this study, we have examined the ability of Hydrilla, Ceratophyllum, and Myriophyllum to use HCO3-ions for photosynthesis and also the photosynthetic responses of these plants to varying irradiance. The possible ecological implications of these factors are discussed.
Chlorophyll a concentrations in Lake Pearl, Florida, increased as the percentage of the lake's total volume infested with aquatic macrophytes decreased. Using data from 32 Florida lakes having a wide range of limnological characteristics, we demonstrated that predictions of chlorophyll a concentrations could be improved by including a term for the percentage of the lake's total volume infested with macrophytes in existing nutrient–chlorophyll models. Our best-fit multivariate regression equation was[Formula: see text]where CHLA is the chlorophyll a concentration (milligrams per cubic metre), TN is the total nitrogen concentration (milligrams per cubic metre), TP is the total phosphorus concentration (milligrams per cubic metre), and PVI is the percentage of the lake's total volume infested with macrophytes. By use of this equation, we assessed the potential effect of aquatic macrophytes on chlorophyll yields and Secchi disc transparencies in lakes of different trophic status.
PURPOSE: The research reported herein investigated the effects of a range of lime (as calcium hydroxide; Ca(OH) 2) dosage levels on the growth of Sago pondweed in outdoor experimental mesocosms. BACKGROUND: Lime (CaCO 3 and Ca(OH) 2) application has been used primarily as a lake rehabilitation technique for limiting algal growth by controlling phosphorus availability in the water column and its release from the sediment (Prepas et al. 1990). At supersaturated concentrations, calcium co-precipitates with phosphorus and settles from the water column. As a deposited layer on the sediment, it can adsorb additional phosphorus (at pH > 8), preventing it from diffusing into the water column for algal uptake. More recently, researchers have found that lime additions can suppress submersed macrophyte growth as well (Babin et al. 1992, Chambers et al. 2001, Prepas et al. 2001). However, the mode of growth suppression, dosage levels, and exposure time requirements are not entirely known. Since lime application drives the pH upward, it may stress macrophyte metabolic activities by inducing inorganic carbon limitation for photosynthesis in hardwater systems. Lime additions to aquatic systems at varying concentrations have not resulted in clear macrophyte community responses. For instance, single dosages of lime at modest concentration levels (<110 mg Ca L-) to hardwater lakes were accompanied by control of macrophyte biomass (species) for over a year (Reedyk et al. 2001). However, Chambers et al. (2001) indicated that exposure time, in addition to concentration, might be important in the control of macrophytes in aquatic systems located in the Canadian Great Plains. These findings suggest that lime application may be a very promising technique for integrated control of both macrophyte and algal production in eutrophic hardwater systems. More information is needed regarding the mode of action, dosage requirements, and impacts on different aquatic macrophyte species in order to develop sound integrated management and control strategies using lime.
We developed an approach for assessing the trophic status of lakes having growths of aquatic macrophytes because conventional criteria for classifying trophic state emphasize conditions in the open water and ignore the nutrients, plant biomass, and production associated with macrophytes. We propose that a potential water column nutrient concentration be determined through adding the nutrients contained in macrophytes to those in the water. Potential nutrient concentrations can be used in existing indices to classify lake trophic status. This approach permits a first approximation of the potential impact of macrophytes on lake trophic state.
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