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.

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Abstract. 1. Plants may compensate for the effects of herbivory, especially under favourable growing conditions, limited competition, and minimal topdown regulation. These conditions characterise many disturbed wetlands dominated by introduced plants, implying that exotic, invasive weeds in these systems should exhibit strong compensatory responses.2. The Australian native Melaleuca quinquenervia is highly invasive in the Florida Everglades, U.S.A., where it experiences limited competition or herbivory from native species, making it a likely candidate for compensation. The introduced biological control agent Oxyops vitiosa feeds exclusively on the seasonal flushes of developing foliage at branch apices, which represents &15% of the total foliar biomass.3. The hypothesis that M. quinquenervia compensates for folivory by O. vitiosa was tested in a series of field-based experiments. Trees experiencing folivory over four consecutive years maintained similar levels of foliar biomass after attack yet possessed twice the number of leaf-bearing terminal stems as undamaged trees. The biomass of these stems was similar among attacked and unattacked trees, indicating that herbivore-damaged trees produce greater quantities of smaller terminal branches. However, undamaged trees were 36 times more likely to reproduce than herbivore-damaged trees.4. In a separate herbivore exclusion study, a single bout of herbivory on previously undamaged M. quinquenervia trees caused an 80% reduction in reproductive structures the following season. Herbivore-damaged trees also possessed 54% fewer fruits than undamaged trees. An increase in the herbivory frequency (two bouts per year) or magnitude (100% simulated herbivory) did not result in a further reduction in fitness.5. It has been concluded that M. quinquenervia partially compensates for herbivory by producing new stems and replacing foliage, but this compensation results in a substantial reduction in reproduction.

There has been some debate recently about the cause of the decline of water hyacinth on Lake Victoria. While much of this evidence points to classical biological control as the major factor, the El Niño associated weather pattern of the last quarter of 1997 and the first half of 1998 has confused the issue. We argue first that the reductions in water hyacinth on Lake Victoria were ultimately caused by the widespread and significant damage to plants by Neochetina spp., although this process was increased by the stormy weather associated with the El Niño event; second that increased waves and current on Lake Victoria caused by El Niño redistributed water hyacinth plants around the lake; and third that a major lakewide resurgence of water hyacinth plants on Lake Victoria has not occurred and will not occur unless the weevil populations are disrupted. We conclude that the population crash of water hyacinth on Lake Victoria would not have occurred in the absence of the weevils, but that it may have been hastened by stormy weather associated with the El Niño event.