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National Irrigation Water Quality Program Guidelines (Powell et al. 1997). A recent ecological risk assessment for a natural community of aquatic plants concluded that, at median spring and fall concentrations of 5.9 and 3.6 mg BIL, patterns o(}eaf tissue discoloration (yellowing) may indicate adverse ecological impacts on the vegetation (Powell et al.1997). Several factors affect plant uptake of boron, including soil texture, pH, macronutrients, temperature, light, evapotranspiration rate, and plant growth stage (Butterwick et al. 1989; Glandon and McNabb 1978). Frick (1985), for instance, found that a concentration of 20 mg B/L was sufficient to inhibit the growth of duckweed at pH 7.0 but that 100 mg BIL was required to produce the same effect at pH 5.0. Once boron is incorporated into plant tissues, it becomes relatively immobile. Leaves generally accumulate the greatest concentra- tions of boron in plants (Gupta et al. 1985). Plant species do not all draw on the same boron supplies. Emergent plants absorb most of their boron from the hydrosoil; floating-leaf species absorb a large proportion of boron from sediments and water. Submerged plants, which lack or have greatly reduced root systems, obtain most of their boron from the water (Hutchinson 1975). Generally, floating-leaf species contain more boron than submerged or emergent plants, and dicotyledons usually contain more boron than monocotyledons (Boyd and Walley 1972; Cowgi111974). In aquatic macrophytes, boron concentrations are usually less than 20 mglkg dw. Based on samples from 22 species of aquatic macrophytes collected from natural environments, the mean tissue level of boron was 11.3 mg/kg dw (Powell et al. 1997). In green algae (Maeso et al. 1985) and blue-green algae (Martinez et al. 1986), adverse sublethal effects are apparent at boron concentrations of 50 mglkg and higher; in duckweed (Lemna spp.), growth inhibition was observed at 100 mg/kg and above (Frick 1985). Boron-contaminated irrigation water is one of the main causes of boron toxicity to plants. 0fJ Evapotranspiration from irrigated fields concentrates boron in the soil and leads, eventually, to toxicity (Gupta et al. 1985). At some places in the Southwestern United States, naturally elevated boron concentrations in surface water used for irrigation are high enough to be toxic to plants of commercial importance (Benson et al. 1984). High con- centrations of boron were found in aquatic plants growing in irrigation drain water at Kesterson Reservoir in the San Joaquin Valley of California. Widgeon grass contained 120-780 mg Blkg dw, and in one pond, Hothem and Ohlendorf (1989) found concentrations (1,630 mg/kg) high enough to impair avian reproduction if widgeon grass from that pond were a sole-source food supply. Widgeon grass seeds contained 430-3,500 mg B/kg dw (Schuler 1987) and algae contained 390-790 mglkg (Hoffman et al. 1991) at Kesterson Reservoir. Levels of boron in plant tissues were elevated compared to mean concentrations found in water (20 mg/L) and sediment (20 mg/kg), indicating that boron was bioconcentrating in aquatic plants. Toxic effects of boron to various plant species, as reported in the literature, are summarized in table 7 at the end of this chapter. I I I I I I I I I I I Macroinvertebrates I I I I I I I I Little information is available on the toxicity of boron to aquatic invertebrates (table 7). In tests with Daphnia magna, the no-observed- adverse-effect level (NOAEL) and the lowest- observed-adverse-effect level (LOAEL) were found to be about 6 and 13 mg BIL, respectively (Lewis and Valentine 1981; Gersich 1984). Hothem and Ohlendorf (1989) found that the boron concentration in adult damselflies was 27 percent lower than in nymphs. This result suggests that a greater proportion of boron in nymphs may be incorporated in the exoskeleton. Maier and Knight (1991) found a significantly decreased growth rate by Chiro1l011l11S deconts larvae at boron concentrations of 20 mg B/l and .~.