Abstract-This study determined the relative sensitivity of five species of aquatic macrophytes and six species of algae to four commonly used herbicides (atrazine, metribuzin, alachlor, and metolachlor). Toxicity tests consisted of 96-h (duckweed and algae) or 14-d (submerged macrophytes) static exposures. The triazine herbicides (atrazine and metribuzin) were significantly more toxic to aquatic plants than were the acetanilide herbicides (alachlor and metolachlor). Toxicity studies ranked metribuzin Ͼ atrazine Ͼ alachlor Ͼ metolachlor in decreasing order of overall toxicity to aquatic plants. Relative sensitivities of macrophytes to these herbicides decreased in the order of Ceratophyllum Ͼ Najas Ͼ Elodea Ͼ Lemna Ͼ Myriophyllum. Relative sensitivities of algae to herbicides decreased in the order of Selenastrum Ͼ Chlorella Ͼ Chlamydomonas Ͼ Microcystis Ͼ Scenedesmus Ͼ Anabaena. Algae and macrophytes were of similar overall sensitivities to herbicides. Data indicated that Selenastrum, a commonly tested green alga, was generally more sensitive compared to other plant species. Lemna minor, a commonly tested floating vascular plant, was of intermediate sensitivity, and was fivefold less sensitive than Ceratophyllum, which was the most sensitive species tested. The results indicated that no species was consistently most sensitive, and that a suite of aquatic plant test species may be needed to perform accurate risk assessments of herbicides.

Aquatic plant toxicity tests are frequently conducted in environmental risk assessments to determine the potential impacts of contaminants on primary producers. An examination of published plant toxicity data demonstrates that wide differences in sensitivity can occur across phylogenetic groups of plants. Yet relatively few studies have been conducted with the specific intent to compare the relative sensitivity of various aquatic plant species to contaminants. We compared the relative sensitivity of the algae Selenastrum capricornutum and the floating vascular plant Lemna minor to 16 herbicides (atrazine, metribuzin, simazine, cyanazine, alachlor, metolachlor, chlorsulfuron, metsulfuron, triallate, EPTC, trifluralin, diquat, paraquat, dicamba, bromoxynil, and 2,4-D). The herbicides studied represented nine chemical classes and several modes of action and were chosen to represent major current uses in the United States. Both plant species were generally sensitive to the triazines (atrazine, metribuzin, simazine, and cyanazine), sulfonureas (metsulfuron and chlorsulfuron), pyridines (diquat and paraquat), dinitroaniline (trifluralin), and acetanilide (alachlor and metolachlor) herbicides. Neither plant species was uniformly more sensitive than the other across the broad range of herbicides tested. Lemna was more sensitive to the sulfonureas (metsulfuron and chlorsulfuron) and the pyridines (diquat and paraquat) than Selenastrum. However Selenastrum was more sensitive than Lemna to one of two thiocarbamates (triallate) and one of the triazines (cyanazine). Neither species was sensitive to selective broadleaf herbicides including bromoxynil, EPTC, dicamba, or 2,4-D. Results were not always predictable in spite of obvious differences in herbicide modes of action and plant phylogeny. Major departures in sensitivity ofSelenastrum occurred between chemicals within individual classes of the triazine, acetanilide, and thiocarbamate herbicides. Results indicate that neither species is predictively most sensitive, and that a number of species including a dicot species such as Myriophyllum are needed to perform accurate risk assessments of herbicides.

This study determined the relative sensitivity of five species of aquatic macrophytes and six species of algae to four commonly used herbicides (atrazine, metribuzin, alachlor, and metolachlor). Toxicity tests consisted of 96‐h (duckweed and algae) or 14‐d (submerged macrophytes) static exposures. The triazine herbicides (atrazine and metribuzin) were significantly more toxic to aquatic plants than were the acetanilide herbicides (alachlor and metolachlor). Toxicity studies ranked metribuzin > atrazine > alachlor > metolachlor in decreasing order of overall toxicity to aquatic plants. Relative sensitivities of macrophytes to these herbicides decreased in the order of Ceratophyllum > Najas > Elodea > Lemna > Myriophyllum. Relative sensitivities of algae to herbicides decreased in the order of Selenastrum > Chlorella > Chlamydomonas > Microcystis > Scenedesmus > Anabaena. Algae and macrophytes were of similar overall sensitivities to herbicides. Data indicated that Selenastrum, a commonly tested green alga, was generally more sensitive compared to other plant species. Lemna minor, a commonly tested floating vascular plant, was of intermediate sensitivity, and was fivefold less sensitive than Ceratophyllum, which was the most sensitive species tested. The results indicated that no species was consistently most sensitive, and that a suite of aquatic plant test species may be needed to perform accurate risk assessments of herbicides.

The acute toxicity of malathion to glochidia, juvenile, and adult freshwater mussels was determined at pH 7.5 in soft water and at pH 7.9 in moderately hard reconstituted fresh water at 25ЊC and 32ЊC. Nine species were tested in one or more life stages. Glochidia tests were conducted for 4, 24, or 48 h, while juvenile and adult exposures lasted 96 h. Overall, Utterbackia imbecillis was the least sensitive species for all exposure conditions and life stages. The LC50 values for glochidia tested at 25ЊC ranged from 7 mg/L for Lampsilis siliquoidea (4 h) to 324 mg/L for U. imbecillis (48 h). At 32ЊC, glochidia LC50s were 119 mg/L for Villosa lienosa (48 h) and 374 mg/L for U. imbecillis (24 h). Tests with juvenile mussels produced 96-h LC50s ranging from 24 mg/L for Lampsilis straminea claibornensis at 25ЊC to 219 mg/L for U. imbecillis at 25ЊC. The 96-h LC50s for three species of adult mussels were greater than the highest malathion exposure concentration of 350 mg/L. These values are considerably higher than the reported 48-h LC50 of 1 g/L for Daphnia magna and the 96-h LC50 of 0.76 g/L for Gamarus fasciatus but are similar to 96-h LC50s for some fish. Expected environmental concentrations should not be lethal to unionids.

This study evaluated the potential effects of different concentrations of bleached/unbleached kraft mill effluent (B/UKME) on several reproductive endpoints in adult largemouth bass (Micropterus salmoides). The kraft mill studied produces a 50/50 mix of bleached/unbleached market pulp with an estimated release of 36 million gal of effluent/day. Bleaching sequences were C90d10EopHDp and CEHD for softwood (pines) and hardwoods (mainly tupelo, gums, magnolia, and water oaks), respectively. Bass were exposed to different effluent concentrations (0 [controls, exposed to well water], 10, 20, 40, or 80%) for either 28 or 56 days. At the end of each exposure period, fish were euthanized, gonads collected for histological evaluation and determination of gonadosomatic index (GSI), and plasma was analyzed for 17beta-estradiol, 11-ketotestosterone, and vitellogenin (VTG). Largemouth bass exposed to B/UKME responded with changes at the biochemical level (decline in sex steroids in both sexes and VTG in females) that were usually translated into tissue/organ-level responses (declines in GSI in both sexes and in ovarian development in females). Although most of these responses occurred after exposing fish to 40% B/UKME concentrations or greater, some were observed after exposures to 20% B/UKME. These threshold concentrations fall within the 60% average yearly concentration of effluent that exists in the stream near the point of discharge (Rice Creek), but are above the <10% effluent concentration present in the St. Johns River. The chemical(s) responsible for such changes as well as their mode(s) of action remain unknown at this time.