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In 1998, we conducted a field-validation study of the chronic 28-day whole-sediment toxicity test with Leptocheirus plumulosus in Baltimore Harbor, MD, an area where this amphipod is indigenous. This study included an evaluation of the effect of sieving on sediment chemical concentrations and the use of field replicates, or separate grabs from the same site, which provided an estimation of within-site chemical and toxicologic variability. Six stations in Baltimore Harbor, MD, were included in this evaluation. Chemical analysis of two separate unsieved field replicates from the six sites indicated that, overall, the chemical concentrations of replicates within each site were similar, especially for metals. Organic contaminants particularly total PCBs, had the highest variability between replicates. Chemical variability did not appear to be related to differences in organic carbon content or grain size or to variability in toxicologic end points. Results supported the use of composite samples in sediment toxicity tests. In addition, in most cases, sieving had little effect on sediment chemistry. For the metals and trace elements, only selenium showed a substantial change after sieving, with some samples increasing after sieving and others decreasing. Concentrations of acid-volatile sulfide (AVS) increased 194.6% at one station after sieving, although in most other cases, AVS and simultaneously extracted metals remained relatively unchanged. As expected, concentrations of organics generally decreased after sieving, but in the majority of cases this decrease was small (i.e., coefficient of variation < or = 25%). Total benzene hexachloride and total chlordanes had the greatest changes, whereas polychlorinated biphenyl concentrations decreased at only two stations after sieving. Concentrations of polyaromatic hydrocarbons showed little change after sieving. These changes in sediment chemistry due to sieving must be viewed in the larger context of the potentially confounding effects that indigenous organisms may have on the interpretation of test results from whole-sediment toxicity tests.

A 96-h exposure to aqueous cadmium (Cd) is the recommended reference toxicity test for 10-day sediment bioassays with the estuarine amphipod, Leptocheirus plumulosus (US EPA 1994). This water-only test was used to assess the influence of organism size, sex, and nutritional status on the sensitivity of laboratory-cultured L. plumulosus to Cd. In addition, the response of field-collected amphipods was compared to similarly sized laboratory animals to assess potential seasonal changes in Cd sensitivity. Lipid content of test organisms was measured in these seasonal experiments and those evaluating effects of nutritional status because of its potential as an indicator of physiological condition. LC50 values of laboratory animals size-sorted on nested 500-, 710-, and 1000-micron mesh sieves, increased with size class: 0.36, 0.65, and 0.88 mg Cd/L, respectively. Gravid females were less sensitive than males or mature females to aqueous Cd. Studies on the influence of the molt cycle on Cd toxicity indicated enhanced sensitivity of immediate postmolt animals that may explain some of the observed differences in Cd tolerance. Nutritional effects were investigated by comparing the sensitivity of fed and starved laboratory-reared amphipods. Starved juveniles and adults were significantly smaller than their fed counterparts and exhibited a 28-43% reduction in lipid content, respectively. However, comparison of LC50 values indicated no significant differences in sensitivity to Cd between starved and fed juveniles (0.23 vs 0.30 mg Cd/L) or adults (0.37 vs 0.52 mg Cd/L). Field-collected amphipods were typically more sensitive to Cd than laboratory animals, regardless of the season, although their lipid content varied, ranging from 6.6% in August to 13.7% in November. Results are discussed with respect to the use and interpretation of toxicity tests with this species.

The effect of mercuric chloride (HgCl2) and monomethyl mercury chloride (CH3HgCl) on unidirectional 22Na and 45Ca influx were measured in the freshwater isopod Asellus aquaticus. Flux measurements involved short-term (20 min) exposure to 22Na or 45Ca following 2 h pre-exposure to Hg. Experiments were conducted at two different Na and Ca concentrations, 0.025 mmol L-1 and 0.25 mmol L-1. HgCl2 and CH3HgCl inhibited Na influx at both Na concentrations although Na influx at 0.25 mmol L-1 was always higher. This reflected the uptake kinetics of the Na pump which was determined to be saturable with a kmax values of 32 mumol Na g-1 h-1 and a Km value of 0.8 mmol L-1. CH3HgCl generally resulted in relatively greater inhibition of Na influx. At 0.025 mmol Na L-1 all CH3HgCl concentrations tested were inhibitory to Na influx, i.e., the lowest observed effect concentration (LOEC) was less than 0.04 mumol L-1. Ca influx was inhibited by HgCl2 at all concentrations tested (LOEC less than 0.04). The degree of inhibition was unaffected by Ca concentration, which was seen as evidence for non-competitive inhibition. However, CH3HgCl, which inhibited Ca influx from 0.025 mmol Ca L-1 by greater than 68% at molar Hg concentrations greater than or equal to 1 mumol L-1 showed no significant inhibitory affect at 0.25 mmol Ca L-1. Elimination of CH3HgCl inhibition at the higher Ca concentration suggests competition in this case, possibly at the level of access to the Ca pump.

Effects of tributyltin (TBT) on survival, growth, morphometry, and RNA-DNA ratio in larval striped bass (Morone saxatilis) were assessed in three experiments. In Experiment I, 13 day old larvae were exposed to 0, 0.067, 0.766, or 2.284 micrograms TBT/L for 6 days. All larvae exposed to 2.284 micrograms/L died by day 5; exposure to 0.766 micrograms/L significantly reduced survival relative to controls (59.8% vs. 81.8%). Significant, concentration-dependent decreases in body depth occurred in larvae exposed to 0.067 and 0.766 micrograms/L. In Experiment II, all 16 day old larvae exposed to 1.498 micrograms/L died by day 6. Survival, weight, and morphometry parameters were not significantly different in larvae exposed to 0, 0.052, or 0.444 micrograms/L for 6 days. In Experiment III, survival was similar in 21 day old larvae exposed to 0, 0.055, 0.281, or 0.514 micrograms/L for 7 days. Notochord length and dry weight decreased significantly in larvae exposed to 0.514 micrograms/L. Weight and morphometry parameters were more sensitive indicators of sublethal stress than RNA-DNA ratio. Maximum TBT concentrations reported in Chesapeake Bay marinas are likely to cause increased larval mortality. Longer-term studies are needed to assess effects at less than 0.050 micrograms/L, which may be more representative of habitat conditions.

Adult intermolt Gammarus pulex were exposed to a range of cadmium concentrations in an artificial stream water containing 20 mg calcium 1(-1). 96 hr and 48 hr LC-50 values obtained were 0.12 and 0.68 mg Cd 1(-1), respectively. The influence of external calcium level on cadmium toxicity was investigated at a cadmium concentration of 0.5 mg 1(-1). Calcium had an antagonistic effect on cadmium toxicity, and at a calcium concentration of 200 mg 1(-1) experimental animals failed to reach 50% mortality after a period of 120 hr. Over a 48 hr period of Cd uptake a solution containing 5 mg Cd 1(-1), animals in 200 mg calcium 1(-1) accumulated only half as much cadmium as animals in 20 mg calcium 1(-1). Post-molt animals were very much more sensitive to cadmium than intermolt specimens. Exposure to 0.1 mg Cd 1(-1) resulted in death in approximately 24 hr and the mean mortality time for animals in 0.01 mg Cd 1(-1) was 45 hr. These data are likely to be explained in terms of competition between cadmium and calcium for binding sites associated with key physiological processes. The results place these animals among the most sensitive organisms yet studied with regard to cadmium toxicity, and emphasize the importance of the molt in assessing toxic effects on crustaceans.