The acute toxicity (96-hr LC50) of un-ionized ammonia to mysids (Mysidopsis bahia) and larval inland silversides (Menidia beryllina) was influenced by pH and salinity in a species specific manner. With mysids, NH3 was most toxic at pH 7.0 and less toxic at pH 8.0 and 9.0. In contrast, NH3 toxicity to inland silversides was greatest at pH 7.0 and 9.0 and lowest at pH 8.0. A drop in salinity from 31 g/kg to 11 g/kg uniformly increased toxicity to mysids over this pH range. In contrast, in silversides at 11 g/kg, NH3 toxicity was less at pH 7.0, greater at pH 8, and slightly less at pH 9, relative to the toxicity at 31 g/kg. Temperature had no significant effect on the acute toxicity of un-ionized ammonia with acclimated mysids tested at 18, 25 and 32.5 degrees C, but did have a small effect with acclimated larval sheepshead minnows (Cyprinodon variegatus) tested at 13, 25 and 32.5 degrees C. The chronic toxicity value (the geometric mean of the highest no-effect concentration and lowest effect concentration) at pH 8.0, 25 degrees C and 31 g/kg salinity is 0.061 mg NH3/L for inland silversides and 0.232 mg NH3/L for mysids; the acute:chronic ratio is 21.3 and 7.2, respectively.

Sediment cores were imaged using a Computer‐Aided Tomography (CT) scanner at Massachusetts General Hospital, Boston, Massachusetts, United States. Procedures were developed, using the attenuation of X‐rays, to differentiate between sediment and the water contained in macrobenthic tubes and tunnels. The effects of sediment type on the ability to discriminate tubes as small as 1.5 mm were examined. Soft sediments with mean X‐ray attenuations (SXA) from 450 to 576 CT numbers were successfully scanned in cores of 15.2 cm diameter by 30 cm depth. We demonstrated the accessibility and availability of CT technology to ecological studies by negotiating a reduced research rate ($200 per core) for sediment scanning at a nearby small hospital. Additionally, we were able to transfer these image data from the local hospital environment to a personal computer, by developing specialized computer software. These steps allowed greater opportunity for data exploration, manipulation, and statistical evaluation than would be available in a medical facility. CT analysis was applied to intact sediment cores from five stations along a 31‐km pollution gradient in Narragansett Bay, Rhode Island, United States. The percentage of tube and tunnel area (PTTA) within the top 18 cm of sediment from each station was measured and ranged from 0.07% to 1.13%. PTTA increased along this gradient with distance from the pollution sources (r2 = 0.81, P < 0.01). The mean X‐ray attenuation for sediment (excluding animals, their tubes and tunnels, and shells) was determined at each station. It also showed a highly significant relationship along this gradient (r2 = 0.98, P < 0.01) and ranged from 271 to 576 CT numbers. The measurement of PTTA may be an effective management tool to assess and monitor the effects of organic carbon loading on benthic communities in Narragansett Bay and similarly impacted estuaries.

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Embryos and yolk-sac larvae of the winter flounder Pseudopleuronectes americanus were exposed to 0 (control), 54, 92, 180, and 386 •g/liter silver (Ag +) during an 18-day flow-through bioassay. Percent hatch was significantly reduced only at the 386-•g/liter exposure level (24%). Embryos exposed to 180 and 386 •g/liter hatched earlier than those exposed to lower concentrations, and many exhibited physical abnormalities. Larva mortality was significantly higher at 386, 180, and 92 •g/liter (100, 97, and 31%, respectively) than in the control (6%). The mean total lengths of larvae at hatch were significantly greater in control (3.56 ram) than in 386 •g/liter (1.98 ram) and 180 •g/liter (3.25 ram). At yolk-sac absorption, the mean total length of control larvae (4.27 ram) was significantly greater than that of larvae exposed to 180 •g/liter (4.15 ram). The highest concentration of silver not significantly reducing either survival or growth was 54 •g/ liter.

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