Learning and attention reveal a general relationship between population activity and behavior.

Prior studies have demonstrated that correlated variability changes with cognitive processes that improve perceptual performance. We tested whether correlated variability covaries with subjects' performance-whether performance improves quickly with attention or slowly with perceptual learning. We found a single, consistent relationship between correlated variability and behavioral performance, regardless of the time frame of correlated variability change. This correlated variability was oriented along the dimensions in population space used by the animal on a trial-by-trial basis to make decisions. That subjects' choices were predicted by specific dimensions that were aligned with the correlated variability axis clarifies long-standing paradoxes about the relationship between shared variability and behavior.

HPSEC as a prepative fractionation technique for studies of natural organic matter (NOM).

High pressure size exclusion chromatography has been investigated as a preparative fractionation technique for studies of natural organic matter. Five different experiments were conducted to examine the effect of gel-solute interactions in the selected high pressure size exclusion chromatography system, and to evaluate the reproducibility of the method: (1) different flow rates (2) different sample concentrations, (3) reinjection of collected fractions, (4) addition of acetonitrile to the mobile phase, and (5) reinjection of hydrophobic and hydrophilic sub fractions. The results show that gel-solute interactions are a minor problem, and that the selected high pressure size exclusion chromatography system separates the natural organic matter molecules mainly on the basis of molecular size. The optical properties (ultraviolet/visible and fluorescence) vary systematically between the collected fractions. The main distinguishing features are relatively stronger absorbancies at shorter (<290 nm) wavelengths for smaller molecules, strong specific ultraviolet/visible absorbancy for natural organic matter molecules of intermediate molecular mass and very low fluorscence of larger molecules.

The effect of mercury and PCBs on organisms from lower trophic levels of a Georgia salt marsh.

We examined several indicators of salt marsh function, focusing on primary producers, microbes, and grass shrimp, at a Superfund site (LCP) contaminated with mercury and polychlorinated biphenyls (PCBs) and a reference site (Cross-River) in Georgia. Primary production of Spartina alterniflora was assessed by measuring peroxidase activity (POD), glutathione concentration (tGSH), photosynthesis (A(net)), and transpiration (E). Microbial populations were assessed by measuring living-fungal standing crop (as ergosterol) and Microtox(R). Grass shrimp (Palaemonetes pugio) reproductive potential was determined by measuring individual egg mass, average egg area, brood size, and brood mass of gravid females. Comparison of the sites suggested that P. pugio reproduction was affected at the LCP site, but we were unable to document clear negative effects on other organisms we investigated. Due to natural environmental gradients, the Cross-River site may not have been a perfect control for the LCP site. Therefore, data from just the LCP site were reanalyzed using multiple regression. Fungal biomass was related to methylmercury concentrations, but the direction of the relationship differed between wholly dead shoots (positive) and partially dead shoots (negative). S. alterniflora POD was positively related to methylmercury concentrations. S. alterniflora A(net) and E were negatively related to elevation and salinity, respectively. Despite high levels of contamination at the LCP site, our results provided only suggestive evidence for impacts on organisms at lower trophic levels.

Uptake of 244Cm, 238Pu and other radionuclides by trees inhabiting a contaminated flood plain.

The plant uptake of 244Cm, 137Cs, 238Pu and 90Sr was measured for trees in a flood plain forest whose soils were contaminated by aqueous discharges from a nuclear-fuel chemical separations facility. Uptake of the naturally occurring radionuclide 226Ra was also measured. The relative availability of the nuclides was 238Pu less than 244Cm less than 137Cs less than 226Ra less than or equal to 90Sr. The concentration ratios for 238Pu and 244Cm, 3 X 10(-4) and 3.6 X 10(-3), respectively, were similar to those reported for other plant-soil systems. The ratios for 137Cs and 90Sr, 0.11 and 3.9, were similar to those reported for other southeastern soils. However, the ratio for 226Ra, 2.1, was greater than that normally reported. These ratios, which were determined in the field, were generally similar to those reported for greenhouse studies on the same soil.

Plutonium contents of broadleaf vegetable crops grown near a nuclear fuel chemical separations facility.

Among agricultural crops, broadleaf vegetables are particularly prone to intercept and retain aerially released contaminants. The plutonium concentration of four broadleaf crops (broccoli, cabbage, lettuce and turnip greens) was determined, when grown in close proximity to a nuclear-fuel chemical-separations facility. Concentrations varied among species, apparently influenced by the crop morphology, with Pu concentrations increasing in the sequence: cabbage less than broccoli less than turnip greens less than lettuce. Washing of the crops significantly reduced the Pu concentration of lettuce, but had no effect on Pu concentration of broccoli and cabbage. The vast majority of Pu found in the crops was due to direct deposition of recently released Pu and resuspension of Pu-bearing soil particles, and was not due to root uptake. Resultant doses from consumption are small relative to the annual background dose.

Seasonal cycling of cesium-137 in a reservoir.

Studies of a reservoir in the southeastern United States show that cesium-137, introduced into the system from a leak in a nuclear fuel element, cycles between the water and sediment on a seasonal basis. The cycling, which coincides with the annual periods of thermal stratification in this monomictic lake, has been occurring for over 10 years.

Elemental mercury evolution mediated by humic Acid.

Elemental mercury is formed in aqueous solution by the chemical reduction of mercuric ion in the presence of humic acid. The reduction proceeds via first order kinetics (rate constant, 0.009 hour-(1)) and is depndent on pH. The reaction mechanism involves interaction of the ionic metal species with the free radical electrons of the humic acid.