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Carbon (δ13C) and nitrogen (δ15N) isotopes were used to evaluate spatial, temporal, and interspecific differences in trophic relationships of four fish species (Paralichthys dentatus, Anchoa mitchilli, Leiostomus xanthurus, and Bairdiella chrysoura) in Maryland's coastal bays. The δ13C values for all species were more enriched in 2017 than in 2018, a year of higher-than-average rainfall that likely caused higher amounts of terrestrial carbon to enter the estuary. There were significant differences among species in the δ13C values, with L. xanthurus being the least depleted (−17.2‰ in 2017; −18.8‰ in 2018). Spatially, the δ13C values of the species, particularly P. dentatus and B. chrysoura, were more depleted in the northern bays, which have a higher nutrient content and receive more freshwater inflow directly from tributaries, than the southern bays. The observed δ13C values (−19.5 ± 0.2‰ to –17.2 ± 0.3‰), however, indicate that marine phytoplankton was the primary carbon source of the fishes. Overall, A. mitchilli was the most enriched in δ15N (13.0‰), and L. xanthurus was the most depleted (10.2‰). δ15N was more enriched in fish from the more human-impacted northern bays than in fish from the southern bays, though this might also have stemmed from the differences in the diet composition of the species in the northern and southern bays. A. mitchilli had the highest trophic level, while L. xanthurus and P. dentatus had the lowest trophic levels. Niche breadth was widest in L. xanthurus compared to the other fish species, suggesting a higher variability in diets among L. xanthurus individuals, leading to specialized diets. There was a high niche overlap between B. chrysoura, A. mitchilli, and L. xanthurus, which indicates they fed on similar prey resources. [ABSTRACT FROM AUTHOR]

The suspended particulate organic matter (SPOM) in transitional waters such as the Maryland Coastal Bays (MCBs) is derived from allochthonous and autochthonous sources. Little is known, however, about the contribution of terrestrially derived organic matter to SPOM in the MCBs. The sources of SPOM in the MCBs were evaluated using stable isotope ratios of nitrogen (δ15N) and carbon (δ13C), and C/N molar ratios. The values of SPOM δ15N, δ13C and C/N ratios from samples collected seasonally (July 2014 to October 2017) at 13 sites ranged from −0.58 to 10.51‰, −26.85 to −20.33‰, and 1.67 to 11.36, respectively, indicating a mixture of terrestrial SPOM transported by tributaries, marine organic matter from phytoplankton, and sewage. SPOM δ13C levels less than −24‰, suggesting the dominance of terrestrially derived carbon, occurred mainly at sites close to the mouths of tributaries, and were less depleted at sites near the ocean. The mean value of SPOM δ13C was higher in October 2014 (−22.76‰) than in October 2015 (−24.65‰) and 2016 (−24.57‰) likely due to differences in river discharge. Much lower values (<4‰) of δ15N observed in February 2016 coincided with a high freshwater inflow that accompanied a major storm, indicating a strong influence of untreated sewage. Results from a two end-member mixing model suggest that on average, the SPOM in the MCBs is composed of 44% terrestrial materials with the highest percent contributions in October 2015 and 2016 (61%), and lowest (28%) in July 2015. The contribution of terrestrial materials to the SPOM was highest (58%) near the mouth of St. Martin River and lowest (25%) near the Ocean City inlet. SPOM composition and distribution in MCBs are, therefore, a function of land use, freshwater inflow, and water circulation that influence in situ phytoplankton production, and the transport and distribution of terrestrially derived materials. [ABSTRACT FROM AUTHOR]

Although most researches with non-essential metals (NEMs) have been done with single or individual metals, in reality, organisms are often exposed to multiple contaminants at the same time through the air, food and water. In this study, we tested the toxicity of four NEMs, As, Cd, Pb, and Hg, individually and as a composite mixture using the microtox bioassay. This assay uses the reduction of bioluminescence of the bacterium Vibrio fischeri as a measure of toxicity. The concentrations of each chemical in the mixture were based on multiples of their maximum contaminant levels (MCLs) set by the U.S. EPA. The highest concentration of exposure was 20 times the MCL, which translated into 200, 100, 40 and 300 ppb for As, Cd, Hg and Pb, respectively. The ratio for the mixture from these concentrations was 10:5:2:15 for As, Cd, Hg and Pb, respectively. Among the individual metals tested, the ranking of toxicity was Hg>Pb>Cd>As based on the EC50 values of 109, 455, 508 and 768 ppb for Hg, Pb, Cd and As, respectively. The EC50 for the composite mixture was 495% MCL which translated into nominal concentrations of 49, 25, 10 and 74 ppb for As, Cd, Hg, and Pb, respectively. Overall, the EC50 value of each NEM within the mixture was lower than the EC50 of the individual chemical; an evidence of synergism for the mixture. The individual toxic units (TU) were 0.06, 0.05, 0.09, and 0.16 for As, Cd Hg, and Pb, respectively and the summed toxic unit (TU) was 0.37 (less than 1). This study provides needed scientific data necessary for carrying out complete risk assessment of As, Cd, Hg, and Pb mixtures of some priority compounds.

DNTs are considered possibly carcinogenic to humans (Group 2B) because there is inadequate evidence in humans for carcinogenicity though there is sufficient evidence in experimental animals. In this study, MCF-7 (breast) and MRC-5 (lung) cells were exposed to a serial dilution of 2,4 and 2,6 DNTs (control, 1-500 ppm) in 96 well tissue culture plates. After various time intervals (24, 48, 72 and 96 hrs) the plates were washed, and 100microl fluorescein diacetate solution (10 microg/ml in PBS) was added column wise to each well, and incubated at 37 C for 30 - 60 min before reading the fluorescence with a spectrofluorometer at excitation and emission wavelengths of 485 and 538 nm respectively. Spectrofluorometeric readings were converted to percentages of cell survival. Regression analysis was conducted to determine the relationship between cell survival and exposed concentration. Linear equations derived from the regression analysis were used to calculate the LC50 values. Results indicated that 2,6 DNT was more toxic to breast cells; LC50 values were 445 and 292 ppm at 24 and 48 hours respectively compared to 2,4 DNT showing LC50 values of 570 and 407 ppm at 24 and 48 hours, respectively. No significant differences in toxicity existed between the two chemicals with regard to lung cells. Contrary to the above observation, 2,4 DNT was more toxic to breast cells; LC50 values were 407 and 238 ppm at 24 and 48 hours respectively compared to lung cells showing LC50 values of 527 and 402 ppm at 24 and 48 hours respectively. No significant difference existed for 2,6 DNT between the two cell lines. Lungs cells were more resistant to the two chemicals.