Colloidal Size Spectra, Composition and Estuarine Mixing Behavior of DOM in River and Estuarine Waters of the Northern Gulf of Mexico

Flow field-flow fractionation (FlFFF) coupled on-line with UV absorbance and fluorescence detectors was used to examine the colloidal composition and size distribution of optically active dissolved organic matter (DOM) in the lower Mississippi River (MR), the East Pearl River (EPR), the St. Louis Bay (SLB) estuary, and coastal waters of the northern Gulf of Mexico. In addition to field studies, laboratory mixing experiments using river and seawater end-members were carried out to study the processes controlling the estuarine mixing behavior and size partitioning of colloids with different sizes and composition. The colloidal size spectra of chromophoric DOM and humic-like DOM showed one dominant peak in the 0.5-4 nm size range, representing \u3e75% of the total FlFFF-recoverable colloids. In contrast, protein-like DOM showed a bi-modal distribution with peaks at 0.5-4 nm and 4-8 nm, as well as a major portion (from similar to 41% in the EPR to similar to 72% in the Mississippi Bight) partitioned to the \u3e20 nm size fraction. Bulk DOM was lower in abundance and molecular-weight in the MR compared with the EPR, while the proportion of colloidal protein-like DOM in the \u3e20 nm size range was slightly larger in the MR compared with the EPR. These features are consistent with differences in land use, hydrological conditions, and water residence time between the two river basins, with more autochthonous DOM in MR waters. In the SLB estuary, different DOM components demonstrated different mixing behaviors. The abundance of colloidal chromophoric DOM decreased with increasing salinity and showed evident removal during estuarine mixing even though the bulk DOM appeared to be conservative. In contrast, colloidal humic-like DOM behaved conservatively inside SLB and during laboratory mixing experiments. The ratio of colloidal protein-like to humic-like DOM generally increased with increasing salinity, consistent with increasing autochthonous protein-like DOM and removal of terrestrially-derived humic-like DOM in estuarine and coastal waters. Similar mixing behavior for the bulk DOM and colloids was observed in short-term laboratory mixing experiments, suggesting that physicochemical processes are the major controlling factor for colloidal removal in the estuary. For the first time, this study showed direct evidence of contrasting estuarine mixing behavior for different size fractions of optically active colloidal DOM. (C) 2016 Elsevier Ltd. All rights reserved

Tracing Bioavailability of ZnO Nanoparticles Using Stable Isotope Labeling

Zinc oxide nanoparticles (ZnO NPs) are widely used in commercial products and knowledge of their environmental fate is a priority for ecological protection. Here we synthesized model ZnO NPs that were made from and thus labeled with the stable isotope Zn-68 and this enables highly sensitive and selective detection of labeled components against high natural Zn background levels. We combine high precision stable isotope measurements and novel bioimaging techniques to characterize parallel water borne exposures of the common mudshrimp Corophium volutator to (ZnO)-Zn-68 NPs, bulk (ZnO)-Zn-68, and soluble (ZnCl2)-Zn-68 in the presence of sediment. C. volutator is an important component of coastal ecosystems where river-borne NPs will accumulate and is used on a routine basis for toxicity assessments Our results demonstrate that ionic Zn from ZnO NPs is bioavailable to C volutator and that Zn uptake is active. Bioavailability appears to be governed primarily by the dissolved Zn content of the water, whereby Zn uptake occurs via the aqueous phase and/or the ingestion of sediment particles with adsorbed Zn from dissolution of ZnO particles. The high sorption capacity of sediments for Zn thus enhances the potential for trophic transfer of Zn derived from readily soluble ZnO NPs. The uncertainties of our isotopic data are too large, however, to conclusively rule out any additional direct uptake route of ZnO NPs by C. volutator