A Model for Methylmercury Uptake and Trophic Transfer by Marine Plankton.

Methylmercury (MeHg) concentrations can increase by 100 000 times between seawater and marine phytoplankton, but levels vary across sites. To better understand how ecosystem properties affect variability in planktonic MeHg concentrations, we develop a model for MeHg uptake and trophic transfer at the base of marine food webs. The model successfully reproduces measured concentrations in phytoplankton and zooplankton across diverse sites from the Northwest Atlantic Ocean. Highest MeHg concentrations in phytoplankton are simulated under low dissolved organic carbon (DOC) concentrations and ultraoligotrophic conditions typical of open ocean regions. This occurs because large organic complexes bound to MeHg inhibit cellular uptake and cell surface area to volume ratios are greatest under low productivity conditions. Modeled bioaccumulation factors for phytoplankton (102.4-105.9) are more variable than those for zooplankton (104.6-106.2) across ranges in DOC (40-500 µM) and productivities (ultraoligotrophic to hypereutrophic) typically found in marine ecosystems. Zooplankton growth dilutes their MeHg body burden, but they also consume greater quantities of MeHg enriched prey at larger sizes. These competing processes lead to lower variability in MeHg concentrations in zooplankton compared to phytoplankton. Even under hypereutrophic conditions, modeled growth dilution in marine zooplankton is insufficient to lower their MeHg concentrations, contrasting findings from freshwater ecosystems.

Two-dimensional liquid chromatography of PDMS-PS block copolymers.

In this study, liquid chromatography at critical conditions of polystyrene (PS) and polydimethylsiloxane (PDMS) is used as the first dimension for the two-dimensional analysis of polydimethylsiloxane-block-polystyrene copolymers. Comprehensive two-dimensional liquid chromatography with size exclusion chromatography as the second dimension reveals information about the molar mass distributions of all separated fractions from the first dimension. Furthermore, fractions eluting at the critical conditions were collected and subjected to analysis in the second dimension at the critical adsorption point of the other block. These fractions were analyzed by Fourier transform infrared spectroscopy to determine their chemical compositions. The combination of the above approaches and the calibration of the evaporative light scattering detector for the first-dimension analysis yield deep insights into the molecular heterogeneity of the block copolymer samples. The composition of the samples and the chemical composition of the real block copolymer are also calculated by combining the results obtained at both critical conditions.

Mercury sources and fate in the Gulf of Maine.

Most human exposure to mercury (Hg) in the United States is from consuming marine fish and shellfish. The Gulf of Maine is a complex marine ecosystem comprising twelve physioregions, including the Bay of Fundy, coastal shelf areas and deeper basins that contain highly productive fishing grounds. Here we review available data on spatial and temporal Hg trends to better understand the drivers of human and biological exposures. Atmospheric Hg deposition from U.S. and Canadian sources has declined since the mid-1990s in concert with emissions reductions and deposition from global sources has increased. Oceanographic circulation is the dominant source of total Hg inputs to the entire Gulf of Maine region (59%), followed by atmospheric deposition (28%), wastewater/industrial sources (8%) and rivers (5%). Resuspension of sediments increases MeHg inputs to overlying waters, raising concerns about benthic trawling activities in shelf regions. In the near coastal areas, elevated sediment and mussel Hg levels are co-located in urban embayments and near large historical point sources. Temporal patterns in sentinel species (mussels and birds) have in some cases declined in response to localized point source mercury reductions but overall Hg trends do not show consistent declines. For example, levels of Hg have either declined or remained stable in eggs from four seabird species collected in the Bay of Fundy since 1972. Quantitatively linking Hg exposures from fish harvested from the Gulf of Maine to human health risks is challenging at this time because no data are available on the geographic origin of seafood consumed by coastal residents. In addition, there is virtually no information on Hg levels in commercial species for offshore regions of the Gulf of Maine where some of the most productive fisheries are located. Both of these data gaps should be priorities for future research.

Monitoring chemical contaminants in the Gulf of Maine, using sediments and mussels (Mytilus edulis): An evaluation.

The objective of this paper is to determine whether contaminant data on mussels and sediments can be used interchangeably, or not, when assessing the degree of anthropogenic contamination of a water body. To obtain adequate coverage of the entire Gulf of Maine, Bay of Fundy sediment samples were collected, analyzed and combined with similar data from four coastal monitoring programs. This required careful interpretation but provided robust results consistent with published literature. A strong correspondence was found between sediment and mussel concentrations for polycyclic aromatic hydrocarbons, moderate to weak correspondence for polychlorinated biphenyls, and except for mercury and zinc, little to no correspondence was found for metals. We conclude that mussel contaminant data are likely sufficient for providing information on the spatial and temporal distribution of chemical contaminants, in coastal waters, under a broad range of environmental conditions and contaminant levels, and unlike sediments, provide direct information on contaminant bioavailability.

Pleistocene glaciation events shape genetic structure across the range of the American lobster, Homarus americanus.

A north/south discontinuity along the northeastern coast of North America in the genetic structure of the American lobster (Homarus americanus) was detected using a suite of 13 microsatellite loci assessed using spatial analyses. Population genetic data laid over existing data on physiographic changes and sea-surface temperatures were used to reconstruct the Pleistocene distribution of this species. A postglacial northern-edge colonization model best explains the relative genetic homogeneity of the northern region compared to the southern region centred in the Gulf of Maine. Population genetic analyses identified significant structure (range of standardized theta 0-0.02) but no significant evidence for isolation by distance. The novel application of spatial genetic analyses to a marine species allowed us to interpret these results by providing a greater insight into the evolutionary factors responsible for shaping the genetic structure of this species throughout is natural range.

Bioaccumulation of methylmercury within the marine food web of the outer Bay of Fundy, Gulf of Maine.

Mercury and methylmercury were measured in seawater and biota collected from the outer Bay of Fundy to better document mercury bioaccumulation in a temperate marine food web. The size of an organism, together with δ13 C and δ15 N isotopes, were measured to interpret mercury levels in biota ranging in size from microplankton (25µm) to swordfish, dolphins and whales. Levels of mercury in seawater were no different with depth and not elevated relative to upstream sources. The δ13 C values of primary producers were found to be inadequate to specify the original energy source of various faunas, however, there was no reason to separate the food web into benthic, demersal and pelagic food chains because phytoplankton has been documented to almost exclusively fuel the ecosystem. The apparent abrupt increase in mercury content from "seawater" to phytoplankton, on a wet weight basis, can be explained from an environmental volume basis by the exponential increase in surface area of smaller particles included in "seawater" determinations. This physical sorption process may be important up to the macroplankton size category dominated by copepods according to the calculated biomagnification factors (BMF). The rapid increase in methylmercury concentration, relative to the total mercury, between the predominantly phytoplankton (<125µm) and the zooplankton categories is likely augmented by gut microbe methylation. Further up the food chain, trophic transfer of methylmercury dominates resulting in biomagnification factors greater than 10 in swordfish, Atlantic bluefin tuna, harbour porpoise, Atlantic white-sided dolphin and common thresher shark. The biomagnification power of the northern Gulf of Maine ecosystem is remarkably similar to that measured in tropical, subtropical, other temperate and arctic oceanic ecozones.

Comprehensive high temperature two-dimensional liquid chromatography combined with high temperature gradient chromatography-infrared spectroscopy for the analysis of impact polypropylene copolymers.

Impact polypropylene copolymers (IPC) are extremely complex materials that can only be effectively analysed by multidimensional analytical approaches. IPC consists of isotactic polypropylene (iPP) as the major phase, ethylene-propylene (EP) copolymers of various compositions and small amounts of polyethylene. The molecular heterogeneity of two IPC samples having different ethylene contents was studied by using a novel cross-fractionation technique, developed from a combination of various analytical separation methods into an effective characterisation tool for complex polyolefins. The initial step involves the fractionation of the sample into EP rubber, EP segmented copolymer, and iPP, by preparative temperature rising elution fractionation (TREF). The resulting fractions are still distributed with regards to chemical composition and molar mass. The separation with respect to these parameters is conducted by comprehensive HT 2D-LC. This is the first time that the individual components in all TREF fractions of an IPC are separated and analysed mutidimensionally, by both SEC-FTIR, high-temperature (HT) HPLC-FTIR, and HT 2D-LC. Molar mass analysis of the chemically homogeneous fractions from HT HPLC is accomplished by HT SEC in the second dimension of HT 2D-LC. The chemical composition of all species is determined by coupling FTIR spectroscopy to HT HPLC via an LC-transform interface. This novel approach reveals the capability of this hyphenated technique to determine the exact chemical composition of the individual components in the complex TREF fractions of IPCs. The HT HPLC-FTIR results confirm the separation mechanism in the given chromatographic system using a 1-decanol to TCB solvent gradient and a Hypercarb stationary phase. The components of differing chemical composition are separated according to the nature and length of the propylene/ethylene segments, with their arrangement in the chains strongly affecting their adsorption/desorption on the stationary phase. FTIR analysis provides information on the ethylene and propylene contents of the fractions as well as on the ethylene and propylene crystallinities.

Two-dimensional liquid chromatography of polystyrene-polyethylene oxide block copolymers.

In this study, liquid chromatography at critical conditions of polystyrene (PS) and polyethylene oxide (PEO) is used as the first dimension for the two-dimensional analysis of PS-b-PEO copolymers. Comprehensive two-dimensional liquid chromatography, with size exclusion chromatography as the second dimension, reveals information about the molar mass distributions of all separated fractions from the first dimension. Furthermore, fractions eluting at the critical conditions of one block were collected and subjected to analysis in the second dimension at the critical conditions of the other block. These fractions were analysed by FTIR to determine their chemical compositions. The combination of the above approaches and the calibration of the evaporative light scattering (ELS) detector for the first-dimensional analysis yield deep insights into the molecular heterogeneity of the block copolymer samples. The composition of the samples and the chemical composition of the real block copolymer are also calculated by combining results obtained at both critical conditions.

Comprehensive two-dimensional liquid chromatography for the separation of protonated and deuterated polystyrene.

Liquid chromatography at critical conditions (LCCC) has been shown to be a powerful method for the separation of complex polymers regarding chemical composition, functionality, or molecular topology. LCCC has never been used, however, to separate polymers according to the degree of deuteration. This is a very challenging task since polymers shall be separated that are identical regarding molar mass, endgroups and chemical composition. In the present work, critical conditions were established in such a way that one component of a complex mixture elutes at critical conditions, whereas the other component shows size exclusion chromatography (SEC) behaviour. Blends of protonated (h) and deuterated (d) polystyrene (PS) were separated by LCCC at critical conditions of both h-PS and d-PS. Depending on the molar masses of the blend components, baseline separation could be achieved. In order to improve the separation further, comprehensive two-dimensional liquid chromatography was carried out on a number of model blends. In the first dimension LCCC was used, which separated the blends according to isotopic effects whereas in the second dimension the separation took place with respect to hydrodynamic volume. In order to further improve the separation of a number of blends a separation protocol was used where one component shows SEC conditions whereas the other component shows liquid adsorption chromatography (LAC) conditions. This separation protocol was achieved by varying the column temperature.

Methylmercury in marine ecosystems: spatial patterns and processes of production, bioaccumulation, and biomagnification.

The spatial variation of MeHg production, bioaccumulation, and biomagnification in marine food webs is poorly characterized but critical to understanding the links between sources and higher trophic levels, such as fish that are ultimately vectors of human and wildlife exposure. This article discusses both large and local scale processes controlling Hg supply, methylation, bioaccumulation, and transfer in marine ecosystems. While global estimates of Hg supply suggest important open ocean reservoirs of MeHg, only coastal processes and food webs are known sources of MeHg production, bioaccumulation, and bioadvection. The patterns observed to date suggest that not all sources and biotic receptors are spatially linked, and that physical and ecological processes are important in transferring MeHg from source regions to bioaccumulation in marine food webs and from lower to higher trophic levels.