Genomic population structure of Striped Bass (Morone saxatilis) from the Gulf of St. Lawrence to Cape Fear River.

Striped Bass, Morone saxatilis (Walbaum, 1792), is an anadromous fish species that supports fisheries throughout North America and is native to the North American Atlantic Coast. Due to long coastal migrations that span multiple jurisdictions, a detailed understanding of population genomics is required to untangle demographic patterns, understand local adaptation, and characterize population movements. This study used 1,256 single nucleotide polymorphism (SNP) loci to investigate genetic structure of 477 Striped Bass sampled from 15 locations spanning the North American Atlantic coast from the Gulf of St. Lawrence, Canada, to the Cape Fear River, United States. We found striking differences in neutral divergence among Canadian sites, which were isolated from each other and US populations, compared with US populations that were much less isolated. Our SNP dataset was able to assign 99% of Striped Bass back to six reporting groups, a 39% improvement over previous genetic markers. Using this method, we found (a) evidence of admixture within Saint John River, indicating that migrants from the United States and from Shubenacadie River occasionally spawn in the Saint John River; (b) Striped Bass collected in the Mira River, Cape Breton, Canada, were found to be of both Miramichi River and US origin; (c) juveniles in the newly restored Kennebec River population had small and nonsignificant differences from the Hudson River; and (d) tributaries within the Chesapeake Bay showed a mixture of homogeny and small differences among each other. This study introduces new hypotheses about the dynamic zoogeography of Striped Bass at its northern range and has important implications for the local and international management of this species.

A new approach to the biological monitoring of freshwater systems: Mapping nutrient loading in two South African rivers, a case study.

Excessive addition of nitrogen (N) has threatened aquatic ecosystems for decades. Traditional water quality and biological monitoring assessment tools are widely used for monitoring nutrient loads and ecosystem health, but most of these methods cannot distinguish between different types and sources of pollution. This is a challenge, particularly when dealing with non-point sources of anthropogenic nitrogen inputs into freshwater systems. Recent laboratory studies using stable isotopic ratios (δ15N and C/N) of aquatic macrophytes (duckweed: Spirodela spp.) have shown successful differentiation and mapping between different N-sources and further, showed abilities to act as early warning indicators for environmental N-loading. Therefore, the aim of this study was to field test the potential of stable isotopic values of transplanted Spirodela spp. to map temporal and spatial N-loading variation and determine the main sources of N-loading in two river systems in the Eastern Cape Province of South Africa, using previously grown, isotopically calibrated and transplanted Spirodela plants, collected over a 13-month sampling period. Nitrogen isotopic values (δ15N) of Spirodela plants traced environmental N-loading and identified pollution hotspots and sources through time and space over a wide range of nutrient gradients. δ15N isotopic values of Spirodela spp. provided detailed dynamics on N-loading, therefore supporting its utilisation in the biological monitoring of ecosystem health and the early detection of eutrophication in freshwater systems.

Photoreducible Mercury Loss from Arctic Snow Is Influenced by Temperature and Snow Age.

Mercury (Hg) is an important environmental contaminant, due to its neurotoxicity and ability to bioaccumulate. The Arctic is a mercury-sensitive region, where organisms can accumulate high Hg concentrations. Snowpack mercury photoredox reactions may control how much Hg is transported with melting Arctic snow. This work aimed to (1) determine the significance of temperature combined with UV irradiation intensity and snow age on Hg(0) flux from Arctic snow and (2) elucidate the effect of temperature on snowpack Hg photoreduction kinetics. Using a Teflon flux chamber, snow temperature, UV irradiation, and snow age were found to significantly influence Hg(0) flux from Arctic snow. Cross-correlation analysis results suggest that UV radiation has a direct effect on Hg(0)flux, while temperature may indirectly influence flux. Laboratory experiments determined that temperature influenced Hg photoreduction kinetics when snow approached the melting point (>-2 °C), where the pseudo-first-order reduction rate constant, k, decreased twofold, and the photoreduced Hg amount, Hg(II)red, increased 10-fold. This suggests that temperature influences Hg photoreduction kinetics indirectly, likely by altering the solid:liquid water ratio. These results imply that large mass transfers of Hg from snow to air may take place during the Arctic snowmelt period, altering photoreducible Hg retention and transport with snow meltwater.

Mercury bioaccumulation in dragonflies (Odonata: Anisoptera): examination of life stages and body regions.

Dragonflies (Odonata: Anisoptera) are an important component of both aquatic and terrestrial food webs and are vectors for methylmercury (MeHg) biomagnification. Variations in mercury content with life stage and body regions may affect the relative transfer of mercury to aquatic or terrestrial food webs; however, there has been little research on this subject. Also, little is known about mercury bioaccumulation in different body regions of dragonflies. To address these knowledge gaps, dragonfly naiads, adults, and exuviae were collected at 2 lakes in Kejimkujik National Park, Nova Scotia, Canada, and mercury concentrations in different life stages and body regions were quantified. Mean whole body concentrations of MeHg were substantial in naiads (232 ± 112 ng g(-1) dry wt, n = 66), emerging adults (236 ± 50 ng g(-1) dry wt, n = 10), and mature adults (231 ± 74 ng g(-1) dry wt, n = 20). Mean MeHg concentrations in exuviae (5.6 ± 4.3 ng g(-1), n = 32) were 40-fold lower than in naiads and adults. Emerging adults had 2-fold to 2.5-fold higher Hg(II) concentrations than naiads, mature adults, and exuviae. In body regions of both naiads and adults, some abdomens contained significantly higher concentrations of Hg(II) than heads or thoraces, and this trend was consistent across families. Across families, Aeshnidae had significantly higher concentrations of MeHg and total Hg than Gomphidae and Libellulidae, but not higher than Cordulidae. The Hg(II) concentrations were lower in Aeshnidae and Libellulidae than in Gomphidae and Cordulidae. Shedding of exuviae presents a possible mechanism for mercury detoxification, but mercury concentrations and burdens in exuviae are low in comparison with naiads and adults. Dragonfly adults retain a high potential for transferring substantial amounts of MeHg to their predators.

Trophic level stability-inducing effects of predaceous early juvenile fish in an estuarine mesocosm study.

BACKGROUND: Classically, estuarine planktonic research has focussed largely on the physico-chemical drivers of community assemblages leaving a paucity of information on important biological interactions. METHODOLOGY/PRINCIPAL FINDINGS: Within the context of trophic cascades, various treatments using in situ mesocosms were established in a closed estuary to highlight the importance of predation in stabilizing estuarine plankton abundances. Through either the removal (filtration) or addition of certain planktonic groups, five different trophic systems were established. These treatments contained varied numbers of trophic levels and thus different "predators" at the top of the food chain. The abundances of zooplankton (copepod and polychaete), ciliate, micro-flagellate, nano-flagellate and bacteria were investigated in each treatment, over time. The reference treatment containing apex zooplanktivores (early juvenile mullet) and plankton at natural densities mimicked a natural, stable state of an estuary. Proportional variability (PV) and coefficient of variation (CV) of temporal abundances were calculated for each taxon and showed that apex predators in this experimental ecosystem, when compared to the other systems, induced stability. The presence of these predators therefore had consequences for multiple trophic levels, consistent with trophic cascade theory. CONCLUSIONS/SIGNIFICANCE: PV and CV proved useful indices for comparing stability. Apex predators exerted a stabilizing pressure through feeding on copepods and polychaetes which cascaded through the ciliates, micro-flagellates, nano-flagellates and bacteria. When compared with treatments without apex predators, the role of predation in structuring planktonic communities in closed estuaries was highlighted.

Spatial attachment-site preferences of macroectoparasites on Atlantic sturgeons Acipenser oxyrinchus in Minas Basin, Bay of Fundy, Canada.

Although parasite habitat preference is well studied, it is rarely rigorously evaluated statistically because of many zero intensities. Attachment-site preference and intensities of 2 macroectoparasite species ( Caligus elongatus and Calliobdella vivida ) of Atlantic sturgeon, Acipenser oxyrinchus Mitchill, in Minas Basin, Bay of Fundy, Canada, were characterized with the use of zero-inflated negative binomial statistical models that included a fork-length offset to control for body size. Three other parasites were encountered, sometimes in high numbers on various body sites, but too few counts overall prevented construction of meaningful statistical models. Of 26 sturgeons, prevalence of (1) C. elongatus (Copepoda) was 85%, mainly on caudal fins and nonfin body sites; (2) C. vivida (Hirudinea) was 81%, mainly on the pelvic and pectoral fins, and dorsal and ventral-lateral body sites; (3) Dichelesthium oblongum (Copepoda) was 31% within the gills or burrowed into the musculature at the base of fins; (4) Argulus stizostethii (Crustacea: Branchiura) was 8%; and (5) Nitzschia sturionis (Monogenea) was 12%. Only D. oblongum was associated with visible damage, mainly as lesions on gills and soft tissues. Characterizing parasite prevalences within the Bay of Fundy is important because some parasites affect fish health and population biology.