Cumulative Effects of Watershed Disturbances and Run-of-river Dams on Mercury Cycling: Case Study and Recommendations for Environmental Managers.

Run-of-river power plants (ROR) represent the majority of hydroelectric plants worldwide. Their environmental impacts are not well documented and are believed to be limited, particularly regarding the contamination of food webs by methylmercury (MeHg), a neurotoxin. RORs are typically installed in small rivers where combined effects of watershed disturbances with dam construction can complicate environmental management. We report a multi-year case study on the Saint-Maurice River (Canada) where an unpredicted temporary increase in MeHg accumulation in predator fish was observed after the construction of two ROR plants. The associated pondages acted as sedimentation basins for mercury (Hg) and organic matter from a watershed disturbed by a forest fire and by logging. This fresh organic carbon likely fueled microbial MeHg production. Hg methylation was more associated with environmental conditions than to the presence of Hg, and main methylating microbial groups were identified. A constructed wetland was a site of significant Hg methylation but was not the main source of the fish Hg increase. Organic carbon degradation was the main driver of MeHg accumulation at the base of the food chain whereas trophic levels explained the variations at the top of the food chain. Overall, carbon cycling was a key driver of Hg dynamics in this system, and ROR plants can cause temporary (ca. 12 years) Hg increase in food webs when developed in disturbed watersheds, although this increase is smaller than for large reservoirs. Recommendations for future ROR construction are to establish a good environmental monitoring plan with initial high temporal resolution and to consider recent and potential watershed disturbances in the plan.

Role of organic matter and microbial communities in mercury retention and methylation in sediments near run-of-river hydroelectric dams.

Run-of-river power plants (RoRs) are expected to triple in number over the next decades in Canada. These structures are not anticipated to considerably promote the mobilization and transport of mercury (Hg) and its subsequent microbial transformation to methylmercury (MeHg), a neurotoxin able to biomagnify in food webs up to humans. To test whether construction of RoRs had an effect on Hg transport and transformation, we studied Hg and MeHg concentrations, organic matter contents and methylating microbial community abundance and composition in the sediments of a section of the St. Maurice River (Quebec, Canada). This river section has been affected by the construction of two RoR dams and its watershed has been disturbed by a forest fire, logging, and the construction of wetlands. Higher total Hg (THg) and MeHg concentrations were observed in the surface sediments of the flooded sites upstream of the RoRs. These peaks in THg and MeHg were correlated with organic matter proportions in the sediments (r2 = 0.87 and 0.82, respectively). In contrast, the proportion of MeHg, a proxy for methylation potential, was best explained by the carbon to nitrogen ratio suggesting the importance of terrigenous organic matter as labile substrate for Hg methylation in this system. Metagenomic analysis of Hg-methylating communities based on the hgcA functional gene marker indicated an abundance of methanogens, sulfate reducers and fermenters, suggesting that these metabolic guilds may be primary Hg methylators in these surface sediments. We propose that RoR pondages act as traps for sediments, organic matter and Hg, and that this retention can be amplified by other disturbances of the watershed such as forest fire and logging. RoR flooded sites can be conducive to Hg methylation in sediments and may act as gateways for bioaccumulation and biomagnification of MeHg along food webs, particularly in disturbed watersheds.

Disentangling trophic relationships in a High Arctic tundra ecosystem through food web modeling.

Determining the manner in which food webs will respond to environmental changes is difficult because the relative importance of top-down vs. bottom-up forces in controlling ecosystems is still debated. This is especially true in the Arctic tundra where, despite relatively simple food webs, it is still unclear which forces dominate in this ecosystem. Our primary goal was to assess the extent to which a tundra food web was dominated by plant-herbivore or predator-prey interactions. Based on a 17-year (1993-2009) study of terrestrial wildlife on Bylot Island, Nunavut, Canada, we developed trophic mass balance models to address this question. Snow Geese were the dominant herbivores in this ecosystem, followed by two sympatric lemming species (brown and collared lemmings). Arctic foxes, weasels, and several species of birds of prey were the dominant predators. Results of our trophic models encompassing 19 functional groups showed that <10% of the annual primary production was consumed by herbivores in most years despite the presence of a large Snow Goose colony, but that 20-100% of the annual herbivore production was consumed by predators. The impact of herbivores on vegetation has also weakened over time, probably due to an increase in primary production. The impact of predators was highest on lemmings, intermediate on passerines, and lowest on geese and shorebirds, but it varied with lemming abundance. Predation of collared lemmings exceeded production in most years and may explain why this species remained at low density. In contrast, the predation rate on brown lemmings varied with prey density and may have contributed to the high-amplitude, periodic fluctuations in the abundance of this species. Our analysis provided little evidence that herbivores are limited by primary production on Bylot Island. In contrast, we measured strong predator-prey interactions, which supports the hypothesis that this food web is primarily controlled by top-down forces. The presence of allochthonous resources subsidizing top predators and the absence of large herbivores may partly explain the predominant role of predation in this low-productivity ecosystem.

Apodization technique for fiber grating fabrication with a halftone transmission amplitude mask.

Experimental results of fiber Bragg gratings fabricated with halftone amplitude transmission masks and 10-cm-long phase masks are presented for the first time to our knowledge. The performance of the devices is evaluated in terms of their spectral characteristics and deviation from linear group delay. Good out-of-band sidelobe suppression of -27 dB and group-delay ripple of ?9.5 ps is achieved for fully apodized grating devices.

Bragg gratings in defect-free germanium-doped optical fibers.

Bragg gratings have been written in germanium-doped optical fibers that have been treated to remove the UV absorption bands associated with oxygen-deficient defects. When one is using high-intensity 193-nm light from an ArF excimer laser to fabricate the gratings, the refractive index increases and the grating transmission spectra are similar to those obtained in standard (untreated) fiber.

Variable-spectral-=response optical waveguide Bragg grating filters for optical signal processing.

A simple method is described that permits the spectral response (spectral width, shape, and center resonant wavelength) of an optical waveguide Bragg grating to be controlled accurately in a prescribed manner. The control methodology consists of bonding the Bragg grating along the length of a mechanical support structure, which is then loaded with an appropriate force distribution. The function of the support structure is to transfer the strain induced by loading to the grating, thus modifying the grating's spectral response in accordance with the variation in effective optical pitch induced by the strain transfer. We design and demonstrate a support structure that provides independent control over the spectral width and center wavelength of a Bragg grating.

Photosensitivity in Ge-doped silica optical waveguides and fibers with 193-nm light from an ArF excimer laser.

Photosensitivity in optical fibers and waveguides has been associated with the bleaching of an absorption band located near 5.0 eV (or 242 nm). We present new results for Bragg grating formation and UV bleaching experiments carried out using 193-nm light from an ArF excimer laser instead of the usual laser sources operating near 242 or 248 nm.

Chirped in-fiber Bragg gratings for compensation of optical-fiber dispersion.

A linearly chirped in-fiber Bragg grating is reported that can compensate at 1549 nm for the dispersion [ approximately -19 ps/(nmkm)] of standard telecommunications optical fiber optimized for 1300-nm operation.

Photosensitization of optical fiber and silica-on-silicon/silica waveguides.

Localized heating with a flame is shown to be a simple and effective method for substantially augmenting the photosensitivity of high-silica optical waveguides to (UV) light. The method increases the photosensitivity of standard (Ge-doped core) telecommunications fiber by a factor greater than 10 (photoinduced Deltan(uv) > 10(-3)) and renders strongly photosensitive the cores of high-quality Ge:SiO(2)-on-Si and Ge:SiO(2)-on-SiO(2) planar waveguides that were negligibly photosensitive before treatment. We have written large-modulation-depth Bragg gratings, in both fiber and planar optical waveguides photosensitized by our method, using KrF (249-nm) radiation incident upon the waveguides through a zero-order-nulled phase mask. It is noteworthy that photosensitization by our method is achieved with a negligible increase in loss at the three principal optical communication windows.

Single-excimer-pulse writing of fiber gratings by use of a zero-order nulled phase mask: grating spectral response and visualization of index perturbations.

Optical fiber Bragg reflectors have been written by irradiating the fiber from the side through a phase mask with a single pulse of high-power 249-nm excimer-laser light. Efficient tapping of light to the radiation modes has been achieved for light at wavelengths shorter than the Bragg wavelength. The photoinduced periodic refractive-index perturbations have been observed directly with an optical microscope and are shown to have the same period as the phase mask and to be highly localized on one side, the irradiated side of the fiber core-cladding boundary.

Long-length, long-period rocking filters fabricated from conventional monomode telecommunications optical fiber.

Using fiber photosensitivity, we have made efficient, application-specific rocking filters from conventional monomode telecommunications optical fiber. Filters with a length of 10.89 m, a bandwidth of 58 nm, and a transmission of 99% at the resonant wavelength of switching 200-fs solitons.

Ultraviolet light photosensitivity in Ge-doped silica fibers: wavelength dependence of the light-induced index change.

A novel technique is reported for detecting permanent and transient light-induced refractive-index changes (photosensitivity) in optical fibers. The index change is detected by irradiating one arm of an unbalanced Mach-Zehnder fiber interferometer with UV light, thereby changing its optical path length. From a measurement of the change in the spectral response of the Mach-Zehnder interferometer, the change in the fiber core index as a function of wavelength can be determined. The equilibrium change in the core index is found to have an almost constant value of approximately 2.3 x 10(-5) over the measured wavelength range of 700 to 1400 nm.

Ultraviolet-light photosensitivity in Er(3+)-Ge-doped optical fiber.

Light-induced refractive-index change in Er(3+)-Ge-doped optical fiber is reported for the first time to the authors' knowledge. Evidence of the change is observed when UV light (lambda = 249 nm) irradiates one arm of an unbalanced Mach-Zehnder interferometer made from Er(3+)-Ge-doped fiber. From a measurement of the change in the spectral response of the interferometer with UV exposure, the change in fiber core index as a function of wavelength is determined. The equilibrium change in core index is found to vary between 2.3 x 10(-5) and 3.7 x 10(-5) over the measured wavelength region of 800 to 1700 nm. Also for the first time to our knowledge, fused couplers made of Er(3+)-doped fiber are reported. These identical fiber couplers are used in a novel all-fiber unbalanced Mach-Zehnder interferometer.

Compact, low-loss, fused biconical taper couplers: overcoupled operation and antisymmetric supermode cutoff.

The automated manufacture and characterization of compact four-port highly overcoupled fused couplers is reported. During the pulling stage of the manufacturing process, the light power in such devices is observed to cycle back and forth up to 1000 times between the two output ports for elongations smaller than 20 mm. For sufficiently long pulls, antisymmetric supermode cutoff has been observed for the first time to our knowledge, manifested by a sudden cessation of the coupling process. The measured wavelength dependence of the coupling ratio together with the device dimensions are consistent with the observation of cutoff as observed in pull-signature characteristics.

Modal-insensitive biconical-taper couplers.

A method for fabricating optical-fiber fused biconical-taper couplers that have coupling ratios insensitive to the modal distribution of light in the input fiber arms is demonstrated. Comparison measurements of the performance of these couplers and those made by the previous technique are presented and discussed.