Current state and trends in Canadian Arctic marine ecosystems: II. Heterotrophic food web, pelagic-benthic coupling, and biodiversity

As part of the Canadian contribution to the International Polar Year (IPY), several major international research programs have focused on offshore arctic marine ecosystems. The general goal of these projects was to improve our understanding of how the response of arctic marine ecosystems to climate warming will alter food web structure and ecosystem services provided to Northerners. At least four key findings from these projects relating to arctic heterotrophic food web, pelagic-benthic coupling and biodiversity have emerged: (1) Contrary to a long-standing paradigm of dormant ecosystems during the long arctic winter, major food web components showed relatively high level of winter activity, well before the spring release of ice algae and subsequent phytoplankton bloom. Such phenological plasticity among key secondary producers like zooplankton may thus narrow the risks of extreme mismatch between primary production and secondary production in an increasingly variable arctic environment. (2) Tight pelagic-benthic coupling and consequent recycling of nutrients at the seafloor characterize specific regions of the Canadian Arctic, such as the North Water polynya and Lancaster Sound. The latter constitute hot spots of benthic ecosystem functioning compared to regions where zooplankton-mediated processes weaken the pelagic-benthic coupling. (3) In contrast with another widely shared assumption of lower biodiversity, arctic marine biodiversity is comparable to that reported off Atlantic and Pacific coasts of Canada, albeit threatened by the potential colonization of subarctic species. (4) The rapid decrease of summer sea-ice cover allows increasing numbers of killer whales to use the Canadian High Arctic as a hunting ground. The stronger presence of this species, bound to become a new apex predator of arctic seas, will likely affect populations of endemic arctic marine mammals such as the narwhal, bowhead, and beluga whales.     

Limits to scale invariance in alluvial rivers

Assumptions about fluvial processes and process–form relations are made in general models and in many site-specific applications. Many standard assumptions about reach-scale flow resistance, bed-material entrainment thresholds and transport rates, and downstream hydraulic geometry involve one or other of two types of scale invariance: a parameter (e.g. critical Shields number) has the same value in all rivers, or doubling one variable causes a fixed proportional change in another variable in all circumstances (e.g. power-law hydraulic geometry). However, rivers vary greatly in size, gradient, and bed material, and many geomorphologists regard particular types of river as distinctive. This review examines the tension between universal scaling assumptions and perceived distinctions between different types of river. It identifies limits to scale invariance and departures from simple scaling, and illustrates them using large data sets spanning a wide range of conditions. Scaling considerations and data analysis support the commonly made distinction between coarse-bed and fine-bed reaches, whose different transport regimes can be traced to the different settling-velocity scalings for coarse and fine grains. They also help identify two end-member sub-types: steep shallow coarse-bed ‘torrents’ with distinctive flow-resistance scaling and increased entrainment threshold, and very large, low-gradient ‘mega rivers’ with predominantly suspended load, subdued secondary circulation, and extensive backwater conditions. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd     

3D MHD Coronal Oscillations about a Magnetic Null Point: Application of WKB Theory

This paper is a demonstration of how the WKB approximation can be used to help solve the linearised 3D MHD equations. Using Charpit’s method and a Runge?–?Kutta numerical scheme, we have demonstrated this technique for a potential 3D magnetic null point, B=[x,ε y,?(ε+1)z]. Under our cold-plasma assumption, we have considered two types of wave propagation: fast magnetoacoustic and Alfvén waves. We find that the fast magnetoacoustic wave experiences refraction towards the magnetic null point and that the effect of this refraction depends upon the Alfvén speed profile. The wave and thus the wave energy accumulate at the null point. We have found that current buildup is exponential and the exponent is dependent upon ε. Thus, for the fast wave there is preferential heating at the null point. For the Alfvén wave, we find that the wave propagates along the field lines. For an Alfvén wave generated along the fan plane, the wave accumulates along the spine. For an Alfvén wave generated across the spine, the value of ε determines where the wave accumulation will occur: fan plane (ε=1), along the x-axis (0<ε<1) or along the y-axis (ε>1). We have shown analytically that currents build up exponentially, leading to preferential heating in these areas. The work described here highlights the importance of understanding the magnetic topology of the coronal magnetic field for the location of wave heating.     

Flow resistance and hydraulic geometry in bedrock rivers with multiple roughness length scales

Many models of incision by bedrock rivers predict water depth and shear stress from discharge; conversely, palaeoflood discharge is sometimes reconstructed from flow depth markers in rock gorges. In both cases, assumptions are made about flow resistance. The depth–discharge relation in a bedrock river must depend on at least two roughness length scales (exposed rock and sediment cover) and possibly a third (sidewalls). A conceptually attractive way to model the depth–discharge relation in such situations is to partition the total shear stress and friction factor, but it is not obvious how to quantify the friction factor for rough walls in a way that can be used in incision process models. We show that a single flow resistance calculation using a spatially averaged roughness length scale closely approximates the partitioning of stress between sediment and rock, and between bed and walls, in idealized scenarios. Both approaches give closer fits to the measured depth–discharge relations in two small bedrock reaches than can be achieved using a fixed value of Manning's n or the Chézy friction factor. Sidewalls that are substantially rougher or smoother than the bed have a significant effect on the partitioning of shear stress between bed and sidewalls. More research is needed on how best to estimate roughness length scales from observable or measurable channel characteristics. © 2019 John Wiley & Sons, Ltd.     

Conventional Oil—The Forgotten Part of the Water-Energy Nexus

The impacts of unconventional oil and gas production via high-volume hydraulic fracturing (HVHF) on water resources, such as water use, groundwater and surface water contamination, and disposal of produced waters, have received a great deal of attention over the past decade. Conventional oil and gas production (e.g., enhanced oil recovery [EOR]), which has been occurring for more than a century in some areas of North America, shares the same environmental concerns, but has received comparatively little attention. Here, we compare the amount of produced water versus saltwater disposal (SWD) and injection for EOR in several prolific hydrocarbon producing regions in the United States and Canada. The total volume of saline and fresh to brackish water injected into depleted oil fields and nonproductive formations is greater than the total volume of produced waters in most regions. The addition of fresh to brackish “makeup” water for EOR may account for the net gain of subsurface water. The total amount of water injected and produced for conventional oil and gas production is greater than that associated with HVHF and unconventional oil and gas production by well over a factor of 10. Reservoir pressure increases from EOR and SWD wells are low compared to injection of fluids for HVHF, however, the longer duration of injections could allow for greater solute transport distances and potential for contamination. Attention should be refocused from the subsurface environmental impacts of HVHF to the oil and gas industry as a whole.     

Spatiotemporal analysis of residential flood exposure in the Atlanta,Georgia metropolitan area

Natural Hazards - This research examines changes in residential built-environment flood exposure within the current boundaries of the Atlanta, Georgia metropolitan statistical area, by estimating...     

Numerical modelling of climate change impacts on Saint‐Lawrence River tributaries

The impacts of climate‐induced changes in discharge and base level in three tributaries of the Saint‐Lawrence River, Québec, Canada, are modelled for the period 2010–2099 using a one‐dimensional morphodynamic model. Changes in channel stability and bed‐material delivery to the Saint‐Lawrence River over this period are simulated for all combinations of seven tributary hydrological regimes (present‐day and those predicted using three global climate models and two greenhouse gas emission scenarios) and three scenarios of how the base level provided by the Saint‐Lawrence River will alter (no change, gradual fall, step fall). Even with no change in base level the projected discharge scenarios lead to an increase in average bed material delivery for most combinations of river and global climate model, although the magnitude of simulated change depends on the choice of global climate model and the trend over time seems related to whether the river is currently aggrading, degrading or in equilibrium. The choice of greenhouse gas emission scenario makes much less difference than the choice of global climate model. As expected, a fall in base level leads to degradation in the rivers currently aggrading or in equilibrium, and amplifies the effects of climate change on sediment delivery to the Saint‐Lawrence River. These differences highlight the importance of investigating several rivers using several climate models in order to determine trends in climate change impacts. Copyright © 2010 John Wiley & Sons, Ltd.     

Paleowind velocity and paleocurrents of pluvial Lake Manly,Death Valley,USA

Pluvial lake deposits are found throughout western North America and are frequently used to reconstruct regional paleoclimate. In Death Valley, California, USA, we apply the beach particle technique (BPT) of Adams (2003), Sedimentology, 50, 565–577 and Adams (2004), Sedimentology, 51, 671–673 to Lake Manly deposits at the Beatty Junction Bar Complex (BJBC), Desolation Canyon, and Manly Terraces and calculate paleowind velocities of 14–27 m/s. These wind velocities are within the range of present-day wind velocities recorded in the surrounding area. Sedimentary structures and clast provenance at Desolation Canyon and the Manly Terraces indicate sediment transport from north to south. Lake level, based on the elevation of constructional features, indicates that the hill west of the BJBC was an island and that the BJBC spits formed during simple lake regression. The data are consistent with the hypothesis that the present wind regime (velocity and direction) formed the pluvial Lake Manly features.