교량 수리학 - Bridge Hydraulics

Boreal regions and their freshwater ecosystems may be susceptible to future climate change under projected warmer conditions. Northwest Ontario is a boreal region adjacent to the climatically sensitive prairie-forest ecotone (PFE). Pollen records spanning the Holocene from near the Manitoba/Ontario border to lakes up to ~300 km east of the PFE indicate a warmer and possibly wetter mid-Holocene period across northwest Ontario from ~8000 to 4500 cal. yr BP. To date, only one Holocene-scale record of changes in effective moisture, as indicated through diatom-inferred changes in lake level (Experimental Lakes Area (ELA) Lake 239), is available from this region. Our study expands the regional context of Holocene climate changes, with the analysis of diatom assemblages in sediment cores from two additional lakes, which span a distance of over 200 km across the present-day boreal forest, from 80 km west of ELA Lake 239 to ~150 km to the northeast. In cores from both lakes, benthic taxa predominate in the early-to-mid-Holocene period, with a low abundance of planktonic taxa, suggesting lower lake levels by ~2–5 m. Increases in the abundance of planktonic taxa to >50% occurred in both lakes beginning ~4500–4000 cal. yr BP suggesting positive water balance over the last 4000 years in comparison with the mid-Holocene period. This new evidence supports a regional mid-Holocene period of aridity, with reduced water levels across the boreal region of northwest Ontario. If future climate change results in lower effective moisture, then conditions could become similar to the mid-Holocene period aridity, leading to real challenges for the management of water resources across the region.

Diatom-based evidence of regional aridity during the mid-Holocene period in boreal lakes from northwest Ontario (Canada)

There has been much debate over the relative importance of environmental selection and spatial variation on community organization in microorganisms To assess the importance of environmental or spatial variables in diatom species assemblages in Gall Lake, northwest Ontario, 41 surface-sediment samples were collected in a two-dimensional gridded pattern along and across depth contours A depth-constrained cluster analysis separated the diatom flora into three communities: a shallow-water benthic zone (B1); a deeper-water benthic zone (B2); and a planktonic zone (P) Redundancy analysis (RDA) confirmed that water depth was a major predictor of variation in the flora Further RDAs and variation partitioning using orthogonal polynomials and Moran’s eigenvector maps showed that spatial location had minimal effect on the diatom assemblages Principal components analysis grouped the diatom flora not only by assemblage, but also by water depth, regardless of two-dimensional spatial separation, suggesting the importance of the environmental gradients associated with lake depth These findings indicate that environment is a more important explanatory variable than spatial variables for diatoms within lakes, suggesting dispersal plays a limited role in intra-lake diatom distributions

Water depth is a strong driver of intra-lake diatom distributions in a small boreal lake

The North American boreal forest has been developing since the end of the last glaciation approximately 10,000 yr ago. With climate warming and human occupation, it is anticipated that fire danger, ignition, and activity will be increasing, compromising forests’ benefits for generations to come. In this study, we show, however, that a century of rapid climate changes and human densification has had the opposite effect in the boreal eastern interior of the North American continent, reducing biomass burning to values below two millennia of historical levels. A multi-millennial fire history was reconstructed for eight forested landscapes from the Lake of the Woods Ecoregion (LWE) located at the boreal–prairie ecotone. Fire history was reconstructed using a combination of archival (period 1920–2010), tree-ring (stand initiations and fire scars: period 1690–2010), and lake sediment charcoal (2500 BP to present) records. The archival record revealed recent large fires (>200 ha) in 1948, 1980, and 1988. An additional 19 fires were identified by the fire-scar record. Fire events in 1805, 1840, 1863, and the 1890s were identified in numerous locations around multiple lakes suggesting that they were of large extents. In accordance with the tree-ring record, the charcoal accumulation rate (CHAR) peak record generally identified the major fires but tended to lag from the tree-ring records by several decades. Within LWE, the long-term charcoal record revealed that CHAR was higher for each lake in the earlier portion of the record including the warm Medieval Climate Anomaly (AD 900 to AD 1000), followed by a progressive decrease toward the cool Little Ice Age period. This decline was abruptly interrupted in the midto late 19th century with large synchronized fires, also reported over western and central North America, and resumed approximately four decades later. Fire disturbance level is today below the historical range, despite the accentuated climate warming. Aging of the forest landscape may create biodiversity loss notably in fire-adapted species while at the same time setting the tone for major fires in upcoming decades if no action is taken for managing fuels.

https://esajournals.onlinelibrary.wiley.com/doi/pdf/10.1002/ecs2.2287

Recent fire activity in the boreal eastern interior of North America is below that of the past 2000 yr

We compile a multi-proxy Holocene record from North Raft Lake located in the sub-Arctic Hudson Bay Lowlands, Canada. Specifically, we use subfossil chironomid, diatom, and pollen assemblages, non-pollen palynomorphs, sedimentary chlorophyll a, magnetic susceptibility, and organic matter content to characterize terrestrial and lake processes and to track paleoclimate following land emergence from the postglacial Tyrrell Sea. Following recession of the Tyrrell Sea (~ 6600 cal yr BP), pollen assemblages are indicative of a salt marsh environment with the establishment of an opportunistic chironomid assemblage. By ~ 6200 cal yr BP, isostatic uplift reduced the marine influence and pollen and palynomorph assemblage changes indicate that North Raft Lake became a closed-basin freshwater system. At this time, pollen assemblages signify the establishment of a forested peatland dominated by Picea, and chironomid taxa indicate warming water temperatures. The North Raft Lake pollen/palynomorph record captures a warm and moist mid-Holocene (Holocene Thermal Maximum) period starting ~ 6200 cal yr BP. During this period, an increase in pollen concentration and a shift to higher abundances of Larix indicate watershed-scale succession and enhanced local ecosystem productivity. Shifts in chironomid taxa at ~ 5000 cal yr BP are indicative of abrupt limnological changes suggesting wet conditions that caused an expansion of littoral habitat. The gradual transition into Neoglacial cooling is signaled by an increase in the abundances of cold-stenothermal chironomid taxa and a small decline in pollen-reconstructed temperatures. Marked changes occur in the mid-twentieth century with the appearance of diatoms in notable abundances for the first time in the lake’s sedimentary record, the arrival of new chironomid littoral taxa, and unprecedented increases in sedimentary chlorophyll a and organic matter content. Throughout the North Raft Lake Holocene sediment record, the pollen trends suggest slow centennial-scale changes in temperature and precipitation, whereas chironomid assemblages indicate abrupt mid-Holocene and twentieth century limnological changes, stressing that biota in lakes of the Hudson Bay Lowlands may change substantially under future scenarios of global climate warming.

Post-glacial lake development and paleoclimate in the central Hudson Bay Lowlands inferred from sediment records

Enhanced algal abundance in northwest Ontario (Canada) lakes during the warmer early-to mid-Holocene period

Consensus among multiple trophic levels during high- and low-water stands over the last two millennia in a northwest Ontario lake

Despite a recent proliferation of bridges in coastal zones, little is known of the long-term impacts of bridge construction on phytoplankton abundance and community composition in nearshore waters. Here, we used high performance liquid chromatographic (HPLC) analysis to quantify historical changes in phytoplankton using fossils from cores on either side of a 57-year-old bridge that reduced fetch by 76% and created a discrete enclosed inner embayment. We hypothesized that phytoplankton abundance should be greater in the inner bay after bridge construction due to reduced hydrological flow and increased nutrient influx associated with terrestrial development and more profound anoxia in the enclosed basin. As expected, primary producer abundance, especially cryptophytes, increased in the inner bay during the 1950–1960s, although the effect was transient despite continuous fourfold elevation in sediment organic matter content (%C, %N) after ca. 1960 in response to wastewater influx. The pulse in cryptophytes appeared to correspond to influx of humic-rich waters derived from locally exploited peatlands, whereas the phytoplankton community of the outer basin largely reflected historical variations in climate, with warmer April months associated with increased abundance of diatoms and cryptophytes. Overall, sediment stable isotope values in the inner basin varied in response to a complex combination of changes in wastewater treatment (δ15N), release of untreated fish plant effluents (δ15N), and connectivity with the open ocean (δ13C). Taken together, these findings suggest that the effects of bridge construction on coastal phytoplankton production and community composition operated through multiple pathways, rather than just via modifications of basin hydrology.

/pdf/effects-of-bridge-construction-and-wastewater-effluent-on-3ve4xdb3jx.pdf

Effects of Bridge Construction and Wastewater Effluent on Phytoplankton Abundance and Sediment Geochemistry in an Atlantic Temperate Coastal Bay Since 1930

Moumita Karmakar

Papers

Diatom-based evidence of regional aridity during the mid-Holocene period in boreal lakes from northwest Ontario (Canada)

Enhanced algal abundance in northwest Ontario (Canada) lakes during the warmer early-to mid-Holocene period

Consensus among multiple trophic levels during high- and low-water stands over the last two millennia in a northwest Ontario lake

Effects of Bridge Construction and Wastewater Effluent on Phytoplankton Abundance and Sediment Geochemistry in an Atlantic Temperate Coastal Bay Since 1930

Changes in the prairie–forest ecotone in northwest Ontario (Canada) across the Holocene