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Shiv Nadar University

We examine among- and within-catchment variability in century-long patterns of phytoplankton abundance and sedimentary C and N isotopic content in two temperate oligotrophic estuaries of the Eastern Canadian Coast. Two upstream sites are compared with one downstream site within a large, relatively unimpacted catchment. The downstream site is compared to another site situated in an adjacent catchment that experienced more urban development. Variations in δ13C, δ15N, and organic content in downstream sites coincided with either water flow restriction events following bridge construction in the 1950s, or wastewater treatment plant startup in the 1960s. Furthermore, downstream sites showed algal abundance peaks between the 1920s and 1990s, in contrast to the upstream site where algal abundance increased throughout the twentieth century. We then quantified the relative importance of two sets of environmental factors as correlates of phytoplankton abundance and composition. We show that downstream algal communities (especially cryptophytes and cyanobacteria) were influenced by land-use factors. In contrast, upstream algal communities were more influenced by climatic variables, namely August air temperature rather than land-use change (e.g., pig farm, golf course). We propose that sand-bar built estuaries tend to be more sensitive to land transformation events at downstream reaches, where land-derived materials tend to accumulate. In contrast, upstream reaches of estuaries drain relatively smaller surface areas, making them potentially more sensitive to climatic signals. © 2021, Coastal and Estuarine Research Federation.

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Center for Public Affairs and Critical Theory

Estuaries and Coasts

Differential Impacts of Climate and Land-use on Twentieth-century Phytoplankton Composition among and within Two Temperate Coastal Estuaries of Atlantic Canada

Pollen and diatom assemblages from well-dated sediment cores from three lakes forming a west-to-east transect across the boreal forest in northwest Ontario (Canada) were used to evaluate the timing and nature of the movement of the prairie-forest ecotone (PFE) across the Holocene. Changes in vegetation, temperature, and effective moisture were inferred from pollen and pollen-based transfer functions. Analyses indicated site-specific vegetational and climate changes across short spatial distances, with prolonged prairie-like conditions during the middle Holocene at the westernmost site. Increased reconstructed temperatures at this westernmost site occurred from ~9000 to 3000 cal yr BP, alongside increases in diatom-inferred lake levels beginning at ~6000 cal yr BP. The abundance of Quercus peaked concurrently with rising lake levels before declining to trace levels by ~3000 cal yr BP. Increases in the abundance of non-arboreal pollen between ~8500 and ~4500 cal yr BP at the more eastern lakes suggest relatively delayed and truncated PFE influence, before the reestablishment of primarily boreal taxa by ~4500 cal yr BP, coincident with diatom-inferred increases in lake levels. This study shows that the PFE moved both farther east and north than previously determined, but generally agrees with established patterns in vegetation from other studied regions along the PFE. Copyright © University of Washington. Published by Cambridge University Press, 2021.

Quaternary Research (United States)

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

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 (δ 15 N), release of untreated fish plant effluents (δ 15 N), and connectivity with the open ocean (δ 13 C). 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. © 2018, Coastal and Estuarine Research Federation.

Fulltext

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

This study investigates regional changes in primary producers in boreal head-water lakes during the warmer early-to-mid-Holocene (EMH) period, across the present-day boreal forest in northwest Ontario, a region that is adjacent to the prairie-forest ecotone. We quantified changes in algal abundance and composition over the Holocene period using pigments, spectrally-inferred chlorophyll a and diatom assemblages in well-dated sediment cores from three lakes. All three indicators showed a coherent pattern of enhanced primary producers in two of the study lakes (Gall Lake and Lake 239) during the EMH, whereas only diatom assemblages suggested higher levels of nutrients in Meekin Lake. Overall, this study supports a regional pattern of enhanced primary producers during the EMH, likely as a function of lower water-levels and warmer temperatures. Elevated concentrations of cyanobacterial pigments also occurred in two of the three lakes during the EMH, whereas pigments from purple-sulphur bacteria provide evidence of enhanced deep-water anoxia in one lake. These findings suggest that future climatic warming in boreal regions could include regional eutrophication and associated increases in cyanobacteria. © 2015 Elsevier Ltd.

Quaternary Science Reviews

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

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. © The Author(s) 2015.

Holocene

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

We investigated the modern distribution of fossil midges within a dimictic lake and explored downcore patterns of inferred lake depths over the last 2000. years from previously published proxies. Modern midge distribution within Gall Lake showed a consistent and predictable pattern related to the lake-depth gradient with recognizable assemblages characteristic of shallow-water, mid-depth and profundal environments. Interpretations of downcore changes in midge assemblages, in conjunction with quantitative lake-depth inferences across a priori defined (based on diatom data) ~. 500-yr wet and dry periods, demonstrated that both invertebrate and algal assemblages exhibited similar timing and nature of ecological responses. Midges were quantified by their relative abundance, concentrations and an index of Chaoborus to chironomids, and all showed the greatest differences between the wet and dry periods. During the low lake-level period of the Medieval Climate Anomaly (MCA: AD 900 to 1400), profundal chironomids declined, shallow-water and mid-depth chironomids increased, chironomid-inferred lake level declined and the Chaoborus-to-chironomid index decreased. The coherence between multiple trophic levels provides strong evidence of lower lake levels in Gall Lake during the MCA. © 2013 University of Washington.