Browsing by Subject "bioturbation"

Now showing 1 - 3 of 3
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    Arctic worming: Human-facilitated earthworm invasion transforms soil organic matter budgets and pools in Fennoscandian forests
    (2018-12) Wackett, Adrian
    Earth’s high latitude ecosystems are already under dire threat from climate warming, permafrost thaw, and intensifying natural resource exploitation. In addition to these ongoing concerns, accelerating urbanization, agricultural expansion, and greater opportunities for recreation in high-latitude regions are likely to introduce a less conspicuous but potentially potent threat: non-native geoengineeing earthworms. Earthworms were eradicated from northern N. America (including Minnesota) during the last glaciation, and it is now established that humans are (re-) introducing exotic earthworm species into these forests with dramatic consequences on soil and ecosystem functioning. However, their invasiveness and capacity to modify high-latitude boreal and arctic forests like those in Fennoscandia remains largely unknown. Here I explore two inter-related hypotheses concerning earthworms and soils in Fennoscandian forests: H1) despite their different human history and proximity to the native range of Lumbricidae earthworms (Southern and Central Europe), I predicted that Pleistocene glaciations also extirpated earthworms from Fennoscandia, suggesting that European earthworms (if present) are also non-native and invasive in these landscapes; and H2) if introduced by humans, the invasive earthworms transform Fennoscandian forest soil morphologies and soil organic matter (SOM) dynamics by removing thick organic layers at the forest floor and forming A-horizon (mineral topsoil) in their wake. To address H1, I tested a series of sub-hypotheses stating that: 1) earthworms did not colonize the Fennoscandian landscape via dispersal by brackish seawater, nor were they introduced by early indigenous peoples (Sami) who followed the retreating glaciers into northern Fennoscandia; and 2) earthworms are spreading into Fennoscandian forests from ‘worm point sources’ created by modern human-mediated dispersal vectors such as farming, fishing, gardening, and logging (among others). Although I could not dispel the possibility that small epigeic-type earthworms may have entered the Fennoscandian landscape considerably earlier via water-mediated dispersal and/or by ‘hitch-hiking’ along with Sami settlers, I found that more impactful ‘geoengineering’ species are only present in arctic landscapes associated with more modern (i.e. last two centuries) human disturbance (supporting H1) and are radiating outward from these anthropogenic point sources into virgin arctic and boreal forests. Furthermore, in line with H2, expansion of these geoengineers into adjacent forests consistently induced changes to forest soil morphologies and nutrient cycling regimes, including rapid reduction of the SOM pool in organic horizons and re-allocation of this SOM into mineral horizons: wherein it is sorbed onto mineral surfaces and/or occluded within aggregates. This belowground transformation likely has significant aboveground consequences as well as implications for the long-term carbon balance of boreal and arctic ecosystems, which store more than half (~ 53%) of earth’s soil carbon. Furthermore, based on results from N. America, this ‘unseen’ invasion may also have cascading effects on fungal and microbial associations, plant communities, and overall ecosystem functioning. Considering that the arctic is already being disproportionately affected by climate change and that human activities in these regions are likely to accelerate as these regions warm, additional research assessing the ecological impacts of arctic worming is urgently needed.
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    Dreissenid-Mediated Energy and Nutrient Cycling in Profundal Regions of the Laurentian Great Lakes
    (2023-08) Huff, Audrey
    In the Laurentian Great Lakes, invasive zebra and quagga (dreissenid) mussels have dramatically altered biotic community structure, primary productivity, and biogeochemistry since their introduction in the 1980s. Recently, quagga mussel (Dreissena rostriformis bugensis) populations have been expanding deeper into profundal regions of Lakes Michigan, Huron, and Ontario. These dense offshore populations have substantially altered offshore energy and nutrient cycling, but there are key gaps in our understanding of deep-water quagga mussel physiology and their impacts on pelagic biogeochemistry. Specifically, there is a lack of information on (1) quagga mussel tissue nutrient sequestration and regeneration rates, including variability in tissue stoichiometry (C:N:P molar ratios) and its influence on mussel excretion rates and excretion stoichiometry, (2) quagga mussel impact on offshore sediment geochemistry, including sediment mixing rate, sediment oxygen penetration, and dissolved nutrient dynamics at the sediment-water interface, and (3) quagga mussel population dynamics, including size distribution and growth rates, in deep, offshore lake regions. Presented here are the results of field (chapter 2), experimental (chapter 3), and modelling (chapter 4) studies I conducted to address these knowledge gaps about quagga mussel physiology and ecological impacts. To determine variability of quagga mussel tissue stoichiometry and its impact on mussel excretion (chapter 2), I measured mussel tissue and excretion carbon, nitrogen, and phosphorus content along depth (20 – 130m) and trophic gradients in Lakes Michigan and Huron during spring mixing and summer stratification periods of 2019. I found that mussel tissue C:N:P ratios varied substantially in Lakes Michigan and Huron, suggesting that quagga mussels have flexible internal homeostasis. I also found that tissue C:N:P stoichiometry was a significant driver of mussel excretion rates and excretion stoichiometry. When mussels had lower tissue C:P ratios than available seston, excretion C:nutrient (C:N and C:P) ratios decreased. Next, to investigate the influence of quagga mussels on offshore sediment geochemistry (chapter 3), I conducted a six-week microcosm experiment. I incubated quagga mussels, Diporeia spp. (previously the dominant Great Lakes’ macroinvertebrate), and oligochaete worms (the second most common benthic macroinvertebrate in the Great Lakes). Species were incubated separately and in combination to determine varying organism impacts on sediment mixing and biogeochemistry as well as potential community interaction effects. To simulate deep, offshore conditions, I used low particulate organic matter (POM) sediment in the microcosms and kept them in the dark and at 4°C. I found that sediment mixing depth and intensity varied significantly among species, but that there were no significant differences in sediment oxygen penetration depth or nutrient dynamics. Additionally, I found no evidence for species interaction effects. Finally, I used a Dynamic Energy Budget (DEB) model to explore quagga mussel physiology and growth rates under variable temperatures and food quantities (chapter 4). First, I simulated quagga mussel growth at annual temperatures and food availability representative of oligotrophic, mesotrophic, and eutrophic conditions in nearshore, mid-depth, and offshore regions of the Great Lakes. I then simulated mussel growth under three climate warming scenarios (+0.5°C, +1°C, and +2°C water temperatures). Corresponding changes in lake stratification regime under warming scenarios included an increase in the duration of summer stratification and a decrease in the duration of winter stratification. I found that quagga mussel growth increased with warmer water temperatures and altered stratification regimes. I also found that relative importance of water temperature and food availability varied over trophic status and mussel age, with mussel sensitivity to food limitation increasing as mussels grew larger over time. The combined results from these three studies indicate that quagga mussel impacts on pelagic energy and nutrient dynamics are mostly due to direct mechanisms – including carbon and nutrient ingestion, sequestration, and regeneration – rather than altered sediment geochemistry. My results provide detailed information on quagga mussel physiology, including variability of internal stoichiometry and growth under a wide range of environmental conditions, which strongly influences mussel nutrient recycling. Together, these results improve the current understanding of quagga mussel biology and will help to inform estimates of quagga mussel impacts on biogeochemical cycling in the Great Lakes and other invaded ecosystems.
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    Sediment mixing and dissolved oxygen and nutrient dynamics in a six week microcosm experiment containing Great Lakes macroinvertebrates
    (2024-05-06) Huff, Audrey H; Rigdon, Matt; Zalusky, John; Katsev, Sergei; Ozersky, Ted; huff0114@umn.edu; Huff, Audrey E; University of Minnesota Duluth Large Lakes Observatory
    Physical and geochemical dynamics at the sediment-water interface in a six-week microcosm experiment including Great Lakes macroinvertebrates (dreissenids, Diporeia, and oligochaete worms) with varying functional biology. Microcosms included single and multi-taxon treatments and data includes sediment mixing rate (as luminophore profiles), weekly NO2,3 and NH3 fluxes between the sediment and overlying water, weekly sediment dissolved oxygen microprofiles, and pore water NO2,3, NH3, and P profiles.