Dreissenid-Mediated Energy and Nutrient Cycling in Profundal Regions of the Laurentian Great Lakes.

다국어 초록 (Multilingual Abstract)

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 sixweek 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.