Browsing by Subject "Nutrient Cycling"

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    Consumer nutrient stoichiometry : patterns, homeostasis, and links with fitness.
    (2010-10) Hood II, James Michael
    The linkages between food webs and nutrient cycles are heterogeneous and often influenced by human activities. Ecological stoichiometry provides one framework for understanding and predicting these linkages. Yet, as it has been extended underlying assumptions are often not evaluated. This dissertation shows that examination of implicit and explicit assumptions reveals unknown mechanisms, interactions, and linkages. For instance, theory assumes that invertebrate stoichiometry does not vary with diet stoichiometry (i.e., strict homeostasis), even though many invertebrates are not strictly homeostatic. Chapters one and two examine the role of stoichiometric homeostasis in shaping the fitness of Daphnia species. Chapter one shows that the long-­‐term phosphorus (P) use efficiency of stoichiometrically flexible Daphnia species is higher in habitats with temporally variable diets, resulting in higher fitness relative to strictly homeostatic species. Chapter two shows that the P cost of a unit of growth increased with growth rate and structures tradeoffs among growth rate, sensitivity to P limitation, and stoichiometric flexibility. Stoichiometric theory can be extended to novel ecosystems, such as streams, to predict the role of consumers in food web and nutrient cycles. To do To do this, the balance between consumer and diet stoichiometries is a logical starting point. Chapter three examines intra-­‐specific variation in consumer-­‐resource stoichiometries at a suite of sites within a river network. In contrast to previous work, this chapter describes wide intra-­‐ specific variation in consumer stoichiometry, similar in magnitude to the variation among invertebrate taxa. Intra-­‐specific variation in nitrogen and phosphorus content was related to both ontogeny and diet. These results suggest that the role of a species in stream nutrient cycles could vary spatially with diet and temporally through ontogeny.Chapter four examines the influence of diet stoichiometry on nutrient release ratios of four stream detritivores. Predictions of nutrient release ratios from bulk diet stoichiometries were misleading for these detritivores, which selectively consumed a nutrient rich portion of the bulk diet. Selective feeding greatly reduced stoichiometric mismatches between these consumers and their diets. Taken together, this dissertation demonstrates that examination of stoichiometric assumptions improves our understanding of consumer-­‐resource dynamics, competition, and the role of consumers in nutrient cycles.
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    Starvation in the Depths: How Quagga Mussels Persist in the Most Challenging Habitat of the Laurentian Great Lakes
    (2021-08) Zalusky, John
    The invasive quagga mussel (Dreissena rostriformis bugensis) can significantly alter the biogeochemical cycling of ecologically important nutrients, such as P and N, in lacustrine ecosystems. While D. rostriformis bugensis has invaded large portions of the Great Lakes, its distribution within waterbodies has fluctuated over time. Anticipating the effects of D. rostriformis bugensis on invaded ecosystems requires lake-wide monitoring because the effects of D. rostriformis bugensis are dependent on the size and distribution of mussel populations throughout a waterbody. During the summer of 2018 and 2019, I conducted a benthic survey consisting of ponar grabs and benthic imaging technology (BIS) to assess the distribution, areal density, and areal biomass of D. rostriformis bugensis populations in Lakes Huron and Michigan. I compared the results of my survey to other contemporary surveys of both lakes and found that, recently, D. rostriformis bugensis populations have expanded into deeper regions of Lake Michigan and contracted into more shallow regions of Lake Huron. In addition, ongoing recruitment by juvenile D. rostriformis bugensis has occurred in portions of both lakes that are deeper than other recent reports, which indicates the biomass of these populations will grow in the future. To better understand the ability of quagga mussels to invade profundal habitats and their response to food availability, I investigated the effects of starvation on quagga mussels under laboratory conditions. In an 8-month trial, I compared survival, growth, tissue elemental composition as well as respiration and nutrient transformation rates of starved mussels to mussels fed a high-quality diet. My results indicate that quagga mussels are highly starvation tolerant at temperatures characteristic of the hypolimnia in the Great Lakes, with greater than 95% survival rates after 231 days under starvation conditions. Starved mussels had lower respiration and excretion rates of P and N compared to well-fed mussels. I show that quagga mussels are highly resilient to disruptions in food supply, which helps explain their invasion success in the cold and food-limited profundal zone of the Great Lakes.