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Climate Sensitivity of Lentic Mountain Ecosystems
Get PDFLentic freshwater ecosystems are threatened by unprecedented global change. Often considered sentinels for change, lentic ecosystems like lakes and ponds are particularly sensitive and vulnerable to the effects of global change because they respond rapidly to changes in the environment and integrate information from their surrounding catchment within their ecosystem. These sensitive lentic ecosystems are increasingly threatened by climate-driven shifts like warming air temperatures, increasing hydroclimatic variability, and changing ice phenologies. Additionally, global-change phenomena such as increasingly extreme wildfire activity further threaten these ecosystems. In particular, mountain lentic ecosystems are experiencing some of the world’s greatest rates of change in air temperature and precipitation regimes, respond strongly to climate forcing, and may be particularly sensitive to global change. In this dissertation, I investigated the climate sensitivity of lentic ecosystems in three primary ways. First, we quantified lake exposure to wildfire smoke across North America, and reviewed the known and theoretical impacts of that exposure. Then, we investigated how wildfire smoke affects lake and pond temperature and ecosystem metabolism across a watershed. Finally, we addressed the impact of hydroclimatic variability on lake zooplankton assemblage, abundance, and diversity. We found that the physical, biological, and chemical processes in lakes likely all respond to wildfire smoke exposure, and that in small, oligotrophic mountain lakes and ponds, smoke reduces water temperatures and ecosystem metabolism. These studies highlight that as wildfires increase in frequency and intensity, smoke from those fires have the potential to impact lentic ecosystem processes from local to continental scales. We also found that while much of the research on climate impacts focuses on the effects of warming, climate change-driven extremes in hydroclimate significantly determines lake zooplankton community abundance, biomass, and diversity. With increasingly extreme variability in hydroclimate, mountain lake zooplankton communities may undergo major shifts in assemblage and abundance. As we face increasing challenges driven by both climate change and human behavior, lakes and ponds can serve as key indicators of change in an ever-changing world
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The Role of Abiotic and Biotic Processes in Regulating Benthic Ecosystem Function along a Productivity Gradient
Get PDFThe benthic zone of lakes can be a hotspot for lake energy flows and nutrient cycling. Indeed, the notion of benthic “ooze” and its role in the recycling of nutrients forms a conceptual cornerstone of ecology (Lindeman 1942). Benthic habitats can vary from soft, muddy sediments rich in organic matter and nutrients to bare rock covered by thin biofilms. These habitats form the linkage between lake and watershed, and support a range of ecosystem processes—from exchange of groundwater nutrients, internal nutrient cycling, mineralization of organic matter, heterotrophic respiration, and benthic algal production—that each vary in importance between different waterbodies. Clear Lake, CA, and Lake Tahoe represent two systems that span a gradient in trophic level, depth, and underlying geology. The former is shallow, and extremely productive, while the latter is deep and has nutrient concentrations near detection limit. Both, however, are large lakes and are extremely important water resources in California. We set out to characterize two very different, but nevertheless important, benthic processes that are adversely affecting water quality in each lake.Benthic algae, or periphyton, blooms may be increasing in oligotrophic, clear water lakes globally in response to increases in water temperature and nutrient loads, often associated with climate change. In many regions, climate change is altering the duration of snow cover, the frequency and severity of drought, and storm intensity, all of which have the potential to affect periphyton growth rates. Lake Tahoe, located in the Sierra Nevada Mountains of California and Nevada, has anecdotal reports that the frequency and severity of nearshore periphyton blooms have increased in recent years. Such blooms decrease both water quality and the aesthetic value of nearshore areas. While recent studies have documented changes in periphyton biomass, it is difficult for in situ studies to disentangle the individual and interactive effects of temperature and nutrients on periphyton growth rates. Consequently, we used laboratory experiments to examine 1) seasonal variability in periphyton biomass and metabolism, 2) the role of temperature, nutrients, and their interactive effects on determining periphyton metabolic rates, and 3) seasonal variability of the aforementioned temperature and nutrient effects. By measuring changes in dissolved oxygen in sealed incubation chambers containing rocks collected from the nearshore, we quantified rates of gross primary production (GPP), ecosystem respiration (ER), and net ecosystem production (NEP). Our experiments had two nutrient treatments (ambient and augmented with nitrogen and phosphorus) and four temperature treatments (ambient, +3, +6, and +9 °C above ambient) to simulate warming during different seasons, and nutrient loading associated with increased runoff. We found that temperature drove the majority of variability in periphyton metabolic rates and increased GPP, ER, and NEP by 4-7% per 1°C of warming. The effect of warming varied seasonally with larger effects in colder months. Warming also stimulated ER more so than GPP, with decreasing trends of NEP with warming during summer months. Nutrient additions had variable effects on rates of GPP, ER and NEP, but increased ER rates by 6-7% at maximum nutrient concentrations observed in the field. However, in contrast to our expectations, we found little evidence for interactive effects between temperature and nutrients for GPP and NEP, and a small, negative, inhibitory interaction for ER. In all, these experiments provide evidence that warming can increase periphyton metabolic rates in oligotrophic systems even in the absence of increased nutrient loads in cases where periphyton are not nutrient limited. These effects on littoral zones of lakes will have important consequences for ecosystem structure and function, and management efforts to curb nutrients will be crucial for limiting periphyton blooms under climate change.
Eutrophication continues to be a problem in aquatic systems globally, resulting from excessive nutrients such as phosphorus and nitrogen. Nutrient sources can be external (i.e. runoff) or from within the lake (internal loading). In the case of phosphorus, internal loading can occur several ways: 1) chemically reduced conditions at the sediment-water interface of lake bottoms (usually induced by anoxic conditions) facilitate the reductive dissolution of phosphorus from the sediments to the overlying water, 2) mineralization of organic matter, and 3) physical resuspension of shallow sediments, and 4) elevated pH (commonly the result of intensive algal productivity) that results in ion exchange in sediments. Efforts to restore Clear Lake, a hypereutrophic lake located in Lake County, CA, have largely focused on external nutrient inputs as the cause of excessive phosphorus concentrations. This study provided the first direct measure of internal phosphorus fluxes from the nutrient-rich lake-bottom sediments to the overlying water column using incubations of intact sediment cores. Similar to other studies, soluble reactive phosphorus (SRP) flux rates from sediments ranged from 8.8 to 26.7 mg SRP m-2 d-1 in anoxic treatments and from -4.2 to 0.9 mg SRP m-2 d-1 in oxygenated treatments. We found little seasonal variation in anoxic SRP flux rates between winter and summer incubations despite large temperature differences between these two seasons. We hypothesize that early summer drawdown of redox-sensitive sediment P pools can potentially limit sediment phosphorus flux rates later in the season despite the positive effects of warmer summer temperatures on SRP flux. Thus, characterizing phenology of sediment P pools is important for estimating annual internal phosphorus loads in lakes. Large internal fluxes of P in this system were largely ignored during establishment of nutrient TMDLs for the watershed. This study highlights the importance of measuring internal fluxes of P and its seasonality when setting nutrient TMDLs or designing restoration strategies for a watershed
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The Spatial Ecology of Macroinvertebrates in Sierra Nevada Lakes and Ponds
Get PDFDetermining the factors that drive patterns of diversity across space and time is a major goal of ecology. Mountain lakes and ponds are uniquely suited for testing how environmental gradients shape the spatial and temporal patterns of population and community diversity. Additionally, mountains contain steep climate gradients and highly variable local contexts, both of which are likely to mediate how communities respond to climate warming. In this dissertation, I used a suite of theoretical and empirical frameworks to understand the factors shaping spatial patterns of population genetic diversity, patterns of community assembly among lakes and ponds, and community responses to a rapidly changing climate. For all chapters, the Sierra Nevada mountains of California served as the natural laboratory, while both historical and contemporary collections of littoral macroinvertebrates served as the subjects.In Chapter 1, I used a common lentic mayfly species (Callibaetis ferrugineus hageni) to explore how spatial patterns of genetic and genomic variation are influenced by the interaction of steep climate gradients, the landscape matrix, and local environmental quality. By using a combination of landscape genetic network models and genomic tools, I found that both at-site and among-site factors influence patterns of gene flow among lakes, while higher elevation populations showed enhanced signatures of local adaptation. Taken together, landscape-restricted gene flow likely allows for selection pressures to facilitate local adaptation in the mayfly populations of higher elevation lakes. In Chapter 2, I used a historical dataset of macroinvertebrates to understand how daily and seasonal temperature variation influence community assembly in lakes versus ponds. Relative to lakes, I found that ponds filter for taxa with wider thermal tolerance, which then constrains the diversity of functional traits in pond communities. My results also suggest that daily temperature variation may be a particularly important abiotic filter for pond communities, though future incorporation of in situ data will help confirm the relative roles of daily, seasonal, and interannual climate variation across categories of ecosystem size. In Chapter 3, I used both historical and contemporary datasets of lake macroinvertebrates to ask 1) if lake communities are responding to climate warming through colonizations by warm-adapted taxa, local extinctions of cold-adapted taxa, or both (i.e. thermophilization) and 2) if current responses to climate warming result in reduced beta diversity (i.e. homogenization). Over 20 years of declining winter snowpack and rising air temperatures, I found that colonizations by predators drove thermophilization in contemporary macroinvertebrate communities, but that thermophilization did not lead to significant homogenization in either taxonomic or functional measures.Collectively, my dissertation demonstrates the unique ways in which mountain environments shape freshwater populations and communities, while also mediating community responses to rapid climate warming
Wildfire Smoke Effects on Lake‐Habitat Specific Metabolism: Toward a Conceptual Understanding
Get PDFThe impacts of wildfire smoke on lake habitats remains unclear. We determined the metabolic response to smoke in the epi-pelagic and two littoral habitats in Castle Lake, California. We compared light regime, gross primary production, ecosystem respiration, and net ecosystem production in years with and without smoke. During the smoke period incident ultraviolet-B (UV-B) radiation and photosynthetically active radiation (PAR) decreased by 53% and 28%, respectively, while the water column extinction coefficient of UV-B and PAR increased by 20% and 18% respectively. Epi-pelagic productivity increased during smoke cover because of decreased solar inputs. PAR values remained sufficient to saturate productivity, suggesting observed differences were primarily the result of changes in UV-B. Littoral-benthic productivity did not change, possibly reflecting adaptation to high-intensity UV-B light in these habitats. Our results highlight the importance of understanding how prolonged wildfire smoke alters the amount of energy produced from specific habitats in lakes.Fil: Scordo, Facundo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto Argentino de Oceanografía. Universidad Nacional del Sur. Instituto Argentino de Oceanografía; Argentina. University of Nevada; Estados UnidosFil: Sadro, Steven. University of California at Davis; Estados UnidosFil: Culpepper, Joshua. University of California at Davis; Estados Unidos. University of Nevada; Estados UnidosFil: Seitz, Carina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto Argentino de Oceanografía. Universidad Nacional del Sur. Instituto Argentino de Oceanografía; Argentina. University of Nevada; Estados UnidosFil: Chandra, Sudeep. University of Nevada; Estados Unido
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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