Theses and Dissertations at Montana State University (MSU)
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Item Long-term and over winter phytoplankton community dynamics in Lake Bonney, Antarctica(Montana State University - Bozeman, College of Agriculture, 2017) Patriarche, Jeffrey Dennis; Chairperson, Graduate Committee: John C. PriscuLake Bonney is a hypersaline permanently ice-covered lake in the Taylor Valley, Antarctica that hosts simplified microbial food-webs. Studied since the 1960s, there are many aspects which are poorly understood. Logistical constraints have prevented sampling during the austral winter, a 4-month period of 24-hour darkness. Our knowledge of how the resident photosynthetic microorganisms respond during this period is limited. With inputs from ephemeral glacial-melt streams the lake level (stage) of Bonney has risen more than 3 m since 2004. With no outflow streams, the only known water loss is via ablation of the permanent ice-cover. A study of the spatial and temporal changes in the phytoplankton community structure during this period of rapid lake level rise is lacking. During the summers (November-January) from 2004-05 to 2014-15 an in situ submersible spectrofluorometer was deployed in Lake Bonney to quantify the chlorophyll-a concentrations (microgram L -1) of four functional groups of microalgae (green algae, brown/mixed algae, cryptophytes, cyanobacteria) using known excitation/emission spectra. During the 2013-14 field season this same instrument was mounted on autonomous cable-crawling profilers deployed in both east and west lobes of Lake Bonney, obtaining the first ever daily profiles of chlorophyll-a concentration at an annual scale. Following a summer of rapid lake level rise (2010-11), an increasing trend in depth integrated chlorophyll-a concentration was observed in Lake Bonney. During the same period, the nutrient poor surface water has become increasingly dominated by green algae. Dramatic shifts were also observed in the phytoplankton communities during the polar night. The highest concentrations of mean chlorophyll-a were measured during the 24-hour darkness. Algal spectral groups containing species capable of a mixotrophic metabolism (brown/mixed and cryptophytes) increased in concentration and relative abundance when photosynthetically active radiation was unavailable. This work provides valuable contributions to our knowledge of long-term and year-round phytoplankton community dynamics in Lake Bonney, and improves our understanding of the metabolic strategies employed by organisms in this high latitude permanently ice-covered lake.Item Bacterioplakton dynamics in stratified lakes of the Taylor Valley, Antarctica during the transition to polar night(Montana State University - Bozeman, College of Agriculture, 2010) Vick, Trista Juliana; Chairperson, Graduate Committee: John C. Priscu.Limnological research on the lakes of the McMurdo Dry Valleys (MCM), Antarctica, is typically carried out during the austral spring-summer (October January) when logistical support is readily available; the current study marks the first sampling effort during the summer-fall transition (January-April). Sampling during the darkness of winter is logistically difficult and expensive, and my study is an important step towards understanding the year-round ecology of the dry valley lakes. Bacterial productivity, measured as protein synthesis and DNA replication, and bacterial cell numbers were measured 10-12 times between October 2007 and April 2008 in Lakes Fryxell (FRX) and the east and west lobes of Lake Bonney (ELB and WLB). Lake Fryxell was the most productive (bacterial) lake on average by an order of magnitude (average = 1.24 mg C m -²d -¹; range = 0.00 to 3.29 mg C m -²d -¹), and also contained the greatest bacterial biomass (~10 ⁶ cells ml -¹) by 1 to 3 orders of magnitude. If bacterial production were directly linked to organic carbon supplied by photosynthetic primary production, a decrease in bacterial production would be expected during the sunset; however, no statistically significant change in bacterial production (a=0.05) was observed during the summer-fall transition. A distinct decoupling of bacterial protein production and DNA replication was detected in FRX and ELB of the lakes as the season progressed, and was present in WLB throughout the season, indicating either a shift towards a lower growth-rate in response to decreasing light or nutrient supply, or a mechanism for dealing with the perennially low temperatures, low light, and nutrient poor conditions in the lakes. Overall, it appears that bacterial communities remain active during the darkness of winter, when the lakes enter a period of "net heterotrophy", which cannot be sustained unless the carbon balance of the TV lakes is reset by climatic events.Item Stream nitrogen uptake dynamics from ambient to saturation across development gradients, stream network position, and seasons(Montana State University - Bozeman, College of Agriculture, 2010) McNamara, Rebecca Anne; Chairperson, Graduate Committee: Brian L. McGlynn; Wyatt Cross (co-chair)The balance between stream nitrogen (N) loading and retention determines stream network nutrient export dynamics. Nutrient retention can be altered due to changes in hydrology, nutrient loading, and biological community response to increased nutrient availability. We quantified physical and biological contributions to total nutrient retention and determined biological uptake kinetics from ambient to saturation across six stream reaches across the West Fork Gallatin Watershed (a 1st to 4th order, headwater, 240 km² watershed) which has experienced rapid exurban, resort development leading to increased watershed nutrient loading over the last four decades. We conducted 17 stream tracer experiments (constant-rate and instantaneous additions) using both conservative (chloride (Cl)) and biologically active (nitrate (NO 3-N)) tracers across a range of watershed areas (WA), stream discharges, seasons, development intensities, and ambient NO 3-N concentrations, reflecting varying degrees of development and upland nutrient loading (i.e., wastewater effluent). Ambient uptake (U amb) was calculated and Michaelis-Menten kinetic models were used to quantify maximum areal uptake rates (U max) and half-saturation constants (K m) for each experimental reach. In the West Fork Gallatin Watershed, the majority of added NO 3-N was physically retained within stream reaches of smaller WA with decreasing physical retention as WA increased. However, as WA increased, biological retention of added NO 3-N became increasingly important and exceeded physical retention in the two largest watersheds. Further, U amb and U max values increased with greater WA, and U max was greatest in the summer and lowest in the winter. Our results demonstrated that nutrient uptake variability between stream reaches was related to WA, discharge, ambient NO 3-N concentration, and season. Although some streams in the watershed no longer appear to be functioning at pre-development levels, none demonstrated saturation with respect to NO 3-N, yet with continued development and increased loading, nutrient saturation could occur. We suggest that quantifying physical and biological contributions to total retention and determining uptake kinetics from ambient to saturation over space, time, and development intensities can yield new insight into the capacity of stream networks to buffer nutrient loading.