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Terrestrial-Aquatic Linkages: Watershed (Geologic and Vegetation) Runoff Influences on Primary Production of a Sub Alpine Lake
AuthorMejica, Brooke N.
Natural Resources and Environmental Science
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Nutrient transfer via runoff from terrestrial to aquatic ecosystems has the potential to enrich aquatic productivity and change algal composition. There has been increased cultural eutrophication of fresh water bodies from increased combustion and atmospheric deposition of nutrients, land development and associated erosion, agricultural fertilizers in runoff, and other non-point sources. Therefore, it is become increasingly important to understand nutrient transfer in natural systems, as this can inform effective watershed management. The goals of this study were to enhance our understanding of nutrient transfer via runoff from a watershed to a lake that has had relatively little changes over time, and to determine the influence of runoff on primary production. The first chapter provides an introduction to the project. The objectives of the second chapter were to assess macro- and micro-nutrient sources in the terrestrial area of Castle Lake, a small sub-alpine lake basin of the Upper Sacramento River Watershed, and to estimate nutrient input and timing through measurements of runoff and springs. Inflows were categorized by water source type (streams, waterfalls, other overland flow, litter interflow, or springs), vegetation type (conifer forest, conifer forest-shrub, alder, mixed, or no vegetation), and underlying bedrock (ultramafic, mafic, felsic, or a combination of these). Macro- and micro-nutrients were analyzed and differences between these ecotypes and seasons (fall rainstorms, winter snowmelt cycles, and spring snowmelt) were assessed. Nutrient concentrations were greater in litter interflow and riparian interflow samples from both the alder and coniferous areas. These areas had a relatively thick organic litter and organic soil layer that runoff was able to pass through prior to collection. N and P concentrations as well as loading were generally higher during fall rainstorms and winter snowmelt cycles than spring snowmelt. Underlying geologic parent material did seem to affect runoff and spring water quality. Areas with more mafic compositions exhibited higher concentrations of magnesium and heavy elements (chromium and nickel). Conversely, lighter elements (boron, chloride, and potassium) were also more concentrated in areas with more mafic compositions, possibly reflecting higher weathering rates of these ultramafic and mafic rocks. The objectives of the third chapter were to quantify the influence of natural runoff on pelagic primary productivity through a series of bioassay experiments. Essential nutrients and their respective concentrations were identified in a pulse of fall litter runoff and from a groundwater spring. The influences of these inflows on lake primary productivity rates were determined via a series of bioassay experiments. A comparison of traditional bioassays using nitrogen, phosphorus and other essential nutrients (molybdenum, boron, silica, and a mixture of trace elements) that have been shown to previously limit or were likely to limit productivity was utilized to contextualize the linkages and allow for the interpretation of results. Compared to the primary productivity (PPr) rates of lake water, four-hour bioassays in fall 2010 exhibited decreased PPr in all treatments [additions of: nitrogen + phosphorus, forest litter interflow, alder spring water, silica + nitrogen + phosphorus, or a mixture of macro and micronutrients (silica + molybdenum + boron + manganese + sulfur + magnesium + calcium + zinc + nitrogen + phosphorus), each added to pooled mixed layer water]. Decreased PPr may have been due to too short an incubation time, heterotrophic bacteria outcompeting phytoplankton initially, inhibitory effects of humic substances, including allelopathy in the runoff and spring treatments, inhibitory effects of silica or sodium in the silica and mixture treatments, and/or more precipitation overall in 2010 and increased runoff from a storm reducing nutrient limitation just prior to our experiments. A four-day bioassay in fall 2011 displayed increased PPr with the addition of forest litter interflow, forest-shrub litter interflow, or alder spring water. Increasing concentrations of litter interflow displayed increased PPr as limiting nutrients were added in greater concentrations. 1% spring water by volume resulted in similar PPr as did 5% forest litter interflow by volume while increasing concentrations of spring water displayed sequentially lower PPr. This may have been due to elevated silica, chromium, or nickel concentrations inhibiting photosynthesis. The various responses observed were likely from different phytoplankton species responding to the changing nutrient pools between treatments.