Browsing by Author "Pilla, Rachel M."

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    Anoxia begets anoxia: A positive feedback to the deoxygenation of temperate lakes
    Lewis, Abigail S. L.; Lau, Maximilian P.; Jane, Stephen F.; Rose, Kevin C.; Be'eri-Shlevin, Yaron; Burnet, Sarah H.; Clayer, François; Feuchtmayr, Heidrun; Grossart, Hans-Peter; Howard, Dexter W.; Mariash, Heather; Delgado Martin, Jordi; North, Rebecca L.; Oleksy, Isabella; Pilla, Rachel M.; Smagula, Amy P.; Sommaruga, Ruben; Steiner, Sara E.; Verburg, Piet; Wain, Danielle; Weyhenmeyer, Gesa A.; Carey, Cayelan C. (Wiley, 2023)
    Declining oxygen concentrations in the deep waters of lakes worldwide pose a pressing environmental and societal challenge. Existing theory suggests that low deep-water dissolved oxygen (DO) concentrations could trigger a positive feedback through which anoxia (i.e., very low DO) during a given summer begets increasingly severe occurrences of anoxia in following summers. Specifically, anoxic conditions can promote nutrient release from sediments, thereby stimulating phytoplankton growth, and subsequent phytoplankton decomposition can fuel heterotrophic respiration, resulting in increased spatial extent and duration of anoxia. However, while the individual relationships in this feedback are well established, to our knowledge, there has not been a systematic analysis within or across lakes that simultaneously demonstrates all of the mechanisms necessary to produce a positive feedback that reinforces anoxia. Here, we compiled data from 656 widespread temperate lakes and reservoirs to analyze the proposed anoxia begets anoxia feedback. Lakes in the dataset span a broad range of surface area (1–126,909 ha), maximum depth (6–370 m), and morphometry, with a median time-series duration of 30 years at each lake. Using linear mixed models, we found support for each of the positive feedback relationships between anoxia, phosphorus concentrations, chlorophyll a concentrations, and oxygen demand across the 656-lake dataset. Likewise, we found further support for these relationships by analyzing time-series data from individual lakes. Our results indicate that the strength of these feedback relationships may vary with lake-specific characteristics: For example, we found that surface phosphorus concentrations were more positively associated with chlorophyll a in high-phosphorus lakes, and oxygen demand had a stronger influence on the extent of anoxia in deep lakes. Taken together, these results support the existence of a positive feedback that could magnify the effects of climate change and other anthropogenic pressures driving the development of anoxia in lakes around the world.
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    Earlier winter/spring runoff and snowmelt during warmer winters lead to lower summer chlorophyll-a in north temperate lakes
    Hrycik, Allison R.; Isles, Peter D. F.; Adrian, Rita; Albright, Matthew; Bacon, Linda C.; Berger, Stella A.; Bhattacharya, Ruchi; Grossart, Hans-Peter; Hejzlar, Josef; Hetherington, Amy Lee; Knoll, Lesley B.; Laas, Alo; McDonald, Cory P.; Merrell, Kellie; Nejstgaard, Jens C.; Nelson, Kirsten; Noges, Peeter; Paterson, Andrew M.; Pilla, Rachel M.; Robertson, Dale M.; Rudstam, Lars G.; Rusak, James A.; Sadro, Steven; Silow, Eugene A.; Stockwell, Jason D.; Yao, Huaxia; Yokota, Kiyoko; Pierson, Donald C. (Wiley, 2021-10)
    Winter conditions, such as ice cover and snow accumulation, are changing rapidly at northern latitudes and can have important implications for lake processes. For example, snowmelt in the watershed-a defining feature of lake hydrology because it delivers a large portion of annual nutrient inputs-is becoming earlier. Consequently, earlier and a shorter duration of snowmelt are expected to affect annual phytoplankton biomass. To test this hypothesis, we developed an index of runoff timing based on the date when 50% of cumulative runoff between January 1 and May 31 had occurred. The runoff index was computed using stream discharge for inflows, outflows, or for flows from nearby streams for 41 lakes in Europe and North America. The runoff index was then compared with summer chlorophyll-a (Chl-a) concentration (a proxy for phytoplankton biomass) across 5-53 years for each lake. Earlier runoff generally corresponded to lower summer Chl-a. Furthermore, years with earlier runoff also had lower winter/spring runoff magnitude, more protracted runoff, and earlier ice-out. We examined several lake characteristics that may regulate the strength of the relationship between runoff timing and summer Chl-a concentrations; however, our tested covariates had little effect on the relationship. Date of ice-out was not clearly related to summer Chl-a concentrations. Our results indicate that ongoing changes in winter conditions may have important consequences for summer phytoplankton biomass and production.
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    Transparency, Geomorphology and Mixing Regime Explain Variability in Trends in Lake Temperature and Stratification across Northeastern North America (1975–2014)
    Richardson, David C.; Melles, Stephanie J.; Pilla, Rachel M.; Hetherington, Amy L.; Knoll, Lesley B.; Williamson, Craig E.; Kraemer, Benjamin M.; Jackson, James R.; Long, Elizabeth C.; Moore, Karen; Rudstam, Lars G.; Rusak, James A.; Saros, Jasmine E.; Sharma, Sapna; Strock, Kristin E.; Weathers, Kathleen C.; Wigdahl-Perry, Courtney R. (MDPI, 2017-06-20)
    Lake surface water temperatures are warming worldwide, raising concerns about the future integrity of valuable lake ecosystem services. In contrast to surface water temperatures, we know far less about what is happening to water temperature beneath the surface, where most organisms live. Moreover, we know little about which characteristics make lakes more or less sensitive to climate change and other environmental stressors. We examined changes in lake thermal structure for 231 lakes across northeastern North America (NENA), a region with an exceptionally high density of lakes. We determined how lake thermal structure has changed in recent decades (1975–2012) and assessed which lake characteristics are related to changes in lake thermal structure. In general, NENA lakes had increasing near-surface temperatures and thermal stratification strength. On average, changes in deepwater temperatures for the 231 lakes were not significantly different than zero, but individually, half of the lakes experienced warming and half cooling deepwater temperature through time. More transparent lakes (Secchi transparency >5 m) tended to have higher near-surface warming and greater increases in strength of thermal stratification than less transparent lakes. Whole-lake warming was greatest in polymictic lakes, where frequent summer mixing distributed heat throughout the water column. Lakes often function as important sentinels of climate change, but lake characteristics within and across regions modify the magnitude of the signal with important implications for lake biology, ecology and chemistry.