Lakes responses to regional and global environmental changes

Abstract

Freshwater is a vital natural resource currently under threat from multiple anthropogenic stressors. The effects of watershed and atmospheric disturbances often present themselves in lakes, given their deep-lying position within a watershed. Water transparency in many north-temperate lakes is rapidly decreasing due to increasing loads of terrestrial dissolved organic matter (“DOM”); commonly referred to as lake browning. At the same time, the planet is undergoing rapid and widespread changes in climate. Lake surface temperatures are warming at record rates following increases in air temperatures. Further, wide-spread declines in surface wind speeds have been observed across much of the northern hemisphere; a phenomenon referred to as “atmospheric stilling”. Watershed and atmospheric changes can fundamentally alter lake ecosystem structure and function, which in turn will reduce many of the ecosystem services lakes provide such as safe drinking water, angling, and recreational opportunities. This dissertation investigates how changes in water transparency and climate interact to alter physical and chemical lake features, and further examines how these changes will ultimately alter habitat availability and growth rates of common sport fish. Though a clear pattern has been shown between lake surface temperatures and climate warming, less is known about how the multiple factors of climate change affect summer thermal stratification. Summer stratification depth and strength are two characteristics that regulate many in-lake processes including the creation of a distinct vertical thermal gradient, which strongly regulates the distribution of many organisms. Understanding the role climate plays on stratification depth and strength can be difficult because non-climate factors like water transparency can strongly influence stratification features. In the first study, I utilized a lake in a protected National Park as a case study in order to isolate the effects of climate change on stratification depth and strength. Crater Lake is a near-pristine lake which has exhibited little changes in water transparency and watershed land use over the last 25 years. I examined long-term trends in water temperature profiles and meteorological conditions to determine how summer stratification characteristics changed from 1993 to 2017. I next calibrated a hydrodynamic model and performed scenarios to investigate how changes in climate variables (mainly air temperature and windspeed) alter stratification characteristics. Summer depth and strength of stratification were regulated by different climate variables. I found that the depth of stratification decreased by 55% across the 25-year period, and that this decline was most likely driven by a decrease in wind speeds. While there was no clear long-term pattern in summer stratification strength, I found variability in stratification strength was largely driven by variation in air temperature, with warmer air temperatures resulting in stronger stratification. Notably, I found that spring time conditions in both wind speed and air temperature strongly influenced summer stratification characteristics. Since wind speeds are declining elsewhere and air temperatures are warming across the globe, other large lakes may be experiencing similar stratification changes. North temperate lakes have been browning across North America and Europe following recovery from acid deposition, and changes in precipitation and land use practices. Browning associated decreases in water transparency can strongly influence lake productivity due to the light absorbing nature of DOM. However, limiting nutrients (i.e., phosphorus and nitrogen) could be associated with DOM, which may help compensate for the negative light absorbing effects of DOM on lake productivity. I utilized two spatial surveys across the United States and a long-term lake survey of 28 lakes in the Adirondacks to understand how limiting nutrients and DOM characteristics were related. Across space, limiting nutrients were strongly positively related with DOM concentrations (quantified here as dissolved organic carbon concentration; “DOC”) and DOM specific absorbance. Adirondack lakes strongly increased in DOC concentration and became browner from 1994 to 2012, but limiting nutrients did not increase. Instead, phosphorus concentrations largely stayed the same while nitrogen concentrations decreased. Further, modeling of lake photosynthetic potential in each of the 28 lakes indicates that most lakes have likely decreased in whole-lake productivity from 1994-2012. Contrasting trends in DOM and limiting nutrients suggests that the strongest effect of lake browning will likely be a decrease in lake productivity through time. Many north temperate lakes are warming and browning at the same time. Both warming temperatures and decreases in water transparency have the potential to threaten lake dissolved oxygen (“DO”) levels. The relative importance of each driver and the combined effects of warming and browning are not well understood. The third study investigates how warming air temperatures and browning affect DO levels via hydrodynamic and biogeochemical modeling scenarios across a 30-year period (1990 – 2019). I calibrated a hydrodynamic and coupled biogeochemical model to Lake Giles in Pennsylvania. Lake Giles has a robust long-term data record and is a small oligotrophic lake, which is representative of many north temperate lakes. After model validation, I recreated Lake Giles across three different initial DOC concentrations to represent a wide initial range of DOC. For each of the three lakes, I simulated trends in browning and climate warming across the 30-year period and calculated annual summer DO metrics. Lakes with more DOC tended to have less DO. Browning tended to reduce DO in clear lakes quicker than in lakes with higher initial DOC concentrations. Climate warming had a smaller negative effect on DO concentrations. Browning increased the prevalence of anoxia and hypoxia in the summer, and the combined browning and warming scenarios generated the highest levels of summer anoxia and hypoxia. An increase in DOC from 1 to 5 mg L-1 shifted the onset of anoxia and hypoxia up to one and half months earlier. This study has major water quality implications, as the onset of anoxia and hypoxia can release nutrients stored in the sediments, which can fuel algae blooms in the surface waters. Browning traps more heat at the lake surface, which leads to warmer surface waters and cooler deep waters in the summer. Fish are ectotherms, and as such many functions including metabolism and growth are temperature dependent. Browning therefore has the potential to alter growth rates in fish. In this last study, I again used Lake Giles as a case study. I used a hydrodynamic model to recreate Lake Giles at different DOC concentrations from 1 to 15 mg L-1 during a typical year. I then examined how the growth of two common fish with different temperature preferences varied in response to browning-induced changes in water temperature. Largemouth Bass (Micropterus salmoides) are a common warm-water species present in many north temperate lakes. As DOC concentrations increased, summer surface temperatures warmed and shifted closer to bass optimal temperatures. Thus, increases in DOC led to larger growth rates for bass, most noticeable in the 1 to 3 mg L-1 range. Brook Trout (Salvelinus fontinalis) are a common cold-water sport fish that inhabits deeper parts of the lake in the summer. Browning led to a slight decrease in deep water temperatures, especially in the spring. As a result, trout growth in the spring slightly reduced with browning, but the overall affects were minimal. Though trout growth was minimally affected by DOC, I estimated that trout would have to move shallower in the water column to track their preferred temperature, which could lead to changes in behavior and interspecific competition. Overall, I found differential effects of lake browning on fish growth via temperature depending on fish type (i.e., warm-water vs cold-water fish). Lakes may become dominated by warm water species as lakes continue to brown if food supplies remain suitable.