Alpine Lake Ecosystems Research Articles

Impacts of climate warming and atmospheric deposition on recent shifts in chironomid communities in two alpine lakes, eastern China

Climate warming and atmospheric deposition are altering alpine lake ecosystems at unprecedented rates, whereas their direct and indirect effects on primary consumer communities are unclear. This study presents sedimentary multi-proxy records including chironomids, diatoms, elements and stable isotopes of carbon and nitrogen in 210Pb-dated cores from two alpine lakes located above the timberline in the Taibai Mountain, eastern China. Before ∼2000 CE, chironomid communities were co-dominated by Heterotrissocladius marcidus-type and Micropsectra atrofasciata-type in the two lakes. Thereafter, Tanytarsus glabrescens-type increased rapidly to be a dominant species. Redundancy analyses (RDAs) revealed that chironomid fauna shifts were significantly correlated with rising diatom concentrations in both lakes, declining Ti content in the upstream lake and δ13C depletion in the downstream lake. Although temperature, precipitation and δ15N were not significant explanatory variables in RDAs, climate warming and atmospheric deposition likely promoted terrestrial and aquatic primary production, indicated by synchronous increases in organic matter contents and diatom concentrations in the two sediment cores. Since diatoms contain essential polyunsaturated fatty acids that are essential for chironomids, rising diatom concentrations can promote food quantity and quality. In addition, increased primary production would create organic substrates for chironomid larvae. Recent shifts in chironomid fauna driven by indirect effects of global warming and atmospheric deposition might be a widespread phenomenon in alpine lakes, probably triggering regime shifts in headwater lake ecosystems.

Lakes-scale pattern of eukaryotic phytoplankton diversity and assembly process shaped by electrical conductivity in central Qinghai-Tibet Plateau.

Phytoplankton are the main primary producers in aquatic ecosystems and play an important role in food web and geochemical cycles. Its diversity, community structure, and assembly process are influenced by several factors. Alpine lake ecosystems are relatively weak and extremely sensitive to global climate change. However, the impact of climate change on phytoplankton in Qinghai-Tibet Plateau lakes and their responses are still unclear. In this study, we analyzed the diversity, environmental drivers, and assembly process of phytoplankton community in the central QTP lakes. The phytoplankton of these lakes can be primarily distinguished into freshwater and brackish types, with significant differences in species diversity and community dissimilarity. Both shared nearly same key environmental factors that significantly affecting phytoplankton such as EC, and brackish lakes were also positively correlative with TN. Stochastic process was predominant in phytoplankton assembly. Additionally, freshwater and brackish lakes were dominated by dispersal limitation and heterogeneous selection respectively. Alpine lakes had significant EC thresholds, and their diversity and assembly processes changed significantly around the thresholds. The present findings have important implications for understanding and predicting the response of lake phytoplankton communities to climate change and for making decisions to protect the ecological resources of alpine lakes.

Climate, atmospheric deposition and catchment process interact to trigger recent diatom community reorganization in alpine lakes of the eastern monsoonal region of China

Atmospheric deposition and climate change are altering alpine lake ecosystems at an accelerating rate. Biotic responses to the interactions of multiple stressors are underexplored in alpine lakes. This study explored diatom responses to climate change and nutrient supply based on multi-proxy sedimentary records in Erye and Sanye Lakes of the Taibai Mountain at the boundary between the subtropical and temperate climate zones of eastern China. Diatom communities of the two lakes shifted from large-sized benthic taxa to small fragilarioid species at the beginning of the 21st century, synchronous with an increase in diatom accumulation rate. Redundancy analyses showed that diatom communities were significantly correlated with catchment processes and nutrient inputs in both lakes, as well as climate warming in the upstream lake. Partial least squares path modelling revealed that catchment processes were more important than direct effects of climate warming and nutrient supply for explaining the observed shifts in diatom flora in the two alpine lakes. The comparison with diatom records in similar alpine lakes in the East Asian monsoon region reveals that responses of alpine lake ecosystems to warming and atmospheric deposition vary with different catchment settings and lake morphometry. Given that these alpine lakes serve as important water sources, recent biotic reorganization suggests that the protection of headwater systems should be given high priority, especially in rapidly developing and densely-populated regions.

Bacterial community composition of the sediment in Sayram Lake, an alpine lake in the arid northwest of China

Sediment bacterial communities play a critical role in biogeochemical cycling in alpine lake ecosystems. However, little is known about the sediment microbial communities in these lakes. In this study, the bacterial community composition (BCC) and their relationships with environmental factors of the sediment in Sayram Lake, the largest alpine and cold-water inland lake, China was analyzed using Illumina MiSeq sequencing. In total, we obtained 618,271 high quality sequences. The results showed that the bacterial communities with 30 phyla and 546 genera, were spread out among the 5 furface sediment samples, respectively. The communities were dominated by Proteobacteria, Acidobacteria, Planctomycetes, Gemmatimonadetes, Chloroflexi, Actinobacteria, Verrucomicrobia and Bacteroidetes, accounting for 48.15 ± 8.10%, 11.23 ± 3.10%, 8.42 ± 2.15%, 8.37 ± 2.26%, 7.40 ± 3.05%, 5.62 ± 1.25%, 4.18 ± 2.12% and 2.24 ± 1.10% of the total reads, respectively. At the genus level, the communities were dominated by Aquabacterium, Pseudomonas, Woeseia, MND1, Ignavibacterium and Truepera, accounting for 7.89% ± 8.24%, 2.32% ± 1.05%, 2.14% ± 0.94%, 2% ± 1.22%, 0.94% ± 0.14% and 0.80% ± 0.14% of the total reads, respectively. Statistical analyses showed the similarity of the sediment bacterial communities at our field sites was considerably low, far below 35%, and total organic carbon (TOC) was the dominant environmental factor affecting the spatial changes of BCC in the sediment. Thus, this study greatly improving our understanding of the microbial ecology of alpine lake in the arid and semi-arid ecosystems today so seriously threatened.

Factors affecting seasonal variation of microbial community structure in Hulun Lake, China

Microbial communities play an important role in water quality regulation and biogeochemical cycling in freshwater ecosystems. However, there is a lack of research on the seasonal variation in lake water microorganisms in cold environments. In this study, 16S rRNA gene high-throughput sequencing was used to explore the microbial community and its influencing factors in Hulun Lake water during different seasons. The results showed that Proteobacteria, Actinobacteria, and Bacteroidetes were the most important phyla in the microbial community of Hulun Lake, but they had significant seasonal differences in their distribution. In addition, significant seasonal differences were observed in the α diversity of microorganisms, with bacterial diversity being higher in winter than in summer. Changes in environmental variables were significantly correlated with changes in the microbial community, and the rapid changes in temperature, pH, and dissolved oxygen are potentially the major factors influencing seasonal bacterial diversity trends. The findings of the present study enhance our understanding of the microbial communities in alpine lake ecosystems and are of great significance for the management and protection of lake ecosystems.

Interactions between climate change and early agriculture in SW China and their effect on lake ecosystem functioning at centennial timescales over the last 2000 years

The responses of alpine lake ecosystems to combined climate and human impacts are ecologically important to understand. It is necessary to differentiate natural, climate-forced ecosystem changes from those driven by human disturbance with a long-term (i.e. 102–103-yr) perspective. Here a14C, 210Pb and 137Cs-dated sediment core covering approximately 2500 years was analyzed for diatoms, pigments, cladocerans and geochemistry in Lugu lake-catchment in southwest China to examine the response of the lake to climate change and human impacts. Between 1000 and 1300 AD, a warming climate coupled with declining summer monsoon precipitation resulted in the increase of oligotrophic taxa and low catchment runoff. Prior to 1300 AD the lake ecosystem was primarily responding to natural, regional climate-related processes. After 1300 AD, the clear ecological shifts and reorganizations in community structure across multiple trophic levels suggest that major transitions took place in the Lugu ecosystem. An increase in Poaceae (grass) pollen from 1300 AD indicates human settlement in the area, and is followed by rapid decline in Tsuga, most likely a consequence of logging. Use of change point analyses of the multi-proxy data demonstrate that the lake ecosystem has been highly variable since 1300 AD and exhibiting enhanced vulnerability to both natural and anthropogenic disturbance. Our results suggest that agriculture-climate interactions played a major role in the ecological transitions over the last ∼700 years. This conclusion provides insight into how the anthropogenic land-use intensity, vegetation cover and surface erosion fluctuated with natural temperature and precipitation change over the past 2000 years.

Concordance of chemically inferred and assayed nutrient limitation of phytoplankton along a depth gradient of alpine lakes in the Canadian Rockies

Deposition of nutrients over mountainous regions in many parts of the world is increasing due to human activities. To assess the level of agreement between chemically-inferred and experimental evidence of the potential sensitivity of alpine lake ecosystems along the Eastern Front range of the Canadian Rockies to increased deposition of nitrogen (N) and phosphorus (P), nutrient limitation was investigated in 29 sites using water chemistry and nutrient amendment bioassays of phytoplankton. The two lines of evidence agreed in 55% of all cases with discrepancies mainly involving instances of co-limitation. Dissolved inorganic N:total P (DIN:TP) mass ratios frequently suggested P limitation while the corresponding bioassays supported meta-analytical evidence of the prevalence of co-limitation of freshwater primary producers. Based on previous evidence of low DIN:TP mass ratios and experimental N amendments stimulating algal communities in shallower alpine sites, we hypothesized that nutrient limitation of phytoplankton shifts from being P- to N-driven with decreasing lake depth. However, we found little evidence of N limitation from bioassays of phytoplankton from the shallower (< 2-m deep) lakes. Most phytoplankton were co-limited by P plus N amendments (42%), while 38% responded to only P, 10% to only N, and 10% to neither nutrient. Overall, DIN:TP mass ratios and bioassay results showed that chromophytes and chlorophytes drove the greater responsiveness of whole communities to P relative to N. Since the short-term bioassay experiments often demonstrated the alpine phytoplankton to be NP-co-limited, our findings highlight a need for further research into potential synergistic fertilizing effects of anthropogenic N emissions and P deposition from forest fires on lake ecosystems at high elevations.

Microbial community composition in alpine lake sediments from the Hengduan Mountains.

Microbial communities in sediments play an important role in alpine lake ecosystems. However, the microbial diversity and community composition of alpine lake sediments from the Hengduan Mountains remain largely unknown. Therefore, based on the Illumina MiSeq platform, high‐throughput sequencing analysis of the 16S rRNA gene was performed on 15 alpine lake sediments collected at different locations in the Hengduan Mountains. The abundance‐based coverage estimate (ACE), Chao1, and Shannon indices indicated that the microbial abundance and diversity of these sediments were high. There are some differences in the composition of microbial communities among sediments. However, in general, Proteobacteria accounted for the largest proportion of all sediments (22.3%–67.6%) and was the dominant phylum. Followed by Bacteroidetes, Acidobacteria, Chloroflexi, and Planctomycetes. In addition, the operational taxonomic unit (OTU) interactions network had modular structures and suggested more cooperation than competition in the microbial community. Besides, we also found that temperature has a significant contribution to the sample–environment relationship. This study revealed the diversity and composition of microbial communities in alpine lake sediments from the Hengduan Mountains, and describe the correlation between microbial community structure and different environmental variables.

Biogeochemical responses to climate change and anthropogenic nitrogen deposition from a ∼200-year record from Tianchi Lake, Chinese Loess Plateau

Human activities over the last ∼100 years have fundamentally changed the biogeochemistry of the global nitrogen cycle. For example, increased nitrogen deposition from industrial and agricultural sources has been linked to lake acidification and nutrient fertilization, and thus it has the potential to significantly influence lake ecosystems. Records of anthropogenic nitrogen deposition from alpine lakes are sparse in China, which limits our understanding of its effects on remote alpine lake ecosystems. In this study, we analyzed multiple geochemical proxies at Tianchi Lake (2430 m a.s.l.), which is part of the Liupan Mountains National Nature Reserve in the central Chinese Loess Plateau (CLP). We measured total nitrogen (TN) concentrations, C/N ratios and nitrogen stable isotope ratios (δ15N), as well as a biological proxy (spectrally-inferred chlorophyll a), from a well-dated sediment core spanning the past ∼200 years. Our aim was to examine anthropogenic nitrogen deposition to the lake and to assess its implications for lake primary production. We found that δ15N was stable prior to ∼1980, but it decreased significantly thereafter. This is consistent with documented changes in anthropogenic nitrogen deposition over the past ∼200 years, and it indicates that the δ15N record of Tianchi Lake likely reflects anthropogenic nitrogen deposition, at least in this region. Prior to ∼1980, the trend in primary production of Tianchi Lake agrees with the trend of reconstructed regional temperature, when reactive nitrogen emissions in China were very low. This suggests that temperature was likely the main factor driving lake primary production before ∼1980. Primary production has increased significantly since ∼1980, consistent with the continued rising temperature and with enhanced nitrogen deposition, indicating that both factors are simultaneous drivers. Although the current data for Tianchi Lake are insufficient to unequivocally determine which of the two stressors is more important after the 1980s, temperature likely played the more significant role in driving primary production. This is because the lake is phosphorus-limited today, and in addition there is a strong association between temperature and chlorophyll a over the past ∼200 years. Continuing global warming and increasing nitrogen deposition in the future will likely further affect the fragile alpine ecosystems in the region.

Winter Conditions Are Changing Rapidly in Alpine Lake Ecosystems

LimnoAlp Workshop; Lake Cadagno, Switzerland, 10–15 September 2017

Effects of water level regulation in alpine hydropower reservoirs: an ecosystem perspective with a special emphasis on fish

Sustainable development of hydropower demands a holistic view of potential impacts of water level regulation (WLR) on reservoir ecosystems. Most environmental studies of hydropower have focused on rivers, whereas environmental effects of hydropower operations on reservoirs are less well understood. Here, we synthesize knowledge on how WLR from hydropower affects alpine lake ecosystems and highlight the fundamental factors that shape the environmental impacts of WLR. Our analysis of these impacts ranges from abiotic conditions to lower trophic levels and ultimately to fish. We conclude that the environmental effects are complex and case-specific and thus considering the operational regime of WLR (i.e. amplitude, timing, frequency, and rate of change) as well as the reservoir’s morphometry, geology and biotic community are prerequisites for any reliable predictions. Finally, we indicate promising avenues for future research and argue that recording and sharing of data, views and demands among different stakeholders, including operators, researchers and the public, is necessary for the sustainable development of hydropower in alpine lakes.

A PCB Sensor Suite for Monitoring the Effects of Annual Variations in Precipitation Rates on Alpine Lakes in Rocky Mountain National Park

Alpine Lakes are fragile ecosystems of immense beauty and value. They occur in high elevations and are often subjected to wide variations in annual precipitation rates. As such, their electrochemistry is affected by these annual variations in precipitation rates, which in turn, affects the entire ecosystem. Low-cost sensors for monitoring these changes will help in improving the management of these ecosystems. Low-cost Printed Circuit Board (PCB) sensors are being applied to many useful environmental and agricultural applications, including measuring soil moisture content, detecting pollution, monitoring drought conditions in estuaries, and monitoring for saltwater intrusion into coastal freshwater bodies. These sensors consist of a low-cost PCB with patterned Cu electrodes designed so that the circuit board electromagnetically interacts with the surrounding media, where its electrochemistry affects the measurable electrical impedance of the patterned traces. Exposed electrodes can be used to measure ion content in aqueous solutions from dissolved ionic compounds. Insulated electrodes can be used to measure moisture content of object and materials, or to identify dissolved ions in aqueous solution by searching for their relaxation frequency. In this multiyear study, a 2-layer PCB sensor suite consisting of a Au coated dual exposed electrode sensor on one side and a solder mask insulated interdigitated fringing field sensor on the opposite side is used. SMA connectors are attached to the opposite end of the sensor board for easy connection too instrumentation. The resistance of the exposed electrode sensor is measured using an Agilent 4192A LF Impedance Analyzer. The complex impedance of the fringing field sensor is measured using an Agilent E5061B Network Analyzer. For this study, PCB sensor technology is being investigated for monitoring the electrochemical properties of water samples from four alpine lake ecosystems in the Loch Vale Drainage at Rocky Mountain National Park. These alpine lakes are The Loch, Sky Pond, Andrews Tarn and Lake of Glass. Water samples were collected from these ecosystems in August of 2016, taken into the laboratory and tested with the PCB sensor suite to determine a baseline for the five year study.

Impacts of Climate Warming on Alpine Lake Biota Over the Past Decade

ABSTRACTAlpine temperatures have risen at twice the rate compared to the northern-hemispheric average during the past century. This can be expected to affect Alpine lake ecosystems via, for example, intensified thermal stratification, shorter ice cover periods, and altered catchment processes. Our study assesses changes in some main constituents of the planktic and benthic communities of five mid-Alpine lakes in the Niedere Tauern region in Austria in relation to climatic warming, by comparing community and environmental data from 1998–1999 to data from 2010–2011. Although lake chemistry remained relatively stable between the study periods, we observed an increase in lake water temperatures and a decrease in ice cover durations. Several of the dominant diatom species and chrysophyte cyst types show relatively clear changes; the responses of the whole communities, however, are less evident. Yet, in particular, diatoms show distinct assemblage changes along the climatic gradients in the two lakes with the largest decrease in ice-cover duration. Chironomid communities appear to be less sensitive compared to diatoms and chrysophyte cysts, which are known for reacting quickly to changes in their environment. Finally, Alpine lakes, which are moderately nutrient-enriched because of human activities in the catchment area, are likely to experience increases in their productivity with climate warming.

Otolith size differences during early life of dwarf and cannibal Arctic char (Salvelinus alpinus)

AbstractIn Arctic and alpine lakes, Arctic char [Salvelinus alpinus (L.)] often form two distinct morphs: invertebrate feeders (‘dwarfs’) and piscivores (‘giant or cannibals’). Here, we test for early life history growth variation in dimorphic Arctic char as a proximate explanation for the observed life history variation between the two forms. Char were sampled in 11 alpine and Arctic Norwegian lakes. Dwarfs (defined as sexually mature char less than 15 cm long; N = 304) had a mean total length of 105 mm, whereas the typical cannibal (body length above 20 cm; N = 153) was 388 mm long. A positive correlation between egg size and otolith hatching ring were ascertained in a separate hatching experiment with brown trout (Salmo trutta) and it is assumed that this relationship also is valid for Arctic char, and otolith size was used as a proxy for length. Otolith hatching ring from Arctic char cannibals was larger (mean ± SD; 187 ± 24 μm) than those from dwarfs (mean ± SD; 164 ± 23 μm). There were only minor size differences between dwarfs and cannibals during the next three years, after which dwarfs usually matured. Two mutually nonexclusive, proximate explanations for the differentiation into separate morphs (dwarfs and cannibals) are therefore maternal effects and/or genetic based differentiation. The high catchability of large piscivorous char and low production in alpine and Arctic lake ecosystems may make these stocks particularly vulnerable to overexploitation.

Dust mediated transfer of phosphorus to alpine lake ecosystems of the Wind River Range, Wyoming, USA

Alpine lakes receive a large fraction of their nutrients from atmospheric sources and are consequently sensitive to variations in both the amount and chemistry of atmospheric deposition. In this study we explored the spatial changes in lake water chemistry and biology along a gradient of dust deposition in the Wind River Range, Wyoming. Regional differences were explored using the variation in bulk deposition, lake water, sediment, and bedrock geochemistry and catchment characteristics. Dust deposition rates in the Southwestern region averaged 3.34 g m -2 year -1 , approximately three times higher than deposition rates in the North- western region (average 1.06 g m -2 year -1 ). Dust-P deposition rates ranged from 87 l gPm 2 day -1 in the Northwestern region to 276 l gP m 2 day -1 in the Southwestern region. Subalpine and alpine lakes in the Southwestern region had greater total phosphorus (TP) concentrations (5-13 l gL -1 ) and greater sediment phosphorus (SP) concentrations (2-5 mg g -1 ) than similar lakes elsewhere in the region (1-8 l gL -1 TP, 0.5-2 mg g -1 SP). Lake phosphorus concentrations were related to dissolved organic carbon (DOC) across vegetation gradients, but related to the percent of bare rock, catchment area to lake area, and catchment steepness across dust deposition gradients. Modern phytoplankton and zooplankton biomasses were two orders of magnitude greater in the Southwest than in the Northwest, and alpine lakes in the Southwest had a unique diatom species assemblage with relatively higher concentrations of Asterionella formosa, Pseudostauros-

Surface Water Temperature and Ice Cover of Tatra Mountains Lakes Depend on Altitude, Topographic Shading, and Bathymetry

This study reflects the growing demand for better understanding the response of alpine lake ecosystems to climate forcing. We combined continuous monitoring of water temperature with GIS-derived data, and modeled the lake surface water temperature (LSWT) and ice-cover characteristics of 18 Tatra Mountains lakes against altitude, lake morphometry, and local topography. The general trend in LSWTs was similar across all studied lakes and showed a high degree of coherence over the whole study period. The daily LSWTs were governed primarily by altitude and topographic shading represented by lake-specific total duration of direct solar radiation (TDDSR). Day-to-day variability of LSWTs was controlled mainly by the maximum depth of the lakes. The surface temperature of deeper lakes was more stable than the temperature of shallow ones. Topographic shading appeared to play an important role in the development and duration of ice-cover. Lakes with low TDDSR retained ice-cover longer than well insolated ones.This is the first time that the effect of topographic shading was explicitly considered in relation to the surface temperature and ice-cover timing of remote lakes. Including direct solar radiation as a model parameter would considerably improve predictions of temperature characteristics of high-altitude lakes. This may have potentially important implications for climate change studies as it could allow for site-specific modifications of temperatures in high-altitude lakes.

A model for high-altitude alpine lake ecosystems and the effect of introduced fish

We discuss a simplified mathematical model for alpine lake ecosystems, describing the summer (i.e. ice-free period) dynamics of phosphorus, phytoplankton, three zooplankton compartments and fish abundance. Model output is compared with measurements of total phosphorus, chlorophyll-a and zooplankton biomass recorded in twelve high-altitude mountain lakes in the Gran Paradiso National Park (northwestern Italy) during the summer season from 2006 to 2009. Model results are consistent with measured data, indicating the appropriateness of this modeling approach for quantitatively studying mountain lake ecosystems and their response to environmental changes. The comparison between the results obtained for lakes without fish and those where the allochthonous brook trout (Salvelinus fontinalis) was introduced clearly indicates the strong impact of fish stocking in alpine lakes.

Impact of introduced fish on Common frog (Rana temporaria) close to its altitudinal limit in alpine lakes

Alien fish species have been introduced into naturally fishless mountain lakes worldwide, with negative consequences for native fauna. This study describes the impact of introduced Brook trout (Salvelinus fontinalis) on Common frog (Rana temporaria) in the Gran Paradiso National Park (Western Italian Alps). We conducted an extensive, six years long (2006-2011) monitoring campaign over 12 fishless and 8 stocked lakes, close to the new altitudinal limit ofR. temporaria(2811 m a.s.l.). This new altitudinal limit is reported for the first time in this study. The presence of introduced fish was a factor of ecological exclusion, negatively affecting the presence of frogs and compromising the suitability of alpine lakes as breeding sites. These results are likely to depend on larval predation or to selective avoidance of stocked lakes byR. temporaria. Eradication of introduced fish is proposed as a conservation measure to recover amphibian populations in alpine lake ecosystems.

Biogeochemical responses of two alpine lakes to climate change and atmospheric deposition, Jasper and Banff National parks, Canadian Rocky Mountains

The sensitivity of remote alpine ecosystems to global change has been documented by 20th century changes in climate, glacial recession, and terrestrial and aquatic ecosystems. Here we present sedimentary records of biogeochemical responses in two alpine lake ecosystems to multiple environmental drivers over the last ∼500 years in Banff and Jasper National Parks (Alberta, Canada). We combine paleoecological measures of primary production and diatom community structure with geochemical proxies of reactive N (Nr) deposition to describe the nature and rate of recent ecosystem changes. Curator Lake in Jasper shows a strong diatom response to the limnological effects of climate warming (e.g., thermal stratification), but little evidence of changes in Nr cycling over the last ∼500 years. The response of McConnell Lake in Banff to climate change is strongly mediated by glacial activity within the catchment and changing inputs of Nr. Our findings highlight the range of limnological responses that may be expressed by similar ecosystems subjected to comparable abiotic stressors, while further documenting the magnitude of the ecological footprint associated with recent environmental change in mountain park environments.

Critical Nitrogen Deposition Loads in High-elevation Lakes of the Western US Inferred from Paleolimnological Records

Critical loads of nitrogen (N) from atmospheric deposition were determined for alpine lake ecosystems in the western US using fossil diatom assemblages in lake sediment cores. Changes in diatom species over the last century were indicative of N enrichment in two areas, the eastern Sierra Nevada, starting between 1960 and 1965, and the Greater Yellowstone Ecosystem, starting in 1980. In contrast, no changes in diatom community structure were apparent in lakes of Glacier National Park. To determine critical N loads that elicited these community changes, we modeled wet nitrogen deposition rates for the period in which diatom shifts first occurred in each area using deposition data spanning from 1980 to 2007. We determined a critical load of 1.4 kg N ha−1 year−1 wet N deposition to elicit key nutrient enrichment effects on diatom communities in both the eastern Sierra Nevada and the Greater Yellowstone Ecosystem.