Effects of hydropower management on the sediment composition and metabolism of a small Alpine lake

Measuring long-term ecological changes in densely populated landscapes using current and historical high resolution imagery

In the context of catchment development and climate change, anthropogenic activities have significantly altered the succession and functioning of freshwater ecosystems. Combining the sedimentary records and modern survey data, we reconstructed a 250-year history of ecological changes in Chenghai Lake, aiming to assess the long-term ecological changes in Changhai Lake in response to multiple environmental stresses, such as eutrophication and hydrological fluctuation.Three stages were identified for the process of nutrient enrichment leading to a long-term increase in primary production. Nutrient level was relatively low before 1970, increased gradually between 1970 and 2000, with an accelerating increase after 2000. The water regulation project enhanced water turbulence and river flux during 1993-2000, which promoted the growth of turbulence-tolerant Aulacoseira and influx of benthic Nitzschia. The organic carbon cycling in Chenghai Lake was mainly driven by the autogenetic inputs. The eutrophication process dominated the long-term shifts of diatom assemblages followed by hydrological fluctuation. Our results illustrated that ecological restoration and catchment management of Chenghai Lake not only need to focus on the control of nutrient enrichment and pollutant input, but also should consider hydrological regulation and water level fluctuation.

Water-level fluctuations (WLF) often represent one of the greatest impacts on the development of lake ecosystems. In the year 1931, the Trentino Hydroelectric Company (SIT) requested the hydroelectric use of water bodies in the Upper and Middle Sarca basin. The largest and deepest lakes were dammed to increase lake volume and exploited since mid 1950s. This research uses the sediment record of two small lakes in close proximity to each other (Garzone and Serodoli) in northern Italy, used for hydroelectric power generation to determine the dependence of diatom-assemblage dynamics on WLF that have taken place over the last 60 years. Historical WLF are clearly reflected in the lithological composition and grain-size variations of the sediment cores. During the regression and transgression phases, the boundaries between the erosion, transport and accumulation zones fluctuated, causing redistribution of previously accumulated sediments, and their return into the lake’s cycling of biogeochemical matter. The water-level changes not only caused distinct taxonomic shifts in the diatom communities, which were dominated by different species in the core sections of each lake, but also significant shifts in the composition of the diatoms’ ecological, morphofunctional and life-form groups. Diatoms with a low- and high-profile attachment type were the dominant morphological forms in the upper core levels, where the most extreme fluctuations in water level had occurred. Increased turbulent mixing caused by WLF favours the presence of large, heavily silicified centric diatoms, while more stable levels would select for smaller centric diatoms through thermal stratification.

In recent decades, intense human activities have caused a decline in many lake ecosystems in Yunnan Province, rendering the transformation of the lake from a clear macrophyte-dominated state to a turbid phytoplankton-dominated state. Improved understanding of the ecological changes in lake ecosystem has significant implications for management. In this study, a small lake in Dali Prefecture of Yunnan Province, i.e., Lake Xihu, was selected. Combined with diatom records and physicochemical proxies from the lake sediments, this paper focuses on the long-term ecological changes in Lake Xihu, Dali since the mid-1960s. The results show that the Lake Xihu, Dali has undergone a significant shift in stable states over the past 50 years. Prior to 2000, the benthic-epiphytic species (i.e., Cocconeis placentula, Staurosira construens, Gomphonema angustum, and Achnanthidium minutissimum) dominated in diatom assemblages, indicating oligotrophic conditions; since 2000, benthic-epiphytic species (i.e., Encyonopsis microcephala and Navicula cryptocephala) and planktonic species (i.e., Cyclotella atomus, Cyclotella meneghiniana, Stephanodiscus hantzschii, and Aulacoseira granulata) dominated successively, indicating mesotrophic to eutrophic conditions. Principal component analysis based on the diatom assemblages in temporal scale showed the response of diatoms succession to nutrients. Redundancy analysis also confirmed that nutrient enrichment was the main driving force for the succession of diatom assemblages in Lake Xihu, Dali. In the past 50 years, climate change and human activities (i.e., agricultural reclamation, fertilization, animal husbandry, and fishery) have enhanced the accumulation of nutrients in the lake. The continuous loading of nutrients promoted the propagation of planktonic algae, and also the productivity of the lake, rendering the transformation to a turbid phytoplankton-dominated state.

Asia's densely populated agricultural landscapes are undergoing unprecedented ecological changes caused by population growth and adoption of industrial technologies such as fossil fuel and chemical fertilizer. Covering nearly 6 × 10 6 km 2 , these landscapes now release more than half of global greenhouse gas emissions from agricultural land and biomass fuel. Measuring ecological processes and their changes in these highly heterogeneous village landscapes is made difficult by their very small scale of management, with households typically managing many small plots using a wide variety of inputs and methods. This chapter describes the global extent of village landscapes, characterizes their spatial heterogeneity, establishes appropriate scales for ecological change measurement, and demonstrates methods developed to measure long-term ecological changes across village landscapes in China. Reliable measurements of ecological change in village landscapes can be made by integrating high-resolution (≤1 m) landscape change measurements with household-level resource management data. These methods link local land use practices with regional and local ecological change, potentially aiding land use decision-making, but require far greater research effort than conventional land use measurements based on 30-1000 m resolution imagery and county or provincial data. Therefore, a multi-scale sampling and analysis system was developed to integrate local and regional data for estimating regional change across village landscapes in China. The strengths and weaknesses of this approach in measuring and mediating the impacts of ecological changes in densely populated landscapes are discussed in light of preliminary results indicating that population increase and modernization are increasing carbon sequestration across these landscapes.

Five short cores (top 40–45 cm of sediment) from 4 lakes of the Plitvice Lakes system (Croatia) were measured for 210Pb, 137Cs, a14C, δ13C, and δ18O in order to study the influence of environmental changes on the sediment system in small and large lakes. Sediment chronology based on the constant flux (CF) 210Pb model was the most reliable. Lake sediments consisted mainly of autochthonous carbonates with higher sedimentation rates in small lakes. Sediments from 2 large lakes, Prošće and Kozjak, showed constant stable isotope profiles for the carbonate fraction and full agreement between the 137Cs and 210Pb chronologies. Sediments from 2 small lakes, Gradinsko and Kaluderovac, showed synchronous increases in 14C and δ13C and disturbed 137Cs records. All lakes showed an increase in a14C in the carbonate sediments above the first occurrence of 137Cs, which was interpreted as a damped (~10 pMC increase in a14C) and decades-delayed consequence of the bomb-induced increase in a14C in atmospheric CO2. For the small lakes, increased δ13C in the last 2 decades and part of the a14C increase is probably due to an increase in primary productivity, which enhanced biologically induced calcite precipitation with concomitant changes in the carbon isotopic composition of carbonate sediments. δ13C values of a near-shore sediment core close to the confluence of one of the tributaries of Lake Kozjak showed that the carbonates in this core are a mixture of autochthonous and eroded allochthonous mineral carbonate. This core had a higher fraction of organic material. The sedimentation rate at this core site was high, but rates could not be quantified by 210Pb, 137Cs, or 14C.

Sustainable campsite management in protected areas: A study of long-term ecological changes on campsites in the boundary waters canoe area wilderness, Minnesota, USA

Long-term ecological changes in fishes and macro-invertebrates in the world's warmest coral reefs

The south-east margin of Tibet is highly sensitive to global environmental change pressures, in particular, high contemporary reactive nitrogen (Nr) deposition rates (ca. 40kgha(-1) yr(-1) ), but the extent and timescale of recent ecological change is not well prescribed. Multiproxy analyses (diatoms, pigments and geochemistry) of (210) Pb-dated sediment cores from two alpine lakes in Sichuan were used to assess whether they have undergone ecological change comparable to those in Europe and North America over the last two centuries. The study lakes have contrasting catchment-to-lake ratios and vegetation cover: Shade Co has a relatively larger catchment and denser alpine shrub than Moon Lake. Both lakes exhibited unambiguous increasing production since the late 19th to early 20th. Principle component analysis was used to summarize the trends of diatom and pigment data after the little ice age (LIA). There was strong linear change in biological proxies at both lakes, which were not consistent with regional temperature, suggesting that climate is not the primary driver of ecological change. The multiproxy analysis indicated an indirect ecological response to Nr deposition at Shade Co mediated through catchment processes since ca. 1930, while ecological change at Moon Lake started earlier (ca. 1880) and was more directly related to Nr deposition (depleted δ(15) N). The only pronounced climate effect was evidenced by changes during the LIA when photoautotrophic groups shifted dramatically at Shade Co (a 4-fold increase in lutein concentration) and planktonic diatom abundance declined at both sites because of longer ice cover. The substantial increases in aquatic production over the last ca. 100years required a substantial nutrient subsidy and the geochemical data point to a major role for Nr deposition although dust cannot be excluded. The study also highlights the importance of lake and catchment morphology for determining the response of alpine lakes to recent global environmental forcing.

Mountain ecosystems are vulnerable because of land-use and climate change. In this study, we performed a re-visitation study using historical and newly collected vegetation plots to explore the primary trends in the floristic, ecological, and structural features of Mediterranean Pinus mugo krummholz over past decades. The plant community composition over time (1992 vs. 2016–17) was analyzed by a detrended correspondence analysis followed by a statistical comparison of time steps and an analysis of the contribution of each species to temporal differences. Ecological and structural changes were analyzed by a permutational multivariate analysis of variance followed by a post hoc comparison. We observed relevant changes in the floristic composition, structure, and ecological characteristics of Pinus mugo scrub. Some subalpine and treeline species that characterize the early stages of Pinus mugo succession declined as several warm-adapted species increased. Furthermore, these changes were most likely due to the natural evolution of high-mountain krummholz combined with a thermophilization process occurring in alpine habitats. In contrast, a small group of cold-adapted species also increased, probably because the patchy spatial pattern of Pinus mugo scrubs gives rise to “mesic patches” in a matrix of arid grasslands. The re-visitation approach adopted for long-term analysis in this study can potentially be applied to other mountainous regions to better understand long-term ecological changes in high alpine vegetation.

Water level fluctuations alter reservoir ecosystems causing direct and indirect effects on fish populations. The dewatering of eggs, a direct impact of lake level drawdowns, can affect reproductive success of species that spawn in littoral zones, such as Yellow Perch. I examined relationships between water level fluctuations and spawning characteristics of Yellow Perch in a Central Appalachian hydropower reservoir, where water levels were permitted to be drawn down to 4 m and 2.1 m below the full pool elevation in March and April, respectively. Daily presences of egg masses were recorded on artificial spawning structures at two sites for the spring spawning seasons of 2019 and 2020. Spawning structures were placed at different distances from the shoreline, spanning water depths with and without the potential for dewatering based on the lowest permitted levels for lake elevation drawdowns. Generalized Estimation Equations (GEE) were used to analyze egg mass presence and six covariates: Secchi disk depth, distance to the shore, water temperature, water depth, lunar illumination, and lake level fluctuation. I also examined the proportion of egg masses in potential dewatering zones based on the minimum lake elevation drawdowns permitted for March and April. Data supported an additive effects model of Year + Water depth + Lunar illumination + Water temperature. The predicted probability of egg mass presence was negatively associated with water depth and lunar illumination, and positively associated with water temperature. A year effect, in part, reflected a between-year difference in the timing of spawning, where the number of egg masses during April exceeded that of March in 2019, a relationship that was reversed in 2020. During the 27-day spawning period in 2019, 52% (54 of 104) of egg masses had the potential to be dewatered, whereas 70% (30 of 43) had the potential to be dewatered in the 22-day spawning period of 2020. Our results have direct implications for fishery and hydropower management, as data on the characteristics and timing of spawning of yellow perch relative to water level fluctuations inform decisions regarding management of fish populations and lake level drawdown regulations.

We tested whether increased phosphorus and nitrogen concentrations would affect a lake trout (Salvelinus namaycush) population in a small oligotrophic lake with a benthically dominated food web. From 1990 to 1994, nitrogen and phosphorus were added to Lake N1 (4.4 ha) at the arctic Long-Term Ecological Research site in Alaska. We used mark/recapture methods to determine the lake trout population size, size structure, recruitment, and individual growth from 1987 to 1999. Data were also collected on water chemistry and food availability. Fertilization resulted in increased pelagic primary productivity, chlorophyll a, turbidity, snail density, and hypoxia in summer and winter. Lake trout density was not affected by the manipulation however growth and average size increased. Recruitment was high initially, but declined throughout the fertilization. These results suggest that lake trout were affected through increased food availability and changes to the physical characteristics of the lake. During fertilization, hypoxia near the sediments may have killed over-wintering embryos and decreased habitat availability. Although lake trout responded strongly to increased nutrients, loss of recruitment might jeopardize lake trout persistence if arctic lakes undergo eutrophication.

In the Northern Calcareous Alps (NCA) there are countless small lakes with small orographic catchments that are often located only slightly below the respective summit regions. On the one hand, the lakes are located in karstable aquifers and their existence is likely to be related to karstification. Then, they are expected to be directly connected to the karst water body. These lakes are classified as karst lakes. On the other hand, the alpine environment is also influenced by glacial processes and lakes might be related to glacial erosion and deposition. For these glacial lakes, the share of groundwater inflow and outflow is regarded as subordinate even within high permeable karst lithologies. Here we compare two alpine lakes of potentially different origin in the NCA in Salzburg with the aim to provide a basis for an aerial survey of the numerous small alpine lakes in the NCA region and their characterization using the guiding parameters elaborated here. We consider (a) the lake geometry, (b) potential inflow and outflow systems, and (c) physicochemical parameters and hydrochemistry of the Filblingsee and the Eibensee, both located in the Fuschlsee region. Filblingsee was initially considered as a typical karst lake and Eibensee as a moraine-dammed glacial lake. Some clear differences arise in lake geometry, which in the karst lake shows a nearly round surface and concentric depth profile, while the glacial lake is elongated in the direction of glacier flow and has the deepest areas just upstream of the moraine dam. Both lakes show very little to no surficial inflow. Inflow and outflow occur in groundwater in both cases but are not directly tied to a highly permeable karst system. The depth profiles of the field parameters of the two lakes differ only slightly and show a dominant groundwater inflow in mid-depth regions but no flow through at the lake bottom. Water chemistry in both lakes and their potential outflows correspond to the respective aquifer in terms of solution load. Filblingsee can be characterized as a hanging lake in a secondarily sealed doline, Eibensee lies in a glacially excavated depression sealed by glacial sediments. While the inflow and outflow conditions and the hydrochemistry of both lakes are very similar, the lake geometry is a clear distinguishing feature that can be attributed to the different genesis of the two lakes. This can therefore be used as a guiding parameter for the classification of the numerous small alpine lakes in the NCA.

Alpine lakes located in high-altitude mountainous regions act as vital sentinels of environmental change. Remote-sensing-based identification of these lakes is crucial for understanding their response to climate variations and for assessing associated disaster risks. However, the complex terrain and weather conditions in these areas pose significant challenges to accurate detection. This paper proposes a method that leverages the high precision of deep learning for small lake and lake boundary extraction combined with deep learning to eliminate noise and errors in the identification results. Using Sentinel-2 data, we accurately identified and delineated alpine lakes in the eastern Himalayas. A total of 2123 lakes were detected, with an average lake area of 0.035 km². Notably, 76% of these lakes had areas smaller than 0.01 km². The slope data is crucial for the lake classification model in eliminating shadow noise. The accuracy of the proposed lake classification model reached 97.7%. In the identification of small alpine lakes, the recognition rate of this method was 96.4%, significantly surpassing that of traditional deep learning approaches. Additionally, this method effectively eliminated most shadow noise present in water body detection results obtained through machine learning techniques.

Small lakes in big landscape: Multi-scale drivers of littoral ecosystem in alpine lakes