Thèses sur le sujet « Subalpine lakes »

Lakes have historically been overlooked as important nutrient processors within their watersheds. In general ecologists have focused on streams as zones of uptake and transformation, while viewing lakes as simple nutrient traps. However, recent research has highlighted the large influence that lakes may have on water chemistry within their watersheds. Within the field of limnology, researchers have traditionally focused on the pelagic zone for in-lake production. Further research in shallow lakes has highlighted the role benthic production within the littoral zone plays in the lake ecosystem. The greater influence of lakes is highlighted when comparing watersheds containing lakes with watersheds composed of solely stream channels. To assess the influence that lakes have on water chemistry and nutrient transport, both field and modeling analyses were performed for Bull Trout Lake, Idaho. In 2008 a large field sampling effort was conducted along with a 15N tracer experiment to characterize the limnology of Bull Trout Lake (Idaho) and nitrogen uptake and transport through the lake. Following the termination of the field season a multi-lake ecosystem model was developed with the use of a one-dimensional lake water quality model. Results from both experiments demonstrated the role of Bull Trout Lake as a nutrient processor and source within its watershed and further suggested the added influence additional lakes might have on water chemistry. The outcomes of the tracer study indicated that pelagic primary producers have the first opportunity to assimilate nitrogen delivered by the inflow stream; however, nutrients incorporated into plants within the littoral zone are held on to longer. Further the tracer experiment demonstrated the small role that large organisms have in ecosystem nutrient dynamics. The multi-lake model demonstrated the effect of BTL as a nutrient source within the watershed and indicated that although multiple lakes in sequence may have additive effects, most of this influence is expressed in the first two lakes of a series. Our research provides examples of valuable tools in limnological research. While whole-lake tracer studies have rarely been performed, they are extremely effective in understanding ecosystems. Additionally, even though lake models may be simplifications of natural systems, they can provide an efficient means of understanding lake functioning and testing hypotheses.

Quantifying carbon emissions of water bodies at regional scale is required as recent studies revealed their contribution in carbon cycling is significant. This demands to scale up water bodies carbon emissions from local to regional scale using as accurate approach as possible. In this study data of carbon (CO2-C) fluxes for 80 sampled lakes were used to scale up to more than 3000 lakes and ponds over the catchment. The most appropriate method for upscaling was the one in which two factors of water body size and location (altitude) were involved and the uncertainties were quantified in an advanced approach (Monte Carlo model). Based on the estimates obtained in this method, the annual carbon emission from all water bodies (~ 500 km2) was about 2900 ton C yr-1 . About 62% of this annual emission was related to the large lake Torneträsk (334 km2) and another 38% to all other lakes and ponds (166 km2). Water bodies in subalpine region dominated (90%) total water bodies area and were the major contributor (97%) to the total carbon emissions of all water bodies. The remaining small contribution (3%) was for water bodies in the alpine region, which contains only 10% of total water bodies area. These data indicate that all water bodies smaller than the large lake Torneträsk especially the ones in the subalpine region have considerable contribution to the annual carbon emission of all water bodies. Considering water body size and altitude factors in the advanced upscaling method was appropriate to obtain accurate estimates.

Il cambiamento climatico è considerato una delle minacce più gravi per la terra e gli ecosistemi acquatici e cresce la preoccupazione per gli effetti che il riscaldamento globale può avere sulle comunità biologiche. Negli ultimi anni, diversi studi hanno evidenziato la sensibilità dei corpi idrici alle fluttuazioni climatiche. Gli organismi zooplanctonici, caratterizzati da piccole dimensioni e rapido ciclo riproduttivo, sono sensibili ai cambiamenti ambientali. Inoltre, rappresentano dei componenti chiave delle catene trofiche acquatiche, collegando i produttori primari ai grandi consumatori secondari come pesci. Lo scopo della presente ricerca di dottorato è di ampliare le conoscenze sulle comunità zooplanctoniche dei grandi laghi subalpini, rilevando tendenze e cambiamenti nella struttura delle reti trofiche pelagiche e le loro potenziali cause. L’analisi di dati limnologici a lungo termine combinata a uno studio paleolimnologico del lago d’Iseo ha permesso di distinguere gli effetti dovuti al cambiamento climatico a quelli dovuti a impatti più locali, come l’eutrofizzazione. I risultati evidenziano che le rete trofiche dei laghi profondi subalpini sono fortemente influenzate dal clima. In particolare, la variabilità climatica invernale provoca un effetto a cascata che coinvolge l'intero ecosistema lacustre, modificando le caratteristiche chimiche, fisiche e biologiche del lago. Gli effetti del riscaldamento delle temperature invernali possono ripercuotersi sulle dinamiche di popolazione dello zooplancton fino all'estate, influenzando il funzionamento dell’intera catena trofica. L'indagine paleolimnologica, attraverso lo studio combinato di fattori biotici e abiotici, ha permesso di considerare gli effetti dei più importanti fattori di cambiamento negli ecosistemi lacustri, suggerendo che i fattori climatici dovrebbero essere considerati di fondamentale importanza nel determinare le dinamiche di sviluppo delle comunità planctoniche e la struttura e il funzionamento delle reti trofiche pelagiche.
Climate change is considered to be one of the most severe threats to earth and aquatic ecosystems and the concern about the effects that global warming can have on biological communities is growing. Even if the number of studies concerning climate change is worldwide increasing the effects of climate change are challenging to monitor and understand because of the multitude of responses within an ecosystem. In the last years, analyses of long-term data sets provide increasing evidence on the sensitivity of water bodies to climatic fluctuation. Small size and fast reproducing zooplankton organisms are sensitive to environmental change. Moreover, they represent key components of aquatic food webs, connecting primary producers to big secondary consumers as fishes. The purpose of the present PhD research is to expand the knowledge on the zooplankton communities of the large and deep lakes south of the Alps, detecting trends and significant change or shift in the pelagic food webs structure due to global or local impacts. The analysis of long-term series of limnological observations and a paleolimnological study on lake sediment have been combined in order to disentangle the effects of climate change from that due to more local and anthropic stressor as eutrophication. The obtained results suggested that in deep subalpine lakes the effect of climate change on pelagic food webs components is complex, including several direct and indirect effects, and it is mediated by nutrient availability. In particular, in deep lakes winter climatic variability trigger a cascading effect that involves the entire lake ecosystems, modifying lake chemical, physical and biological characteristic. The effects of warmer winter temperature can reverberate on zooplankton seasonal patterns until summer, affecting the food webs functioning. The paleolimnological investigation, through the combined study of biotic and abiotic factor, allowed clarifying the synergic effects of the most important drivers of change in lake ecosystems, suggesting that climatic factors should be considered with nutrient availability as a determinant element in controlling the temporal development of plankton communities and pelagic food web structure.

This research combines observation, experimentation, and modeling to evaluate the influence of lakes on dissolved organic matter (DOM) quantity, quality and export in subalpine watersheds of the Sawtooth Mountain Lake District, central Idaho. First, I conducted an empirical study of the hydrologic and biogeochemical controls on DOM dynamics in stream-lake fluvial networks. I hypothesized that lakes would decrease temporal variability (i.e., buffer) and alter the characteristics of DOM from inflow to outflow. I tested these hypotheses by evaluating DOM temporal patterns and measuring annual export in seven-paired lake inflows and outflows. I then evaluated how ultraviolet (UV) exposure affected DOM characteristics during snowmelt and baseflow, and how UV alters baseflow DOM bioavailability and nutrient limitation. Given that increased water residence time increases UV exposure, I hypothesized that lake outflow DOM would be more photorecalcitrant than DOM from lake inflows. I further hypothesized that UV exposure would increase DOM quality, heterotrophic processing, and nutrient demand. Results indicate that lakes can buffer stream temporal variability by acting as a DOM sink during snowmelt and a DOM source during baseflow. Lake outflow DOM photodegradation was similar to lake inflows during snowmelt (p=0.66). Conversely, outflow DOM was 2X more photorecalcitrant than inflow DOM during baseflow (ANOVA, p=0.03) and was strongly related to water residence time (WRT). During baseflow, light exposure increased inflow and outflow DOM bioavailability (p=0.059 and 0.024, respectively) and nutrient limitation (p=0.03 and 0.09, respectively). Combined, these results indicate that WRT in subalpine lakes strongly influences DOM temporal variability and DOM degradation and processing. Thus, lakes can provide temporal stability of DOM and potentially increase both carbon and nutrient uptake by heterotrophs in lake outflows. I then evaluated how global changes could alter hydrologic and nutrient dynamics in a subalpine lake. Model results indicate that the magnitude and timing of snowmelt runoff can have a substantial effect on water and nutrient exports. In phosphorus (P)-limited lakes, increases in inorganic N concentrations within and exported from lakes are likely to occur with increased temperatures and lake WRT. Increases in atmospheric N deposition will further enhance inorganic N exports in P-limited subalpine lakes.

碩士
國立聯合大學
土木與防災工程學系碩士班
98
In the lake ecosystem, Light attenuation coefficient (Ke) does not only impact the planktonic algae, aquatic plant growth, and reproduction, but also determines the zooplankton and fish predation. The attenuation of sunlight includes absorbs and scatters in the lake, therefore the light attenuation coefficient in water column to certain extent lake biology, hydrology, and water chemistry elements of a comprehensive response. The purpose of present study is to understand light attenuation property in the Yuan-Yang Lake and Emerald Peak Lake. Field measurements were conducted from April 2008 to March 2010 in the Yuan-Yang Lake. Single-variable regression analysis showed that Ke and the inverse of Secchi disk depth (1/SD) yield the best correlation (R2=0.576), indicating the 1/SD might be a good variable for predicting water column light attenuation coefficient in the Yuan-Yang Lake. The regression between Ke and total suspended solids (TSS) yields a good correlation (R2=0.575), and regression between Ke and chlorophyll-a concentration yields a poor correlation (R2=0.256). We also conducted the multiple-regression analysis for the Ke, chlorophyll-a (Chl), and total suspended solids (TSS) in the Yuan-Yang Lake. The multi-variable regression can be expressed by Ke = 1.172 + 0.610‧Chl + 0.544‧TSS (R2 = 0.44). The model was applied to validate the other set of data and showed that the RMSE (Root Mean Square Error) and MAE (Mean Absolute Error) values between the observed and predicted results are 2.35m-1 and 1.47 m-1, respectively. Field measurements were also conducted from November 2009 to May 2010 in the Emerald Peak Lake. The single-variable regression analysis showed that Ke and the inverse of Secchi disk depth (1/SD) yield the best correlation (R2=0.645), indicating the 1/SD might be a good variable for predicting water column light attenuation coefficient in the Emerald Peak Lake. The regression between Ke and chlorophyll-a concentration yields a good correlation (R2=0.516), and regression between Ke and total suspended solids (TSS) yields a poor correlation (R2=0.202). We implemented the multiple-regression analysis for the Ke, chlorophyll-a, and total suspended solids (TSS) in the Emerald Peak Lake. The multi-variable regression can be expressed by Ke= 6.051 + 0.026‧Chl - 0.134‧TSS (R2 = 0.47). The results revealed that the background light attenuation coefficients, K(w + DOC), are 6.051m-1 and 1.172m-1 in the Yuan-Yang Lake and Emerald Peak Lake, respectively. It showed that the background light attenuation coefficient in the Emerald Peak Lake exhibits the significant influence on the Ke value comparing to the Yuan-Yang Lake.

Sediment records of two Italian subalpine lakes (Lake Garda and Lake Ledro) were analyzed in order to reconstruct their ecological evolution over the past several hundred years. A multi-proxy and multi-site approach was applied in order to disentangle the effects of local anthropogenic forcings, such as nutrients, and climate impacts on the two lakes and their catchments. Biological indicators (sub-fossil pigments, diatoms and Cladocera) were used to reconstruct changes in the aquatic food web and to define the lake reference conditions, while geochemical methods, i.e. wavelength-dispersive X-ray fluorescence spectroscopy (WD-XRF), were used to provide quantitative information on the different physical or chemical processes affecting both lake and catchment systems. Sub-fossil pigments and diatoms, together with their respective inferred TP values, suggested very stable oligotrophic conditions in both lakes until the 1960s. The period following was affected by nutrient enrichment, which led to a drastic shift in the phytoplanktonic community. The response of sub-fossil pigments and diatoms to major climatic anomalies such as the Medieval Climatic Anomaly (MCA) and the Little Ice Age (LIA) were not pronounced, and the taxonomic composition remained relatively stable. On the contrary, these proxies showed an indirect response to climate variability since the beginning of the nutrient enrichment phase in the 1960s. In Lake Garda, the winter temperature regulates the water column mixing, which in its turn controls the degree of nutrient fertilization of the entire water column, and the related phytoplankton growth. In Lake Ledro a rapid reorganization of planktonic diatoms was observed only during the temperature recovery after the LIA, while recent temperature effects are masked by the prevailing nutrient effects. In Lake Garda, Cladocera remains responded in quantitative and qualitative terms to climatic changes, whereas in Lake Ledro they appeared to be mainly affected by variations in hydrological regimes, i.e. flood events. Cladocera remains corroborated the nutrient enrichment after the 1960s in both lakes as inferred by diatoms and pigments. In Lake Garda, the geochemical data showed a pronounced shift in elemental composition since the mid-1900s, when major elements and lithogenic tracers started to decrease, while some elements related to redox conditions and other (contaminant) trace elements increased. The general trends since the mid-1900s agree with the biological records. However, some differences recorded in the two different basins of Lake Garda reflected the effects of local conditions, both related to hydrology and sedimentation patterns. Lake Ledro showed higher short-term variability for most elements, even though some features were comparable to Lake Garda. The geochemical record of Lake Ledro revealed a major influence of human-induced lake-level fluctuations and catchment properties. This paleolimnological study allows us to place temporally restricted limnological surveys into a longer-term secular perspective, which is highly valuable for the definition of lake reference conditions. Because the restoration targets are usually based on the lake reference conditions, this study highlighted also the necessity to pay particular attention to the lake-specific sensitivity patterns. The multi-proxy and multi-site approach showed that the lake conditions of large and deep lakes in northern Italy, such as Lake Garda, are mainly driven by nutrient enrichment and/or climate change. In contrast, smaller lakes with larger catchment areas, such as Lake Ledro, are seemingly more impacted by conditions and processes occurring in the drainage basin.

Because very little is known about past climate change in the Ashnola region of southwestern British Columbia , a stratigraphic analysis of fossil head capsules of chironomids (Diptera: Chironomidae) was performed for two lakes located at treeline (approx. 2250m) in southernmost B.C.: North Crater Lake and Lake-of-the-Woods. Distinct changes in the chironomid (non-biting midge) communities implied changes in climate throughout the Holocene (10 000 yr. B. P. to present). Prior to 10 000 yr. B.P., cold conditions are indicated by the lack of temperate taxa and the presence of cold-stenotherms such as Sergentia and Heterotrissocladius. The early Holocene (after 9500 yr. B.P.) in both lakes shows rapid increases in the abundance and diversity of warm-adapted taxa (e.g., Dicrotendipes, Microtendipes, Polypedilum and Cladopelma), while cold-indicators disappear. This is indicative of warm, dry (i.e., xerothermic) conditions. Just prior to Mazama ash deposition, and later in the mid-Holocene, several warm-adapted taxa decrease in both lakes. This coincides with the warm, yet moist conditions of the mesothermic. The late-Holocene assemblages (~ 4500 yr. B. P. to present) indicate continued cooling with a continued reduction in diversity and abundance of warm-adapted taxa. Furthermore, cold-stenothermsreappear in Lake-of-the-Woods. To quantitatively assess these inferred climate changes, temperature reconstructions were performed using a newly-developed chironomid-paleotemperature inference model. Reconstructed temperatures are generally in agreement with those changes inferred qualitatively from the stratigraphic diagrams. For both lakes, mean summer temperatures were cold in the late-glacial (8-10 °C). Inferred temperatures increase rapidly in the early- Holocene, with the highest inferred temperatures (12-16 °C) occurring during the xerothermic interval (9500 to 7000 yr. B.P.). Both lakes show a slight drop in temperature just prior to the Mazama eruption. This cooling trend continues thereafter at North Crater Lake, to present day. The cooling trend begins slightly later at Lake-of-the- Woods, after 5400 yr. B. P. For both lakes, this inferred cooling trend is consistent with the time of neoglaciation in B.C., as inferred from glacial and paleobotanical evidence. For comparison, paleotemperature reconstructions were done for Cabin Lake and 3M Pond, located slightly northwest of my sites. The reconstructed temperatures for these lakes show similar trends as those inferred for North Crater Lake and Lake-of-the-Woods. As diversity often changes with changing community assemblages, diversity was also assessed quantitatively using the Shannon-Wiener Diversity Index. The inferred diversity changes within cores, parallel the inferred climate and temperature changes. Diversity is low during cold intervals, such as in the late-glacial. Diversity increases in the early-Holocene, corresponding with the warm reconstructed temperatures. Diversity drops in the late-Holocene (after 5400 yr. B.P.), and this is consistent with the cooler conditions of the neoglacial. Community trajectory analysis for both lakes showed that while species assemblages of the late-glacial did not reassemble in the neoglacial, distinct late-glacial, early Holocene and late-Holocene stages of development were indicated throughout the history of each lake.

碩士
國立聯合大學
土木與防災工程學系碩士班
106
In recent years, anthropogenic and natural pollution resources were increasing gradually. For instance, the nutrients caused by such activity of agriculture or tourism flow into lakes with the runoff which results from rainstorm. Subsequently, they have direct or indirect effects on the water quality of lakes and, thereby, cause eutrophication. In this study, a simulation concerning the conditions of hydrodynamic and water quality of Tsuei-Feng Lake, a subalpine lake, was conducted using a three-dimensional model of hydrodynamic and water quality. In addition, the calibration and validation of the model were conducted with the observational data (i.e., water level, temperature, and water quality) collected from January 2010 to July 2011. The results demonstrate that there the wimulations reproduce the observational data. After model calibration and validation, it was further used to do the sensitivity analysis of parameters. The results show that growth rate of algae has the strongest effect on concentration of chlorophyll a. Also, the model was used to explore the influence of wind stress on the temperature and water quality. Consequently, to explore, three simulation scenarios were set in the model applications. The results show that wind stress has a significant influence on surface flow field, water temperature, and water quality of Tsuei-Feng Lake. The study also displayed showed the results of pollution reducing by 50% and 90%. Especially, if the pollution was reduced by 90%, the concentration of chlorophyll a would decrease to less than 10 μg/L, even lower than 4 μg/L; namely, the lake would perform the state of mesotrophic/oligotrophic status according to the water quality. Besides, the effects of flow differences on lake water quality were discussed. The simulated results show that when the lake flow rate increases from 0.98 to 9.6 m3/s, the concentration of chlorophyll a obviously decreases from 146 to 98 μg/L. In the other words, the concentration of polluted water can be reduced by high flow rate.

碩士
國立聯合大學
土木與防災工程學系碩士班
102
Eutrophication refers a large amount of nitrogen, phosphorus, and other nutrients in the lakes, reservoirs, and estuaries under the natural and anthropogenic conditions so that algae and plankton are rapid growth within a certain time, resulting in the decreasing dissolved oxygen in water column, death of aquatic organisms due to hypoxia, and water quality deterioration. It contains a series of complex physical, chemical, and biological processes. Water quality model is an important tool for assessing water quality and ecosystem in the lakes. The changes of external environment and the internal interaction in the water quality and ecosystem can be evaluated by models. Water quality model is also the best tool for lake management. 　　In the present study, a three-dimensional model is established and applied to simulate the hydrodynamic and water quality conditions in the subalpine Yuan-Yang Lake. The observational data were collected from June 2009 to May 2010 and adopted for model calibration and verification. The results indicate that the simulated and observed water level, water temperature, and water quality are in good agreement. The validated model was then applied to investigate the physical and biochemical processes in the lake. The model sensitivity shows that the algae maximum growth rate is an important parameter to affect chlorophyll a concentration. When the wind stress increases, current field at the surface layer, water temperature, and water quality will be significantly influenced. The nitrogen and phosphorus loads reduced 50% were used to investigate the effects of non-point source reduction in the watershed on chlorophyll a concentration. The modeling results reveal that the effect of a 50% phosphorus loading reduction on chlorophyll a concentration is more significant, therefore the Yuan-Yang Lake is subjected to phosphorus control. The model was also applied to probe the effect of climate change on water quality. The simulated results indicate that chlorophyll a concentration will increase 25.77 % and 7.7 % for the medium-term and long-term climate change conditions, respectively, due to the decrease of surface runoff. It means that the water quality in the lake will get worse in the future.