The Sources of Carbon and Nitrogen in Mountain Lakes and the Role of Human Activity in Their Modification Determined by Tracking Stable Isotope Composition

Lobelia Lakes' Vegetation and Its Photosynthesis Pathways Concerning Water Parameters and the Stable Carbon Isotopic Composition of Plants' Organic Matter

Most of the aquatic vegetation produces organic substances via the C3 photosynthetic pathway (mosses, isoetids-Lobelia dortmanna L., Luronium natans (L.) Raf., and vascular plants) or Crassulacean acid metabolism (CAM, e.g., Littorella uniflora (L.) Asch. and Isoëtes lacustris L.) or by their ability to use HCO3- via carbon concentration mechanisms (CCMs-some elodeids and charophytes). Differentiating these predominant photosynthetic pathways in aquatic vegetation based on their organic matter (OM) carbon stable isotopes (δ13CORG) is a complex task, in contrast to terrestrial plants. This study investigates the OM deposition, characterized by δ13CORG values in 10 macrophyte species with different photosynthetic pathways (C3, CAM, and CCM) collected from 14 softwater Lobelia lakes in northern Poland. The higher δ13CORG values distinguish the CCM group, indicating their use of 13C-enriched HCO3¯ in photosynthesis. CAM species show slightly higher δ13CORG values than C3, particularly in lower pH lakes. Principal component analysis of isotopic and environmental data did not yield clear distinctions by the groups, but still, they significantly differ in light of analyzed parameters and isotopic signals (PRMANOVA = 5.08, p < 0.01; K-W H = 27.01, p < 0.001). The first two PCA dimensions showed that the water pH and Ca2+ concentration positively influenced δ13C values. The influence of light conditions on δ13CORG values revealed by third PCA components seems to also be important. In summary, northern Polish Lobelia lakes serve as a key differentiation point between vegetation employing CCMs and those relying on C3/CAM photosynthesis without HCO3- utilization, providing insights into transitions in plant communities within these ecosystems.

Efficiency of reactive nitrogen removal in a model Mediterranean high-mountain lake and its downwater river ecosystem: Biotic and abiotic controls

High-mountain lakes and rivers are usually oligotrophic and strongly influenced by atmospheric transport processes. Thus, wet deposition of reactive N species (Nr), mainly in the form of nitrate (NO₃^-), is a major source of N input in these high-mountain ecosystems. Bacterial denitrifiers are thought to be largely responsible for reduction of NO₃^- to nitrous oxide (N₂O) and molecular dinitrogen (N₂) as main biological pathway of N removal in these ecosystems. Nitrifiers, through the oxidation of ammonium to NO₃^-, can also be a source of NO₃^- and N₂O. However, there is uncertainty regarding the abiotic and biotic factors controlling Nr elimination from aquatic ecosystems at different altitudes and seasons. We examined the efficiency of Nr removal as N₂O and N₂ (total removal) or N₂ only (clean removal) in a model lake and its downwater river ecosystem (Sierra Nevada, Spain) representative of Mediterranean high-mountain freshwater ecosystems along an altitudinal gradient during the warm period of the year. Denitrification activity and the abundance of nitrifiers and denitrifiers in sediments were measured at thaw, mid ice-free and late ice-free periods. We found the efficiency of total and clean removal of Nr increased from the downwater river to the high-mountain lake. Regardless of the location, the efficiency of total removal of Nr decreased over the ice-free period whereas that of clean removal of Nr peaked at mid ice-free period. The efficiency of total removal of Nr was mainly controlled by the abundance of archaeal nitrifiers and bacterial denitrifiers. Abiotic (ammonium and NO₃^- concentration) and biotic (mainly nosZI-type denitrifiers) factors drove changes in the efficiency of clean removal of Nr. Our results suggest that abiotic and biotic factors can control the efficiencies of Nr removal in Mediterranean high-mountain lakes and their downwater rivers, and that these efficiencies increase with altitude and vary over the ice-free period.

An Evaluation of the Impact of Hiking Tourism on the Ecological Status of Alpine Lakes—A Case Study of the Valley of Dolina Pięciu Stawów Polskich in the Tatra Mountains

Eutrophication is one of the major threats to the quality of water in high mountain lakes. The inflow of elements having biological origin may significantly aggravate the ecological status of those ecosystems. For this reason, the aim of this study was to map and assess the impact of anthropogenic pressure on alpine lakes in the valley of Dolina Pięciu Stawów Polskich (known in English as the Valley of Five Polish Lakes) in the Tatra National Park in Poland. The effects of tourism and easy access to alpine lakes on changes in their ecological status were determined. Tourist trails were evaluated based on the difficulty of access to the water surface of analyzed alpine lakes, with a method developed for assessing anthropogenic pressure on aquatic ecosystems. The method, deployed for the first time in 2019, was modified and adapted to the local environmental conditions in the research area. The results of this study indicate that tourism pressure contributes to the growth of submerged vegetation in alpine lakes. The presence of aquatic plants (including vascular plants) shows ecosystem response to water enrichment with biogenic substances. The present findings were used to formulate practical recommendations and propose modifications to the evaluated hiking trails. The research method developed in the study can support analysis and control of tourist traffic, thus reducing anthropogenic pressure on alpine lakes in national parks located in mountain areas.

Effects of catchment area and nutrient deposition regime on phytoplankton functionality in alpine lakes

High mountain lakes are a network of sentinels, sensitive to any events occurring within their waterbodies, their surrounding catchment and their airshed. In this paper, we investigate how catchments impact the taxonomic and functional composition of phytoplankton communities in high mountain lakes, and how this impact varies according to the atmospheric nutrient deposition regime. For two years, we sampled the post snow-melt and the late summer phytoplankton, with a set of biotic and abiotic parameters, in six French alpine lakes with differing catchments (size and vegetation cover) and contrasting nitrogen (N) and phosphorus (P) deposition regimes. Whatever the nutrient deposition regime, we found that the lakes with the smallest rocky catchments showed the lowest functional richness of phytoplankton communities. The lakes with larger vegetated catchments were characterized by the coexistence of phytoplankton taxa with more diverse strategies in the acquisition and utilization of nutrient resources. The nutrient deposition regime appeared to interact with catchment characteristics in determining which functional groups ultimately developed in lakes. Photoautotroph taxa dominated the phytoplankton assemblages under high NP deposition regime while mixotroph taxa were even more favored in lakes with large vegetated catchments under low NP deposition regime. Phytoplankton functional changes were likely related to the leaching of terrestrial organic matter from catchments evidenced by analyses of carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotope ratios in seston and zooplankton. Plankton δ¹⁵N values indicated greater water-soil interaction in lakes with larger vegetated catchments, while δ¹³C values indicated the effective mineralization of the organic matter in lakes. The role played by catchments should be considered when seeking to determine the vulnerability of high altitude lakes to future changes, as catchments' own properties will vary under changes related to climate and airborne contaminants.

Cyanobacteria blooms: A neglected facilitator of CH4 production in eutrophic lakes

Lakes are regarded as one of the important sources of atmospheric CH₄. However, the role of cyanobacteria blooms (CBBs) play in the CH₄ production in eutrophic lakes is not fully clear. In this study, the spatial distribution characteristics of CH₄ concentrations in surface water and sediment columns were investigated in Zhushan Bay of Taihu lake, China. Results showed that CH₄ concentrations in CBBs accumulated zones were much higher than that in the open lake areas, with the highest values of 3.79 μmol·L^-1 and 2261.88 μmol·L^-1 in surface water and sediment columns, respectively. CH₄ concentrations were strongly influenced by various factors. In surface water, the occurrence of CBBs greatly contributed to CH₄ productions, as evidenced by the well-predicting for CH₄ concentrations using Chl-a and NH₄⁺ concentrations. In the sediments, the Ignition Loss and C:N ratio values were two indicators of CH₄ contents, suggesting that the methanogenesis processes were influenced by not only the quantities, but also the qualities of organic matter. The labile substrates produced during the CBBs decomposition processes promoted the CH₄ production and migration from sediments to the water column, resulting in the coherence in CH₄ concentrations between the sediments and the surface water. The high-resolution determinations of CH₄ concentrations in surface water and sediments clarified that the CBBs were a neglected facilitator of CH₄ productions, which should be considered in the future estimation of CH₄ emissions in eutrophic lakes.

The Impact of Recreational Activities on Aquatic Vegetation in Alpine Lakes

Tourism pressure on protected areas with attractive landscapes leads to environmental modifications. Eutrophication poses the main threat to the quality of water in alpine lakes. Even small inflows of biogenic elements can disrupt the ecological balance of these ecosystems. The aim of this study was to verify the hypothesis that recreational activities and easy access to alpine lakes contribute to changes in their ecological status. This hypothesis was verified by analyzing the presence of hydromacrophytes in two lakes of the Tatra National Park (Poland). The analysis was carried out along segments of hiking trails which were evaluated for accessibility to the studied lakes. Underwater surveys were carried out during scuba diving expeditions in 2012–2016. Submerged vegetation was evaluated in both lakes. This is the first study in Poland and one of the few projects in the world to have relied on such extensive support from scuba divers to generate highly accurate measurements. The study demonstrated that lake bottoms were extensively covered by Potamogeton friesii in the vicinity of trail segments with easy access to lakes. Our results show that tourism pressure contributes to the growth of aquatic vegetation in some areas of alpine lakes. We relied on our findings to propose several modifications to the routes of the evaluated tourist trails to minimize the impact of anthropogenic pressure on the studied lakes.

Limited acid deposition inferred from diatoms during the 20th century - A case study from lakes in the Tatra Mountains

Mountain lakes are usually sensitive to the effects of global and regional environmental changes. Since the second half of the 20th century, surface-water acidification has become a significant ecological problem, and many lakes in Europe and North America have anthropogenically acidified. Additionally, following reduction in emissions of sulfur (S) and nitrogen (N) compounds, recovery from acidification has been observed in many lakes. In this study, we used changes in diatom communities to reconstruct the pH histories based on changes recorded in nine Tatra lakes (Western Carpathians, Poland) since approximately 1850AD. Overall, results indicate that acidic precipitation had little influence on lake-water pH in the Tatra Mountain lakes. Changes in diatom-inferred pH (DI-pH) generally were small and showed little evidence of acidification during the time of the highest air pollution (since the 1960s), and have shown little change since the reduction of acidic deposition since the 1990s. Lakes that showed some evidence of acidification included dystrophic lakes with low acid neutralizing capacity. However, as illustrated by the PCA trajectories of the diatom assemblages, the majority of the lakes currently contain diatom assemblages that are unlike the diatom floras that existed ca. 1850.

Denitrification and Biodiversity of Denitrifiers in a High-Mountain Mediterranean Lake

Wet deposition of reactive nitrogen (Nr) species is considered a main factor contributing to N inputs, of which nitrate ([Formula: see text]) is usually the major component in high-mountain lakes. The microbial group of denitrifiers are largely responsible for reduction of nitrate to molecular dinitrogen (N₂) in terrestrial and aquatic ecosystems, but the role of denitrification in removal of contaminant nitrates in high-mountain lakes is not well understood. We have used the oligotrophic, high-altitude La Caldera lake in the Sierra Nevada range (Spain) as a model to study the role of denitrification in nitrate removal. Dissolved inorganic Nr concentration in the water column of la Caldera, mainly nitrate, decreased over the ice-free season which was not associated with growth of microbial plankton or variations in the ultraviolet radiation. Denitrification activity, estimated as nitrous oxide (N₂O) production, was measured in the water column and in sediments of the lake, and had maximal values in the month of August. Relative abundance of denitrifying bacteria in sediments was studied by quantitative polymerase chain reaction of the 16S rRNA and the two phylogenetically distinct clades nosZI and nosZII genes encoding nitrous oxide reductases. Diversity of denitrifiers in sediments was assessed using a culture-dependent approach and after the construction of clone libraries employing the nosZI gene as a molecular marker. In addition to genera Polymorphum, Paracoccus, Azospirillum, Pseudomonas, Hyphomicrobium, Thauera, and Methylophaga, which were present in the clone libraries, Arthrobacter, Burkholderia, and Rhizobium were also detected in culture media that were not found in the clone libraries. Analysis of biological activities involved in the C, N, P, and S cycles from sediments revealed that nitrate was not a limiting nutrient in the lake, allowed N₂O production and determined denitrifiers' community structure. All these results indicate that denitrification could be a major biochemical process responsible for the N losses that occur in La Caldera lake.

Lake biota response to human impact and local climate during the last 200 years: A multi-proxy study of a subalpine lake (Tatra Mountains, W Carpathians)

Element content, loss-on-ignition, chironomid analysis and (210)Pb dating were applied on a sediment core from a subalpine Tatra lake (Popradské pleso) to reveal the response of aquatic biota to eutrophication induced by human activities in the lake catchment. The lead dating indicates that the 0-8 cm section of the core represents the past ca 200 years, ending at ~1814 AD. Comparing the key changes of the proxies with human activities that are historically well documented, four phases of the recent lake development were distinguished: (1) a pre-tourism phase, (2) a phase of increasing touristic activity and early cottage development, (3) a phase of eutrophication, and (4) a phase of post-eutrophication. Neither touristic activity, nor early cottage development around the lake (1st and 2nd phases) had considerable influence on the chironomid assemblage structure or organic content of the lake. The most significant change both in chironomid assemblage structure and loss-on-ignition occurred during the 3rd phase, when a big tourist hotel was built close by the lake and started contaminating it via direct wastewater input. However, the structure of the chironomid assemblage has not changed significantly over time and the dominating taxa remained the same during the whole period. Parallel with the nutrient signal of the paleo assemblage, a secondary signal has been identified as the ratio of rheophilic taxa on total abundance that did not correlate with the sediment's organic content, and is most likely driven by local climatic oscillations. Changes of most of metal elements concentrations reflected rather bigger scale changes of industrial activities than local scale human disturbances. Our results indicate that hydromorphological properties can moderate the impact of organic pollution on the lake biota.

Methane and Carbon Dioxide in the Sediment of a Eutrophic Reservoir: Production Pathways and Diffusion Fluxes at the Sediment-Water Interface

The estimated diffusion fluxes of methane (CH₄) and carbon dioxide (CO₂) at the sediment-water interface in the Rzeszów Reservoir in southeastern Poland are presented. The relevant studies were conducted during 2009, 2010, and 2011. Calculated fluxes ranged from 0.01 to 2.19 mmol m^-2 day^-1 and from 0.36 to 45.33 mmol m^-2 day^-1 for methane and carbon dioxide, respectively. While the values for calculated diffusion fluxes of methane are comparable with those reported for other eutrophic reservoirs, much higher values were obtained here for carbon dioxide. The resulting values of δ¹³C-CH₄ and the fractionation coefficients between methane and carbon dioxide (αCH₄-CO₂) suggest that methane in the sediment of the Rzeszów Reservoir is produced by acetate fermentation, while the hydrogenotrophic methanogenic process is of successively greater importance with increasing depth. In the top layer of the sediment, 24-72 % of CO₂ came from methanogenesis, while the contribution made by the degradation of organic matter by methanogenesis to CO₂ was greater in the deeper layer.