Application of the Isolation Forest Algorithm to Substantiate the Uniqueness of Water Bodies in the Group of Karst Lakes

Improving the scientific foundations for the development and expansion of the network of specially protected natural areas requires the search for algorithms that could be used to identify unique ecosystems. Algorithmization of the anomaly identification process provides an opportunity not only to process large amounts of data but also leads to obtaining objective and comparable estimates. The purpose of this research is to identify the most optimal mechanisms for identifying anomalous values for the morphometric characteristics of karst lakes, which may indicate the uniqueness of the entire lake ecosystem. Within the framework of this article, the study was carried out based on a mathematical analysis of samples built for various characteristics based on the WORLDLAKE database. Statistical methods and the Isolation Forest (iForest) machine learning algorithm were used as methods of analysis. As a result of applying the iForest algorithm to a sample of morphometric parameters of karst lakes, consisting of 738 objects, 43 anomalous water bodies were identified. An expert assessment of the final set of lakes for the uniqueness of their ecosystems showed that the chosen method for identifying anomalous values is well suited for the task at hand. Many lakes with an anomaly index above 60% can be recognized as unique due to the unusualness of their abiotic characteristics; a number of them also have a peculiar biota. The anomalous objects included such well-known lakes as Tserik-Kol’, Crveno, Salda Lake, Trihonida, Vegoritida, Petron, etc. Moreover, for most of them, anomalies were detected for several parameters at once. Thus, the applied algorithm for identifying anomalous morphometric characteristics of lakes made it possible to obtain interesting samples for further expert analysis of the entire lake ecosystem for its uniqueness.

Integrated land and water-borne geophysical surveys shed light on the sudden drying of large karst lakes in southern Mexico

Karst water resources play an important role in drinking water supply but are highly vulnerable to even slight changes in climate. Thus, solid and spatially dense geological information is needed to model the response of karst hydrological systems to such changes. Additionally, environmental information archived in lake sediments can be used to understand past climate effects on karst water systems. In the present study, we carry out a multi-methodological geophysical survey to investigate the geological situation and sedimentary infill of two karst lakes (Metzabok and Tzibaná) of the Lacandon Forest in Chiapas, southern Mexico. Both lakes present large seasonal lake-level fluctuations and experienced an unusually sudden and strong lake-level decline in the first half of 2019, leaving Lake Metzabok (maximum depth ∼25 m) completely dry and Lake Tzibaná (depth ∼70 m) with a water level decreased by approx. 15 m. Before this event, during a lake-level high stand in March 2018, we collected water-borne seismic data with a sub-bottom profiler (SBP) and transient electromagnetic (TEM) data with a newly developed floating single-loop configuration. In October 2019, after the sudden drainage event, we took advantage of this unique situation and carried out complementary measurements directly on the exposed lake floor of Lakes Metzabok and Tzibaná. During this second campaign, we collected time-domain induced polarization (TDIP) and seismic refraction tomography (SRT) data. By integrating the multi-methodological data set, we (1) identify 5–6 m thick, likely undisturbed sediment sequences on the bottom of both lakes, which are suitable for future paleoenvironmental drilling campaigns, (2) develop a comprehensive geological model implying a strong interconnectivity between surface water and karst aquifer, and (3) evaluate the potential of the applied geophysical approach for the reconnaissance of the geological situation of karst lakes. This methodological evaluation reveals that under the given circumstances, (i) SBP and TDIP phase images consistently resolve the thickness of the fine-grained lacustrine sediments covering the lake floor, (ii) TEM and TDIP resistivity images consistently detect the upper limit of the limestone bedrock and the geometry of fluvial deposits of a river delta, and (iii) TDIP and SRT images suggest the existence of a layer that separates the lacustrine sediments from the limestone bedrock and consists of collapse debris mixed with lacustrine sediments. Our results show that the combination of seismic methods, which are most widely used for lake-bottom reconnaissance, with resistivity-based methods such as TEM and TDIP can significantly improve the interpretation by resolving geological units or bedrock heterogeneities, which are not visible from seismic data. Only the use of complementary methods provides sufficient information to develop comprehensive geological models of such complex karst environments

Regional Pelagic Rotifer Biodiversity in a Tropical Karst Lake District

The species richness, composition, abundance, and biomass of pelagic rotifers were determined in 17 karst lakes of the “Lagunas de Montebello” National Park, Chiapas, Mexico. The species richness of the region (21 species) and single lakes (1–12 species) was smaller than that of other Mexican, tropical, and temperate lakes. It is worth noting the high dissimilarity in species composition—about half (52%) of the species were observed in only 1–3 lakes. A total of eight rotifer families, all from the Monogononta subclass, were recorded. Keratella americana was the species with the highest occurrence (13 lakes), followed by Ptygura sp. (8 lakes). The abundance (0 to 536 ind L−1) and biomass (0 to 21 µg L−1) of rotifers were low. The highest values of species richness, abundance, and biomass were found in eutrophic lakes, and the lowest in oligotrophic lakes. The low values of rotifer biodiversity, abundance, and biomass in the Montebello lakes are probably the product of the interaction of different factors—such as environmental homogeneity (all water bodies are karst lakes), the low availability of “good-quality” food, and predation by cyclopoid copepods in the eutrophic lakes, and the low availability of food, and competitive interference by calanoid copepods and cladocerans in the oligotrophic lakes.

Chemistry of Bottom Sediments from Selected Karst Lakes in the Vicinity of Staszów (Nida Basin)

The aim of this study was to reveal the chemical composition of bottom sediments in karst lakes located in the northern part of Połaniec Basin (eastern part of the Nida Basin, vicinity of Staszów municipality). A field investigation was carried out in 2010 on four lakes: Duży Staw, Donica, Dziki Staw and Łajba. The contents of a very wide range of chemical elements were determined, which is a rarity. On the basis of our research, it was found that the analysed lakes differed considerably between each other in spite of their close location. In general, sediments contained fair amounts of organic substances as well as Fe, Al, Ca and Mg due to the type of source rock and soligenic lake water supply. Increased concentrations of V, Cr and Ni might be explained by the weathering of rock material. In turn, higher contents of Zn, Pb, Cu and Co in the bottom sediments from Duży Staw indicated anthropopression. The measurement results obtained in this study will allow a future comparative analysis with bottom sediments from other lakes.

Water- and land-borne geophysical surveys before and after the sudden water-level decrease of two large karst lakes in southern Mexico

The present geophysical study was motivated by the need to determine suitable coring locations for paleolimnological studies in two karst lakes (Metzabok and Tzibaná) of the Lacandon Forest in Chiapas, southern Mexico. We used seismic and transient electromagnetic methods to map the sediment thickness below the lake floor. When lakes were filled in March 2018, we collected seismic data with a sub-bottom profiler (SBP) and transient electromagnetic (TEM) data with a floating single-loop configuration. The latter aimed at assessing the TEM method as an alternative to seismic methods for the investigation of lake sediments and geology. After the first campaign, water levels of both studied lakes dropped dramatically by July 2019, leaving Lake Metzabok (maximum depth ~ 25 m) dry and Lake Tzibaná (~ 70 m) with a water level decreased by approx. 30 m. After the sudden drainage of the lakes, we complemented water-borne measurements by a survey carried out on the exposed lake floor in October 2019, when lake levels were still low. During this second campaign, we collected time-domain induced polarization (TDIP), and seismic refraction tomography (SRT) data on the desiccated bed of Lake Metzabok and some dry parts of Lake Tzibaná. By comparing the various data sets, we find that (i) SBP and TDIP phase images consistently resolve the thickness of the fine-grained lacustrine sediments covering the lake floor, (ii) TEM and TDIP resistivity images consistently detect the upper limit of the limestone bedrock and the geometry of fluvial deposits of a river delta, and (iii) TDIP and SRT images suggest the existence of a layer that separates the lacustrine sediments from the limestone bedrock and consists of collapse debris mixed with lacustrine sediments. While our results do not imply that resistivity-based methods could generally replace seismic reflection surveys for lake-bottom reconnaissance, they clearly show that TEM and TDIP surveys can provide important complementary information and resolve additional geological units or bedrock heterogeneities.