Cryptodepression

Abstract: Magnetic properties of sediments from two different environmental settings in Indonesia have been studied using rock magnetic methods and scanning electron microscopy (SEM). In the first setting, magnetic measurements were conducted on core sediments from two maar lakes in East Java (Lakes Lading and Bedali) that represent very confined environments where sediments are derived mainly from rocks and soils within the craters. In the second setting, similar measurements were obtained on core sediment from Lake Matano, a cryptodepression lake in tectonically active South Sulawesi where the area around the lake is dominated by highly magnetic lateritic soils. The results show that the predominant magnetic mineralogy in sediments from Lakes Lading, Bedali, as well as Matano is pseudo-single domain (PSD) magnetite (Fe3O4). Compared to that of Lake Matano, the maar lake sediments of Lakes Lading and Bedali have higher magnetic susceptibility as well as high intensity of ARM and SIRM. Variations in magnetic susceptibility in all core sediments are controlled mainly by the concentration of magnetic minerals. The homogeneity of magnetic minerals in these three lakes sediment provides an excellent setting for interpreting paleoclimatic signals as they will be recorded as anomalies of magnetic susceptibility.

Abstract: Vrana Lake is a coastal karst lake on Cres Island within the eastern Adriatic coast. It appears to have formed as a cryptodepression on karstified carbonate rocks, and is filled with fresh water. It is classified as an oligotrophic hydrosystem and serves as a natural water supply reservoir (max. depth 74.5 m; surface area 5.5 km2; water volume 220 × 106 m3). The aim of this study was to assess the influence of seasonally and spatially varying ecological conditions (i.e., thermal stratification, dissolved oxygen, orthophosphate and chlorophyll a concentrations) on the vertical distribution of the rotifer plankton community in this geomorphologically and hydrologically specific coastal lake. The rotifer community was surveyed monthly over a one year period. During the study period, a total of 21 rotifer taxa were identified. Among them Ascomorpha spp. (including A. ecaudis, A. ovalis and A. saltans), Filinia terminalis, Keratella cochlearis, Synchaeta gr. tremula-oblonga and Polyarthra vulgaris prevailed, representing 92% of the total rotifer abundance. Measured seasonal and spatial patterns of the rotifer assemblage along a vertical profile of Vrana Lake were considerably affected by temperature, food availability and biotic (competition, predation) interactions in the zooplankton community.

Abstract: A strong and potentially dangerous decreasing trend in the level of water in Vrana Lake over the last three decades was analysed. This freshwater lake is a unique karst hydrology feature located on a small Adriatic island of Cres (405.71 km2), which is entirely composed of carbonate rocks. The lake is situated in a large cryptodepression and has a bottom reaching a depth of 61.3 m below mean sea level. The lake is a complex hydrological-hydrogeological system, with an average water volume of c. 220×106 m3. The larger geographical region has been affected by an increase in air temperature over the last c. 40 years. This exceptionally clean freshwater lake is the only source of potable water for the whole Cres archipelago. A dangerous drop in the lake water level started in 1983. The threatening decreasing trend is driven by booth global climate change and anthropogenic (the overexploitation of water) factors.

Abstract: GREAT Bear Lake is situated on the Arctic Circle at a point where the Pre-Cambrian Shield emerges from the adjoining Cretaceous formations. The present lake surface is 143 m above sea level, and because the maximum depth is 542 m there is a cryptodepression of 309 m. The shape of the lake could be described as amoeboid with five arms joining a central “body”. McTavish Arm, with a mean depth of 102 m, contains the deepest water at a point close to the eastern shore. The total surface area is 31,153 km2 and the volume 2,200 km3. Surface temperatures seldom exceed 4° C and, although the bottom temperature remains close to 3.56° C throughout the year, complete turnover does not occur annually1. The lake is extremely oligotrophic with oxygen saturations up to 112 per cent (14 mg/l.) in the upper 50 m in summer and minimum values very close to saturation (12.6 mg/1.) near the bottom at the end of winter. Total dissolved solids are low: 80 p.p.m. with a specific conductance of 155 µmhos. Ice cover reaches a thickness of 2 m by the end of winter and break-up does not occur until the middle of July; this is followed by an open-water season of 3–4 months, the lake becoming ice-covered during November.