Search

Monitoring purposes determine the selection of variables, location of sampling sites, and sampling frequency. The selection should provide the best signal to noise ratio for the parameters of interest. For trend and surveillance monitoring, the deepest point of a lake, where different inputs to the lake are integrated, is frequently selected. However, the representativeness of a single site is often questioned, especially for large lakes. Based on data collected from 10 sampling points during 11 survey expeditions in August 2001.2011 to the large shallow Lake Võrtsjärv, Estonia, we studied the spatial and annual variability of environmental and phytoplankton variables and analysed the representativeness of a permanent sampling station for the whole lake conditions. The two southernmost stations under the influence of the main tributary deviated clearly from the homogeneous group of the other eight stations, which we termed. Võrtsjärv Proper.. Among the stations of Võrtsjärv Proper, the year-to-year variability dominated strongly over the spatial variability, the latter being almost negligible for most of the variables. Surface water temperature and water level explained approximately half of the total variability in parameters commonly used in ecological status assessment of lakes. This has serious implications for using these variables to detect human impacts in Võrtsjärv. Our study showed that the deep sampling site, which was characterized by the lowest average variability of the parameters measured and was representative of more than 90% of the lake aquatory, possesses all necessary qualities required of a permanent surveillance monitoring station. [ABSTRACT FROM AUTHOR]

Copyright of Estonian Journal of Ecology is the property of Teaduste Akadeemia Kirjastus and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

On the basis of underwater radiation measurements and laboratory analyses of water samples, the effect of coloured dissolved organic matter (CDOM) on the light field in the largest Estonian lake, Lake Peipsi, was investigated. CDOM variation and its optical properties were compared with two other large European lakes, Vänern and Vättern in Sweden, as well as with 41 small lakes in Estonia, Sweden, and Finland. The light absorption coefficient at 380 nm for filtered water varied from 4.1 to 16.7 m-1 with the highest values close to the inflow of the Suur-Emajõgi River. CDOM values in L. Peipsi are in the same range as measured earlier in small Estonian and Finnish lakes. Various optical water types (‘brown’, ‘moderate’, ‘turbid’) were sampled over L. Peipsi. It is shown that the use of GF/F filters results in on average 4% higher absorption values than the use of filters with pore size 0.2 μm, and the effect might be up to ±20% in clear waters. Comparison of spectral measurements of diffuse attenuation coefficient with three-band spectrometer's data showed high consistence of datasets. This allows supplementing the existing database of optical properties of lakes in Estonia with new data from L. Peipsi. [ABSTRACT FROM AUTHOR]

The hydrochemical regime and the biota in two lakes of Vooremaa landscape protection area, Central Estonia, were studied in 2000–2001 within the frames of the EC project ECOFRAME aimed to work out water quality criteria for shallow lakes in Europe. Lake Prossa is a macrophyte-dominated lake with an area of 33 ha and a mean depth of 2.2 m. Most of the bottom in this lake is covered by a thick mat of charophytes all the year round. Lake Kaiavere located at a distance of 10 km from the former is a much larger (250 ha, mean depth 2.8 m) plankton-dominated lake. Despite the different area and the Chara meadows in Lake Prossa, the nutrient dynamics were very similar in both lakes. The first vernal phytoplankton peak was expressed in reduced Secchi depth in both lakes. After that peak, the water became clear in L. Prossa but remained turbid in Lake Kaiavere. In the Chara-lake the mean individual weight of zooplankton reached its maximum in both years in June. In the year 2000 it coincided with the peak of the zooplankton/phytoplankton ratio and with the minimum of soluble reactive phosphorus both contributing to the initiation of the clear-water period. Towards autumn, the individual mean weight of zooplankton decreased in the Chara-lake and remained smaller than in the plankton-dominated lake. Hence, zooplankton grazing could initiate the clear-water phase in the Chara-lake but other factors were needed for its maintenance. Another parameter that showed a clear difference between these two lakes was the carbonate alkalinity. It was rather stable or even increasing during spring in the plankton-dominated lake, while it decreased by nearly 50% between April and July in the Chara-lake. The calculated saturation level of calcite remained still lower in the Chara-lake showing that the co-precipitation of algae with autochthonous calcite could not explain the observed increase in water transparency above Chara meadows. The reduced sediment resuspension and the possible allelopathic influence of charophytes on phytoplankton remain the main explanations for the maitenance of the extensive clear-water period in the Chara-lake. [ABSTRACT FROM AUTHOR]

Summary Two-year comparison of methane (CH 4 ) and carbon dioxide (CO 2 ) fluxes from different habitat zones in Lake Võrtsjärv revealed strong zonal, seasonal and annual variabilities. On average, largest emissions of both gases per unit area occurred in the helophyte zone. Fluxes of CO 2 exceeded those of CH 4 in both pelagic and littoral zones. In both years, emission peaks occurred in early spring and late autumn, but were considerably higher in 2010, which caused a more than twenty times higher net efflux. Annual differences in carbon emission were mostly accounted for by variable pelagic CO 2 fluxes caused by different ice and snow conditions in spring and phytoplankton growth patterns in summer and autumn. On an annual basis, carbon uptake predominated over release in the pelagic zone in 2009, whereas an opposite balance occurred in 2010. An almost continuous emission from the helophyte zone outbalanced the pelagic uptake in 2009, resulting in a positive net efflux from the lake as a whole. In 2010, when the pelagic area was also mostly emitting carbon, the efflux from the helophyte zone constituted 36% of the total. Multiple linear regression analysis showed that the pelagic flux of CO 2 could be best described by opposite changes in phytoplankton biomass and pH ( R 2 = 0.68), whereas the best descriptors for the whole lake methane emission ( R 2 = 0.52) were sediment temperature (positive), saturation level of dissolved oxygen and depth (both negative). Strong annual differences in net C emissions of Võrtsjärv confirm the necessity of inclusion longer observation period, compared to widely used one year/ice-free period, in cases of lakes in climate zones with large seasonal variabilities. The results also confirm the need for including GHG emissions from helophytes ( e.g . Phragmites australis ) to the lake’s net GHG budget, even if they cover a relatively small area. [ABSTRACT FROM AUTHOR]