Several lake ice phenology studies from satellite data have been undertaken. However, the availability of long-term lake freeze-thaw-cycles, required to understand this proxy for climate variability ...and change, is scarce for European lakes. Long time series from space observations are limited to few satellite sensors. Data of the Advanced Very High Resolution Radiometer (AVHRR) are used in account of their unique potential as they offer each day global coverage from the early 1980s expectedly until 2022. An automatic two-step extraction was developed, which makes use of near-infrared reflectance values and thermal infrared derived lake surface water temperatures to extract lake ice phenology dates. In contrast to other studies utilizing thermal infrared, the thresholds are derived from the data itself, making it unnecessary to define arbitrary or lake specific thresholds. Two lakes in the Baltic region and a steppe lake on the Austrian–Hungarian border were selected. The later one was used to test the applicability of the approach to another climatic region for the time period 1990 to 2012. A comparison of the extracted event dates with in situ data provided good agreements of about 10d mean absolute error. The two-step extraction was found to be applicable for European lakes in different climate regions and could fill existing data gaps in future applications. The extension of the time series to the full AVHRR record length (early 1980 until today) with adequate length for trend estimations would be of interest to assess climate variability and change. Furthermore, the two-step extraction itself is not sensor-specific and could be applied to other sensors with equivalent near- and thermal infrared spectral bands.
•A novel automated two-step extraction method for lake ice phenology is proposed.•The first step makes use of NIR and the second step uses TIR derived LSWT data.•LSWT thresholds are derived from the data itself.•This avoids the definition of arbitrary or lake specific thresholds.•The method was validated for European lakes located in different climate regimes.
The benthic foodweb structure of Lake Võrtsjärv, a large (270 km2), shallow, and turbid Estonian lake, was evaluated based on C and N stable-isotope signatures (δ13C, δ15N). Variation in δ13C between ...sampling sites was not related to site proximity to the littoral zone or the more vegetated southern part of the lake, but rather appeared to be influenced by in-situ site peculiarities. δ13C was stable temporally and between functional feeding groups, a result implying that the whole benthic food web of the lake relies largely on the same C source admixture, essentially particulate organic matter (POM). Thus, the foodweb composition of Lake Võrtsjärv is remarkably homogeneous given the lake’s large surface area. Apparent trophic-level δ15N fractionation between total collectors and total predators (mean 1.7‰) was lower than the value of 3.4‰ generally adopted in foodweb studies, but the higher value was valid for specific prey–predator links. The low δ13C signature of some chironomid samples indicated probable assimilation of methane-oxidizing bacteria (MOB) by these sediment-dwelling invertebrates. However, the lack of similar13C depletion in benthic filterers (mussels) indicated that the MOB layer is essentially confined to the sediments and does not reach the water column, which probably constrains transfer of methane-derived C through the food web to fish in this lake. Our study demonstrates that the benthic food web of shallow turbid lakes like Võrtsjärv is simplified and is mostly sustained by phytoplanktonic C sources.
Recent reports of increasing iron (Fe) concentrations in freshwaters are of concern, given the fundamental role of Fe in biogeochemical processes. Still, little is known about the frequency and ...geographical distribution of Fe trends or about the underlying drivers. We analyzed temporal trends of Fe concentrations across 340 water bodies distributed over 10 countries in northern Europe and North America in order to gain a clearer understanding of where, to what extent, and why Fe concentrations are on the rise. We found that Fe concentrations have significantly increased in 28% of sites, and decreased in 4%, with most positive trends located in northern Europe. Regions with rising Fe concentrations tend to coincide with those with organic carbon (OC) increases. Fe and OC increases may not be directly mechanistically linked, but may nevertheless be responding to common regional‐scale drivers such as declining sulfur deposition or hydrological changes. A role of hydrological factors was supported by covarying trends in Fe and dissolved silica, as these elements tend to stem from similar soil depths. A positive relationship between Fe increases and conifer cover suggests that changing land use and expanded forestry could have contributed to enhanced Fe export, although increases were also observed in nonforested areas. We conclude that the phenomenon of increasing Fe concentrations is widespread, especially in northern Europe, with potentially significant implications for wider ecosystem biogeochemistry, and for the current browning of freshwaters.
Key Points
Fe has increased significantly in 28% of the 340 studied waters by on average 64% between 1990 and 2013
Most increasing trends and the highest rate of change were found in the boreal region of northern Europe
Hydrological changes may have contributed to the observed Fe trends. Trends may also be linked to declining atmospheric S deposition
Summary
To investigate the difference in growth rates of eels in the studied water bodies (Võrtsjärv, Ülemiste, Saadjärv, Vagula, Kaiavere and Kuremaa lakes, all in Estonia), 828 individual eel ...otoliths from the years 1999–2004 and 2008–2012 were examined for age determination. Anguilla anguilla captured in Lake Võrtsjärv mostly by professional fyke nets (mesh size >36 mm in the cod end) showed the best growth rate (gR = 6.9 cm/year) and had a high Fulton condition factor (K = 0.19), indicating suitable growth conditions in the lake. However samples caught using the same type of gear from the smaller lakes situated near Võrtsjärv, showed much slower gR (3 cm/year) and lower K (0.17) growth rates. The slowest gR (2.6 cm/year) was observed for specimens caught in Lake Kuremaa. Stocking densities are much higher in the small lakes of the region and have a direct effect on the growth rates.
The grazing impact of different sized zooplankton on 'edible' and total phytoplankton biomass and primary production was measured in L. Võrtsjärv during a seasonal study in 1998 and 2000. The ...organisms of 48-100 μm size class, composed of ciliates and rotifers, contributed significantly to the total grazing of zooplankton community throughout the study period (average 68%). The average daily filtering and grazing rate of the whole zooplankton community (micro- and macro-zooplankton) remained low, corresponding to a filtration of 44% of the water volume, 4% of the total phytoplankton biomass and 29% of primary production. However, a strong grazing pressure on small-sized phytoplankton (<30 μm) was estimated in most of the study period (average 44% d-¹). Among size classes of 'edible' phytoplankton, the size range 5-15 μm was the most important algal food for the dominant zooplankton grazers (herbivorous ciliates, Polyarthra spp., Chydorus sphaericus and Daphnia cucullata) in L. Võrtsjärv.
Phytoplankton constitutes a diverse array of short-lived organisms which derive their nutrients from the water column of lakes. These features make this community the most direct and earliest ...indicator of the impacts of changing nutrient conditions on lake ecosystems. It also makes them particularly suitable for measuring the success of restoration measures following reductions in nutrient loads. This paper integrates a large volume of work on a number of measures, or metrics, developed for using phytoplankton to assess the ecological status of European lakes, as required for the Water Framework Directive. It assesses the indicator strength of these metrics, specifically in relation to representing the impacts of eutrophication. It also examines how these measures vary naturally at different locations within a lake, as well as between lakes, and how much variability is associated with different replicate samples, different months within a year and between years. On the basis of this analysis, three of the strongest metrics (chlorophyll-
a
, phytoplankton trophic index (PTI), and cyanobacterial biovolume) are recommended for use as robust measures for assessing the ecological quality of lakes in relation to nutrient-enrichment pressures and a minimum recommended sampling frequency is provided for these three metrics.
We analyzed data from 81 shallow European lakes, which were sampled with standardized methods, for combined effects of climatic, physical, and chemical features of food-web interactions, with a ...specific focus on zooplankton biomass and community structure. Multiple-regression analysis showed that total phosphorus (TP) generally was the most important predictor of zooplankton biomass and community structure. Climate was the next most important predictor and acted mainly through its effect on pelagic zooplankton taxa. Benthic and plant-associated taxa (typically almost half the total zooplankton biomass) were, however, affected mainly by macrophyte coverage. Neither climate nor TP affected the relation between small and large taxa, and we found only a weak trend with increasing TP of increasing mean crustacean body mass. Dividing the data set into three climate zones revealed a pronounced difference in response to lake productivity between cold lakes, with long periods of ice cover, and the two warmer lake types. These "ice" lakes differed from the others with respect to the effect of TP on chlorophyll a, the zooplankton: chlorophyll a ratio, the chlorophyll a:TP ratio, and the proportion of cyclopoids in the copepod community. Our data suggest that bottom-up forces, such as nutrient concentration, are the most important predictors of zooplankton biomass. In addition, climate contributes significantly-possibly by affecting top-down regulation by fish-and may interact with productivity in determining the zooplankton standing biomass and community composition. Hence, the present study suggests that food-web dynamics are closely linked to climatic features.
To study the role of large and shallow hemiboreal lakes in carbon processing, we calculated a 3-year carbon mass balance for Lake Võrtsjärv (Estonia) based on in situ measurements. This balance took ...into account hydrological and biogeochemical processes affecting dissolved inorganic (DIC), dissolved organic (DOC) and particulate organic (POC) carbon species. Accumulation varied greatly on a seasonal and yearly basis. The lake exported carbon during most of the year except during spring floods and in late autumn. In-lake processes were responsible for exporting POC and storing DOC while DIC switched between storage and export. The carbon cycle was alternatively dominated in 2009 by biogeochemical processes and in 2011 by riverine fluxes, whereas in 2010 the two process types were of the same magnitude. These results suggest that the role of large shallow lakes like Võrtsjärv in the global C cycle is equally driven by hydrological factors, in particular seasonal water level changes, and by biogeochemical in-lake reactions.
► DOM was related to catchment. ► Fulvic acids were dominated. ► Allochthonous DOM was dominated.
UV–Vis spectroscopy was applied to characterize the nature of dissolved organic matter (DOM) in the ...inflows and in the outflow of the large and shallow temperate Lake Võrtsjärv and to determine its temporal and spatial dynamics in 2008–2010. The concentration of dissolved organic carbon (DOC) was influenced by water temperature, precipitation, stream gradient, discharge, and percentage of peatland in the catchment area and its mean annual values by rivers varied from 9.2 to 27.7mgL−1. Higher DOC values in the outflow in summer/autumn corresponded to periods of intensive phytoplankton development whereas lower values were observed in winter or early spring when biological processes are at a standstill. The results of specific ultraviolet absorption at 254nm (SUVA254) and 285nm (SUVA285) showed domination of fulvic acids in the rivers. According to our spectroscopic results, the aromaticity of DOM increased with the increasing percentage of peatland and forest and with the decreasing percentage of arable land in the river catchments.
Lake Peipsi (3555 km2, mean depth 7.1 m) located on the border of Estonia and Russia is the largest transboundary lake in Europe. L. Peipsi consists of three parts. The shared largest northern part ...L. Peipsi s.s. (2611 km2, 8.3 m) and the southern L. Pihkva (708 km2, 3.8 m) which belongs mainly to Russia are connected by the river-shaped L. Lämmijärv (236 km2, 2.5 m). The catchment area (44,245 km2 without lake area) is shared between Estonia (33.3%), Russia (58.6%) and Latvia (8%). Intensive eutrophication of L. Peipsi started in the 1970s. The biomass of N2-fixing cyanobacteria was low at heavy nutrient loading in the 1980s. After the collapse of soviet-type agriculture in the early 1990s, the loading of nitrogen sharply decreased. A certain improvement of L. Peipsi s.s. was noticed at the beginning of the 1990s together with the temporary reduction of phosphorus loading from Estonian catchment while in recent years a destabilisation of the ecosystem has been observed. This deterioration has been expressed mainly as intensive blue-green blooms and fish-kills in summer. Reappearance of blooms has been explained by the decrease in N/P loading ratio due to reduced N discharge while in some periods increased phosphorus loading could have supported this trend.
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