A comparison of values of photosynthetically absorbed radiation calculated with spectral and broa... more A comparison of values of photosynthetically absorbed radiation calculated with spectral and broad-band models was performed. Special attention was paid to turbid lake waters (Secchi depth ranging from 0.3 to 3 m). Ten measurement series of bio-optical character-istics were carried out in June and August 2003 at three Estonian lakes, Peipsi, Võrtsjärv and Harku. Photosynthetically absorbed radiation was calculated from measured quantum irradiance and a radiation model. The results obtained showed that in case of moderately clear waters (Secchi depth about 3 m) it is especially important to use a correct spectral model, as the errors of the broad-band model can be more than 40%–50%. For very turbid waters these errors usually do not exceed 20%. In our measurement results there was a rather remarkable variability of the bio-optical characteristics of the lakes. The relation-ships between depth-averaged assimilation number and primary production and between depth-integrated primary pro...
In the18th and 19th centuries the first data on fisheries, hydrology, and aquatic vegetation in L... more In the18th and 19th centuries the first data on fisheries, hydrology, and aquatic vegetation in Lake Võrtsjärv were published by Baltic German naturalists. The comprehensive investigation by the Lake Commission of the Estonian Naturalists' Society in 1911–13 led by M. von zur Mühlen is a milestone in the scientific level. During more than 30 years after the publication of a profound monograph in 1920 by M. von zur Mühlen and G. Schneider only a few investigations were carried out on L. Võrtsjärv. The bulk of the present long-term data set on L. Võrtsjärv has been collected during the existence of the Limnological Station since the 1960s.
Nowadays automated monitoring buoys are widely applied. Day-to-day variation and distribution of ... more Nowadays automated monitoring buoys are widely applied. Day-to-day variation and distribution of dissolved oxygen (DO) in different lake types is already well known. However, data on dissolved CO2 are still rare, because the relatively low reliability and accuracy and high price of CO2 sensors. Recently, the usage of CO2 sensors has increased although mostly in the upper mixed layers of lakes. Continuous profile measurements of CO2 are still very scarce. In year 2014 we measured DO and CO2 every 10 to 30 minutes during one week at up to four different depths in all 8 Estonia lake types, according to European Water Framework Directive. In case of fully mixed lakes only two depths were measured. In case of stratified lakes two sensors were placed in the epilimnion, one in the metalimnion and one in the hypolimnion. CO2 differed largely between lakes. The highest CO2 value in the surface layer (11.9 mg/L) was measured in highly calcareous polymictic Lake Äntu Sinijärv, but the record h...
Lake Peipsi (3555 km2, mean depth 7.1 m) located on the border of Estonia and Russia is the large... more 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 deteri...
The final publication is available at www.springer.com To study the role of large and shallow hem... more The final publication is available at www.springer.com 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...
Tracing of O2 fluxes and ecological parameters fluctuations within very short periods of time hav... more Tracing of O2 fluxes and ecological parameters fluctuations within very short periods of time have been facilitated by the development of high-frequency measurements in lakes. The parallel advent of robust metabolic models which necessitate relatively few variables has made possible the assessment of lake metabolism on a very fine time scale. One of these models, BaMM, is a Bayesian statistical model that can estimate simultaneously GPP, R and their uncertainties. We wanted to assess the metabolism of Lake Võrtsjärv, a large, shallow North-European lake during most of the growing season (from May to August) using BaMM. With high-frequency measuring BUOY positioned in the lake we have managed to measure water temperature, irradiance, and dissolved O2 with 10 min-frequency. These parameters were then entered into the model with each day separated, starting at 7:00am of day d and finishing at 6:55am of day d+1. Preliminary results show that BaMM successfully modelled integrated daily G...
We reviewed 219 papers and built an inventory of 532 items of ecological evidence on multiple str... more We reviewed 219 papers and built an inventory of 532 items of ecological evidence on multiple stressor impacts in rivers, lakes, transitional and coastal waters, as well as groundwaters. Our review revealed that, despite the existence of a huge conceptual knowledge base in aquatic ecology, few studies actually provide quantitative evidence on multi-stress effects. Nutrient stress was involved in 71% to 98% of multi-stress situations in the three types of surface water environments, and in 42% of those in groundwaters. However, their impact manifested differently along the groundwater-river-lake-transitional-coastal continuum, mainly determined by the different hydro-morphological features of these ecosystems. The reviewed papers addressed two-stressor combinations most frequently (42%), corresponding with the actual status-quo of pressures acting on European surface waters as reported by the Member States in the WISE WFD Database (EEA, 2015). Across all biological groups analysed, higher explanatory power of the stress-effect models was discernible for lakes under multi-stressor compared to single stressor conditions, but generally lower for coastal and transitional waters. Across all aquatic environments, the explanatory power of stress-effect models for fish increased when multi-stressor conditions were taken into account in the analysis, qualifying this organism group as a useful indicator of multi-stress effects. In contrast, the explanatory power of models using benthic flora decreased under conditions of multiple stress.
We used a Bayesian metabolic model for assessing the gross primary production (GPP), ecosystem re... more We used a Bayesian metabolic model for assessing the gross primary production (GPP), ecosystem respiration (ER) and their uncertainties in Lake Võrtsjärv, a large eutrophic lake in Estonia (North-eastern Europe). Diel cycle modeling was based on high-frequency (10-min) measurements of irradiance, water temperature and dissolved oxygen during most of the growing season (from May to August 2011). Posterior distribution of production and respiration was successfully simulated with the model and displayed with highly credible intervals (2.5 and 97.5 percentiles). Considering the mean GPP and ER values, the lake was autotrophic from May to June, at equilibrium in July, and heterotrophic in August. However, adding the uncertainty to metabolism estimates revealed that an ambiguous metabolic state (no clear monthly predominance of auto- or heterotrophy) represented between 12 to 32% of the period. It is thus incautious to conclude about lake metabolic state in these conditions. A comparison with the existing classical model based on dissolved oxygen measurements showed that metabolic dynamics differed between the two approaches. Though the classical model recorded highest ecosystem productivity in midsummer, the Bayesian model predicted that productivity peaked earlier in the season and gradually declined as the irradiance dropped and the water temperature rose. Coupling between GPP and ER during the whole study period was very variable, resulting that, depending on the month, 50-100% of primary production was consumed in the lake. This coupling variability was caused by extensive diel fluctuation of irradiance-dependent production compared to relatively stable water temperature and respiration. The background respiration was high in spring and declined progressively in summer, reflecting lower inputs of allochthonous organic matter to the lake. With a wider use of high-frequency techniques for measuring lake ecological parameters, this kind of performant models that are able to assess lake productivity within small time steps and take into account the uncertainty, will be increasingly needed in the future.
ABSTRACT Lakes are of importance for their heritage, ecological and aesthetical value. However, l... more ABSTRACT Lakes are of importance for their heritage, ecological and aesthetical value. However, lakes are subjected to enormous pressure from industry emissions, sewage, agricultural run-off, hydrological modifications and human induced climate changes. These are responsible for the six major pressures affecting lakes today: 1) excessive inputs of nutrients leading to eutrophication; 2) hydromorphological modifications; 3) acidification; 4) alien species; 5) hazardous substances; 6) climate changes. Effective management is therefore of critical importance for lake preservation as both heritage asset and also as a resource benefiting for human population and requires a focus on criteria indicative of these pressures. Having a comprehensive suite of criteria at a lake manager’s disposal will allow the evaluation of the extent of pressure affecting a lake and its influence on the ecosystem’s structure and functioning. The appropriate unit of management of water quality is at watershed level and it is in this context that criteria relating to pressures in lakes should be incorporated into a decision support system to allow interpretation regarding their influence on the structure and functioning of lake ecosystems. The usefulness of criteria should be reevaluated both in context of their ability to reflect pressure and also their relevance to lake biota
A comparison of values of photosynthetically absorbed radiation calculated with spectral and broa... more A comparison of values of photosynthetically absorbed radiation calculated with spectral and broad-band models was performed. Special attention was paid to turbid lake waters (Secchi depth ranging from 0.3 to 3 m). Ten measurement series of bio-optical character-istics were carried out in June and August 2003 at three Estonian lakes, Peipsi, Võrtsjärv and Harku. Photosynthetically absorbed radiation was calculated from measured quantum irradiance and a radiation model. The results obtained showed that in case of moderately clear waters (Secchi depth about 3 m) it is especially important to use a correct spectral model, as the errors of the broad-band model can be more than 40%–50%. For very turbid waters these errors usually do not exceed 20%. In our measurement results there was a rather remarkable variability of the bio-optical characteristics of the lakes. The relation-ships between depth-averaged assimilation number and primary production and between depth-integrated primary pro...
In the18th and 19th centuries the first data on fisheries, hydrology, and aquatic vegetation in L... more In the18th and 19th centuries the first data on fisheries, hydrology, and aquatic vegetation in Lake Võrtsjärv were published by Baltic German naturalists. The comprehensive investigation by the Lake Commission of the Estonian Naturalists' Society in 1911–13 led by M. von zur Mühlen is a milestone in the scientific level. During more than 30 years after the publication of a profound monograph in 1920 by M. von zur Mühlen and G. Schneider only a few investigations were carried out on L. Võrtsjärv. The bulk of the present long-term data set on L. Võrtsjärv has been collected during the existence of the Limnological Station since the 1960s.
Nowadays automated monitoring buoys are widely applied. Day-to-day variation and distribution of ... more Nowadays automated monitoring buoys are widely applied. Day-to-day variation and distribution of dissolved oxygen (DO) in different lake types is already well known. However, data on dissolved CO2 are still rare, because the relatively low reliability and accuracy and high price of CO2 sensors. Recently, the usage of CO2 sensors has increased although mostly in the upper mixed layers of lakes. Continuous profile measurements of CO2 are still very scarce. In year 2014 we measured DO and CO2 every 10 to 30 minutes during one week at up to four different depths in all 8 Estonia lake types, according to European Water Framework Directive. In case of fully mixed lakes only two depths were measured. In case of stratified lakes two sensors were placed in the epilimnion, one in the metalimnion and one in the hypolimnion. CO2 differed largely between lakes. The highest CO2 value in the surface layer (11.9 mg/L) was measured in highly calcareous polymictic Lake Äntu Sinijärv, but the record h...
Lake Peipsi (3555 km2, mean depth 7.1 m) located on the border of Estonia and Russia is the large... more 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 deteri...
The final publication is available at www.springer.com To study the role of large and shallow hem... more The final publication is available at www.springer.com 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...
Tracing of O2 fluxes and ecological parameters fluctuations within very short periods of time hav... more Tracing of O2 fluxes and ecological parameters fluctuations within very short periods of time have been facilitated by the development of high-frequency measurements in lakes. The parallel advent of robust metabolic models which necessitate relatively few variables has made possible the assessment of lake metabolism on a very fine time scale. One of these models, BaMM, is a Bayesian statistical model that can estimate simultaneously GPP, R and their uncertainties. We wanted to assess the metabolism of Lake Võrtsjärv, a large, shallow North-European lake during most of the growing season (from May to August) using BaMM. With high-frequency measuring BUOY positioned in the lake we have managed to measure water temperature, irradiance, and dissolved O2 with 10 min-frequency. These parameters were then entered into the model with each day separated, starting at 7:00am of day d and finishing at 6:55am of day d+1. Preliminary results show that BaMM successfully modelled integrated daily G...
We reviewed 219 papers and built an inventory of 532 items of ecological evidence on multiple str... more We reviewed 219 papers and built an inventory of 532 items of ecological evidence on multiple stressor impacts in rivers, lakes, transitional and coastal waters, as well as groundwaters. Our review revealed that, despite the existence of a huge conceptual knowledge base in aquatic ecology, few studies actually provide quantitative evidence on multi-stress effects. Nutrient stress was involved in 71% to 98% of multi-stress situations in the three types of surface water environments, and in 42% of those in groundwaters. However, their impact manifested differently along the groundwater-river-lake-transitional-coastal continuum, mainly determined by the different hydro-morphological features of these ecosystems. The reviewed papers addressed two-stressor combinations most frequently (42%), corresponding with the actual status-quo of pressures acting on European surface waters as reported by the Member States in the WISE WFD Database (EEA, 2015). Across all biological groups analysed, higher explanatory power of the stress-effect models was discernible for lakes under multi-stressor compared to single stressor conditions, but generally lower for coastal and transitional waters. Across all aquatic environments, the explanatory power of stress-effect models for fish increased when multi-stressor conditions were taken into account in the analysis, qualifying this organism group as a useful indicator of multi-stress effects. In contrast, the explanatory power of models using benthic flora decreased under conditions of multiple stress.
We used a Bayesian metabolic model for assessing the gross primary production (GPP), ecosystem re... more We used a Bayesian metabolic model for assessing the gross primary production (GPP), ecosystem respiration (ER) and their uncertainties in Lake Võrtsjärv, a large eutrophic lake in Estonia (North-eastern Europe). Diel cycle modeling was based on high-frequency (10-min) measurements of irradiance, water temperature and dissolved oxygen during most of the growing season (from May to August 2011). Posterior distribution of production and respiration was successfully simulated with the model and displayed with highly credible intervals (2.5 and 97.5 percentiles). Considering the mean GPP and ER values, the lake was autotrophic from May to June, at equilibrium in July, and heterotrophic in August. However, adding the uncertainty to metabolism estimates revealed that an ambiguous metabolic state (no clear monthly predominance of auto- or heterotrophy) represented between 12 to 32% of the period. It is thus incautious to conclude about lake metabolic state in these conditions. A comparison with the existing classical model based on dissolved oxygen measurements showed that metabolic dynamics differed between the two approaches. Though the classical model recorded highest ecosystem productivity in midsummer, the Bayesian model predicted that productivity peaked earlier in the season and gradually declined as the irradiance dropped and the water temperature rose. Coupling between GPP and ER during the whole study period was very variable, resulting that, depending on the month, 50-100% of primary production was consumed in the lake. This coupling variability was caused by extensive diel fluctuation of irradiance-dependent production compared to relatively stable water temperature and respiration. The background respiration was high in spring and declined progressively in summer, reflecting lower inputs of allochthonous organic matter to the lake. With a wider use of high-frequency techniques for measuring lake ecological parameters, this kind of performant models that are able to assess lake productivity within small time steps and take into account the uncertainty, will be increasingly needed in the future.
ABSTRACT Lakes are of importance for their heritage, ecological and aesthetical value. However, l... more ABSTRACT Lakes are of importance for their heritage, ecological and aesthetical value. However, lakes are subjected to enormous pressure from industry emissions, sewage, agricultural run-off, hydrological modifications and human induced climate changes. These are responsible for the six major pressures affecting lakes today: 1) excessive inputs of nutrients leading to eutrophication; 2) hydromorphological modifications; 3) acidification; 4) alien species; 5) hazardous substances; 6) climate changes. Effective management is therefore of critical importance for lake preservation as both heritage asset and also as a resource benefiting for human population and requires a focus on criteria indicative of these pressures. Having a comprehensive suite of criteria at a lake manager’s disposal will allow the evaluation of the extent of pressure affecting a lake and its influence on the ecosystem’s structure and functioning. The appropriate unit of management of water quality is at watershed level and it is in this context that criteria relating to pressures in lakes should be incorporated into a decision support system to allow interpretation regarding their influence on the structure and functioning of lake ecosystems. The usefulness of criteria should be reevaluated both in context of their ability to reflect pressure and also their relevance to lake biota
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