Estonian Rivers Research Articles

Long-term data reflect nitrogen pollution in Estonian rivers

Abstract We analysed long-term (1992–2020) changes in fertiliser use, wastewater treatment, and river water nutrient status in Estonia (N-E Europe) in the context of changing socio-economic situations and legislation. We hypothesised that improved regulation of fertiliser usage and wastewater treatment are reflected as declining riverine nutrient concentrations, with the largest relative improvements occurring in catchments with initially high proportions of point source loading. We analysed nutrient dynamics in 16 rivers differing by catchment land use, population and livestock densities. Data on fertiliser use and wastewater treatment originated from the Statistics Estonia database, and riverine nutrient concentrations from the State Environmental Monitoring Database. We clustered the rivers by their catchment properties and analysed trends in their nutrient status. Point source nutrient loading reductions explained most of the decline in riverine nutrient concentrations, whereas application of mineral fertilisers has increased, hindering efforts to reach water quality and nutrient load targets set by the EU Water Framework Directive and the Baltic Sea Action Plan. Highest nitrogen concentrations and strongest increasing trends were found in rivers within the Nitrate Vulnerable Zone, indicating violation of the EU Nitrates Directive. To comply with these directives, resource managers must address non-point source nutrient loading from river watersheds.

Source apportionment of nitrogen in Estonian rivers

The statistical model MESAW (Matrix Equations for Source Apportionment on Watershed) was used to estimate the diffuse unit-area source emission coefficients of nitrogen in Estonian rivers. The input data included monitored riverine loads, point sources and land use categories from a total of 50 rivers/catchment areas. Two independent studies were conducted: the estimation of emission coefficients for the whole of Estonia and for a smaller study area near Tallinn. The results from both cases showed that drained peat soils were the highest diffuse source contributor in unit-area loads. The results show that the unit-area loads from drained peat soils were up to 2.3 times higher than from arable land. Moreover, a comparison of emission coefficients for the whole of Estonia and for the Tallinn catchment area indicated that coefficients can vary significantly between sources and single years. Additional detailed studies and monitoring are needed to support these conclusions.

Temporal trends in nitrogen concentrations in Estonian rivers

The nutrient content in streams and rivers depend on many interacting processes such as hydro-geographical conditions and land use practices. The aim of this study was to investigate the current status of Estonian rivers and determine any trends in the concentrations of total nitrogen (TN) and nitrate-nitrogen (NO3-N) between 1992 and 2013. This study involved 43 monitoring sites and 32 rivers in Estonia. The temporal trends were assessed using the partial Mann- Kendall (PMK) test, which was adapted to account for the influence of water discharge. Most of the studied streams and sites did not show any trend in nitrogen concentrations. The statistically significant downward trend in TN was identified at 13 monitoring stations and upward trend at four monitoring sites. The results for NO3-N showed a statistically significant downward trend at three sampling sites while the upward trend was found at nine monitoring stations, particularly at four sites located within the nitrate vulnerable zone (NVZ). Overall, the increasing nitrate content in surface waters can most probably be attributed to the intensification of agricultural activities in rivers catchments during the last ten years. However, there are still many uncertainties in nutrient loss processes. Thus, the national monitoring programmes should be further developed.

Nutrient retention and export to surface waters in Lithuanian and Estonian river basins

The statistical model MESAW was applied to simultaneously estimate export coefficients and retention of nutrients in four Lithuanian and three Estonian river basins (range 946–8,388 km2). This modelling approach uses non-linear regression to yield the export coefficients for total nitrogen and total phosphorus losses to surface waters, considering different land-use classes and retention of nutrients in the river network. The model was applied to data from 40 water quality monitoring sites and corresponding sub-basin data on land use, point sources and atmospheric deposition. The results showed that the studied river basins had a high nutrient retention capacity: 67–78 and 24–63% of total nitrogen and total phosphorus inputs remained in surface waters. The estimated retention was large in lakes: 27–59% for nitrogen and 11–31% for phosphorus. In-stream retention was apparently much lower, in the range 11–15% for total N and 3–12% for total P. Retention in lakes was lower in Estonia than in Lithuania due to the locations and smaller areas of the lakes in the Estonian basins. In Estonia, the highest relative retention in lakes was 46% for nitrogen and 35% for phosphorus. In-stream retention was also somewhat lower than in Lithuania, possibly because of the lower temperature in Estonian rivers in summer.

Increased organic carbon concentrations in Estonian rivers in the period 1992–2007 as affected by deepening droughts

There are 10,091 km2 of peatlands in Estonia and human activities may have changed the role of northern peatlands from global sinks to global sources of carbon. The aim of this work was to explain the changes in organic carbon exports from eleven Estonian rivers in the period 1992–2007 in terms of land use change, climate change quantified by trends in stream-water discharge and hydrological droughts and reductions in atmospheric sulfate quantified by change in water chemistry. Direct TOC (total organic carbon) measurements had been initiated in 1998. We used CODKMnO4 (permanganate oxygen consumed) as its surrogate for the whole time span (Spearman’s determination coefficient in six small northern Estonian rivers 0.95 > I2 > 0.72; p  2.35; p  0.05). We conclude that the main factor in the increase of organic carbon export is the deepening of droughts driven by climate change, magnified by man-made drainage.

Nitrogen and phosphorus in Estonian rivers discharging into Lake Peipsi: estimation of loads and seasonal and spatial distribution of concentrations

Our study aims to estimate the riverine loads of nutrients to Lake Peipsi sensu stricto and to assess their effect on the lake's water quality. The data of national monitoring of 1992-2007 were used for the determination of seasonal and spatial distribution of nitrogen and phosphorus in the rivers discharging into Lake Peipsi s.s. Statistical conclusions and tests were based on a multi- parametric model. Our calculations revealed that the major Estonian rivers discharged approximately 5600 tonnes of nitrogen and 179 tonnes of phosphorus annually to Lake Peipsi s.s. Nitrogen export coefficients varied from 4 to 8 kg N ha -1 yr -1 , while the range of P losses was from 0.12 to 0.21 kg P ha -1 yr -1 . Seasonal variation in nitrogen concentrations had a sinusoidal pattern, with high concentrations during winter and spring and low values in summer, whereas phosphorus dynamics created a slightly different picture, with a much more irregular variation and a smaller amplitude over the year. Only a few rivers showed a pattern of notably higher phosphorus concentrations during summer low flows and more diluted concentrations during winter and spring. The differences in the concentration of phosphorus were mainly explained by variation in water discharge, while the availability of nitrogen in rivers was determined rather by seasonality. Nutrient concentrations in rivers discharging into Lake Peipsi are probably still influenced by nutrient stores from the period of intensive fertilization. Although a local effect of nitrogen loads on the lake's water quality was detected, the concentration of phosphorus in lake water remained insensitive to year-to-year changes in the riverine load of phosphorus.

Recent trends in nutrient concentrations in Estonian rivers as a response to large-scale changes in land-use intensity and life-styles

The aim of the study was assessment of changes in nutrient concentrations in Estonian rivers as a response to improved wastewater treatment and substantial reductions in the use of fertilisers and number of livestock during the past 15-20 years. A Mann-Kendall test and flow adjusted technique to assess recent trends have been used. Statistical analysis covered time series of 53 sampling sites on 40 rivers and streams in different hydro-geographical regions and varying human pressures. The results indicate a statistically significant downward trend in nitrogen concentration in 18 sampling stations during the studied period; only very few showed an upward trend. These decreases in total nitrogen (TN) relate mainly to (i) substantial reductions in the use of fertilisers, (ii) decreased area of agricultural land, (iii) decreased point source load and (iv) increased self-purification capacity of soil-water systems. The concentration of phosphorus decreased only in 13 locations, despite of efforts to improve the efficiency of wastewater treatment. Moreover, in seven locations the concentration of phosphorus was increasing. This increase in total phosphorus (TP) probably relates to the low treatment efficiency of small wastewater treatment facilities as well as to the raised ground water table due to insufficient maintenance of drainage systems that favour transport of soil P to water bodies. Accordingly, the ratio of nitrogen and phosphorus had both decreasing and increasing trends.

Temporal and spatial patterns of phytoplankton in a temperate lowland river (Emajogi, Estonia)

For the first time, a general survey is presented of the temporal and spatial patterns of the phytoplankton community in the River Emajõgi, one of the largest Estonian rivers. The species composition of phytoplankton in the River Emajõgi resembles that of Lake Võrtsjärv from which the river arises. The cyanobacteria Limnothrix planctonica (Wołoszyńska) Meffert, Limnothrix redekei (Van Goor) Meffert and Planktolyngbya limnetica (Lemmermann) Komárková-Legnerova et Cronberg dominated the phytoplankton biomass. Phytoplankton abundance, biomass and chlorophyll a concentration showed an increasing trend from spring to late autumn. The results of principal component analysis revealed that the first and the second axes explained 43.6% and 14.5% of total variance, respectively. The first principal component was positively correlated with pH, O2, BOD5 as well as with phytoplankton parameters, and negatively correlated with water level, transparency and concentrations of nitrite and carbonate. The second principal component was positively correlated with chemical oxygen demand and negatively correlated with the concentrations of ammonium and total phosphorus. Projection of the cases on the factor plane indicated that the water of the upper reach of the river and the water of the River Pedja differed significantly from each other as well as from the waters of a majority of other rivers.

Source apportionment of nutrients in Estonian rivers

Eutrophication caused by excess nutrient loads is the main problem for Estonian surface waters. The assessment of the type of human activity on the catchment area that may cause an impact on the status of a water body is needed for successful implementation of the Water Framework Directive. The lack of necessary information often makes it difficult to perform this task. The simple export coefficients approach has been used in this investigation for evaluation of the impact of different sources of nutrients on the water quality in Estonian rivers. Attention has mainly been concentrated on the diffuse pollution sources (natural and anthropogenic). Nonlinear regression was used for the estimation of the export coefficients and retention of nutrients. Results revealed that the export coefficients vary within a wide range of limits even in such a small country as Estonia. Soil type, topography, climatic conditions and water flow are the main natural factors influencing export coefficients. It was demonstrated that the use of export coefficients estimated for other regions may lead to wrong conclusions about the impact of different diffuse sources on the water body status.

Critical N:P ratio for cyanobacteria and N2-fixing species in the large shallow temperate lakes Peipsi and Võrtsjärv, North-East Europe

In the 1990s a sharp decrease in nitrogen loading occurred in Estonian rivers, bringing about a reduction of the nitrogen-to-phosphorus ratio (N:P ratio) in the large shallow lakes, Peipsi (3,555 km2, mean depth 7.1 m) and Vortsjarv (270 km2, 2.8 m). The average mass ratio of total nitrogen (TN) and total phosphorus (TP) in Vortsjarv (45) was about twice as high as that in Peipsi (22). In Peipsi, the N2- fixing Gloeotrichia echinulata, Aphanizomenon flosaquae and Anabaena species prevailed in the summer phytoplankton, while in Vortsjarv the dominant cyanobacteria were Limnothrix planktonica, L. redekei and Planktolyngbya limnetica, which cannot fix N2; the main N2-fixing taxa Aphanizomenon skujae and Anabaena sp. seldom gained dominance. In May-October the critical TN:TP mass ratio, below which N2-fixing cyanobacteria (Nfix) achieved high biomasses, was ∼40 in Vortsjarv and ∼30 in Peipsi. The percentages of both total cyanobacteria (CY) and Nfix (CY% and Nfix%) in Peipsi achieved their maximum values at an N:P mass ratio at or below 20 for both TN:TP and Nmin:SRP. In Vortsjarv, the TN:TP supporting a high Nfix% was between 30 and 40 and the Nmin:SRP supporting this high percentage was in the same range as that in Peipsi (<20), though the maximum Nfix% values in Vortsjarv (69%) were much lower than in Peipsi (96%). The Nmin:SRP ratio explained 77% of the variability in Nfix% in May–October. The temperature dependence of Nfix% approximated to the maximum function type, with an upper limiting value at a certain water temperature, and this was most distinct in May–October. The critical TN:TP ratios obtained from our study (roughly 30 for Peipsi and 40 for Vortsjarv) are much higher than the Redfield N:P mass ratio routinely considered (7). Our results represent valuable guidelines for creating effective management strategies for large shallow lakes. They provide a basis for stressing the urgent need to decrease phosphorus loading and to keep the in-lake P concentration low, and not to implement nitrogen reduction measures without a simultaneous decrease of phosphorus concentration.

Reaction of large and shallow lakes Peipsi and Võrtsjärv to the changes of nutrient loading

More than 20-year monitoring of Estonian rivers reveals that the loading of nitrogen to large shallow lakes Peipsi (3,555 km2, mean depth 7.1 m) and Vortsjarv (270 km2, mean depth 2.8 m) decreased substantially in the 1990s. Phosphorus loading decreased to a much smaller extent than nitrogen loading. In L. Vortsjarv both N and P concentrations followed the decreasing trends of loading, which show the high sensitivity of large shallow lakes to catchment processes. Our study showed a positive relationship between P content in sediments and the relative depth of the lake. Assumingly the resilience of a lake in responding to the reduction of nutrient loading decreases together with the decrease of its relative depth. In L. Peipsi the concentration of P has not decreased since the 1990s. Our data show indirectly that P loading from Russia to L. Peipsi may have increased. The N/P ratio has decreased in both lakes. Cyanobacterial blooms have been common in both lakes already at the beginning of the 20th century. The blooms disappeared during heavy nitrogen loading in the 1980s but started again in L. Peipsi in recent years together with the drop of the N/P ratio. In L. Vortsjarv the N/P ratio is higher and the ecosystem is more stable although the share of N2-fixing cyanobacteria increased from the 1990s. Reappearing cyanobacterial blooms in L. Peipsi have caused fish-kills in recent years. In L. Peipsi summer/autumn fish-kills during water-blooms are a straightforward consequence of reduced nitrogen level at remaining high phosphorus level while in L. Vortsjarv the climatic factors affecting water level are more critical––at low water level winter fish-kills may occur. In L. Vortsjarv nutrient loading has decreased and water quality has improved, present ecological status seems to be mostly controlled by climatic factors through changes of water level. The most important measure to improve water quality in L. Peipsi would be the reduction of phosphorus loading from both Estonian and Russian subcatchments.

Origin of phytoplankton and the environmental factors governing the structure of microalgal communities in lowland streams

Information on the structure of microalgal assemblages in the epiphyton and epilithon is necessary to understand the origin of phytoplankton in lowland rivers. To this end, we carried out concurrent investigations on phytoplankton, epiphyton and epilithon in 18 reaches of three Estonian rivers during the midsummers of 2002 and 2003. A total of 251 taxa was recorded, of which 192 were epiphyton species, 158 were epilithon species and 150 were phytoplankton species. Canonical correspondence analysis (CCA), based on the 31 most abundant taxa, indicated differences in the structure of the algal assemblages between the different biotopes (phytoplankton, epiphyton and epilithon) as well as between the studied rivers. The composition of the phytoplankton clearly differed from that of the other biotopes, with prevailing small flagellates, a chrysophyte (Synura uvella) and cryptophytes (Rhodomonas lacustris and Cryptomonas erosa). The epiphyton was characterized by a large number of diatoms, while the epilithic community contained filamentous cyanobacteria (Phormidium tergestinum and Planktolyngya sp.) and a green alga (Stigeoclonium tenue) in addition to diatoms. Based on redundancy analysis (RDA), phosphorous was the most relevant parameter determining the distribution of species in the phytoplankton assemblages. Shading by trees on the river bank, dissolved oxygen concentration and water temperature as well as river width determined the distribution of species in the epiphyton. The data set on the epilithon did not reveal any significant relationships between species distribution and the measured environmental parameters.

Effects of large-scale changes in emissions on nutrient concentrations in Estonian rivers in the Lake Peipsi drainage basin

The fall of the Iron Curtain resulted in dramatic changes in Eastern Europe, including substantial reductions in the use of fertilisers and livestock production, as well as a marked decrease in water consumption by both the general population and industries. This situation has created a unique opportunity to study the way that rivers have responded to these changes. Here, the impact of these reductions on concentrations of nutrients (N and P) at 22 sampling sites on Estonian rivers are examined. There were statistically significant downward trends (one-sided test at the 5% level) in total nitrogen (TN) concentrations at 20 of the 22 sites. These decreases in TN relate to: (i) substantial reductions in the use of organic and inorganic fertilisers, (ii) reduction of cultivated and ploughed areas and increased proportions of grassland and abandoned land and (iii) improvements in farm management practices. For total phosphorus (TP), significant downward trends were detected at only two sites, and there were also two upward trends. The TP trends can be mainly explained by changes in phosphorus discharges from municipal sewage treatment plants. Fifteen downward trends and one statistically significant upward trend were found for the TN:TP ratio. The general decline in this ratio has likely been conducive to blue-green algae blooms in the recipient, Lake Peipsi.

Self-purification processes in Estonian rivers receiving sewage from oil shale processing industry.

Rivers in the northeastern part of Estonia have been for decades under constant input of phenolic leachate water from semi-coke mounds formed as by-product of oil-shale processing. The dynamics of chemical and microbiological parameters were studied along river system polluted by ash-dump leachate in May-September 1995. The structure of the bacterial community was characterized by general microbial functional groups (heterotrophic bacteria), by biodegradative bacterial groups (phenol, m-toluate, benzoate, salicylate, camphor, naphtalene and heptane degrading bacteria), and by community-level physiological profile (Biolog microplates). The taxonomic diversity of the isolated phenol-utilizing bacterial strains was studied using commercial identification systems (Biolog and API). Multivariate statistical techniques were applied to relate different aspects of bacterial community structure to environmental parameters, and to assess the relationships between different estimates of community stricture. Pollution markedly altered all aspects of the studied microbial diversity. The structure and functioning of the bacterial communities changed significantly along the river and revealed remarkable adaptation to phenolic compounds in environment. Based on the data observed the concentration of volatile phenols decreased as an average 85 % along the river system, and the most significant process involved in self-purification is biodegradation of these compounds.

Biophysical impacts of climate change on some terrestrial ecosystems in Estonia

Climate warming due to the enhanced greenhouse effect is expected to have a significant impact on the natural environment and human activity in high latitudes. Because of its geography, wide coastal areas, water resources, forests, and wetlands, the environment of Estonia is sensitive to climate change and sea level rise. Climate change scenarios for Estonia were generated using a Model for the Assessment of Greenhouse-gas Induced Climate Change (MAGICC) and a regional climate change database, Scenario Generator (SCENGEN). Three alternative emission scenarios were combined with data from 14 general circulation model experiments. The assessment results of forest resources using RipFor, a forest-soil-atmosphere model, show that climate warming would enhance forest growth in Estonia resulting in increased productivity (2–9%) of harvestable timber on highly productive sites. Nutrient mobility increases greatly and in highly permeable soils with stable vegetation, increased mobility may result in nutrient losses through leaching. The assessment results of water resources using the simple water balance model, WatBal, show that the runoff regime of Estonian rivers would equilibrate and the groundwater table would rise. Climate warming would not cause any particular problems with water supply but the groundwater quality may suffer from increased leaching. Due to milder winters and increased storminess, the destruction of coastal areas, inundation of wetlands and disappearance of rare plant communities in coastal areas would be the most damaging results of climate change. Most sandy beaches high in recreational value would disappear. However, isostatic uplift and settlements inland from the present coastline reduce the risk of socio-economic decline.

Phytoplankton of Estonian rivers in midsummer

The material for the present phytoplankton study was collected from 427 reaches of 162 Estonian rivers during the period of 1991–1997. Sampling was carried out at the end of June and in July. A total of 402 taxa of algae were identified in phytoplankton. The most numerous were diatoms (180), followed by green algae (125) and cyanobacteria (45). The phytoplankton of the Estonian rivers is sparse and heterogenous: only 13 species of algae were recorded in more than 50% of streams. Pennate diatoms dominate most frequently in phytoplankton biomass, while tiny flagellates dominate in phytoplankton abundance. The nonparametric means (medians) of phytoplankton biomass, abundance and number of taxa per sample in the lower reaches of the Estonian streams were 0.2 mg l−1, 0.4×106 cells l−1 and 19, respectively; the highest mean values (0.3 mg l−1, 0.6×106 cells l−1 and 23, respectively) were observed in the rivers of the drainage basin of Peipsi-Vortsjarv and the lowest ones (0.1 mg l−1, 0.2×106 cells l−1 and 13, respectively), in the streams of Hiiumaa. Phytoplankton biomass was correlated with content of chlorophyll a according to Lorenzen (L chl a) (Spearman r=0.72) and Jeffrey & Humphrey (JH chl a) (Spearman r=0.70) at the confidence level P<0.001. The Spearman correlation coefficients between phytoplankton abundance, and L chl a and JH chl a were 0.63 (P<0.001) and 0.64 (P<0.001), respectively. A low correlation was observed between phytoplankton biomass and BOD5 (Spearman r=0.43; P<0.001). Phytoplankton parameters were not significantly related to flow velocity, water temperature, pH, dissolved oxygen and chemical properties of water. Estonian streams are short. In many places, river banks are bordered with trees and bushes, and nutrients cannot be assimilated by phytoplankton due to deficiency of light. Only two rivers – the Vaike Emajogi and the Elva with their tributaries – are relatively rich in phytoplankton.

Modelling wash-off and leaching of pollutants by spring-time flow

The quality of water in Estonian rivers in the spring is influenced strongly by the conditions in the catchments, and in particular by the amount of ice within the soil. After cold winters, the soil is extensively frozen and surface runoff predominates. After warmer winters, water flows through the soil and leaches pollutants from the soil. Increased pollutant load to Matsalu Bay has been identified by calibrating a water quality model for the period 1959–1966 and then running the same model for a test period (1977–1991). The observed concentrations during the test period were higher than those predicted by the calibrated model, indicating a real increase in pollutant load.