Per Stålnacke

Trends in nutrient concentrations in Latvian rivers and the response to the dramatic change in agriculture

In recent years, the use of fertilisers in the Baltic countries (Estonia, Latvia, and Lithuania) has decreased at an unprecedented rate. The import of mineral fertilisers and feed stuff became almost non-existent, and extensive slaughtering of livestock reduced the amount of manure. In Latvia, the purchase of mineral fertilisers decreased by a factor of 15 between 1987 and 1996 and the number of livestock decreased with a factor of almost 4 during the same time period. Such abrupt and comprehensive changes in land use have never before occurred in the history of modern European agriculture. Here, the impact that this dramatic reduction has had on concentrations of nutrients in Latvian rivers is examined. To discern temporal changes, statistical analyses were undertaken on time series of nutrient concentrations and relationships between concentrations and runoff at 12 sampling sites in ten Latvian rivers covering drainage areas from 334 to 64,000 km 2 . Considering the study period 1987‐ 1998, only four of the 12 sites showed statistically significant downward trends (one-sided test at the 5% level) in the dissolved inorganic nitrogen (DIN ¼ NO3-N þ NO2-N þ NH4-N) data. There are probably two main explanations for the weak DIN trends. Firstly, long water-transit times in the soilwater and groundwater may have caused substantial time lag between changes in input and output of nitrate in the studied catchments. Secondly, the loss of DIN might have been dominated by mineralisation of large pools of organic nitrogen that have accumulated over several years. These inferences are supported by (i) a hydrograph recession analysis and (ii) indications of DIN transformation processes, presumably denitrification, in smaller streams and channels, based on measurements in small agricultural catchments (1 ‐ 4 km 2 ) in Estonia and Latvia. Formal testing of trends in phosphorus data revealed that marked drops occurred in riverine concentrations at six sites in 1987‐ 1998. A joint analysis of concentration time series for all sampling sites for 1987‐ 1998 showed weak statistical significance for downward trends in NH4-N, NO3-N, and DIN ðp o 0:04Þ and substantial significance for PO4-P ðp , 0:01Þ: Thus, the extensive decrease in agricultural intensity that began in the early 1990s has led to only a slow and limited (especially regarding nitrogen) response in Latvian rivers. The difference noted between nitrogen and phosphorus also suggests that factors other than reduced fertiliser application influenced the inertia of the water quality response.

Estimation of riverine loads of nitrogen and phosphorus to the Baltic Sea, 1970-1993

This article presents the results of the first critical examination of time series of riverine nutrient-load data for the entire Baltic Sea drainage area. Water quality data collected by or for the different national environmental agencies were compiled and analysed statistically to identify and remove inconsistent or obviously incorrect observations. Moreover, sampling tours were undertaken to acquire additional information about the present nutrient concentrations in the largest rivers in the study area. Gaps in the time series of approved data were then filled in by employing statistical interpolation and extrapolation methods. Thereafter, the concentration and runoff data were combined to obtain estimates of monthly nutrient loads for the time period 1970–93. The results of the calculations showed that although there had been substantial changes in land use, atmospheric deposition and wastewater treatment in many parts of the study area, the total riverine loads of nitrogen (N) and phosphorus (P) to the Baltic Sea have been fairly constant since 1980, and most likely also since 1970. Moreover, the interannual variation was clearly correlated to the runoff. The mean annual loads for the time period 1980–93 were found to be about 825 000 tonnes N and 41 000 tonnes P, respectively. This implies that (i) several other investigators have strongly underestimated the riverine loads of nutrients, especially the nitrogen, and that (ii) the riverine loads by far exceed the input to the Baltic Sea from other sources, {i.e.} atmospheric deposition, direct emissions from cities and industries along the Baltic Sea coast and nitrogen fixation by marine algae.

Time scales of nutrient losses from land to sea - a European perspective

Abstract Empirical data regarding the time scales of nutrient losses from soil to water and land to sea were reviewed. The appearance of strongly elevated concentrations of nitrogen and phosphorus in major European rivers was found to be primarily a post-war phenomenon. However, the relatively rapid water quality response to increased point source emissions and intensified agriculture does not imply that the reaction to decreased emissions will be equally rapid. Long-term fertilisation experiments have shown that important processes in the large-scale turnover of nitrogen operate on a time scale of decades up to at least a century, and in several major Eastern European rivers there is a remarkable lack of response to the dramatic decrease in the use of commercial fertilisers that started in the late 1980s. In Western Europe, studies of decreased phosphorus emissions have shown that riverine loads of this element can be rapidly reduced from high to moderate levels, whereas a further reduction, if achieved at all, may take decades. Together, the reviewed studies showed that the inertia of the systems that control the loss of nutrients from land to sea was underestimated when the present goal of a 50% reduction of the input of nutrients to the Baltic Sea and the North Sea was adopted.

Riverine input of nutrients to the Gulf of Riga - temporal and spatial variation

Riverine transport is the, most important pathway for input of nutrients to the Gulf of Riga. The present study focused on updating existing estimates of the riverine nutrient contributions and on ...

TRENDS IN NITROGEN TRANSPORT IN SWEDISH RIVERS

Concern about nitrogen loads in marine environments has drawn attention to the existence and possible causes of long-term trends in nitrogen transport in rivers. The present study was based on data from the Swedish environmental monitoring programme for surface water quality; the continuity of these data is internationally unique. A recently developed semiparametric method was employed to study the development of relationships between runoff and river transport of nitrogen since 1971; the observed relationships were then used to produce time series of flow-normalised transports for 66 sites in 39 river basins. Subsequent statistical analyses of flow-normalised data revealed only few significant downward trends (p ≤0.05) during the time period 1971–1994, and the most pronounced of these downward trends were caused by reduced point emissions of nitrogen. The number of significant upward trends was substantially larger (15 for total-N and 18 for NO3-N). Closer examination of obtained results revealed the following: (i) the most pronounced upward trends were present downstream of lakes, and (ii) observed increases in nitrogen transport coincided in time and space with reduced point emissions of phosphorus or organic matter. This indicated that changes in the retention of nitrogen in lakes were responsible for the upward nitrogen trends. The hypothesis that nitrogen saturation of forest soils has caused a general increase in the riverine export of nitrogen from forested catchments in Sweden was not confirmed. Neither did the results indicate that improved agricultural practices have reduced the export of nitrogen from agricultural catchments.

Integrated tool for risk assessment in agricultural management of soil erosion and losses of phosphorus and nitrogen

In recent years, increased attention has been focused on models for risk assessment of source areas in agricultural landscapes. Among the simplest of such models are index tools, which have been developed particularly for phosphorus (P) and to some extent nitrogen (N). However, only a few studies have considered the development of an integrated management strategy that includes erosion and losses of both P and N. Accordingly, the major objective of this study was to initiate the development of an integrated risk assessment tool, consisting of indices for erosion, P and N. The strategy used to create the integrated tool was based on the assumption that all input data at field scale should be readily available either from ordinary agricultural statistics or from the farmer. The results from using the indices in a pilot case study catchment illustrated that losses of P and N had often different critical source areas. The P index was highest for fields with manure application and/or high soil P status or with autumn ploughing, and the N index was highest for fields with excessive N application. The integrated risk was greatest for areas with manure application and some areas with a high erosion risk in combination with high nutrient application rate. Additionally, four different management options were assessed: (1) reduced fertilisation, (2) catch crops, (3) autumn ploughing, and (4) no autumn ploughing. The results verified that reduced nutrient application and stubble during autumn and winter led to the largest decrease in index values, and it was also apparent that management changes in high-risk areas had the greatest impact on the indices. Overall, our findings indicate that the present integrated risk assessment tool with readily available input data can be used to rank farm fields according to risk of soil erosion and losses of P and N.

Nutrient runoff and transfer from land and rivers to the Gulf of Riga

In this paper, we have synthesized and integrated results regarding nutrient loads and eutrophication of the Gulf of Riga (GoR) that were obtained in three projects that were part of a six-year research programme (1993–1998). In particular, we focused our attention on the factors that control the temporal variability in the load of nutrients in the drainage basin and rivers, as well as the effects of such nutrients on the environment of the recipient, the Gulf of Riga.The results indicate that the rivers play a crucial role in the total input of nutrients to the GoR, and exceed the combined contribution from atmospheric deposition, point emissions from cities and industries along the coast, and nitrogen fixation by marine organisms. It was found that natural variability in water discharge was the main factor controlling the temporal variability in the riverine load of nutrients (i.e. differences in load between seasons and years). Moderate nutrient losses to surface waters, especially from agriculture, and weak riverine response to the sudden decrease in agricultural production after the disintegration of the Soviet Union in the early 1990s were also found. It is suggested that this is most likely related to the inertia in, and buffering capacity of, agricultural soils, i.e. depending of factors such as hydrological conditions, the size of ground water aquifers and water-saturated soils, creating favourable conditions for nutrient retention processes in the agricultural landscape. Regardless of the relatively low area-specific riverine inputs, the pollution loads in the rivers have contributed significantly to eutrophication of the Gulf of Riga. This seems to be true despite the relatively rapid distribution of the loads beyond the littoral zone of the gulf, resulting in a moderate all-over eutrophication of the entire recipient, characteried by a continuously shifting gradient between the southern and northern parts of the gulf. Furthermore, the marine eutrophication seems to be less prominent and less confined to the southernmost basin than previously assumed.

Impact of sampling frequency on mean concentrations and estimated loads of suspended sediment in a Norwegian river: implications for water management.

Reliable estimates of mean concentrations and loads of pollutants in rivers have become increasingly important for management purposes, particularly with the implementation of the European Union Water Framework Directive (WFD). Here, the Numedalslågen River (5500 km(2)) in southern Norway was used as a case study to evaluate the effects of sampling frequency on mean concentrations and estimated loads of suspended particulate matter (SPM). Daily monitoring data from five seasons (April/June-October/November 2001-2005) were analyzed, and three different load estimation techniques were tested: rating curves, linear interpolation, and the ratio method. The reliability of mean SPM concentrations improved with increasing sampling frequency, but even weekly sampling gave error rates as high as 70% in seasons with elevated sediment loads. Load estimates varied considerably depending on both the sampling frequency and the calculation method used. None of the methods provided consistently good results, but the lowest error rate was achieved when using the rating curve on data from fortnightly sampling and additional sampling during floods. Sampling at monthly intervals gave the highest error rates and cannot be recommended for any of the calculation methods applied here. SPM concentrations were correlated (r(2)>0.5) with arsenic, lead, nickel, orthophosphate, and total phosphorus in the Numedalslågen River. Therefore, the current findings may also have implications for substances other than SPM. The discussion considers examples from actual use of infrequently collected data, and it is advised that managers account for uncertainties in both concentration means and load estimates when assessing the state of a water body or planning mitigation measures.

Nitrogen source apportionment—a comparison between a dynamic and a statistical model

Abstract A dynamic model, HBV-N, and a statistical model, MESAW, for nitrogen source apportionment were compared regarding model performance, model uncertainty and user applicability. The HBV-N model simulates continuous series of nitrogen concentrations with meteorological data and sub-basin characteristics as input. Diffuse nitrogen emissions are defined as regional model parameters which are calibrated by comparison of observed and simulated nitrogen data. The MESAW model uses nitrogen loads for a fixed time interval at each monitoring site as response variable and sub-basin characteristics as explanatory variables to estimate diffuse nitrogen emissions through non-linear regression analysis. The two models were applied in the Matsalu Bay watershed (3640 km 2 ) in Estonia and the same land use and point sources data were used as input. Both models gave similar levels of diffuse total nitrogen emissions and retention rates, which also fit well with previous estimates made in Estonia and Scandinavia. A sensitivity analysis of the model parameters also showed similar uncertainty levels, which indicated that the model uncertainty was more dependent on the availability of nitrogen data and land cover distribution than the choice of model. Furthermore, the sensitivity analysis showed a parameter interdependency in both models, which implied the risk of compensation between estimated diffuse emissions and retention. In conclusion, however, the study showed that both models were capable of estimating nitrogen leakage from the dominating land classes and giving reliable source apportionment from the available input data. The study indicated that the HBV-N model has its advantage in assessments where detailed outputs are needed and when run-off data are limited, while the statistical MESAW model has its advantage in extensive studies since it is easily applied to large watersheds that have dense monitoring networks.

Riverine Inputs of Nutrients to the Baltic Sea

Rivers play a crucial role in the total input of nutrients to the Baltic Sea. Several studies have indicated that nutrient delivery along this route may exceed the combined contribution from atmospheric deposition on the sea, point emissions along the coast, and nitrogen fixation by marine organisms (Larsson et al. 1985; Rosenberg et al. 1990; HELCOM 1993; Enell and Fejes 1995; Stalnacke et al. 1999a). There are also strong indications that the riverine loads of nutrients have increased dramatically over the past century (Larsson et al. 1985). However, data regarding the historical development of nutrient loading of the Baltic Sea are uncertain, and even loading during the past few years has been a matter of controversy. According to the second pollution load compilation undertaken by HELCOM (1993), rivers were responsible for an annual input of 550,000 tons of nitrogen, whereas Enell and Fejes (1995) estimated that this route, including some point emissions along the coast, contributed about 980,000 tons year−1.