PROCEEDINGS OF THE LATVIAN ACADEMY OF SCIENCES. Section B, Vol. 65 (2011), No. 1/2 (672/673), pp. 40–47. DOI: 10.2478/v10046-011-0017-1 DISSOLVED ORGANIC MATTER CONCENTRATION CHANGES IN RIVER WATERS OF LATVIA Mâris Kïaviòð, Ilga Kokorîte, and Valçrijs Rodinovs Faculty of Geography and Earth Sciences, University of Latvia, Raiòa bulv. 19, LV-1586, Rîga, LATVIA E-mail: [email protected] Contributed by Mâris Kïaviòð Amounts of natural organic matter in surface waters reflect the character and intensity of biologi- cal processes in water bodies, human impact and depend on the physico-geographical environ- ment and land-use in the catchments. Thus, analysis of the concentrations and loadings of organic substances to adjacent water bodies can be used to indicate environmental change and human impacts. This study revealed significant increasing trends of total organic carbon (TOC) and water colour in most of the studied Latvian rivers during the last decade. However, over longer time periods, there have been pronounced oscillations of TOC concentrations, stressing the importance of long-term changes of river discharge. On a yearly basis, there was a positive correlation between parameters of organic matter concentration and discharge in all selected riv- ers. The impact of discharge on concentrations of organic matter can be masked by other factors, such as changes in precipitation, biological processes, soil types and land-use. Key words: TOC, water colour, Latvia, discharge, trend analysis. INTRODUCTION observed organic matter trends are quite contradictory; most studies show increasing DOC trends (Evans et al., 2005; Dissolved organic matter (characterised as total, dissolved Roulet and Moore, 2006; Vuorenmaa et al., 2006), but sig- or particulate organic carbon – TOC, DOC or POC) plays a nificant decreasing trends have also been found (Arvola et significant role in the global carbon biogeochemical cycle, al. 2004). Clair et al. (2008) found a significant decreasing influences mineral weathering, nutrient cycling, metal trend of TOC concentrations during the 1980s, when acid leaching as well as pollutant behaviour and toxicity in wa- deposition was decreasing rapidly, there were no significant ters. Organic substances accumulated in wetlands and soils changes in TOC concentrations during 1995–2005. In- are an important reserve of carbon, while human activities creasing DOC concentrations can be explained by global (extraction of peat, agricultural activities and land-use climate change, such as decrease of acid precipitation or sea changes) and global climate change can release stored car- salt deposition (Hongve et al., 2004) and increased atmo- bon both as greenhouse gases and dissolved organic sub- spheric temperature, which in turn leads to a higher micro- stances to surface waters. Considering this, the flows of dis- bial degradation rate of organic matter (Fenner et al., 2007; solved organic matter are very important indicators of Xiang and Freeman, 2009). Results from experimental stud- climate change. Flows of organic substances are influenced ies (Freeman et al., 2004; Fenner et al., 2007; Hagedorn and by the proportion of area covered by wetlands in the catch- Machwitz, 2007) support the hypothesis that elevated atmo- ment, intensity of eutrophication, bedrock geology, inten- spheric CO2 concentrations, along with increasing tempera- sity of agricultural land-use, direct anthropogenic load (also, tures, can enhance DOC export from catchments due to in- industrial effluents and non-point pollution sources) and creased primary production and DOC exudation from other features of the catchment area, and thus, their charac- decaying plants, as well as due to changes in the composi- ter evidently depends on the studied region (Gergel et al., tion of organic matter. Changes in hydrological regime such 1999). as increased discharge and changes of flow-path, and drought, also have an impact on concentrations and fluxes Internationally, the roles of dissolved organic matter in the of dissolved organic carbon from catchments (Clark et al., global biogeochemical carbon cycle, on biological pro- 2008; Dawson et al., 2008; Jager et al., 2009). Worrall and cesses in hydroecosystems, in reduction of pollutant toxicity Burt (2007) examined long-term data on DOC concentra- and flows of DOC have been investigated (Depetris and tions from 315 monitoring stations in Great Britain and Kempe, 1993; Pettine et al., 1998; Westerhoff and Anning, found that, despite dominant increasing trends, DOC con- 2000; Arvola et al., 2004; Evans et al., 2005). However, the centrations in some rivers in the south-west of the country 40 Proc. Latvian Acad. Sci., Section B, Vol. 65 (2011), No. 1/2. showed a significant decreasing trend. This indicates the water colour, parameters of basic water chemistry and river importance of region or even catchment-specific processes, discharge used in this study were obtained from the Latvian e.g. land-use types, anthropogenic pressure and hydrologi- Environment, Geology and Meteorology Centre for the pe- cal features (Worrall and Burt, 2007; Mattsson et al., 2009; riod 1977–2005. Water colour was determined color- Yallop and Clutterbuck, 2009). In previous studies of dis- imetrically until 1995, but after 1995 spectrophotometri- solved organic matter in Latvia (during 1977–1995), signifi- cally using the Pt/Co scale. COD was determined by cant decreasing trends of chemical oxygen demand and wa- oxidation with K2Cr2O7 and titration with ferrous ammo- ter colour were found and explained by decreasing nium sulphate (Anonymous, 1973). At the studied sites, the anthropogenic load in catchments (Apsite and Klavins, discharge, temperature and basic chemistry were also mea- 1997). In the last decades, the environmental situation in sured. Commencing in 2002, COD measurements were re- Latvia has been connected with evident climate change phe- placed by TOC measurements. For one year (2003), COD nomena, which raises the question of trends of organic car- and TOC measurements were run in parallel in all monitor- bon and their influencing factors. ing stations. Since 2003, TOC was measured using a Shimadzu Total Organic Carbon Analyser TOC — VCSN. The aim of this study was to analyse long-term changes of The relationship between COD and TOC was estimated us- concentration and loadings of organic matter in river waters ing a calibration experiment indicating the following rela- of Latvia and the controlling factors. tionship between recorded COD and TOC: TOC = (0.2928 × COD) + 7.9503; r2 = 0.611, P < 0.05 MATERIALS AND METHODS A similar approach has been used in other studies (e.g. The study site covers the entire territory of Latvia (Fig. 1). 2 Hejzlar et al., 2003; Worrall et al., 2003; Worall and Burt, Latvia, with an area of 64 000 km , is located on the north- 2007; Erlandsson et al., 2008) to calculate DOC or TOC western part of the East European Plain on the coast of the values from historical data of water colour, COD or Baltic Sea. Bedrock is covered by Quaternary deposits con- Mn COD . sisting of moraine material, limnoglacial or fluvioglacial de- Cr posits. The climatic conditions can be characterised as hu- Long-term changes in river discharge, TOC and water col- mid with mean annual precipitation of 600–850 mm our were studied using the non-parametric Mann–Kendall (Klavins et al., 2002). Due to the influence of cyclones, test (Hirsh et al., 1982; Hirsh and Slack, 1984), which can summer temperatures are slightly lower, but winter temper- be applied to data sets with non-normal distributions, miss- atures are higher than the average for temperate zones. The ing values or “outliers”, and serial character (e.g. seasonal mean temperature in January varies from –2.6 °C to changes). The programme MULTIMK/CONDMK was used –6.6 °C, and in July from +16.8 °C to +17.6 °C. The rivers to detect trends, as it allowed including covariates repre- in Latvia have mixed water feeding: rain, snowmelt and senting natural fluctuations (e.g. meteorological and hydro- groundwater. The river discharge in spring is 45–55% of the logical data; Libiseller and Grimvall, 2002). A Mann–Ken- total annual discharge, and winter contributes only 15–20% dall test value > 1.96 indicates P < 0.05; and < –1.96 of annual discharge (Klavins et al., 2002). indicates an increasing trend (P < 0.05). The Mann–Kendall It has been shown that DOC constitutes 95% of the TOC on test was applied to the period 1996–2005, as for this period average for surface waters, and thus the TOC values are water colour was measured by uniform methods. This equivalent to DOC (Vuorenmaa et al., 2006). Data on avoided the effect of different analytical methods on the chemical oxygen demand (COD) (after 2003, TOC is used), study results. Fig. 1. Location of monitoring sites (t):1– the River Tebra 1.5 km upstream the Town Aizpute; 2 – the River Irbe at hydrological station Vièaki; 3 – the River Venta 0.5 km up- stream the Town Kuldîga; 4 – the Lielupe River 0.5 km upstream the Town Kalnciems; 5 – the River Iecava at the river mouth; 6 – the River Lielâ Jugla 0.2 km upstream the Village Zaíi; 7 – the River Gauja 2 km down- stream the Village Carnikava; 8 – the River Salaca 0.5 km upstream the Town Salacgrîva; 9 – the River Aiviekste 0.2 km upstream the river mouth; 10 – the River Daugava 1.0 km upstream the Town Jçkabpils; 11 – the River Dubna 2.5 km upstream the Town Lîvâni; 12 – the River Tulija 0.3 km downstream the Village Zosçni. Proc. Latvian Acad. Sci., Section B, Vol. 65