Role of climate and agricultural practice in determining matter discharge into large, shallow Lake Võrtsjärv, Estonia
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- Nõges, P., Kägu, M. & Nõges, T. Hydrobiologia (2007) 581: 125. doi:10.1007/s10750-006-0504-6
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This article addresses how seasonal and annual matter discharge into large, shallow Lake Võrtsjärv (Estonia) is determined by climate and changes in agricultural practices. Climate variability involved increasing winter air temperatures and large inter-annual temperature and precipitation variation. Agriculture practices have transformed from high fertilizer usage (i.e. swine slurry) in the 1970s and 1980s, leading to high phosphate, ammonium, and BOD5 loadings, to low loadings after the collapse of soviet-type agriculture in the early 1990s. The 28-year monthly record on river flow and concentrations of nutrients (N, P) and dissolved organic matter (BOD5, CODMn) from four main tributaries was analysed with seasonal air temperature and precipitation data. Long-term trends in nutrient and organic matter loadings to Lake Võrtsjärv resulted from agriculture and climate changes. The change could be traced as a linear trend in loadings and a highly inter-correlated cluster of the slurry-related pollutants. Coincidental trends in air temperature and fertiliser use caused strong correlations between air temperature and pollutant loadings to the rivers, which turned non-significant after removing trends showing that the relationships were not based on year-to-year differences. Residual analysis revealed significant positive correlations between precipitation and annual loadings of ammonium, phosphates, and CODMn. Both components forming the load (water discharge and concentrations of substances) increased in wet years. The effect of high winter North Atlantic Oscillation Index (NAO) was expressed as more intensive river flow during winter months and decreased flow during the flood peak.