Peculiarities of the changes of nutrient amounts contained in water of the Nemunas delta polders

Abstract

The studies were carried out in the Nemunas lowland polders drained with pumping stations and by gravity within the period of 1997–2005, when the probability of annual precipitation height was fluctuating from 17 to 76% and the average air temperature exceeded the long term rate by 0.3–1.8°C. The paper presents the changes of nutrients contained in water and their distribution in polder channels in respect of the peculiarities of water circulation there. In spring, further from operating pumping stations in channels the amount of nitrates is decreasing, while ammonia N and P concentrations are higher compared to that at the outlet structure where flow velocities are higher. In summer, the differences in stagnant water almost disappear. In drainage ditches the amount of ammonia nitrogen is 1.5–2 times higher and the amount of phosphorus ions is about 2 times higher than that in main channels. There were almost no differences between average annual Ntotal ions concentrations (1.6–1.8 mg·l-1) contained in water of summer polders drained with pumping stations and by gravity. Differences of P concentrations were insignificant (0.12 and 0.29 mg·l-1 respectively). The highest N runoff was observed in Rusnė winter polder. In Vorusnė summer polder it was about 1.5 times lower. In gravity polders 1.2–1.5 times more nitrogen flow in Plaškiai polder which is of a higher level, while P runoff was about 2 times higher in a lower Tulkiaragė polder. Significant fluctuations were observed in nutrient runoff with pumped water: in Rusnė winter polder Ntotal reached 6.0–19.3 kg·ha-1, Ptotal reached 0.4–2.0 kg·ha-1, while in Vorusnė summer polder the concentrations were 1.5–14.7 kg·ha-1 and 0.1–1.2 kg·ha-1 respectively.