CNP-stoichiometry of transboundary rivers (Dniester, Prut and Siret) at the exit from Ukraine

Abstract

The Prut is the left tributary of the Danube and the second largest river of the Eastern Carpathians. It flows on the territory of Ukraine (within the Ivano-Frankivsk and Chernivtsi regions), Moldova and Romania. At first the Prut flows from the south to the north and it is cutting through the ridges in a deep valley. Prut turns to the southeast, leaving the foothills, then to the south, forming a natural border between Moldova and Romania. The river crosses the border with Romania near the Mamalyga village of the Chernivtsi region.

Siret is the left tributary of the Danube, flowing through Ukraine and Romania. In Ukraine it flows through Vyzhnytsa, Storozhynets and Glyboka districts of the Chernivtsi region. At first it flows to the north, and it turns gradually to the northeast, east and southeast. It is crosses the Ukrainian-Romanian border near the Novy Vovchynets village of the Chernivtsi region.

Dniester is a river in the south-west of Ukraine and in Moldova (on the border of two countries). Most of it is in Ukraine. Here it occupies a significant part of the territories of seven regions of Ukraine (Lviv, Ivano-Frankivsk, Chernivtsi, Ternopil, Khmelnitsky, Vinnitsa and Odessa regions). The Chernivtsi region, however, is precisely the region of Ukraine where one can assess the influence of the «purely Ukrainian side» on the state of this river. The Dniester becomes common and serves as the border between Ukraine and Moldova below the Voloshkove village of the Chernivtsi region.

Thus, the Chernivtsi region can be a testing ground for monitoring the influence of the Ukrainian side on the quality of the water of transboundary rivers. There are Dniester, Prut and Siret.

In 1934 E. Redfield [3] established the statistical stoichiometric ratio                1017C: 15N: 1P for sea water and it proved the Nitrogen limiting role in sea water for the development of plankton [4].

The last years were marked by the revival of the A. Redfield stoichiometric analysis ideas. Thus, P.M Glibert [2] showed that ecological stoichiometry has a significant effect on the resistance of aquatic ecosystems. Data were obtained on the disturbance of the stability of pastoral ecosystems during stoichiometric shifts of biogenic elements in the soils surface layer [6]. Yuanhe Yang et al. [5] have demonstrated the crucial importance for predicting the dynamics of ecosystems under conditions of environmental change is the CNP-elements stoichiometric availability. All this prompted us to supplement the CNP-monitoring of river ecosystems by stoichiometric analysis of the relevant elements.

The purpose of our research was the CNP-monitoring of transboundary rivers (Dniester, Prut and Siret) within the Chernivtsi region by stoichiometric analysis.

Monitoring covered two vectors: comparison of the content of dissolved inorganic compounds PO₄^3-, NH₄⁺, NO₂^-, NO₃^-, CO₂, HCO₃^- in the water of the studied transboundary rivers with maximum permissible concentrations (MPC) of water bodies used for the household-drinking and water used for fishery purposes and assessment of the stoichiometric availability of CNP-elements of the listed compounds.

The research was carried out in the summer in 2014 at the monitoring stations of the main river basins of the Chernivtsi region. Water samples were taken by a bathometer at 16 sites (near 8 forest and 8 meadow floodplains) of each of the 15 monitoring stations. Under laboratory conditions the nitrates contents were determined by nitratemetr H-401; carbonates and hydrogen carbonates were determined titrimetrically; phosphates, ammonia and nitrites were determined photocolorimetrically [1].

As a result of the researches it was established that no excesses of water bodies used for the household-drinking MPC and water used for fishery purposes MPC of the most dissolved inorganic compounds contents of Carbon, Nitrogen and Phosphorus in the water of transboundary rivers of the Dniester, Siret and Prut basins at the exit from Ukraine within the boundary Chernivtsi region. The exception is the excess of the fishery MPC for nitrites against the background of the lack of such under the household-drinking MPC.

The phosphates contents in the river water of the Dniester Basin exceeds their contents in the river water of Prut Basin; and the NH4 + and CO2 contents in the rivers water of the Siret Basin exceeds their contents in the rivers water of the Dniester Basin.

For the first time, median stoichiometric ratios of molar concentrations of dissolved inorganic CNP elements were established. For the rivers of the Dniester Basin it is 807С: 49N: 1P, for the rivers of the Prut Basin it is 932C: 58N: 1P, for the rivers of the Siret Basin it is 1239C: 69N: 1P.

On the basis of DIN: DIP ratio there are a pronounced P-deficiency phytoplankton growth in the rivers of the Siret Basin. In the rivers of the Prut Basin it stays at the lower boundary of the P-deficiency effect, while in the rivers of the Dniester Basin the limiting effect of Nitrogen or Phosphorus on phytoplankton is not predicted.