Figure 1.  Map of the Dniester estuary.

The salinity of the estuary changes from 0.0 to 9.0 psu from the delta of the Dniester River to the Tsaregrad Channel.  The temporal variability of average salinity can reach 2.0-3.5 psu during a low-flow year and 0.1-0.8 psu during full flow years. 

The estuary is located in the region with four pronounced seasons: winter, spring, summer and autumn. The mean annual temperature of the air is 10.4-10.5 ⁰C.  Mean annual precipitation is 2 mm d^-1 and evaporation is 3 mm d^-1.  Maximum evaporation occurs between June and August (Braginskii, 1992).

The Dniester River basin and the estuary area are the regions of high agricultural, hydrotechnical and port economical activities.  Chemical, wood, structural and engineering enterprises are located in the watershed of the estuary.  Agriculture is based on production of corn, vegetables, wine and stock raising.  The population in the coastal zone is about 750,000 people (Swebs, 1988). 

Dniester River nutrient concentrations and loading to the Dniester estuary has changed dramatically last decades.  The mean annual concentration of the nitrogen in 1980-1990 comparison to the 1951-1960 increased by 6.5 times, the mean annual concentration of the phosphorus increased by 7 times of the concentration to the 1951-1960 (Zaitsev, 1993).  The increase of nutrient flux during last decades has lead to broad–scale degradation of the marine environment of the estuary (Braginskii, 1992.

Data of nutrient concentrations for the estuary and the Dniester River summarized by Sirenko L.A. et al. (in Braginskii, 1992) and nutrient concentrations for adjacent sea estimated by Garkavaya et al. (2000) were used in the budget calculations described here.

Water and salt balance

Figure 2 illustrates the annual water and salt budgets for Dniester estuary.  The estuary was budgeted following LOICZ approach as well-mixed system (Gordon et al., 1996).   Groundwater discharge (V_G) of about 0.1x10⁶ m³ d^-1 (Timchenko, 1990) is negligible compared to the river discharge and was assumed to be 0 in the budget.  The average annual precipitation in the estuary is 0.7x10⁶ m³ d ^–1 and the annual evaporation is 1.2x10⁶ m³ d ^–1.  Precipitation minus evaporation (V_P-V_E) is negligible compared to the river discharge thus also assumed 0. The river discharge (V_Q) is 27x10⁶ m³ d ^-1.  Other freshwater inputs (V_O) were assumed 0.  The average salinity of the estuary is about 2.1 psu and the salinity of the adjacent sea is taken about 12.4 psu (Table 1).  Residual flow (V_R) is therefore equal to the river discharge. Volume mixing (V_X) calculated from the salt balance is 19x10⁶ m³ d ^-1.  The water exchange time (t) calculated as V_syst/(V_X + |V_R|) is about 12 days. The water exchange time is consistent with that estimated by Tolmazin 1985 which is about 11 days.

Table 1. Salinity and nutrient concentrations in the river, system and adjacent sea for Dniester estuary.

Budgets of nonconservative materials

Due to lack of necessary data, it was difficult to  estimate the contribution of all human activities to the nutrient input into the Dniester estuary, thus only estimated waste load from household activities (i.e., solid waste, domestic sewage, detergent) was considered in the budget calculations. V_ODIP_O and V_ODIN_O were estimated for the coastal population of 750,000 people using San Diego-McGlone et al, 2000.   It was assumed that 25% of the waste water enters the estuary. 

DIP balance

Figure 3 illustrates the dissolved inorganic phosphorus (DIP) budget.  The budget assumed that nutrient loads are largely delivered through the river.  The non-conservative DIP flux (DDIP) was estimated from the total inputs (river and waste loads) and total outputs (residual and exchange fluxes); DDIP of the system is -49x10⁶ mmol d^-1 (or -0.1 mmol m^-2 d^-1).  The estuary appears to be a net sink of DIP.

DIN balance

Figure 4 shows the dissolved inorganic nitrogen (DIN) budget. The non-conservative DIN (DDIN) of the system is approximately +5x10⁶ mmol d^-1 (or +0.01 mmol m^-2 d^-1). The system seems to be a net source of DIN.

Stoichiometric calculations of aspects of net system metabolism.

The rate of nitrogen fixation minus denitrification (nfix-denit) can be calculated as DDIN_obs minus DDIN_exp, where DDIN_exp is DDIP multiplied by the N:P ratio of the particulate material in the system (assumed to be 16:1 as the Redfield N:P molar ratio for phytoplankton).Thus, (nfix-denit) = +2 mmol m^-2 d^-1 (Table 2).  The estuary appears to be fixing nitrogen in excess of denitrification.

Table 2. Summary of nonconservative nutrient fluxes, apparent net metabolism (p-r) and nitrogen fixation minus denitrification (nfix-denit) for Dniester estuary.

Net ecosystem metabolism, the difference between primary production and respiration (p-r) is estimated as ?DIP multiplied by the C: P ratio of the reacting organic material (assumed to be 106:1). Therefore, (p-r) = +11 mmol m^-2 d^-1.  The estuary appears to be net autotrophic.