Figure 1.  Location of Donuzlav estuary.

The Donuzlav estuary is an area of reproduction and nursery for many valuable fish species, such as mullet, flatfish and sturgeon.  It is also the location of one of the largest Ukrainian underwater sand quarrying operations, which occupies about 7.5 % of the shallow estuarine area.  Approximately 200x10³ m³ to 300x10³ m³ of sand are removed annually (data of 1994).  Investigations showed that the sand withdrawal did not negatively affect pelagic fish (Zuev and Boltachev 1999).  The estuary also contains a naval base and several small towns with a total population of about 6,000 (as of 1992).

The hydro-chemical regime of the Donuzlav estuary is relatively poorly known.  The first studies of the estuary (1963 to 1964) are reported in Shulgina (1966).  Furher studies were made 30 years later (Kovrigina and Kuftarkova 1997).  The estuarine salinity and nutrient data used for the budget calculations were means of measurements collected in May-October 1990 and June-August 1997 (Kovrigina. and Nemirovsky 1999).  The nutrient concentrations of the adjacent sea were taken from Garkavaya et al. (2000).

Water and salt budgets.

Figure 2 shows the water and salt budgets for Donuzlav estuary.  The estuary was treated as single box, single layer model due to developed mixing processes in the system.  The mean salinity of the estuary is about 18.2 psu higher than salinity of the adjacent sea which is 18.0 psu (Kovrigina and Nemirovsky 1999).  As there is no significant river input to the estuary and data on the groundwater discharges are not available, they are assumed to be zero.  The annual precipitation is 50x10³ m³ d^-1 and evaporation is 100x10³ m³ d^-1.  The calculated residual flow is 50x10³ m³ d^-1 (V_R).  Mixing exchange is about 4,500x10³ m³ d^-1 (V_X).  Estimated V_X using an alternative equation without relying on the salinity difference but vertical shear diffusion (Yanagi 2000) is about 4,600x10³ m³ d^-1 which agrees well with the V_X estimated through water and salt balance approach.  Residual flow velocity (U) used in the shear diffusion equation was 10⁴ m d^-1 or 0.1 m sec^-1.  Water exchange time for the Donuzlav estuary is estimated to be about a month (31 days).

Budgets of nonconservative materials

There are no data on waste load into the estuary, so the DIN and DIP export from the 6,000 people living near the coast were calculated using LOICZ approach.  It was assumed that because the sewage-disposal systems are old, 40% of the wastewater is discharged to the estuary.  The calculated DIN and DIP fluxes (V_ODIP_O and V_ODIN_O) from the population are 200 mol d^-1  and 1,000 mol d^-1, respectively.

DIP balance

Figure 3 summarizes the dissolved inorganic phosphorus (DIP) budget.  The estuary is a net sink for the dissolved inorganic phosphorus; DDIP = -210 mol d^-1   (-0.004 mmol  m^-2 d^-1).

DIN balance

Figure 4 summarizes the dissolved inorganic nitrogen (DIN) budget.  The estuary is a net source for the dissolved inorganic nitrogen; DDIN = +2.1x10³ mol d^-1 (+0.04 mmol m^-2 d^-1).

Table 1.  Salinity and nutrient concentrations in the Donuzlav estuary system and adjacent sea.

Stoichiometric calculations of aspects of net system metabolism

Stoichiometric estimates can be based on the molar C:N:P ratio of reacting material in the system.  It is assumed that this material is plankton, with a Redfield C:N:P molar ratio of 106:16:1.

 Stoichiometric analysis of the nonconservative fluxes indicated that the estuary is net nitrogen-fixing: (nfix-denit) = +0.1 mmol m^-2 d^-1 and net autotrophic: (p-r) = +0.4 mmol m^-2 d^-1.

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