Riverine Discharge Of Nutrients In The Black Sea
Riverine DISCHARGE OF nutrients in the Black Sea
from 1968 to 1998
R.Y. Min’kovska, Y.P. Ilyin, A.N. Demidov
Marine Branch of Ukrainian Hydrometeorological, Sevastopol, Ukraine
Main sources of the nutrients input in the Black Sea surface layer are: river discharge, distributed coastal discharge (rainfall and wastewater runoffs, groundwater), atmospheric and deep-water inputs. In average, 88% of phosphates, 57% of total phosphorus, 82% of silicon and 89% of nitrites are coming into the sea with the river water [1]. Therefore, studies and monitoring of this source of nutrient income are of large importance for the ecological assessment of the Black Sea, especially its biological productivity and water eutrophication.
Annually, by different authors’ estimates for different periods, the Black Sea accepts with the river runoff (in thousands tons): 7-75 of phosphates, 930-1016 of silicates and 2,8-10,7 of nitrites. Some averaged data on concentrations of nitrogen and phosphorus forms in river waters and their discharge into the Azov - Black Sea basin for the 1982-1996 years period by MB UHMI studies are presented in Table 1. Also by our data, in average for last 30 years (1968-1998), annual outcomes from large rivers (Danube, Dniester and Southern Bug) were 30000 tons of phosphates and 9000 tons of nitrites. This time interval of three full solar activity cycles contained about the same numbers of warm and cold years, both high and low river discharge periods (Fig. 1). Total average runoff of Danube, Dnieper, Dniester and South Bug rivers was 270km3 per year. Some visible long-term changes on Fig. 1 are presented, but no significant linear trends for this 30-years time interval were discovered. Below are some preliminary results of long-term tendencies in nutrient discharge of large rivers for the same period.
Total phosphorus (TP) concentration, but not phosphate-ion one should describe content of phosphorus in natural ponds. However, in the Black Sea coastal waters and river deltas, variations of phosphates are highly correlated with TP concentration changes, correlation coefficient range is 0,82-0,88 ± 0,02 by our data. Since determination of phosphates is more precise and their time
series are longer, phosphorus variability is studied by means of data on phosphates for these areas. Nitrogen compounds variability is described here by most enduring and trusty series of observations on nitrites.
The inter-annual variability analysis for nutrient concentrations in Ukrainian large rivers’ mouths has allowed to reveal significant (in terms of Fisher’s criterion) tendencies for the surface layer of water (Table 2). Besides, the increasing of phosphate concentrations is registered within Dnieper-Bug and Dniester limans, as well as at the Danube seashore and in the Northwestern Black Sea waters.
Though some decreasing of river water discharge since 1984 (see Fig. 1), rise of annual phosphates runoff is observed because of their concentration increasing in river discharge (see Table 2, the last column). Any significant tendencies were not found in the nitrites runoff for this period. Results for the Danube River nutrient concentrations and runoff are shown on Figures 2, 3.
Traditionally, the increasing of river runoff of nutrients in the sea is explained by the rise of direct antropogenic loads. But, in our opinion, the main reasons of this, at least for the Dnieper, S. Bug and Dniester rivers, are natural physical, chemical and biological processes in river water reservoirs and deltas. In average, antropogenic loads in Ukraine are still constant and even less because of decreasing of population and agricultural areas (both sawn and stock-raising), as well as increasing of waste water treatment plants number and environment protection measures efficiency, stabilization of water reservoirs functioning.
During the period of water reservoirs laying out, flooding of large areas of soils and vegetation was occur and perimeter of waterside was increased. Therefore, zones of shore erosion and wastewater runoff covered more territory. Not only riverbeds but also less flowing ponds accepted this runoff. All of these processes promote increasing of phosphorus and nitrogen compounds contents in water. Water reservoirs became auspicious environment for the organic matter development and new biocenoses were formed, not presented in rivers earlier. Processes of eutrophication and algal blooms were intensified. Riverbeds silting promoted the secondary contamination of water masses, while increasing of river discharges in 2-3times in a wintertime promoted the water income to the tail waters, limans and sea. Since early 70s, stabilization of water reservoirs functioning and nutrient runoff to the sea are taking place.
In average for the last 10-15 years, sun activity and water temperature increasing and land water runoff decreasing were observed, in comparison with the same preceding time interval. Flowing ability was decreased for water reservoirs, stretches, delta branches and ponds, consequently mean water temperature increased there, especially in the summer and fall seasons. Such conditions promoted the primary production growth and its further destruction and consequent accumulation of nutrients in water reservoirs, tail waters, deltas and limans.
The main input in amount of nutrients discharging into the Northwestern Black Sea is connected to the Danube River (70-95% from the total nutrient runoff of Ukrainian large rivers). It flows through the high-developed industrial and agricultural countries, having the watershed area 517000km2. Not only natural factors mentioned above influence on nutrient contamination from the Danube River, but also antropogenic loads. Their negative effects should be strictly limited by international agreements with account of self-cleaning abilities of waters along the ways of contaminant transport. International system of water quality monitoring and management should be developed in order to minimize damages from river and marine water eutrophication consequences, such as excessive algal bloom, anoxia, fish species loss, etc. [2].
Table 1. Average nutrient concentrations in the Azov – Black Sea basin river deltas and their discharge into the sea for the 1982-1996 period (for Ukrainian and Russian rivers only)
Ingredient
Available limiting concentra-tion for fishing ponds, mg/l
Rivers of the Black Sea
Rivers of the Azov Sea
Interval of mean concen-tration values, mg/l
Maximal concentration, mg/l (river)
Nutrient input, thousands tons per year
Interval of mean concen-tration values, mg/l
Maximal concen-tration, mg/l (river)
Nutrient input, thousands tons per year
Nitrites
0,02
0,006-0,2
1,72
(Dniester)
14,6
0,02-0,05
0,2
(Don)
1,32
Nitrates
9,1
0,08-2,0
6,61
(Dniester)
86,0
0,2-1,0
4,05
(Kuban)
15,4
Ammonia nitrogen
0,39
0,03-1,1
2,52
(Rioni)
45,6
0,08-0,4
2,10
(Kuban)
5,41
Total
nitrogen
–
0,2-3,4
17,8
(Danube)
797
0,4-1,4
–
22,1
Phosphates
3,5
0,01-0,3
1,10
(Danube)
61,9
0,02-0,1
1,08
(Don)
3,16
Total
Phosphorus
–
0,03-0,3
1,28
(S. Bug)
74,0
0,04-0,2
0,4
(Don)
3,71
Table 2. Coefficients of significant linear trends, discovered for annual nutrient concentrations in large river deltas (mkg/m3 per year) and phosphates discharge (tons per year) during the 1968-1998 period
River
Concentration rate, mkg/m3 per year
Discharge of phosphates rate, tons/year
Phosphates
Total
Phosphorus
Nitrites
Silicates
Danube
4,1
-
1,2
-
900
Dnieper
2,7
5,0
2,4
42,8
100
Fig. 1. Annual Wolf’s numbers (W), large rivers discharge (Q, km3) and water temperature in the Danube delta (T, °C).
Fig.2. Danube delta annual water runoff (Q, km3), nutrient (PO4 and NO2) concentrations, mkg.dm-3, and nutrient runoffs (Q*PO4 and Q*NO3), tons.
References
1. Hydrometeorology and hydrochemistry of the USSR seas. Vol IV: The Black Sea. Issue 2. – S. - Peterburg, Hydrometeoizdat, 1992, 220 pp.
2. C. Humborg and C. Kolle. Integrated coastal management from the perspective of nutrient control. – Journal of Coastal Conservation, 1999, 5, pp. 135-144.
Related articles::