MAP Technical Reports Series No. 106 UNEP

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Transfer through, and exchange of pollutants between the Black Sea via the Marmara
and the Aegean Sea have been studied by Aubert et al. (1990). The authors note that dissolved
nitrogen and phosphorus increase from the Bosporus to the Dardanelles, but give no figures.
The N and P production from Istanbul alone amounts to at least some 14,000 and 2,500 t/year,
respectively. Of these one can assume that at least 50% is retained in the Marmara Sea.
Accordingly, the load increment from this source to the Mediterranean as a whole remains
modest (about 1-2%), but the relative load to the Northern Aegean Sea will be much higher.
The Atlantic exchange, instead, is a major factor determining the nutrient balance of the
Mediterranean. Béthoux (1979; 1986), evaluating available information, derives values for the
nitrogen and phosphorus exchange through the Strait (Table 17). Regarding deep water outflow
totals he assumes that the bulk of nitrogen and phosphorus is inorganic using average
concentrations of 6 mmol/m 3 for N(NO 3) and 0.28 mmol/m 3 for inorganic phosphorus multiplied
by the respective water load. Regarding inflow estimates for total nitrogen from the Atlantic to
the Mediterranean the author considers three assumptions for DON in excess of measured
inorganic nitrogen, and theorises from the respectively estimated balance deficits about the
possible magnitude of nitrogen fixation by cyanobacteria (Trichodesmium), macrophytes
(Posidonium), and their epiphytes growing in the Mediterranean. Inorganic phosphorus load, on
the other hand, was simply derived as balance deficit using the UNEP river load data. The
respective estimations are summarised in Table 18.
Table 17
Nitrogen Budget for the Mediterrranean
calculated from J.P. Béthoux (1979 & 1986)
Assumption --> 1 2 3 Phosphorus
t/year
t/year
Atlantic Inflow:
- inorganic
- organic
Total
742,000
0
742,000
742,000
742,000
1,484,000
Medit. Outflow: 4,242,000 4,242,000 4,242,000
N-deficit to satisfy 3,500,000 2,758,000 2,016,000
Terrestr. Discharges:
- inorganic (UNEP)
- organic estimated
Total
1,036,000
546,000
1,582,000
1,036,000
546,000
1,582,000
Rainfall: 182,000 182,000 182,000
742,000
1,484,000
2,226,000 77,500*
1,036,000
546,000
1,582,000 359,600
Estimated N2-fixation: 1,736,000 994,000 252,000 *
* Balance values
Béthoux's considerations are interesting but raise some serious questions. Regarding
nitrogen, figures appear to be in tune with our estimates about river load and aeolian depositions,
while the phosphorus budget is hardly sustainable by our figures. Our load