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the required deficit to account for would amount to 187,000 t/y. Assuming an aeolian load of
25,000 to 50,000 t/y (10% of the estimated aeolian nitrogen load), plus some 75,000 t/y of
inorganic phosphorus (Béthoux), plus 75,000 t/y in organic particulate and dissolved form
(modifying assumption 2 of Béthoux's table, using P/N (at)=1/16); then the sum would cover the
estimated deficit. Accordingly, the total phosphorus return flow through the strait would amount
to some 150,000 t/y.
Algal blooms along the Almaria-Oran front are common. Therefore, it is not only
conceivable but likely that part of the required nutrient return flow to the Alboran Sea originates
from the Guadalquivir basin, and that nutrient rich coastal waters from that region is mixed with
nutrient poor atlantic water, or sucked along the coast. However, to verify this assumption, more
detailed studies around the strait areas are required. If proven, then programmes of sanitation
of the Mediterranean should also extend to areas outside areas directly discharging to the
Mediterranean. Of course, this reasoning would also apply to the Black Sea-Marmara exchange.
4.6 Concluding remarks
In summary, and regardless of the many uncertainties that make integral estimates
difficult, the load estimates to the Mediterranean made under b) are plausibly in the right order
of magnitude. If so, this would re-dimension some of the cursory figures reported elsewhere.
Vukadin (1992) estimates the river input of nitrogen and phosphorus into the Adriatic Sea alone
to 250,000 t and 82,000 t (N/P = 3). The same figures are cited by Attenborough (1987) in a
popular book, and gives also input values of nitrogen and phosphorus into the Northern arc of
the Western basin between Genoa and Valencia as 340,000 t N and 115,000 t P (N/P = 3). While
the nitrogen input may be in the right order of magnitude, the phosphorus loads appear to be
grossly overestimated, even if one doubles the input estimates under (b) for which there is no
justification to do so. 7
Another perspective resulting from these estimates concerns the question whether the
Mediterranean as a whole is endangered by eutrophication. The answer follows from estimating
the average areal load due to anthropogenic nutrient inputs. Accepting the figures under b),
though neglecting the Atlantic and Black Sea interchanges, the mean areal nitrogen load to the
Mediterranean as a whole would be 1.5 $ 0.5 g/m 2 .year, and that of phosphorus 0.15 $ 0.05
g/m 2 .year.
Comparing these figures to those known from other marine areas (cf. Table 18) show
that the Mediterranean figures are still very low. Accordingly, it is safe to conclude that the main
body of the Mediterranean as a whole is not yet seriously threatened by eutrophication over the
next decades. The problem instead is local and regional, limited largely to specific coastal and
adjacent offshore areas, where it still can be quite serious, as will be substantiated by the review
of the specific eutrophication incidences around the Mediterranean in the following Chapter 5 of
this report.
However, these local and regional problems must in no case be underestimated as to
their potential socio-economic and sanitary impact on tourism, aquaculture, fisheries, and other
water uses (cf. Chpt. 6 and 7). Without the necessary cure and precaution taken, and
7 We suspect that such phosphorus values have been calculated as ortho-phosphate, and
not as P. This is a common error. Appropriate correction would bring down the loads to acceptable
figures.