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202 R. ZACCONE, R. LA FERLA, M. AZZARO, G. CARUSO AND E. CRISAFI

m, proved to be higher in the Eastern Mediterranean, probably because of the winter
convection processes which entrain organic substances in the deep waters (Fig.2).

Integrated data of CO2 production in the Mediterranean Sea (derived from the
equation mg C d-1 m-3 = 2.467 z-0.631) compared with those found in the Pacific,
Indian and Atlantic Oceans are reported in Table 1.

TABLE 1.
Shallow respiratory rates (integrated from 200 to 1200 m), deep respiratory rates (integrated from 1500 to
2500 m) and deep/shallow ratios (expressed as percentage) estimated in the Mediterranean Sea and in the
Indian, Pacific and Atlantic Oceans (from Azzaro, 1997).

Area xRdz: Shallow xRdz: Deep (1500- xRdz: Deep Reference
(200-1200 m) 200 m) xRdz: Shallow

mgC d-1 m-2 mg C d-1 m-2 %

Mediterranean Sea 44.2 20.6 46.6 Azzaro 1999
Indian Ocean 105.1 22.9 21.8 Naqvi at al. 1996
Pacific Ocean 140.4 29.6 21.1 Packard et al. 1998
Atlantic Ocean 50.4 7.5 14.9 “

The Deep zone/Shallow zone ratio, espressed as a percentage, shows just how
important is the oxidation taking place in the deeper layers of the Mediterranean Sea
with respect to others seas.

According to Christensen et al. (1989), who had already determined a shallow
zone/deep zone ratio of 45.4 in the Western Mediterranean, such evidence may be
imputed to the trapped DOC in waters which sinks during the formation of deep and
intermediate waters, in addition to the normal particulate flow which sinks down
from surface waters.

Temporal variations
Temporal variations in respiratory activity were found in the surface layers of the
Otranto Channel during two surveys carried out in February and August 1994, with
higher values in the euphotic layers during the summer (February: 0.41 ± 0.21;
August: 0.74 ± 0.76 µl O2 h-1 l-1). In the deeper layers winter and summer
respiratory rates were comparable (February: 0.39 ± 0.18; August: 0.43 ± 0.16 µl O2
h-1 l-1) (La Ferla et al. in stampa).

Results of microbial enzymatic activity assay obtained in the Northern Adriatic in
different seasonal periods are strongly affected by the peculiar hydrological
conditions of this basin. In fact, the Northern Adriatic water circulation determines
variations on a seasonal scale, with re-mixing in winter and a stratification period in
the other seasons (Artegiani et al., 1997a; 1997b). The intense input of organic matter
from rivers constitutes an anthropic supply, which determines a decreasing level of
microbial protease activity moving from the coast toward off-shore areas. This trend
can be observed in the summer months in the surface and sub-surface layers. Protease
activity also shows a decreasing trend from the surface to greater depths (Fig.3).