The results on the vertical fluxes of the suspended matter in different seasonal conditions are here presented and discussed.
In the framework of MAST project the program "Hydrodynamics and geochemical fluxes in the Strait of Otranto", some sediment traps and current-meters were moored in two sites: on the Italian shelf (station 301) and on the shelf slope (station 304), with the aim to estimate the vertical fluxes of suspended matter in the Strait area.
In addition to the sediment trap samples, water samples were
collected during six seasonal cruises, on the station grid reported
in the map.
The results on the vertical
fluxes of the suspended matter in different seasonal conditions are
here presented and discussed.
The shelf station (st. 301) was located at 39° 50.00 N, 18° 30.00 E, with a bottom depth of 124 m. A sediment trap (Technicap P.P.S. 4/3, with 0.05 m2 collecting area) was moored at a depth of 118 m, from February 1994 to May 1995. The sampling interval was 7 days (after August 95 the interval was 20 days).
The slope station (st. 304) was located at 39° 50.00 N,18° 48.29 E, with a bottom depth of 610 m. A sediment trap (Technicap P.P.S. 5/2, with 1.0 m2 collecting area) was moored at a depth of 603 m, from May 1994 to May 1995. The sampling interval was 7 days.
The total mass flux in the shelf trap (st. 301) ranges between 844 and 21729 mg m-2 day-1, with highest values in winter. The minimum was measured in summer (June 1994). Carbonates flux was on average 1435 mg m-2 day-1, with maxima in late autumn-early winter 95. The percentages of organic Carbon and Nitrogen show a trend opposite to the total flux and represent on average respectively 1.8 % and 0.3 % of the total flux, with highest values in winter 95. The organic matter flux ranges between 109 and 2298 mg m-2 day-1.
During winter the main fraction of the total flux is represented by the inorganic constituents (carbonates maxima), presumably connected with the active dynamic processes occurring in the coastal area. The organic fraction increases in summer, when biological processes become more important, as source of particulate matter.
The total mass flux values measured in the slope station (st. 304) range from 25 to 1990 mg m-2 day-1, about 10 times lower than in the shelf area. Maxima were observed in spring 94. The carbonate flux shows a mean value of 158 mg m-2 day-1.
Organic Carbon and Nitrogen trends are rather similar to the total flux. The highest percentage occurs in late summer-early autumn (approximately 4.51 and 0.52 %, respectively). The organic matter presents maxima in spring 94 (216 mg m-2 day-1).
Flux trends are in good agreement with the vertical distribution of
suspended matter observed during surveys in continuous transmittance
profiles in different seasonal conditions.
In winter, at surface, the general dynamics is highly active, mostly
in February 94 
, as shown by the varying topography of
transmittance isolines. The presence of a cyclonic circulation pattern
reinforces the outflow of the Adriatic Surface Water in the western side of
the Strait and slightly the inflow of the Ionian Surface Water in the
eastern one.
In spring (May 94)
and in summer (August 94)
in the surface layer no particular flow direction is
evident, probably because of the presence of a large anticyclonic gyre
in the Strait area.
In autumn (November 94)
and in winter (February 95)
the situation is rather
similar to that observed in spring (May 94).
On the contrary the behaviour of the Adriatic Bottom Water is more or
less similar in all seasons, with an increased dynamics during winter.
,
on the italian coastal shelf, at surface, with values up to 70 cm/s.
At the bottom we observed a progressive increase of the total flux, concurrently with the current speed increase. This increased flux is to attribute to the higher suspended matter concentration in the water column, due partly to the river solid run-off and partly to local resuspension phaenomena, possibly occurring in case of so high current speed.
The trend of the flux in station 304
(not directly interested by the
coastal dynamics) only partly shows the influence of the rivers discharge,
being more affected by the local biological processes occurring in the
upper water column.
In conclusion, the fluxes measured in the coastal shelf seem to be related both to the vertical transfer of suspended matter bond to the biological processes (maximum observed in April) and to lateral advective transport, even associated to the riverine run-off. On the other hand, in the open sea area, the observed fluxes seem to be connected mainly to the vertical transfer of particulate matter biologically produced, and only in a minor fraction to horizontal transport pathways.
During winter the main fraction of the total flux is represented by the inorganic constituents (carbonates maxima), related in the shelf area to the active dynamic processes. In the slope area the inorganic fraction is still dominant because of the biological activity is reduced. In both areas the organic fraction increases in summer, when biological processes become relevant as source of particulate matter.
The fluxes measured in the coastal shelf seem to be related both to the vertical transfer of suspended matter bond to the biological processes (maximum observed in April) and to lateral advective transport, even associated to the riverine run-off. On the other hand, in the open sea, the observed fluxes seem to be connected mainly to the vertical transfer of particulate matter biologically produced, and only in a minor fraction to horizontal transport pathways.