CHAPTER 54
Seasonal Influence of the Front Area on Sediment Deposition
in the Northwestern Adriatic Sea
F. Alvisi, M. Frignani, and M. Ravaioli
ABSTRACT
In order to improve our knowledge of short time-scale variations of seasonal particle fluxes into
sediments,28 short sediment cores were collected along six transects in two areas influenced
by the frontal system formed by the Po River waters moving into the Adriatic Sea. Radioisotopes
234Th and 137(S were measured in surficial sediment sections and used as tools to evaluate the
rates of sediment deposition, accumulation and mixing. Sedimentological and mineralogical
parameters were also analysed.
The 137(S distribution points out that the area south of the Po Delta, characterised by muddy
sediment scarcely colonised by macrofauna, is directly influenced by the input of fine material
delivered by the Po River. Here the surficial 234Th activities are lower in winter than in summer.
This suggests a front influence that in the warm season couples a higher concentration of suspended matter with an enhanced capacity of interaction with particle-reactive species. The
lithology of the area in front of the Marche Region is otherwise characterised by an alternance
of silt and day. Together with the presence of thanatocoenoses, this pattern reflects pulsed sedimentation with colonisation stages and sudden inputs of sediment that bury the benthic fauna.
High values of 234Th activities were found at the stations close to the front, where the amount of
particulate material is high. Behind the front line, thorium activities are lower, due to both the
shallow environment and the kind of particles. The maximum values of 23 4 Th activities, characteristic of the two offshore stations, reflect the high depth of the water column, the enhanced
residence time of particles and the very fine grain size of the sedimentary material.
Introduction
,Many key processes in the Adriatic system are
influenced by the seasonal variation of the density structures along the water column. This is, in
fact, completely mixed in winter and strongly
stratified in summer (Artegiani et al.1997a). This
cycle determines the dynamics of biological and
sedimentological processes influencing the vertical and horizontal transport of particles within
the basin (Frascari et al. 1988).
To improve our knowledge of the causes of
phenomena such as algal blooms and mucilage
production in the Northwestern Adriatic Sea, it is
very useful to study the biogeochemical cycles of
nutrients at the frontal system. This is formed by
the contact between the continental waters and
those coming from the North Adriatic, and
delimits a coastal area, directly influenced by the
anthropogenic input, physically separated from
the offshore oligotrophic area. The intensity of
the front is variable as a function of the strength
of the thermohaline gradients between the two
water masses. Therefore, the density gradient
acts as a barrier that influences the dispersal of
the suspended matter and dissolved materials
delivered by the Po River and by the minor rivers
discharging along the Romagna coast.
This work has been carried out within the
framework of the Italian Project Prisma 2, subproject "Biogeochemical cycles", which had the
general aim to evaluate the fluxes of both dissolved and particulate matter across the front
area, with respect to its seasonal location and
intensity. In particular, our purpose was to contribute to the quantitative knowledge of the bioIstituto di Geologia Marina, CNR, Via P. Gobetti 101,40129 Bologna, Italy
F.M. Faranda, 1. Guglielmo, G. Spezie (eds)
Mediterranean Ecosystems: Structures and Processes
@ Springer-Verlag Italia. 2001
Seasonal Influence of the Front Area on Sediment Deposition
in the Northwestern Adriatic Sea
F. Alvisi, M. Frignani, and M. Ravaioli
ABSTRACT
In order to improve our knowledge of short time-scale variations of seasonal particle fluxes into
sediments,28 short sediment cores were collected along six transects in two areas influenced
by the frontal system formed by the Po River waters moving into the Adriatic Sea. Radioisotopes
234Th and 137(S were measured in surficial sediment sections and used as tools to evaluate the
rates of sediment deposition, accumulation and mixing. Sedimentological and mineralogical
parameters were also analysed.
The 137(S distribution points out that the area south of the Po Delta, characterised by muddy
sediment scarcely colonised by macrofauna, is directly influenced by the input of fine material
delivered by the Po River. Here the surficial 234Th activities are lower in winter than in summer.
This suggests a front influence that in the warm season couples a higher concentration of suspended matter with an enhanced capacity of interaction with particle-reactive species. The
lithology of the area in front of the Marche Region is otherwise characterised by an alternance
of silt and day. Together with the presence of thanatocoenoses, this pattern reflects pulsed sedimentation with colonisation stages and sudden inputs of sediment that bury the benthic fauna.
High values of 234Th activities were found at the stations close to the front, where the amount of
particulate material is high. Behind the front line, thorium activities are lower, due to both the
shallow environment and the kind of particles. The maximum values of 23 4 Th activities, characteristic of the two offshore stations, reflect the high depth of the water column, the enhanced
residence time of particles and the very fine grain size of the sedimentary material.
Introduction
,Many key processes in the Adriatic system are
influenced by the seasonal variation of the density structures along the water column. This is, in
fact, completely mixed in winter and strongly
stratified in summer (Artegiani et al.1997a). This
cycle determines the dynamics of biological and
sedimentological processes influencing the vertical and horizontal transport of particles within
the basin (Frascari et al. 1988).
To improve our knowledge of the causes of
phenomena such as algal blooms and mucilage
production in the Northwestern Adriatic Sea, it is
very useful to study the biogeochemical cycles of
nutrients at the frontal system. This is formed by
the contact between the continental waters and
those coming from the North Adriatic, and
delimits a coastal area, directly influenced by the
anthropogenic input, physically separated from
the offshore oligotrophic area. The intensity of
the front is variable as a function of the strength
of the thermohaline gradients between the two
water masses. Therefore, the density gradient
acts as a barrier that influences the dispersal of
the suspended matter and dissolved materials
delivered by the Po River and by the minor rivers
discharging along the Romagna coast.
This work has been carried out within the
framework of the Italian Project Prisma 2, subproject "Biogeochemical cycles", which had the
general aim to evaluate the fluxes of both dissolved and particulate matter across the front
area, with respect to its seasonal location and
intensity. In particular, our purpose was to contribute to the quantitative knowledge of the bioIstituto di Geologia Marina, CNR, Via P. Gobetti 101,40129 Bologna, Italy
F.M. Faranda, 1. Guglielmo, G. Spezie (eds)
Mediterranean Ecosystems: Structures and Processes
@ Springer-Verlag Italia. 2001