Organic matter (OM) supply to marine coastal areas influenced by river runoff is strictly connected to the riverine input with a strong inﬂuence upon the productivity cycles. Temporary sequestration and preservation of OM across the shelf and thus changes in its sedimentological regime could alter OM cycling in coastal marine ecosystem.The identification of the pattern of OM sedimentation in the different zones can thus be useful to evaluate general future scenarios of marine productivity under changing river ﬂow regimes and climatic scenarios.
In fact, the environmental management of coastal areas historically subject to dystrophic crisis caused by the accumulation of riverine-derived organic material, shallow coastal and seafloor morphology and restricted hydrodynamic conditions have particular difficulties to cope not only with the recent and actual phenomena deducted from the measured parameters but also to have inherited past situations that condition the present depositional environment, in particular the benthic sedimentary one. In these environments, the gradual build up over time of organic material still reactive and usable by marine organisms strongly influences the physical-chemical conditions of the sediment and the overlying water. It shows a kind of buffer effect in that it represents the inertia of the system to recover exactly as is, in the opposite direction, the environment subjected to a gradual but progressive disturbance.
The shallow continental shelf of the northwestern Adriatic Sea is a suitable example of this kind of setting. It is one of the most economically important areas of the Mediterranean, in particular because of the signiﬁcance of tourism, intensive ﬁshing and aquaculture. An increasing number of studies were therefore addressed to the problem of organic matter (OM) cycling within different shelf environments as a result of the increasing awareness about its role in hypoxic-anoxic crises and eutrophication as well as in the blooming of mucilaginous aggregates and more generally on water quality. Despite nutrient availability being mainly driven by river discharge and hydrodynamics, it also depends upon the remobilisation of the surface sediments due to physical resuspension and bioturbation as well as to microbially mediated nutrient regeneration.
The results of some projects developed in the NW Adriatic shelf by the CNR of Bologna in collaboration with regional EPA and other Italian institutions will be presented and discussed and among others the areas most prone to accumulation and preservation of OM identified by mean of environmental indicators, the mechanisms of development of bottom anoxia and some of the possible impacts that changes in river regimes and local hydrodynamics could have on the distribution of surface sediment and OM contained and preserved therein. In general, the results showed the importance to reconstruct the depositional dynamics of the seabed, define hotspots accumulation of fine materials and OM by environmental indicators where monitoring efforts can be most efficiently targeted, provide the bases for the development of environmental quality standards to identify potentially DO vulnerable areas in similar shelf settings. Moreover, in order to improve bottom DO concentration of similar areas, a better management of river and canalised waters on land is a way to decrease nutrients and fresh OM input to the shelf. Another improvement of local situations could result from displacing the mussel farms present in the area at different location according to the identified problem.
Keywords: sedimentation rates; organic matter sequestration; riverine input; environmental management; land-sea interaction.