Estuarine circulation in the Taranto Seas

Abstract

The Taranto basin is a shallow water marine system in the South of Italy characterized by the presence of a lagoon environment together with a semi-enclosed bay connected to the Ionian Sea. This marine system experienced over the last few decades strong biochemical pollution and environmental degradation, and it is considered a hotspot study site for economic, ecological and scientific reasons. The aim of this study was to examine, on an annual temporal scale and with high spatial resolution, the main hydrodynamical processes and transport scales of the system by means of a 3D finite element numerical model application, adopting the most realistic forcing available. The model allowed us to assess the role played by baroclinic terms in the basin circulation, describing its estuarine nature. In particular, the main features of water circulation, salinity and temperature distribution, water renewal time and bottom stress were investigated. Our results allowed us to equate this system dynamic to that of a weakly stratified estuary, identifying the main driving sources of this mechanism. The vertical stratification over the whole year was proved to be stable, leading to a dual circulation flowing out on the surface, mainly through Porta Napoli channel, and inflowing on the bottom mainly through Navigabile channel. This process was responsible also for the renewal time faster on the bottom of the Mar Piccolo basin than the surface. Due to the great importance of the Taranto basin for what concerns sediment pollution, also the effect of currents in terms of bottom stress was investigated, leading to the conclusion that only in the inlets area the values of bottom stress can be high enough to cause erosion.

Analysis of data characterizing tide and current fluxes in coastal basins

“…The principal findings due to the estimations of these indicators can be summed up in the following way. The values assumed by the net flow indicator reveals that a double circulation occurred in the connecting channel, with inflowing flux in the deeper layers and outflowing flux in the most superficial ones, confirm-ing both numerical modelling (De Pascalis et al, 2015) and previous field data analysis (Armenio et al, 2016;De Serio and Mossa, 2016a). The second indicator shows that the transit of both tide crests and tide troughs rapidly involved the whole water depth, inducing a mass transport like a progressive wave, which vertically affected the current with a time delay equal to 2-3 h. The third and fourth indicators both confirm a flood-dominant condition in the basin.…”

“…At deeper layers, from the bottom up to z = −4 m the net flow is inflowing, while in the most superficial layer it is directed outward of the basin. This resulting net flow agrees with a previous numerical modelling (De Pascalis et al, 2015), which represented the annual hydrodynamic simulation of this site in the year 2013. Also, the annual field data by Armenio et al (2016) and De Serio and Mossa (2015, 2016a) are consistent with these findings.…”

“…This trend should be interpreted also considering that in the Navigable channel water temperature and salinity are not uni- form along the water column and that a thermohaline gradient occurs. The annual mean salinity distribution investigated in previous research (Umgiesser, 2007;De Pascalis et al, 2015) showed that freshwater inflows in the Mar Piccolo, from both subaerial watercourses and submarine springs, create an increasing gradient between the Mar Piccolo and the Mar Grande. Moreover, they stated that freshwater stays on the surface layer, while near the bottom salinity is increased by about 1 PSU in most of the basin.…”

“…It is composed of two bays, called I Bay and II Bay. Its total surface is about 21.7 km 2 , its maximum depth is around 15m in the I Bay and 10 m in the II Bay (De Serio and Mossa, 2016b;De Pascalis et al, 2015). It is joined to Mar Grande, an external basin, by means of an artificial channel, i.e.…”

“…Moreover, they stated that freshwater stays on the surface layer, while near the bottom salinity is increased by about 1 PSU in most of the basin. Referring to annual averaged temperature trends, De Pascalis et al (2015) noted that the morphological and bathymetric differences between the sub-systems influence their thermal inertia and create an increasing temperature gradient going from the open sea to the inner part of the basin. Consequently, it seems consistent that during the passage from trough to crest (Fig.…”