Dynamics of a Very Special Mediterranean Coastal Area: the Gulf of Naples

5

Dynamics of a Very Special Mediterranean Coastal Area: The Gulf of Naples

Chapter 6

Dynamics of a Very Special Mediterranean Coastal Area: The Gulf of Naples

Daniela Cianelli1,2, Marco Uttieri1, Berardino Buonocore1, Pierpaolo Falco1, Giovanni Zambardino1, and Enrico Zambianchi1

1 Department of Environmental Sciences, University of Naples “Parthenope”, Centro Direzionale di Napoli Isola C4, 80143 Naples, Italy

2 ISPRA – Institute for Environmental Research and Protection, Via di Casalotti 300, 00166 Rome, Italy

Abstract

The Gulf of Naples (GoN) is an important sub-basin of the Mediterranean Sea, with hydrological features typical of both oligotrophic systems (in its offshore area) and eutrophic coastal zones. In addition, the GoN is subject to severe anthropic impacts (e.g., pollutant discharges, improperly treated sewage, maritime traffic, etc.) which might compromise the water quality and the state of the marine ecosystem. Owing to such high variability and complexity in the hydrodynamic and biogeochemical processes, the GoN represents a natural laboratory to investigate the interactions between physics and biology.

Since the late 1970s, an increasing number of investigations has been carried out to study the circulation and hydrology of the GoN, as well as the annual and interannual dynamics of phyto- and zooplanktonic organisms.

As to ecosystem dynamics studies, the GoN hosts a unique site, the Long Term Ecological Research Station MareChiara of the Stazione Zoologica “Anton Dohrn”, where environmental parameters (T, S, O2, nutrients, Chl, HPLC pigments) and plankton communities (phytoplankton, microzooplankton, mesozooplankton) have been sampled at a weekly frequency since 1995, whereas from 1984 to 1991 sampling used to be biweekly.

With specific reference to the circulation, while early studies were limited by technological hindrances, in very recent years the installation of a network of high frequency coastal radars has allowed a very detailed synoptic description of the surface current field in the GoN.

In this review we will first provide a summary of the knowledge of the physical and biological dynamics of the GoN, and then we will focus on the seasonal features of the surface circulation and plankton dynamics. These results confirm the necessity of an integrated multiplatform monitoring of coastal areas for a proper understanding of marine ecosystems.

Introduction

The Mediterranean Sea is a unique marine ecosystem (Turley, 1999): it is one of the largest semi-enclosed seas of the Earth, though its surface and volume only score respectively the 0.82% and the 0.32% of the world ocean. It is connected to three important basins: the Atlantic Ocean through the Strait of Gibraltar; the Red Sea through the Suez Channel; the Black Sea through the Bosphorus Strait. This determines the intrusion and exchange of water masses with different physical and chemical characteristics, as well as the immigration of potentially invasive species. The biodiversity of the Mediterranean Sea is very high, due to paleogeographic, ecological and historical reasons (Nike Bianchi and Morri, 2000). The Mediterranean Sea thus turns out to be a very sensitive ecosystem, whose management and conservation need adequate studies and strategies.

Among the numerous coastal sub-basins of the Mediterranean, the Gulf of Naples (GoN) is probably one of the most challenging and fascinating. The GoN is a marginal area of the southern Tyrrhenian Sea, in the mid-western Mediterranean region, washing the homonymous city of Naples (Campania, Italy) and the neighbouring areas (Figure 1).

The GoN presents hydrological features typical of both oligotrophic and eutrophic systems. The outer part of the GoN is more directly influenced by Tyrrhenian oligotrophic waters (e.g., Povero et al., 1990), whereas its inner part shows hydrographic and biological properties peculiar of coastal eutrophic systems. A boundary between these two subsystems can be identified; its location exhibits a strong seasonal variability (Carrada et al., 1980 and 1981; Marino et al., 1984). The exchanges between these two subsystems are determined by the circulation patterns of the GoN, which are very complex and depend on a number of forcing factors acting over different spatial and temporal scales, as well as by the local physiography and bottom topography (Moretti et al., 1976-1977; De Maio et al., 1985; Gravili et al., 2001; Grieco et al., 2005; Menna et al., 2007b).

Plankton abundance and composition mirror this very high hydrological complexity: a strong spatial and temporal variability is reported, reflecting the mixture of forcings, water masses and biological processes developing in this area (Ribera d’Alcalà et al., 2004; Zingone et al., 2010). Patterns and processes observed in the GoN are similar to those reported in other Mediterranean sites (Ribera d’Alcalà et al., 2004; Zingone et al., 2010), confirming the typical Mediterranean nature of the GoN and its representativeness of the dynamics of the entire basin.

For all these reasons, the GoN can be used as a dynamical test site for the central Tyrrhenian Sea and, to a larger extent, for the Mediterranean Sea.

Besides its natural peculiarities, the GoN is site of an intense antrophic pressure determining a strong impact on the marine ecosystem (Moretti et al., 1981; GNRAC, 2006). The GoN is among the most densely inhabited Italian areas, and along its 195 Km of coasts approximately 30 ports and more than 300 maritime constructions are located (GNRAC, 2006). Human activities range from urban settlements to industrial areas located on the coast, to intense maritime traffic, resulting in the potential discharge of sewage, industrial pollutants and hydrocarbons which might negatively affect the water quality and the state of the ecosystem. In addition, the eastern part of the GoN receives the land runoff of the Sarno, a very polluted river carrying a heavy load of sediment and suspended matter that can influence the physical, chemical and biological quality of the coastal waters. At the same time, the GoN is also an internationally renowned touristy location, not only for its historical (the ancient Roman ruins of Pompei, just to cite one remarkable example) and environmental attractions, but also for swimming and leisure activities spring through fall. The GoN also hosts four marine protected areas, selected on the basis of environmental parameters as well as historical relevance. As a consequence, the maintenance and improvement of the environmental quality of the GoN is of critical importance not only for the welfare of the entire ecosystem, but also for social and economic reasons.

In the last 30 years the GoN has been systematically investigated and monitored. This scientific effort has allowed the continuous observation of physical and biological parameters for a prolonged time, improving the knowledge of the dynamics of the system and at the same time providing information about changes in the environmental quality and possible ecosystem adaptations or modifications. Through the years, classical in situ sampling techniques to investigate the hydrological and bio-chemical properties of the water body have been integrated with the release of drifters and the deployment of current meters to record speed and direction of the currents. A recent improvement in the monitoring network is represented by the installation of a high frequency (HF) radar system network allowing for the real-time measurement of the surface circulation over the entire basin.

This chapter is intended as a review of the dynamics developing in the GoN. In the next sections we will provide a detailed description of the morphology and topography of the basin, and we will follow with a description of the hydrology and the circulation of the GoN. We will then provide a summary of plankton dynamics in the GoN, stressing the close interplay between physics and biology supporting the evolution of processes typical of the Mediterranean Sea. Conclusive remarks will focus on the relevance of an integrated, continuous monitoring of coastal systems for a detailed knowledge of environmental dynamics.

Morphological Characteristics

The GoN is a wide semi-enclosed embayment of the south-eastern Tyrrhenian Sea (Mediterranean Sea). The basin, SW oriented and located over the continental shelf of the southern Italy, is characterized by an average depth of 170 m and an area of approximately 900 km2 (Carrada et al, 1980).

The GoN is limited by the islands of Procida and Ischia and the Campi Flegrei in the northern part, and by the island of Capri and the Sorrento peninsula in the southern part (Figure 1). The interior waters of the GoN are in direct communication with the Tyrrhenian Sea through two main openings called Bocca Grande and Bocca Piccola.

Figure 1. Bathymetry and orography of the area of the GoN.

The Bocca Grande, located between Ischia and Capri islands, is characterized by the presence of two canyons (Magnaghi and Dohrn) where the maximum depths reach around 800 m. The Bocca Piccola separates Capri from the Sorrento peninsula through a 74 m deep sill (cross section 0.4 km2) which slopes down to the 1000 m isobath and represents the passage to the south bonding the GoN with the Gulf of Salerno (Aiello et al., 2001).

The passages to the north between Procida and the coast (Procida channel) and between Ischia and Procida (Ischia channel) have shallow sills of 12 and 22 m respectively (De Maio and Moretti, 1973), connecting the GoN with the Gulf of Gaeta. In the northern zone of the GoN is also located the natural Bay of Pozzuoli where the maximum depth is around 100 m. Two other sub-basins can be identified in the GoN: the Bay of Naples is the coastal area washing the city of Naples, in the NE sector of the basin; the Gulf of Castellammare is located in the SE part of the GoN, fronting Castellammare di Stabia and the neighbouring areas and receiving the freshwater input from the Sarno river.

A relevant topographic feature of the GoN is the presence of the continental shelf, the maximum depth of which ranges between 100 and 180 m. The width of the continental shelf varies from 20 km in the central part of the GoN to some 2.5 km in proximity of Capri and Procida.

The GoN is characterized by active tectonic and volcanic phenomena. A fault line divides the basin into two sectors (Bruno et al., 2003): the western sector shows several volcanic piles, while the eastern one presents alluvial features. The Magnaghi and Dohrn canyons engrave the continental slope and are located in the western and eastern sector respectively (Aiello et al., 2001). Both canyons are NE-SW oriented: the Magnaghi canyon develops in the north-western volcanic zone between Capo Miseno and Ischia, while the Dohrn canyon (depth greater than 150 m, length around 25 km and width around 2 km) extends across the Bocca Grande where it branches off towards the coast. The northern branch of the Dohrn canyon develops over the continental shelf originating the Ammontatura channel, while the southern one extends towards Procida coastline (Aiello et al., 2001). Both canyons control the vertical fluxes acting as a conduit for transport of sediment from the shelf to the slope.

The study area is also characterised by peculiar orographic aspects influencing wind and sea dynamics. In the proximity of the NE coast of the GoN is located the Vesuvius volcano (elevation: 1.281 m), and the area around the city of Naples is punctuated by numerous hills (Posillipo, Vomero, Camaldoli, Capodimonte, Pizzofalcone) with altitudes reaching values greater than 150 m. In the SE-S part of the GoN, in the area of the Gulf of Castellammare and Sorrento peninsula, the Lattari Mountains are also present (Mount Faito: 1.131 m).

The morphology of the coasts varies from N to S. In the northern part of the GoN sandy coasts smoothly degrade over the shelf, while in the Sorrento Peninsula high calcareous cliffs rapidly decline at depths greater than 80 m.

Hydrology

The first physical and hydrological data gathered in the GoN date back to 1913 (Wendicke, 1916), when surveys were conducted during the summer months in 21 stations in the basin and adjacent waters. It then took almost fifty years before a complete set of records could be collected. During the 1957-1958 International Geophysics Year monthly data of numerous parameters (T, S, O2, pH and total phosphorous) were measured at 6 stations, including measurements of biochemical properties of the water (Hapgood, 1960). Soon after, current records began to be collected. De Maio (1959) measured subsurface currents using the parachute method. A few years later, Krauss and Düing (1963) obtained the first vertical current profiles collecting data at 28 stations, while Düing (1965) recorded currents in the Ischia channel and in the Bocca Piccola.

During 1966-67 current measurements were performed by the Institute of Meteorology and Oceanography of the Istituto Universitario Navale of Naples (presently Department of Environmental Sciences of the “Parthenope” University) using drifters released around the coastal zones of the GoN and Ischia (De Maio and Moretti, 1973). Current data were also collected inside the basin in 1973.

In order to collect more hydrological and current data, from 1977 to 1981 a monitoring program was established by the above mentioned research institution (De Maio et al., 1978-1979a, 1978-1979b, 1980-1981, 1983). Marine currents were measured at 11 fixed sites along the coast of the GoN, and hydrological surveys were carried out (De Maio et al., 1985). These first studies have provided for a long period of time the only information on the oceanography of the GoN. More recent modelling studies (Gravili et al.,2001; Grieco et al., 2005) analysed the barotropic dynamical features and the dispersion of passive/reactive tracers in the GoN respectively.