season circulation

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In: Mediterranean Ecosystems: Dynamics, Management & Conservation ISBN 978-1-61209-146-4
Editor: Gina S. Williams
© 2011 Nova Science Publishers, Inc.
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
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
1
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.
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Daniela Cianelli, Marco Uttieri, Berardino Buonocore, et al.
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,
Dynamics of a Very Special Mediterranean Coastal Area: The Gulf of Naples
3
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.
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Daniela Cianelli, Marco Uttieri, Berardino Buonocore, et al.
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
Dynamics of a Very Special Mediterranean Coastal Area: The Gulf of Naples
5
(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., 19781979a, 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.
All these studies highlighted the spatio-temporal complexity of the dynamics in this
coastal area. With specific reference to the hydrological features, the GoN is characterised by
the presence of two main water masses typical of the southern Tyrrhenian Sea (De Maio et
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Daniela Cianelli, Marco Uttieri, Berardino Buonocore, et al.
al., 1978-1979a and 1978-1979b): the Modified Atlantic Water (MAW) and the Levantine
Intermediate Water (LIW). These two waters are representative of the surface and
intermediate branches of the so-called Mediterranean conveyor belt (Malanotte-Rizzoli, 2001;
Robinson et al., 2001): water of Atlantic origin (AW, i.e. Atlantic Water) enters at the surface
through the Strait of Gibraltar and circulates eastwards, replenishing the upper layer of the
entire Mediterranean, mixing with ambient waters and thus modifying its original
characteristics (hence the definition of MAW). The LIW forms in the Levantine basin,
typically in the Rhodes gyre, where intense winds cause strong evaporation whose effect is a
large increase of surface salinity (higher than 39.0 psu, see e.g. Lascaratos, 1993). The
consequent increase of density makes this water sink and settle at intermediate depths; the
LIW flows westwards between 200 and 500 m, circulating in the eastern sub-basin and
entering the western Mediterranean across the sills of the Straits of Sicily (Sparnocchia et al.,
1999).
The MAW is located in the GoN at depths around 50-100 m and is characterized by a
salinity value of 37.5 psu and a seasonally variable temperature value. During winter, due to
the intense mixing of the water column the MAW shows a constant temperature around 14°C.
The LIW enters the lower layers of the Tyrrhenian Sea (400-500 m) through the Straits
of Sicily. It is found in the GoN at the deepest measure sites close to the Bocca Grande and is
located below 200 m during summer and below 300 m during winter. In the GoN the typical
hydrological values of the LIW are: T = 14.2°C, S = 38.65 psu (De Maio et al., 1978-1979a
and 1978-1979b).
Depending on the season, other water masses can be present inside the GoN (Carrada et
al., 1980). The Tyrrhenian Sea presents numerous water masses, deriving from the
modifications of waters entering from adjacent areas (Hopkins, 1988; Povero et al., 1990).
The winter mixing promotes the formation of the Tyrrhenian Intermediate Water (TIW),
which is found in the homogeneous water column down to  150 m and is characterized by
temperature value around 14°C and salinity value around 38.1 psu. Due to the summer
warming and freshening the TIW rises in the water column above 75 m where it becomes
Tyrrhenian Surface Water (TSW), its typical temperature and salinity values being 25.0°C
and 38.3 psu respectively (Hopkins et al., 1994). Another water similar to the TSW has been
identified in the GoN: the Coastal Surface Water (CSW). Its hydrological features are
strongly affected by the river discharges as well as by the urban and industrial sewages, thus
resulting in a water mass fresher and warmer than the TSW.
The hydrology in the GoN presents a seasonal pattern characterized by the summer
stratification of the water column determining the formation of a surface mixed layer 30-40 m
thick; by contrast, the intense winter mixing involves the entire water column which is
homogeneous down to 150 m (Carrada et al., 1980).
Circulation
Observation Methods
Between 1970s and 1980s, the hydrological and dynamical features of the GoN have
been investigated on the occasion of a number of oceanographic cruises. In particular, the
dynamics were studied using drifters, whose trajectories provided details on the
Dynamics of a Very Special Mediterranean Coastal Area: The Gulf of Naples
7
characteristics of the motion, or moored current-meters, recording direction and speed of the
current at a fixed depth.
Since then, no regular sampling programme has been run to enhance the knowledge of
the basin dynamics. In 2004 a network of HF coastal radars has been installed in the GoN, a
system permitting the real-time, synoptic monitoring of the surface current field at the basin
scale. This system is operated by the Department of Environmental Sciences of the Università
degli Studi di Napoli “Parthenope” on behalf of the Centre for the Analysis and Monitoring of
Environmental Risk (AMRA Scarl). As in conventional radars, an HF radar transmits an
electromagnetic wave towards a target and measures the time taken by the reflected echo to
get back to the antenna. A coastal radar system emits electromagnetic waves in the 3-30 MHz
band, and uses the gravity waves propagating over the sea surface as target. When a so-called
Bragg scattering coherent resonance occurs (Crombie, 1955), the backscattered signal shows
a clear peak in the signal spectrum (Barrick et al., 1977). If a surface current field is present
beneath the gravity waves, the peaks in the backscattered signal will be shifted owing to the
Doppler effect (Paduan and Rosenfeld, 1996). As each HF radar antenna only measures the
radial component of surface velocity, at least two transmitting-receiving antennas are required
(Barrick and Lipa, 1986).
The system installed in the GoN is a SeaSonde type manufactured by CODAR Ocean
Sensors (Mountain View, California, USA). It works in the 25 MHz band, measuring surface
currents relative to the first 1 m of the water column. The temporal resolution of the system is
1 h, while the range is approximately 35 Km from the coast. The original network installed in
2004 comprised two remote stations (in Portici and in Massa Lubrense); in this configuration
the spatial resolution was 1250 m. In 2008 a third antenna has been installed in
Castellammare di Stabia; this implementation improved both the spatial coverage and
resolution (1000 m).
The observations accumulated over the years have also received a strong feedback by
modelling results (Gravili et al., 2001; Grieco et al., 2005) which have greatly improved the
understanding of the dynamics governing the circulation of the basin.
Observed Circulation
The circulation of the GoN is the result of the complex interaction between several
factors, acting over different spatial and temporal scales. The forcings acting over the basin
can be differentiated as remote and local (Gravili et al., 2001). The most important remote
forcing is represented by the southern Tyrrhenian Sea, whose circulation can drive the motion
of the water mass inside the GoN (Pierini and Simioli, 1998; Gravili et al., 2001). Among the
local factors, the main one is certainly wind, whose stress strongly affects the surface
circulation (Moretti et al., 1976-1977; De Maio et al., 1985; Menna et al., 2007a and 2007b).
Menna (2007) performed an interannual (2002-2006) investigation highlighting seasonally
dependent wind regimes. In winter, the dominant winds blow from NNE-NE, with speeds up
to 8-10 m/s; occasional gusts from SW can reach higher velocities (> 10 m/s), due to the
passage of depressionary systems over the basin, with rapid decreases of the air pressures and
the setting up of intense winds. In spring, summer and fall an alternation between NE and SW
winds is observed. During this period of the year, owing to the relaxation of larger scale
forcings the breeze regime becomes dominant (Perusini et al., 1992), with a daily alternation
of sea and land breeze wind. The most intense winds are always associated with SW wind (810 m/s or higher).
8
Daniela Cianelli, Marco Uttieri, Berardino Buonocore, et al.
The surface circulation of the GoN is additionally strongly influenced by the interaction
between the wind and the local orography: the Vesuvius and the hills surrounding the city of
Naples can play a relevant role in channelling NE winds (Moretti et al., 1976-1977; De Maio
et al., 1985; Menna et al., 2007b). The deeper circulation is instead more directly influenced
by the interaction between the currents along the water column and the complex bottom
topography of the basin (Gravili et al., 2001; Grieco et al., 2005).
The surface current fields recorded in the GoN often show intricate patterns, with the
concurrent presence of cyclonic and anticyclonic vortices, jet streams, areas of convergence
and divergence, and other oceanic structures. Currents can be divided into two classes,
depending on their period of evolution (De Maio et al., 1985). Motions with period ≤ 24 h
present a marked inertial component offshore and a breeze induced shift close to the coast.
Motions evolving over periods > 24 h are less variable, and even when very weak they are
responsible for long range transport (De Maio et al., 1985).
As in other Mediterranean sites (e.g., Millot and Crepon, 1981), also in the GoN tidal
oscillations have small amplitudes so that transient atmospheric forcings evolving over time
scales comparable to the inertial periods can become drivers of marine dynamics. Date
recorded by moored instruments showed a peak around the local inertial frequency and the
occurrence of wave oscillations with periods of 3-5 days that could be observed only over
periods longer than 20 days (De Maio et al., 1980-1981). Smaller peaks from diurnal and
semidiurnal tidal components have been observed as well (De Maio et al., 1980-1981). A
subsequent reanalysis of the same data set highlighted a slight shift of the inertial peaks,
indicative of the presence of so called “near-inertial waves” (Moretti et al., 1985). Inertial
phenomena are more evident in the warmer period of the year, when the higher temperatures
favour the evolution of a stable thermocline (Moretti et al., 1985; Menna et al., 2008). During
winter, when storms are more frequent and energetic, inertial events are by contrast less
recurrent (Moretti et al., 1985; Menna et al., 2008). Inertial oscillations are more evident in
the outer part of the GoN than close to the coast (Moretti et al., 1985). Roselli et al. (2007)
observed that enhanced inertial oscillations might contribute to the resuspension of particles
in the water column and enhance their sedimentation.
Despite the difficulty in describing and predicting the dynamics of the surface currents,
five main circulation schemes can be depicted. Three of them are directly influenced by the
wind regime acting over the basin, whereas the other two are driven by the southern
Tyrrhenian current. Wind induced surface currents are extremely important in the dynamics
of the basin, as they can reach intensities up to 10 times greater than the mean circulation
(Moretti et al., 1976-1977).
In presence of NNE-NE winds, owing to the sheltering effect of the Vesuvius the wind
stress over the sea surface determines the convergence of water along the coast of Naples and
the subsequent creation of a jet current directed offshore (Moretti et al., 1976-1977; De Maio
et al., 1983 and 1985; Gravili et al., 2001; Grieco et al., 2005) (Figure 2). This condition
favours the renewal of the coastal waters and the exchanges between the inner and the outer
sectors of the GoN (Grieco et al., 2005; Menna et al., 2007b). The position of the jet is not
fixed in space, but is rather subject to latitudinal shifts owing to the different degrees by
which the Vesuvius and the hills surrounding Naples deviate NNE-NE winds (Menna et al.,
2007b).
Dynamics of a Very Special Mediterranean Coastal Area: The Gulf of Naples
9
(a)
(b)
Figure 2. a) Schematic circulation induced in the GoN by a NE wind (from Moretti et al., 1976-1977;
reprinted with permission); b) surface circulation in the GoN induced by a NE wind, detected by HF
radar measurements. An offshore jet is formed, favouring the renewal of coastal waters.
When the wind blows from SW, the circulation pattern is characterised by the presence of
cyclonic and anticyclonic structures inside the GoN (Gravili et al., 2001; Grieco et al., 2005;
Menna et al., 2007b) (Figure 3). At basin scale, currents are mostly directed towards the inner
part, condition determining the piling up of water along the littoral (De Maio et al., 1985). In
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Daniela Cianelli, Marco Uttieri, Berardino Buonocore, et al.
such circumstance, the renewal of coastal waters is inhibited and estimated residence times
are very high (Menna et al., 2007b).
Figure 3. Surface circulation in the GoN induced by a SW wind, detected by HF radar measurements. A
net flux towards the inner part of the GoN accumulates water along the coasts.
During the breeze regime, the wind rotates clockwise during the 24 h due to the
alternation of sea and land breezes (Figure 4). The GoN responds with a coincident clockwise
rotation of the surface current field (Menna, 2007): under the effect of the sea breeze, surface
currents flow from W to E; viceversa, in presence of land breeze surface currents move from
E to W. In between these two extremes, the current field turns clockwise by approximately
90° in six hours so as to adapt to the changing wind conditions.
Dynamics of a Very Special Mediterranean Coastal Area: The Gulf of Naples
11
(a)
(b)
Figure 4. Surface circulation in the GoN induced by the daily alternation of breeze winds, detected by
HF radar measurements. The two panels show the evolution of the circulation field: at 06:00 GMT (a)
the flow is mainly directed offshore; at 18:00 GMT (b) the flow is reversed.
When the dynamics of the GoN are dependent upon the offshore Tyrrhenian currents, two
scenarios can be outlined (De Maio et al., 1983). When the Tyrrhenian current is directed
north-westwards, an intense flux develops almost parallel to the Bocca Grande, entering the
GoN from the Bocca Piccola and moving towards Ischia. The current enters the GoN creating
a basin-scale cyclonic gyre, whereas in the Gulf of Castellammare an anticyclonic vortex is
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Daniela Cianelli, Marco Uttieri, Berardino Buonocore, et al.
formed (Figure 5). With this circulation structure, freshwaters coming from the Sarno river
and from the city of Naples are transported offshore as the result of the combination of
current structure and variability, as well as bottom topography (De Maio et al., 1983).
(a)
(b)
Figure 5. a) Schematic circulation in the GoN as driven by a north-westward Tyrrhenian current,
showing a cyclonic vortex at basin scale and an anticylonic vortex in the Gulf of Castellammare and ;
b) associated salinity distribution of the waters in the GoN (from De Maio et al., 1983; reprinted with
permission).
On the contrary, when the Tyrrhenian current moves south-eastward, the outer part of the
GoN shows a cyclonic gyre, while the Bay of Naples and the Gulf of Castellammare present
anticlonic structures (De Maio et al., 1983) (Figure 6). In this case, the inner part of the GoN
is separated from the offshore sector, condition preventing the renovation of the coastal
waters and favouring stagnation conditions. Less saline waters are thus concentrated along the
Dynamics of a Very Special Mediterranean Coastal Area: The Gulf of Naples
13
littoral area, as the result of the convergence of the currents towards the coast (De Maio et al.,
1983).
(a)
(b)
Figure 6. a) Schematic circulation in the GoN as driven by a south-eastern Tyrrhenian current,
indicating a separation between the inner part of the GoN and the offshore area; b) associated salinity
distribution of the waters in the GoN (from De Maio et al., 1983; reprinted with permission).
TIDES AND SEICHES
In the Mediterranean basin, tidal oscillations are typically quite small with amplitudes of
the order of a few tens of centimetres. The only relevant exceptions refer to the Strait of
Gibraltar, the Straits of Sicily, the Gulf of Gabes and the North Adriatic Sea, where higher
values can be observed (Tsimplis et al., 1995).
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Daniela Cianelli, Marco Uttieri, Berardino Buonocore, et al.
Even in the Tyrrhenian Sea, tides are characterised by reduced amplitudes, and for this
reason only few studies have investigated this issue. Despite this, the analysis of sea level
trends in the GoN has been observed with attention through the years for its geophysical
implications, as the area of the GoN has an intense active geologic and volcanic activity
(Vesuvius, Campi Flegrei, Ischia). The first observations have been recorded in 1890 in Ischia
(Grablovitz, 1911) thanks to the installation of an observatory after the Casamicciola
earthquake in 1883. Systematic observations in the city of Naples, carried out by
governmental authorities, started in 1896 and have continued to date, though with some
discontinuities (Lama and Corsini, 2000 and 2003). Since 1970 a tide gauge network operated
by the INGV-Osservatorio Vesuviano is active in the GoN, aimed at determining soil
deformations (Tammaro et al., 2007).
The analysis of tidal data reveals aspects of great interest for the comprehension of the
dynamics of coastal basins. The harmonic constants of the Tyrrhenian Sea have been first
determined by von Sterneck (1915 and 1922) and by Vercelli (1925). The study by Mosetti
and Purga (1985) then showed constant values of amplitude and phase for all the Tyrrhenian
stations investigated, highlighting behaviour of the tides pulsing as a piston (“come un unico
pistone”; Mosetti and Purga, 1985).
Values and pulsing behaviour are confirmed also inside the GoN by the determination of
the harmonic constants from two tide gauges, installed in Piano di Sorrento (2000-2005) and
Ischia (2002-present) and operated by the Department of Environmental Sciences of the
Università degli Studi di Napoli “Parthenope”. These stations are respectively located in the
eastern and western part of the GoN. Amplitude and phase values of the main components are
reported in Table 1 and are in full agreement with the determinations obtained by other
authors for gauges installed in Naples (von Sterneck, 1922; Polli, 1956; Purga et al., 1979).
Table 1. Amplitude (in cm) and phase (in degrees) of the main tidal components for
Piano di Sorrento and Ischia, compared with the determinations for a site installed in
Naples by von Sterneck (1922) (S), Polli (1956) (P) and Purga et al. (1979) (M)
M2
S2
K1
N2
K2
Piano di
Sorrento
11.8/260°
4.4/282°
2.7/212°
2.4/247°
1.2/274°
Ischia
11.6/260°
4.3/282°
2.8/212°
2.3/247°
1.1/278°
Napoli S
11.0/268°
4.0/283°
2.6/217°
1.9/246°
1.0/298°
Napoli P
11.2/268°
4.3/287°
2.8/217°
2.3/253°
1.2/288°
Napoli M
11.1/263°
4.6/283°
2.8/216°
2.3/252°
1.2/274°
The tides in the GoN are mainly semidiurnal (form factor: F=0.24). The vertical
excursions reach maximum values of around 40 cm during spring tide, while during neap tide
oscillations are weaker. It is worth noticing, however, that in terms of sea level oscillations
Dynamics of a Very Special Mediterranean Coastal Area: The Gulf of Naples
15
(thus also considering eustatic and meteorological factors, as well as the effect of the inverted
barometer) the overall variation can reach values significantly higher, of the order of 1 m.
A special attention must be given to the analysis of the seiches. These oscillations take
place in closed or semienclosed basins, and are generated by sudden variations in the wind
direction and intensity, by abrupt changes in the atmospheric pressure, or may be induced by
oscillations in adjacent basins. Seiches are moreover strongly dependent on the geometry of
the basin and on bottom topography. In Table 2 seiche periods determined by analytical and
experimental elaborations for the GoN (Caloi and Marcelli., 1949) are compared with those
calculated by a numerical model for shallow waters (Gravili et al., 2001) and with
experimental observations in the Bay of Pozzuoli, a sub-basin in the north of the GoN
(Buonocore, 2000).
Table 2. Seiche periods in the GoN
Observed periods
(Caloi and Marcelli,
1949)
(Gulf of Naples)
T = 58 -59 min
T = 48 min
----T = 22 min
T = 17.8 min
Analytically
calculated periods
(Caloi and Marcelli,
1949)
T = 59.6 min
T = 47 min
----T = 21 min
T = 17.3 min
Numerically
calculated periods
(Gravili et al., 2001)
Observed periods
(Buonocore, 2000)
(Bay of Pozzuoli)
--T = 45 min
--T = 27 min
T = 23 min
T = 20 min
T = 58 -59 min
--T = 32 min
T = 25 - 26 min
T = 22 -23 min
T = 18.5 min
The values reported in Table 2 show a good agreement of data relative to the periods of
18 min, 22 min and 26 min approximately, while the period of around 1 h is not detected by
Gravili et al. (2001) owing to the intrinsic characteristics of the model. The period of 32 min
observed in the Bay of Pozzuoli (Buonocore, 2000) is not reported in the other works, which
instead show a period of 45 min. This latter is found in the GoN, which is strongly affected
by the topography of the Bocca Grande (Gravili et al., 2001). Therefore, this period can be
considered as dependent on the location of the observation sites and consequently on the
topography of the GoN.
PLANKTON DYNAMICS
Since the establishment of the Stazione Zoologica of Naples “Anton Dohrn” in 1872,
marine flora and fauna of the GoN have been extensively sampled and analysed; in particular
plankton communities have attracted the attention of scientists. By the end of the XIX
century, Giesbrecht (1892) published a monograph describing the pelagic copepods of the
GoN, while a few years later Lo Bianco (1902 and 1903) provided the earliest information
about zooplankton distribution and its seasonal cycle. Phytoplankton communities began to
be studied later (De Angelis, 1956; Wawrick, 1960), in parallel with a more continuous
collection of data on zooplanktonic organisms (Issel, 1934; Della Croce, 1962; Yamazi, 1964;
Hure and Scotto di Carlo, 1968, 1969, 1970 and 1974).
16
Daniela Cianelli, Marco Uttieri, Berardino Buonocore, et al.
Starting from January 1984, the Stazione Zoologica has been collecting plankton samples
and hydrological data at a fixed station (LTER-MC, Long Term Ecological Research Station
MareChiara; 40° 48.5’ N, 14°15’ E) located in the Bay of Naples two nautical miles from the
coastline (Ribera d’Alcalà et al., 2004: Zingone et al., 2010). Between 1984 and 1991
sampling frequency was fortnightly, while from February 1995 it has been incremented
weekly; only one major break in the samplings was recorded, between August 1991 and
February 1995. Such conspicuous data set allows to depict the seasonal evolution and the
recurrent features in the annual cycle of the plankton biomass of the GoN. The effects of
environmental forcings on phytoplankton photophysiology in the GoN have also been
investigated on the occasion of specifically conceived cruises (Brunet et al., 2003 and 2008).
The GoN can be divided into two subsystems which, on the basis of their hydrological
properties, reproduce typical oligotrophic and eutrophic conditions (Carrada et al., 1980 and
1981; Marino et al., 1984; Zingone et al., 1990 and 1995). Depending on the local circulation
the exchanges between these two subsystems can be either promoted or hampered (Casotti et
al., 2000; Menna et al., 2007b).
The open water system has typical Mediterranean oligotrophic characteristics, being
affected by the seasonal alternation of different water masses of Tyrrhenian origin (as
discussed in the previous sections). The nutrient input in this sector is mostly due to the
penetration of the LIW, which owing to its density is confined to the deeper layers (Zingone
et al., 1995). This area can also receive the riverine inputs from the adjacent Gulf of Gaeta,
entering from the Ischia and Procida channels.The coastal subsystem presents hydrographic
and biological features typical of eutrophic areas (Carrada et al., 1980; Zingone et al., 1990
and 1995). It feels the effect of the intense anthropic pressure insisting on the area and of the
freshwater inputs from the Sarno river, resulting in pulsing bursts of nutrient inputs (Ribera
d’Alcalà et al., 1989; Zingone et al., 1990 and 1995).
These two subsystems co-exist all year round, but the boundary separating them displays
seasonal shifts in position and dimension. It is narrow and positioned in the vicinity of the
coast in summer, while in winter it moves offshore and widens (Carrada et al., 1980 and
1981; Marino et al., 1984). Textbook phytoplankton dynamics for temperate latitudes are
expected to show a spring bloom at the beginning of the thermal stratification and a second
bloom during early fall. Maxima in zooplankton abundance follow and control phytoplankton
biomass (e.g., Mann and Lazier, 1996). This scheme is somehow modified in the
Mediterranean Sea where several cases of winter blooms are tallied (Estrada et al., 1985;
Duarte et al., 1999).
The interannual analysis of the data gathered at LTER-MC shows that the autotrophic
biomass has a first peak in winter, a second one in late spring – early summer and a third one
in fall (Ribera d’Alcalà et al., 2004). The biotic and abiotic conditions leading to these blooms
are different. The interaction between large scale meteorological events and land runoff drive
the winter and fall peaks, while the lateral advection of nutrients is responsible for the late
spring – early summer bloom (Ribera d’Alcalà et al., 2004). Maximum phytoplankton
abundances are tallied in May, though depth-integrated (0-60 m) values are almost stable all
year long (Zingone et al., 2010). The spring-summer peak evolves mainly in the surface layer,
owing to the haline and thermal stratification occurring in this season, while fall maxima are
distributed over a deeper mixed layer (Ribera d’Alcalà et al., 2004). When the water column
is homogenous, diatoms and flagellates alternate as dominant components; in presence of a
Dynamics of a Very Special Mediterranean Coastal Area: The Gulf of Naples
17
stratified water column diatoms are dominant in the first 5-10 m whereas flagellates and
unicellular cyanobacteria spread along the water column (Ribera d’Alcalà et al., 2004).
Recurrent patterns in phytoplankton (Zingone and Sarno, 2001), ciliate (Modigh, 2001)
and zooplankton (Mazzocchi and Ribera d’Alcalà, 1995) have been reported. The spatial and
temporal variability of the dynamic conditions in the GoN supports a diversified plankton
community (Ribera d’Alcalà et al., 1997). At LTER-MC diatoms and nanoflagellates
dominate phytoplankton population for most of the year (Scotto di Carlo et al., 1985; Zingone
et al., 1990; Ribera d’Alcalà et al., 2004), with seasonal alternation in the composition
(Ribera d’Alcalà et al., 2004). Winter assemblage is highly diversified, with most of the
species occurring also in other seasons (Zingone et al., 2010). Some species, including a few
diatoms, are distinctive of the season, indicating the presence of the possible presence of
optimal niches (Zingone et al., 2010). During Indian summer the coastal area is dominated by
diatoms whereas small flagellates are more abundant in the offshore sector; in the
intermediate area, diatoms are abundant, but with a more diversified population with respect
to that of the coastal area (Zingone et al., 1995).
The presence of different water masses in the GoN can explain the differences recorded
in the zooplankton community, which can present a mixed structure of open sea and coastal
species and forms at the same time (Carrada et al., 1980). Also phytoplankton assemblage
(Carrada et al., 1981; Casotti et al., 2000) and bacteria (Casotti et al., 2000) respond to the
different water masses present in the GoN, occupying specific regions on the basis of the
peculiar physical and chemical properties of the medium.
Winter depth-integrated biomasses record moderate concentrations with values
comparable to those recorded in spring or summer, with occasional surface blooms driven by
freshwater stratification (Zingone et al., 2010). In any case, these biomasses can be labelled as
blooms, according to Cloern (1996), Smayda (1997) and Longhurst (2007). These blooms are
not an exclusive feature of the GoN but are reported for other Mediterranean sites, both along
the coasts and offshore (Travers, 1974; Estrada et al., 1985; Mura et al., 1996; Caroppo et al.,
1999; Duarte et al., 1999; Marty et al., 2002; Socal et al., 2002; Bernardi Aubry et al., 2004).
The winter blooming diatoms in the GoN belong to the same genera responsible for similar
phenomena in the Mediterranean (Travers, 1974; Estrada et al., 1985 and 1999; Moran and
Estrada, 2005). These biomass increases are however short lived and are consequently poorly
exploited by pelagic zooplankton, which in this season are not abundant and are mostly
represented by small species not preferentially feeding on diatoms (Ribera d’Alcalà et al.,
2004).
Compared to other Mediterranean sites (Margalef and Blasco, 1970; Gilmartin and
Revelante, 1980; Pucher-Petković and Marasović, 1980), summer blooms at LTER-MC have
a well diversified community (Ribera et al., 2004). This is primarily due to the high dynamic
conditions in the area, as discussed in Ribera d’Alcalà et al. (1997).
Picoplankton represents a minor but more stable fraction of total biomass and production
at LTER-MC (Modigh et al., 1996). Ciliate assemblage is dominated by small naked
choreotrichs, with maxima in biomass recorded in late spring (Ribera d’Alcalà et al., 2004).
Mesozooplankton maxima are scored in April-May and July-September, whit minima in
December-January, with clear seasonal alternations of groups and species (Mazzocchi and
Ribera d’Alcalà, 1995; Ribera d’Alcalà et al., 2004). Copepods are the dominant component
of zooplankton assemblage, with four species being the most abundant: Paracalanus parvus,
Acartia clausi, Centropages typicus and Temora stylifera (Mazzocchi and Ribera d’Alcalà,
18
Daniela Cianelli, Marco Uttieri, Berardino Buonocore, et al.
1995). Summertime zooplankton assemblage presents more homogeneous spatial and
temporal patterns (Ianora et al., 1985), and is dominated by cladocerans (Ianora et al., 1985;
Mazzocchi and Ribera d’Alcalà, 1995) and Paracalanus parvus (Ribera d’Alcalà et al.,
2004). The abundance of cladocerans in the GoN is another typical feature of Mediterranean
neritic regions, as verified in several works (Della Croce and Bettanin, 1965; Thiriot, 19721973; Christou et al., 1995; Fonda Umani, 1996; Siokou-Frangou, 1996; Calbet et al., 2001).
This section has focused on the plankton dynamics in the GoN. For the sake of
completeness, it is worth noticing that through the years great attention has also been devoted
to the study of benthic communities, in particular to the extensive seagrass meadows of
Posidonia oceanica (e.g., Procaccini et al., 2003) present in the area.
CONCLUSION
The Mediterranean Sea is one of the most susceptible areas to the effect of climate
variability (Lionello et al., 2006). The GoN is probably one of the most representative of its
numerous sub-basins; it presents features typical of the Mediterranean coastal sites and plays
a crucial role in assessing the interplay between physical forcing and marine ecosystem
dynamics.
The physical dynamics of the basin is complex and reflects the interactions between
numerous factors acting over different spatial and temporal scales (De Maio et al., 1985).
This, coupled with the presence of different water masses (De Maio and Moretti, 1973),
influences the dynamics of plankton communities developing in this area.
The analysis of the data described in Ribera et al. (2004) suggests that the temporal
development of plankton community in the GoN may be only in part explained in terms of
resources availability. While the GoN, as a typical coastal zone, may be considered a nutrientdepleted area, changes in physical factors, affecting the water column structure and the
residence times of water masses, play a significant role in the interannual variability observed
in the plankton biomass. In particular, the meteo-oceanographic forcing determines the
amplitude of the growth phases, whereas the biological rhythms modulate the temporal
dynamics of the populations (Ribera et al., 2004).
As shown by Zingone et al. (2010), similarly to other Mediterranean inshore and offshore
waters an increase in phytoplankton biomass is recurrent during winter in the GoN. Physical
dynamic plays a main role in controlling recurrence and variability of these blooms. The
typical winter conditions in the GoN (predominance of winds from the NE quadrant and
strong turbulence) favour the renewal of surface waters (Menna et al., 2007b). By inducing
offshore transport, the NE winds tend to disperse the blooms. By contrast, stable conditions
promote the retention of low salinity waters in the surface layers consequently sustaining
biomass accumulation (Zingone et al., 2010).
In spring-summer an alternation of high and low phytoplankton biomasses is recorded,
thus mirroring an intermittent nutrient input from terrestrial sources and the entrance of
oligotrophic Tyrrhenian waters into the GoN (Modigh et al., 1985).
The fall increase of plankton biomass is also supported by a nutrient increase;
hydrological and circulation conditions of this season (De Maio et al., 1985) allow the
dispersion of these nutrients and drive the phytoplankton growth.
Dynamics of a Very Special Mediterranean Coastal Area: The Gulf of Naples
19
On the basis of the investigations conducted so far in the GoN, the integration of physical
and biological data has to be considered as a prerequisite for an exhaustive comprehension of
the marine ecosystem dynamics in this coastal site. Owing to the connection between climate
and human activities the response of marine ecosystems can be difficult to assess and detect,
and for this reason a continuous monitoring of coastal areas ought to be promoted.
ACKNOWLEDGEMENTS
The authors and the Department of Environmental Sciences of the Università degli Studi
di Napoli “Parthenope” gratefully acknowledge the ENEA centre of Portici, the “Villa
Angelina Village of High Education and Professional Training” and “La Villanella” resort in
Massa Lubrense, and Fincantieri - Cantieri Navali Italiani S.p.A. of Castellammare di Stabia
for hosting the coastal radar antennae. This work was partly funded by the MED TOSCA
project, cofinanced by the European Regional Development Fund; by the PROMETEO
project, funded by the Campania Region; by the PRIMI project, financed by the Italian Space
Agency; by the FISR - VECTOR project (subtasks 4.1.5 and 4.1.6). M. U. was supported by a
PROMETEO grant (WP03). Constructive exchanges with and bibliographic suggestions by
M. Ribera d’Alcalà and C. Brunet helped improve the ecological breadth of this contribution.
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