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INTRODUCTION

Double diffusive mixing in the ocean is driven by the
difference between molecular diffusivities of heat and salt
(Stommel et al., 1956), the diffusion of heat being roughly
100 times faster than for salt (Zhang et al., 1998). This
can be illustrated by the case of relatively warm water
parcels that stabilize locally the water-column, tending to
rapidly diffuse their heat content, while the slower
diffusion of the salty content renders the vertical stability
prone to gravitational collapse. This situation leads to a
transfer of salt toward the bottom, denominated as salt-
fingering (SF) after the famously known chimney
structure (Stommel et al., 1956; Stern, 1960; Linden,
1973). Another situation can occur too, when relatively
cold and fresh water overlays on warmer and saltier
parcels. Thermal content diffusion tends to stabilize
upward, bringing salty parcels toward the surface, and an
oscillatory diffusive (DDF) instability is generated. Once
established in the water-column, these diffusive regimes
can be identified in the vertical profiles of density by a
series of well-mixed layers, whose staircases signature
can extend from relatively vertical thin or fine-scale layers
(e.g., 5 to 100-m thick intrusions, Ruddick, Richards
(2003)) to larger structures (e.g. 300-m thick in the
Tyrrhenian Sea in Durante et al. (2019)). This process has
been widely observed since decades in the ocean (e.g.,
Schmitt et al. (2005) in the Atlantic Ocean, Timmermans
et al. (2003) and Lenn et al. (2009) in the Arctic), and
particularly in the Mediterranean Sea (Meccia et al., 2016;
Falco, 2016), but field observations and time series
acquisition remain of importance to investigate properly

the temporal variability associated to these diffusive
phenomena, as pointed out by the study of Durante et al.
(2019). Weak turbulent environment remains a key
condition for a density staircase establishment against
strong mixing processes (Timmermans et al., 2003), but
the compilation of all in-situ observations demonstrates
decades of their persistence in the Mediterranean basin
with spatially distributed coherent patterns (Buffett et al.,
2017). Even if these diffusive phenomena have been
shown to contribute weakly to the ocean circulation (Lenn
et al., 2009; Boog et al., 2021), its influence to the
buoyancy flux can be significant in non-sheared
environment and should be taken in account properly in
the water-column budgets (Inoue et al., 2007). Due to the
direct transfer of salt, they provide toward the deep layers,
and even weakly turbulent, double diffusive processes are
effective and of importance to supply nutrients for the
biological activity in the internal part of the water column
(Fernández Castro et al., 2015). Historically, studies of
this phenomenon focused on the open and interior part of
the ocean basins, but lakes, shallow seas and coastal areas
can be concerned too (Carniel et al., 2008; Schmid et al.,
2010; Umlauf et al., 2018). The Gulf of Naples is a mid-
latitude semi-enclosed shallow basin in the Western
Mediterrean Sea having a subtropical regime and almost
no tides (Figure 1). The area presents a marked salinity
contrast due to the combination of the salty Tyrrhenian
Sea waters, with its own feature of inshore/offshore water
exchange with the open ocean, located on its southern side
(Cianelli et al., 2015), and the freshwater inputs from a
densely inhabited coastal area, on its northern part and
from nearby rivers (Cianelli et al., 2012, 2017). The recent

ARTICLE

A note about density staircases in the Gulf of Naples: 20 years of persistent weak
salt-fingering layers in a coastal area

Florian Kokoszka, Daniele Iudicone, Adriana Zingone, Vincenzo Saggiomo, Maurizio Ribera d’Alcalà,
Fabio Conversano

Stazione Zoologica Anton Dohrn, Naples, Italy

ABSTRACT
This is a short communication about the inter-annual recurring presence at a coastal site in the Gulf of Naples of density stair-

cases visible below the mixed surface layer of the water-column, from the end of summer to the beginning of winter, each year
during nearly two decades of survey (2001 to 2020). We repetitively observed sequences from 1 to 4 small vertical staircases struc-
tures (~3 m thick) in the density profiles (~Δ0.2 kg m–3), located between 10 m to 50 m deep below the seasonal mixed layer depth.
We interpret these vertical structures as the result of double-diffusive processes that could host salt-fingering regime (SF) due to
warm salty water parcels overlying on relatively fresher and colder layers. This common feature of the Mediterranean basin (i.e.,
the thermohaline staircases of the Tyrrhenian Sea), may sign here for the lateral intrusions of nearshore water masses. These stably
stratified layers are characterized by values of the density ratio Rρ from 5.0 to 10.0, slightly higher than the critical range (1.0 –
3.0) generally expected for fully developed salt-fingers. SF mixing, such as parameterized, appears to inhibit weakly the effective
eddy diffusivity with negative averaged value (~-1×10–8 m2 s–1) varying in a main interval between -1×10–7 and -1×10–9 m2 s–1 during
the two decades. Even contributing weakly to the turbulent mixing of the area, we hypothesize that SF could influence the seasonal
stratification by intensifying the density of deep layers. Downward transfer of salt could have an impact on the nutrient supply for
the biological communities, that remains to be determined.

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F. Kokoszka et al.66

study of Kokoszka et al. (2021) in this location shows
weak turbulent observations during the seasonal
destratification, associated to the presence of double-
diffusive layers below the intrusion of warm salty layers
present in sub-surface from late summer to early winter.
We extend this half-year period analysis to the long-term
time series in the Gulf of Naples with the use of
temperature and salinity profiles. These observations were
made in the framework of the long-term ecosystem
research Marechiara (LTER-MC) initiative that produced
a historical time series of the Mediterrean coastal
ecosystem of the Gulf of Naples through a weekly
sampling of the water column started in 1984 and running
until now (Ribera d’Alcalà et al., 2004; Zingone et al.,
2019). We will focus here on the last two decades
(January 2001 to March 2020), and identify the layers of
the water-column prone to salt-finger regimes, to show
their variability, and estimate the associated eddy
diffusivity, to determine their possible contribution to the
vertical mixing in such coastal area

MATERIALS AND METHODS

General hydrology

Conductivity–temperature–depth (CTD) profiles were
carried out at the LTER-MC sampling point in the Gulf of
Naples (75 m deep, 2 km off the coast, see Figure 1) with
a Seabird SBE-911+ mounted on a 12-bottle carousel, with
all sensors calibrated. The raw profiles were processed
using the Seabird data processing software to obtain 1-m
bin-averaged data. The weekly survey refers to the casts
MC465 (January 2001) to MC1359 (February 2020) and
includes a total of 895 CTD profiles. The Gibbs sea water
oceanographic toolbox (Mc-Dougall, Barker, 2011) was
used to calculate the conservative temperature Θ (°C), the
absolute salinity AS (g kg−1), the potential density σ0 (kg
m−3), and the potential temperature θ0 (°C). When
mentioned thereafter, temperature T and salinity S refer to
Θ and AS. Mixed layer depth (MLD, m) was calculated
following the method of de Boyer Mont´egut et al. (2004)

Figure 1. Bathymetry of the Gulf of Naples (GEBCO grid (GEBCO, 2020)) along the Tyrrhenian Sea in the Mediterranean basin). The
75m-deep LTER-MC coastal sampling site (14.25°E, 40.80°N ) is located by the pink dot. Volturno and Sarno’s river mouths are shown
in blue. Thin lines indicate the 50, 200, 300 and 400 m isobaths, thick ones indicate the 100, 500, 1000 and 2000 m isobaths.

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A note about density staircases in the Gulf of Naples: 20 years of persistent weak salt-fingering layers in a coastal area 67

based on threshold values. Given a vertical profile of
density σ0(z), we calculated the depth below zref = 3m
where the profile reached a threshold defined as a
cumulative of 0.03 kg m−3.

Turner’s stability regimes

To produce reliable statistics of the double diffusive
regimes, we followed the recommendation of Inoue et al.
(2007) who compared successfully CTD estimates and
dedicated turbulence measurements. We applied the same
10-m-scale averaging (indicated by an overline symbol) on
temperature and salinity profiles, and conserved only the
parts of the water column where the threshold for the
minimum temperature gradient was |∂θ̅ / ∂z| > 0.05°C m−1

(Zhang et al., 1998; Inoue et al., 2007). This was shown to
improve the statistics by embedding the information
contained in the layer, that determines the processes
occurring at finer scales (Inoue et al., 2007). We applied
the method introduced by Turner (Turner, 1967; 1973) to
localize parts of the water column where vertical gradients
of T and S are favorable to double-diffusive instability.
Combining the vertical gradients and their signs allows
the identification of stability regimes, that can be defined
from the density ratio Rρ = (α∂θ/∂z)/(β∂S/∂z) where
α = –ρ−1(∂ρ/∂θ) is the thermal expansion coefficient,
β = ρ−1(∂ρ/∂S) is the haline contraction coefficient, and
∂θ/∂z, ∂S/∂z and ∂ρ/∂z are respectively the vertical gradients
of temperature, salinity, and density. This ratio is used to
calculate the Turner angles (°) Tu = arctan((1+Rρ)/(1-Rρ))
(Ruddick, 1983). The value of the Turner angle allows to
identify various stability regimes. A diffusive convection
regime (e.g., fresh cold layers over warm salty layer) arises
when –90° < Tu < –45°. A double-diffusive regime (e.g.,
salty warm layer over cold fresh layer) arises when
45° < Tu < 90°. Within each of these regimes, the instability
is higher when |Tu| is close to 90 degrees. A stable regime
occurs when |Tu| < 45°, whereas a gravitationally unstable
regime occurs when |Tu| > 90°. Generally, salt-fingering is
considered active when 1 < Rρ < 3 (Inoue et al., 2007;
Carniel et al., 2008), but as illustrated thereafter in Figure
2, our observations exhibit small density staircases (~3 m)
associated to slightly larger values of Rρ (3.0 - 5.0), which
should sign for a weak salt-fingering regime, but marked by
persisting structures, that were visible repetitively weeks
after weeks. Given that values 1 < Rρ < 10 are frequently
observed (Kelley, 1990), and the large variability of the
worldwide observations (You, 2002; Nakano, Yoshida,
2019), we included then all the cases 1 < Rρ < 30.

Salt-fingering diffusivity and salty flux

From the estimates of Rρ, diffusivities of heat, salt and
density associated with salt-fingering have been extensively
reviewed and are still discussed until now (Kunze, 2003,

Nakano, Yoshida, 2019). Once we have identified parts of
the water column prone to SF regime, we apply the
parameterization of Zhang et al. (1998) to obtain the
effective salt and thermal diffusivities, respectively KS

SF =
K*/(1 + (Rρ/RC)n) and KT

SF = γSF(KS
SF)/Rρ, where n = 6, RC =

1.6, K* = 1×10−4 m2 s−1 a upper limit for the SF diffusivity,
and γSF is computed as γSF = 2.709e−2.512Rρ + 0.5128 (Radko,
Smith, 2012). Finally, we infer the effective eddy diffusivity
of the density, Kρ

SF = (KT
SF Rρ – KS

SF)/(Rρ – 1) (Eq 8 in
Nakano, Yoshida (2019)). As pointed out by the authors in
their review, values of Kρ

SF are negative, indicating that SF
reduces the potential energy of the system by transferring
salt downward in the water-column, and consequently
intensifies density stratification. To illustrate that, we recall
a general expression for the diffusivity (valid for heat, salt,
or density) as a combination of salt-fingering (SF), double-
diffusive (DDF), and other processes than double-diffusion
(e.g., internal wave turbulence): Kρ

TOTAL = Kρ
Turb. + Kρ

SF +
Kρ

DDF (Merryfield et al., 1999; Merryfield, 2000; Inoue et
al., 2007); Kρ

TOTAL is generally dominated by the
contribution of Kρ

Turb., and can be reduced by the negative
values of Kρ

SF. Please note that double-diffusive (DDF) will
not be discussed in this work, and has been reviewed in
detail by the mentioned authors. An estimate of buoyancy
flux for salt is given by Kunze (1987) for SF developed on
“thick” layers (> 1m), as gβFS = 2νgβ(∂S /̅ ∂z)(Rρ

1/2 + (Rρ −
1)1/2)2 (Kunze (1987), and Eq. 97 in Nakano, Yoshida
(2019)), with S ̅a mean value in the layer, g the gravitational
acceleration (9.80 m s−2), ν the kinematic molecular
viscosity (1.05×10−6 m2 s−1). Here FS is the vertical salt flux
(g kg−1 m s−1), βFS the density flux of salt (m s−1), and gβFS

the buoyancy flux (m2 s−3).

RESULTS

Staircase layers during the seasonal cycle

Established from the weekly profiles of the period
2001-2020, the climatological monthly variations of
salinity show a remarkable intrusion in sub-surface
(thick blue contour in Figure 2, top), with values close
to the maximum, between 38.05 and 38.1 g kg−1, visible
from September to November below 10 m depth, and
above the 20 m to 45 m layer of relative less salty water
(< 38.0 g kg−1).

The thickness of this salty tongue increases in time
following the deepening of the seasonal thermocline up to
November (MLD in thick gray line in Figure 2, top),
progressively filling the water column, besides the first 5
m. Temperature (black contours in Figure 2, top) shows a
more classical seasonal cycle, with a mean maximum of
around 26 °C in July and August, decreasing to 24.0 °C in
September, then to 20.0 °C and 18.0 °C in October and
November. From August to November these intrusions of

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F. Kokoszka et al.68

salty water from 10 to 60 m create the unstable conditions
for SF regime, whose water-column occupation is shown
in plain blue in Figure 2 (top), below the MLD (thick gray).

The overview of the mean seasonal hydrological state
allowed us to identify some general vertical distribution
of SF regimes during the seasonal cycle. We illustrate this
situation by showing a typical example of small staircases
(e.g., during the cast MC1126 in Figure 2, bottom). From
around 25 to 45 m deep, both gradients of T and S are
compatible with the host of SF regime. A sharp variation
of nearly 0.2 g kg−1 is visible between 30 and 32 m,

followed by a more moderate one of 0.1 g kg−1 from 32
to 37 m, associated both with a temperature loss of nearly
1.0 °C. The density profile is then marked by a sequence
of thin and curvy staircases, progressing stably toward
depth by steps of 0.20 kg m−3 on vertical scales from 1 to
3 m. In terms of Turner angles, stronger value of 60° is
obtained at 27 m where the instability presumably initiates
from the salty and warm input, and progressively
decreases to around 50° at 40 m where T −S gradients stop
to host the SF regime. Associated values of Rρ vary from
3.0 to 5.0 where density staircases are the sharpest, then

Figure 2. Top: Seasonal water-column occupation by the four stability regimes of Turner, showing vertical layers prompt to possibly host
double-diffusive instabilities (SF or DDF), inferred from the monthly climatological proles established with the weekly CTD data from
2001 to 2020 (MC465 to MC1359). Black and blue: contours of the climatological temperature and salinity proles. Thick blue: 38:05 g
kg–1 haline level, showing the salty intrusion. Light gray: seasonal mixed layer depth (MLD). Bottom: Illustration of a density staircase
situation (cast MC1126). Left: temperature and salinity proles; Center: density; Right: Turner angles and density ratio R (SF regime).

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A note about density staircases in the Gulf of Naples: 20 years of persistent weak salt-fingering layers in a coastal area 69

increase above 10.0 at the host ending. These values are
slightly above the range in which SF are expected to be
the most active (1.0 - 3.0), but density observations are
marked by small curvy staircases, whose vertical structure
have been smoothed by the 1-m scale vertical averaging
of the data. Given these values of Rρ and the shape of
density profiles, we consider that we observe here a
relatively weak SF regime, and this situation tends to
repeat and persist in time during the season.

Unfolding the layers: nearly 20 years of staircase layers
This persistence during the two last decades is clearly

demonstrated in Figure 3 with the vertical distribution of
Rρ showing the vertical hosting of the SF regime below
the mean MLD (gray line), mainly from August to
November. Even being weak in general (Tu in the range
45 - 60 °), the most intense Turner angles values are more
frequent in October and November than during the

summer (see the vertical patterns in Figure 3, and the red
to blue distributions in Figure 4). Mean values of Rρ are
between 5.0 and 10.0, and occurrences < 5.0 are more
frequent in October and November. Estimates of salt and
thermal diffusivities reach mean values centered around
1×10−8 m2 s−1 and 4×10−9 m2 s−1 during these months,
while the intensity is weaker and close to 1×10−11 m2 s−1

in August. This marks a seasonal differentiation in our
observations, the post-summer period being prompter to
host the more intense SF regimes. When estimating the
effective eddy diffusivity for the density, values of KSF are
negative, indicating that SF reduces the potential energy
of the system by transferring salt downward in the water-
column. Mean contribution is low ( -3×10−8 m2 s−1),
compared to the averaged turbulent diffusivity expected
in such coastal system (from 1×10−6 m2 s−1 to 1×10−4 m2

s−1). The range of the associated buoyancy flux for salt is
around -6×10−9 m2 s−3 from September to October (see

Figure 3. Time series of the vertical profiles of Rρ (Top, pink to light blue chart) and the effective eddy diffusivity KSF (Bottom, blue to
orange chart), for the SF regime. For each parameter, decades 2001-2010 and 2011-2020 are split between upper and lower panels,
respectively. Y-axis indicates the depth of profiles, bottom x-axis indicates the sequence of weekly MC casts (MC465 to MC1359), and
years are indicated on top. Gray line: mixed layer depth.

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F. Kokoszka et al.70

the yellow, orange and blue distributions in Figure 4), and
is centered closer to -1×10−8 m2 s−3 in August (red).
Consequently, SF mixing, such as parameterized, appears
to inhibit weakly the turbulent mixing of the area (KρTOTAL

= KρTURB + KρSF + KρDDP), but increase the stability of the
deep layers by intensifying the density stratification due
to the transfer of salt toward the bottom. The inter-annual
values (black plots in Figure 4) of KSF confirms these
averages ranging between -1×10−9 m2 s−1 and -1×10−6 m2

s−1 during the two decades, and the low averaged
buoyancy flux for salt (~-6×10−9 m2 s−3) compared to the
expected total buoyancy fluxes due to heat and freshwater
by atmospheric forcing at the surface (of the order
of~1×10−7 m2 s−3, see Kokoszka et al. (2021)).

DISCUSSION
The long-term monitoring (20 years) of the coastal

station Marechiara in the Gulf of Naples reveals
noteworthy repetitive observations of small staircases
vertical structures (~3 m) in the density field (~∆0.2 kg
m−3), whose presence is associated to surrounding layers
of relatively warm and salty waters in sub-surface (from

10 to 50 m deep) from August to November, each year.
We interpret these observations as the result of double-
diffusive processes, i.e. here salt-fingering instabilities.

Such fine-scale structures may sign here for lateral
intrusions (Merryfield, 2000; Umlauf et al., 2018), or
interleaving (Ruddick, Richards, 2003; Ruddick, Kerr,
2003), whose inshore advection remains to be determined.
These stably stratified layers are characterized by density
ratio Rρ from 5.0 to 10.0, close to some observations made
in the Artic Ocean (Timmermans et al., 2008; Shibley et
al., 2017). As pointed out by Bebieva, Timmermans
(2017), taking in account the horizontal gradients of T and
S (intrusions), a critical value for the instbility does not
necessarily need to be close to 1.0, the higher values of
Rρ being the signature of T-S intrusions. This may be what
we observe here, differing from the T-S staircases, typical
of the neighboring Tyrrhenian Sea (Zodiatis, Gasparini,
1996; Durante et al., 2019). In such configuration,
dedicated parameterization taking account of horizontal
gradients and using intermediate and higher Rρ values
should be investigated.

As showed in the work of Merryfield et al. (1999)
double-diffusive phenomena can have regional influences

Figure 4: Top: Probability density function (PDF) associated with the following parameters in the SF regime: Turner angles, Rρ, effective
salt diffusivity (KS

SF), effective eddy diffusivity (Kρ
SF), and buoyancy flux for salt. Distributions have been established for the months

of August to November, from 2001 to 2019, when SF regime was possible. Bottom: Inter-annual variability of the year-averaged mean
values of Kρ

SF and buoyancy flux for salt gβFS
SF (note the negative values).

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A note about density staircases in the Gulf of Naples: 20 years of persistent weak salt-fingering layers in a coastal area 71

on temperature and salinity. The recent observations of
Ferron et al. (2021) in the Algerian Sea inferred that SF
mixing can significantly modify the basin water masses
within a period of 2 years. Noteworthy, a rise in the values
of the fluxes in the Tyrrhenian interior was observed by
Durante et al. (2021) during the 2003-2016 period, due to
higher salt and temperature gradients. These studies
highlight the temporal and regional variability of SF, and
strengthen the idea that from boundaries to deep sea
locations, observational efforts on SF phenomena could be
valuable to improve the parameterizations modeling the
development and effect of staircases (Kantha et al., 2011;
Radko et al., 2014) in global and coastal circulation models.

Given the averaged values of Rρ (~ 5.0) and the
observed values of turbulent mixing (< Kρ

Turb > between
0.2 to 0.8 × 10−5 m2 s−1, in Kokoszka et al. (2021)), the
inhibition due to negative < KSF > in the mixing mixture
(Inoue et al., 2007) is expected to be negligible, even some
intermittent unstable occurrences (Rρ close to 1.0) can be
present in the SF layer below the MLD. Even if mixing
would be unaffected, the density stratification enforcement
due to the transfer of salt could influence the generation
and propagation of internal waves in such stratified-
compatible layers (Kunze, 2003; Malki-Epshtein, Uppert,
2004; Maurer, Linden, 2014), followed by their breaking
in the deepest layers, more relaxed to the buoyancy-control
of vertical motions. When the SF-compatible layers are
located closer to the bottom (e.g., around 50 m in
November), influence of boundary processes could be at
work too, as suggested by the turbidity observations in
Kokoszka et al. (2021). The step size of the observed
structures could be a clue of the coexistence between
weakly sheared internal wave and double-diffusion
processes, as mentioned in the review of Kunze (2003).
This feature of the shallow non-tidal area of the Gulf of
Naples could provide an interesting in-situ experimental
field to investigate and understand better the dynamic
behind background gradients of tracers and velocity, and
the growing of SF instabilities (Inoue et al., 2008; Ma,
Peltier, 2021).

In general, implications for biological communities
could be important. Compared to fluxes associated with
mechanical forcing or mesoscale eddies, Oschlies et al.
(2003) found the same magnitude attributed to double-
diffusive processes, that showed a salt-finger driven
enhancement of the upper ocean nutrient supply. As
estimated in the work of Fernández Castro et al. (2015),
nitrate diffusion mediated by salt fingers is responsible
for 20% of the new nitrogen supply in several areas of the
Atlantic and Indian Oceans. More recently, Taillandier et
al. (2020) showed that the nitrates supply across
thermohaline staircases in the Western Mediterranean Sea
contributed at 25% to the budget of the Levantine
intermediate water.

The Gulf of Naples stands as a shallow bay connected
to the open Tyrrhenian area, and Cianelli et al. (2017)
have shown the importance of the interplay between
coastal and offshore water masses to promote
phytoplankton diversity.

Their study identified the role of the horizontal mixing
to enhance or dilute the favorable conditions for dominant
species, and under this hypothesis we propose that salty
intrusions (i.e., horizontal gradients) should be
investigated in terms of their stability relative to the
vertically surrounding layers. Given the downward
transfer of salt due to SF regime in this shallow area
where the photic layer prevails in the growing dynamic
of the biological populations (Zingone et al., 1995, 2010),
the importance of such a supply to the communities
inhabiting the deep layers is a primer to determine.
Because SF activity depends on the density ratio rather
than on the stratification stability, its sensitivity to the
future expected warming/freshening trends of the surface
waters in the Mediterranean Sea (Volosciuk et al., 2016)
should be addressed, as shown by the inter-annual decadal
variability already observable at the coastal site in the
Gulf of Naples.

Corresponding author: Florian Kokoszka, florian.kokoszka@szn.it

Keywords: Coastal ecosystem; Mediterranean Sea; time series;
hydrological data; turbulence; stratification; mixing; double-
diffusion; salt-fingers.

Author’s contribution: All the authors have read and approved the
final version of the manuscript and agreed to be accountable for
all aspects of the work.

Availability of data and materials: Data sets for this research are
available on request.

Conflict of interest: The authors declare no potential conflict of
interest.

Acknowledgments: The authors would like to thank the LTER-
MC team that includes, besides the main authors: D. D’Alelio,
C. Balestra, M. Cannavacciuolo, R. Casotti, I. Di Capua, F.
Margiotta, M. G. Mazzocchi, M. Montresor, A. Passarelli, I.
Percopo, M. Saggiomo, D. Sarno, F. Tramontano, G. Zazo, all
based at the Stazione Zoologica Anton Dohrn of Naples. Special
thanks must be given to A. Passarelli and the commandants and
crews of the R/V Vettoria for all their dedicated work at sea. The
research program LTER-MC is supported by the Stazione
Zoologica Anton Dohrn.

Received: 31 July 2021.
Accepted: 29 October 2021.

This work is licensed under a Creative Commons Attribution
NonCommercial 4.0 License (CC BY-NC 4.0).

©Copyright: the Author(s), 2021
Licensee PAGEPress, Italy
Advances in Oceanography and Limnology, 2021; 12:10008
DOI: 10.4081/aiol.2021.10008

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