Detection and mapping of Posidonia oceanica dead matte by high

Rivista Italiana di Telerilevamento - 2009, 41 (2): 139-146
Italian Journal of Remote Sensing - 2009, 41 (2): 139-146
Detection and mapping of Posidonia oceanica dead matte
by high-resolution acoustic imaging
Agostino Tomasello, Filippo Luzzu, Germana Di Maida, Carla Orestano, Maria Pirrotta,
Antonino Scannavino and Sebastiano Calvo
Dipartimento di Ecologia, Università di Palermo, Viale delle Scienze, Edificio 16, 90128 Palermo, Italy.
E-mail: agtoma@unipa.it
Abstract
A high-resolution echosounder was used to acquire seismo-acoustic records of P. oceanica
dead matte patches in the Gulf of Palermo. Seismo-acoustic profiling enabled detection of
dead matte structures that are invisible to visual inspection, being hidden by variable layers
of sandy sediment. The thickness of the dead matte measured reaches a maximum of 2.2
m. The mean volume of the matte per unit area of seabed surface (MEIx) varies from 0.2 to
2.2 m3/m2, with an average value of 1.6 m3/m2 and a total volume estimated at 73,000 m3.
From literature data and from the volume of dead matte calculated in the present work we
estimated that total carbon accumulated in the matte is about 2,484 tons. Seismo-acoustic
technologies proves to be a powerful, non-destructive method to measure the thickness,
extension, volume of dead matte and its potential carbon content.
Keywords: Posidonia oceanica, dead matte, sub-bottom profiler, remote sensing, carbon sink.
Individuazione e mappatura di matte morta di Posidonia oceanica attraverso
immagini acustiche ad alta risoluzione
Riassunto
Un sistema acustico ad alta risoluzione è stato utilizzato per acquisire profili sismoacustici su matte morta di P. oceanica nel Golfo di Palermo. Le indagini hanno permesso
di individuare matte morta altrimenti invisibile tramite ispezioni visive. Lo spessore della
matte raggiunge valori di 2,2 m. Il volume medio di matte per unità di superficie (MEIx)
varia da 0,2 a 2,2 m3/m2, con un valore medio di 1,6 m3/m2 e un volume di 73.000 m3.
Utilizzando dati bibliografici ed il volume di matte morta calcolato nel presente lavoro è
stato stimato che il carbonio totale contenuto nella matte è di 2.484 t. La tecnologia sismoacustica rappresenta uno strumento efficace e non distruttivo per misurare lo spessore,
l’estensione, il volume della matte morta e la quantità potenziale di carbonio in essa
contenuto.
Parole chiave: Posidonia oceanica, matte morta, sub-bottom profiler, telerilevamento,
riserva di carbonio.
Introduction
The endemic Mediterranean seagrass Posidonia oceanica (L.) Delile is a key species in
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Detection and mapping of Posidonia oceanica dead matte
Mediterranean coastal environments, where it forms a very extensive and productive
ecosystem [Ott, 1980; Pergent et al., 1995]. This species is the only marine phanerogam
able to form matte, a biogenic structure resulting from growth of plagiotropic and
orthotropic rhizomes intertwined with roots and autochthonous and allochthonous detritus
[Boudouresque and Meisnez, 1982]. These long-term organic reservoirs, due to their highly
refractory nature, can be preserved over thousands of years, forming structures several
meters thick [Mateo et al., 1997] that represent a potential carbon sink [Mateo et al.,
2006].
The structure of matte provides important information on the health of the meadows, the
sedimentary dynamics and hydrodynamic regime [Pergent-Martini et al., 2005]. Natural
and/or anthropogenic causes can lead to formation of a regressive structure, called “dead
matte”, caused by the regression of the P. oceanica meadow and disappearance of living
shoots in the upper part of the matte [Pergent et al., 1995].
Historically, matte thickness has been evaluated by direct visual inspection or core
sampling [Pergent-Martini et al., 2005]. Earlier remote sensing methods based on acoustic
technologies attempted to estimate the thickness of the matte, but gave results for only the
upper few layers [Rey and Diaz del Rio, 1989]. Recently, a very high-resolution seismoacoustic (Sub-Bottom Profiler) technique was successfully applied to assess the volume
occupied by the matte underlying P. oceanica meadows [Lo Iacono et al., 2008].
Acoustic-based geophysical methods are remote-sensing techniques commonly used in
aquatic environments [Garcia et al., 2004; Nitsche et al., 2004; Schrottke et al., 2006] to
indirectly provide images of seabed features and sub-bottom sediment layers. These methods
offer a cost-effective and non-destructive approach that allows for continuous mapping
of bathymetry and subsurface information, representing a valid alternative to traditional
surveys. In particular, the Sub-Bottom Profiler (SBP) uses a combination of sonic and
ultrasonic waves to provide images of the water-sediment interface and underlying sediment
layers by detecting changes in the mechanical impedance through reflections from the subbottom stratigraphy [Schock et al., 1989; Ballard et al., 1993; Schock, 2004]. In particular,
the use of the very high-resolution sediment sub-bottom profiler, using the parametric effect
[Grant and Schreiber, 1990; Spieß, 1993], can detect small impedance variations, like that
created by the presence of organic matter storage in the matte [Wunderlich and Müller,
2003].
In this study a seismo-acoustic survey was carried out in the Gulf of Palermo (NW coast of
Sicily, Italy) (Fig. 1) where very extensive P. oceanica dead matte patches occur [Tomasello
et al., 2007], as a consequence of several decades of multiple pollution sources due to city
planning disorder, improper waste disposal and untreated wastewaters [Genchi et al., 1982;
Calvo et al., 1994].
The aim of this study is to assess the effective extension and volume of dead matte by
seismo-acoustic techniques and to estimate indirectly the amount of carbon stored.
Materials and methods
Data acquisition
On 28 July 2008 a high-resolution non-linear parametric echosounder Innomar SES-2000
compact was used to acquire seismo-acoustic records of P. oceanica dead matte patches in
the inner sector of the Gulf of Palermo (Fig. 1); the transducer was mounted on the sides of
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Italian Journal of Remote Sensing - 2009, 41 (2): 139-146
R/V “Antonino Borzi” (www.unipa.it/cisac). The track lines of R/V were run by navigation
software using as background a morpho-bathymetric
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map previously acquired by a Multi
Beam Sonar System (Reson SeaBat 8125), on which the dead matte emerging from the
bottom was
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identified [Luzzu, 2009] (Fig. 2).
Seismo-acoustic profiling was carried out using a secondary frequency of 6 kHz. The data
were corrected for loss of energy (spreading corrections) by using a linear time-varying
gain (TVG). Ship movements were corrected by a motion reference unit.
Data processing
The upper and lower depth limits of each acoustic layer were selected in all sub-bottom
profiles using the software Innomar-ISE 2.9 (Interactive Sediment layer Editor). All selected
points were interpolated by the kriging method to reconstruct via DTM (pixel size of 1 m2)
the top and the base of the dead matte. Matte thickness and volume were then calculated.
In particular, we defined a Matte Edification Index (MEIx) obtained by the ratio between
matte and sea bottom surface (m3/m2). Finally, the total dead matte volume of the area was
determined by the sum of the MEIx values in all pixels.
Figure 1 - Study area.
Results and discussion
The area investigated has a depth of -14.8 to -20.8 m and covers a surface of about 45,000
m2. The topography of the seafloor is irregular and includes areas with sandy bottom,
dead matte and circular eroded structures delimited by matte walls of up to 1 m high (Fig.
2). Emerging dead matte covers up to 22,000 m2, corresponding to 49% of the total area
investigated.
Circular-shaped eroded morphologies (termed marmites by French authors) are common
structures within P. oceanica meadows and are caused by a whirling hydrodynamic regime
[Pergent and Pergent-Martini, 1995].
Seismo-acoustic profiling enabled us to identify different acoustic layers, interpreted as
sand and rock, which constitute the present-day base of the matte. The rocky structure was
identified by a high amplitude reflector positioned below the sediment at depths of ranging
from 1.5 to 4 m below the seabed (Fig. 3). This suggests that rock and sand represent the
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Detection and mapping of Posidonia oceanica dead matte
native substrata on which P. oceanica meadow started to settle in the area.
Seismo-acoustic profiling also enabled detection of other dead matte structures that are
invisible to multibeam inspection, being hidden by variable layers of sandy sediment (Figs.
3 and 4).
Figure 2 - Seismo-acoustic tracks (white lines) acquired on P. oceanica
dead matte in the Gulf of Palermo. The black square delimits the area
in which all the analyses were performed. The morpho-bathymetric
map was acquired by Multi Beam Sonar System Reson SeaBat 8125.
Figure 3 - Example of a seismo-acoustic profile in which the three different layers can be recognized:
the base of the matte over sand is marked with a black line; the rocky layer is marked with dots
through point sequence.
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Italian Journal of Remote Sensing - 2009, 41 (2): 139-146
Hidden dead matte covers a total surface of 19,000 m2, which corresponds to 42% of the
area investigated. The cause of complete loss of the meadow and its coverage by sandy
sediment was probably the erosion, transport and deposition of inert solid waste that has
been discharged along the shoreline since the end of the second world war. The thickness
of the dead matte measured seismo-acoustically reaches a maximum of 2.2 m. The vertical
accretion of matte including production, decomposition and erosion events estimated over
centuries or millennia in several sites of Mediterranean sea, ranged from 0.06 to 0.41 cm
year-1 [Mateo et al., 1997; Lo Iacono et al., 2008]. According to these estimates, settlement
of the meadow in the area investigated may thus have occurred between 3,600 and 530
years ago.
Figure 4 - Map distribution of emerging matte (grey) and sediments above the matte revealed by a
sub-bottom profiler (red and yellow).
The volume of the matte per unit area of seabed surface (MEIx) varies from 0.2 to 2.2 m3/m2
with an average value of 1.6 m3/m2 (Fig. 5). Considering the coverage of dead matte in the
area its total volume was estimated at 73,000 m3. The amount of carbon storage in the matte
is 0.034 tons/m3 [Lo Iacono et al., 2008]. The computation excludes the carbon storage in
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Detection and mapping of Posidonia oceanica dead matte
other species (e.g. Cymodocea nodosa or macroalge) living on this substrate. Finally, we
extrapolate that the total carbon accumulated in 4.5 ha of dead matte is about 2,484 tons.
Conclusion
The use of seismo-acoustic technologies proves to be a powerful, non-destructive method
to measure the thickness, extension and volume of matte [Lo Iacono et al., 2008; Endler,
2009]. These technologies can provide an accurate assessment of the potential size of the
carbon stored in the dead matte, confirming the importance of P. oceanica as a sink for this
element. Moreover, seismo-acoustic technologies will be applied to larger scales in the
future, providing a better understanding of the role of P. oceanica in the carbon cycle of the
entire coastal marine Mediterranean ecosystem.
Figure 5 - Distribution map of Matte Edification Index (MEIx).
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Received 25/03/2009, accepted 30/04/2009.
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