677
1 INTRODUCTION
Mangroves are wetlands that occur in tropical and
subtropical areas, bathed by tidal movements, by the
exchange of organic matter between fresh and salty
water. It receives debris from soils, rocks, and mainly
organic matter, usually brought by rivers. A portion is
transformed into smaller particles by the action of
crabs and other animals, serving as food for snails,
larvae, and soil microorganisms, which supply
nutrients to water flora, which will later serve as food
for several animals [9].
Santos’s mangroves are important wetlands
located on the Brazilian coast, in an estuarine rural
tropical area within Santos (Figure 1). The most
extensive mangrove study in the Santos Estuary [9]
classified their characteristics in 1985 as high: 67.85%;
low: 4.14%; degraded: 8.53%; amended: 19.48%. High
mangrove is the vegetation with trunks that support
large leaf mass. Low mangrove is the vegetation with
smaller diameter of tree cup. Degraded mangrove is
partial or total cut areas, exposing dark substrate
(dominance of decomposed organic matter exposure)
with cut logs or small regeneration shrubs. Amended
mangrove presents structural modifications of total or
partial character by the installation of paths or roads,
constructions, or embankments and even agricultural
or aquaculture activity. The study was developed by
means of remote sensing techniques from satellite and
orthophoto images.
Therefore, in this paper only healthy mangroves
were considered, that is, degraded and amended were
disregarded. The overall mangroves area of healthy
mangroves (high and low) along the riparian zones
influencing the Santos Estuary was around 25.20 km
2
.
The study informs the areas of each mangrove swamp
according to its characteristics.
According to [1], it is concluded that mean sea
level rise will have a considerable impact upon the
mangrove areas, with approximately 1.0 m rise
Prediction of Wetland Loss Due to Sea Level Rise
A
round the Largest Port Area in Latin America
P
. Alfredini & E. Arasaki
Polytechnic School of São Paulo University, São Paulo, Brazil
ABSTRACT: Santos’s mangroves are important wetlands located in Brazilian coast, a fishing area inside Santos
Bay. The overall healthy mangroves area along the riparian zones influencing the Santos Estuary is around
25.20 km
2
. The resulting tidal level recorded from Port of Santos tide gauge (from 1940 to 2014), also located in
the estuary, shows consistent increasing trend. One healthy mangrove was selected for a previous qualitative
biological survey to better understand the characteristics of the habitat to be monitored and evaluated about the
possible impacts in the next decades. The mangroves situated a few meters upper from the sea level and some
other areas have the risk to be submerged till 2085 which will seriously affect the riparian mangroves biome.
Indeed, the mangrove area is confined downward by the low tide level and upward by existing structures,
roads, rural and urban areas.
http://www.transnav.eu
the
International Journal
on Marine Navigation
and Safety of Sea Transportation
Volume 15
Number 3
September 2021
DOI: 10.12716/1001.15.03.23
678
estimated from 1990 to 2100. Baixada Santista is a
lowland situated a few meters upper from the sea
level and some areas have the risk to be submerged in
the end of this century. The increasing in the salinity
intrusion upward the estuary due to higher tidal
levels will seriously affect the riparian mangroves
biome. With the confirmation of the 1.0 m sea level
rise scenario, the first estimation was that there will be
a loss of 50% of the riparian zones with healthy
mangroves [2] of Santos Estuary. The sea level rise is a
threat particularly to the wetlands of the South
Atlantic. In general, the sea level rise will flood the
estuarine saline areas, such as mangroves, and the sea
level rise will submerge wetlands causing the death of
vegetation by salt stress [10].
An important observation is that the mangrove
area of Santos Estuary (Figure 1) is confined
downward by the mean sea level and upward by high
tide level, Port of Santos structures, industrial plants
(Cubatão petrochemical and steel complex), roads and
urban areas (including stilt houses).
Figure 1. Mangrove distribution in Santos Estuary (adapted
from [9]).
As a result of the likely reduction of mangrove
areas in the Santos and São Vicente Estuaries, due to
the mean sea level rise, some of the ecological
functions of this coastal ecosystem may be
compromised, including the retention of sediments
and pollutants, export of organic matter and nutrients
to the adjacent coastal waters and restriction of critical
habitat for some species that use the mangrove at
some stage of their life cycle.
Regarding the mangrove, [4] state that the coastal
wetlands can deal with changes in sea level when they
are able to stay at the same elevation relative to the
tidal range. That can happen if sediment increase is
equal to the rise in sea level, or if the wetland is able
to migrate (when the mangrove moves) upward.
According to [15], the vulnerability of mangroves
to climate change is moderate, and, although the
increase in winter temperatures can enhance growth,
the rising sea level and saline intrusion can cause
significant reduction. Potential changes in
hydrological regimes projected to occur over the next
100 years will lead probably to the loss of wetlands,
deterioration of water quality and damage to fisheries
production [11].
The study of [2, 13] consider that the increase does
not show itself which areas will submerge if the sea
level rises a meter or more but is the most important
factor when looking for that answer. In this sense, the
evaluation carried out for the study area is restricted
only to the rising sea level.
Beyond the biological loss and its biodiversity,
there will be serious physical consequences due to the
partial loss of a natural filter for the fine (clay and silt)
sediment transport in suspension, from the rivers into
the estuary and the erosion of the former riparian
areas submerged. Indeed, the complex mangrove
roots trap large quantities of fine sediment in
suspension, like silt and clay. Without this retention, a
larger amount of sediment will be carried to the Port
of Santos nautical areas, silting, and increasing the
dredging volumes of maintenance. The study [8]
described that up to 80 % of the sediment transported
by the tides may be retained in mangrove areas, but
the mechanism of retention of this sediment is
unclear. Hence, meanwhile the sea level rise will
increase the depth, the siltation certainly will
overcome many times this favorable increasing of
depth, and the result will be an increasing cost of
maintenance dredging, with volumes larger than the
current ones.
In an agreement between the Hydraulic Laboratory
and CODESP (Port of Santos Authority) it was
decided to define a characteristic stretch of the
preserved mangrove of Largo de Santa Rita (Ilha
Barnabé) for a previous qualitative biological survey
to better understand the characteristics of the habitat
(Figure 2). Although the Santos Estuary is a studied
region, there was no prior knowledge of the
mangrove preservation status in this area of Ilha
Barnabé. The small bay has depths less than 3 m, and
the land stretch has well-defined limits of port
structures, railways, and highways, making it possible
to estimate the potential loss of mangrove areas in the
coming decades, due to mean sea level rise [3].
Observe in the figure the landfill of the railroad
crossing the mangrove of Ilha Barnabé. The railroad
separated the mangrove plain in the middle of the
seventies, thus more than forty years ago.
Figure 2. The location of the mangrove bank studied. In
detail, aerial photo with at first plan, River Jurubatuba, and
after, Largo de Santa Rita.
The main objective of this paper is to estimate the
potential loss of healthy mangrove area in the coming
decades due to the rise in the mean sea level.
679
2 MATERIAL AND METHODS
2.1 Experiment in Tide Condition of Equinoctial Syzygy
To get advantage of the equinoctial spring tide, on
September 6
th
, 2002, the staff made the topographic
survey of the levels (Chart Datum, CD) of flooding of
the mangrove (Figure 3). The CD of Brazilian Navy
for the Port of Santos was used as level of reference
and horizontal coordinates are referred to the System
UTM – Córrego Alegre, used in Brazil.
Figure 3. Field conditions for the experiment in equinoctial
spring tide.
2.2 Characterization of the Mangrove Structure
Three plots were positioned, each 15 m x 15 m (Figure
3), Plot I nearby the railway and in the central portion
of the mangrove, Plot II near River Jurubatuba in the
north portion of the mangrove and Plot III was the
closest to Santos Port. The species Avicennia
schaueriana, Rhizophora mangle and Laguncularia
racemosa were identified and measurements of the
diameter of each tree were taken, according to the
methodology described in [12]. The average heights
were determined with rods, replacing the rangefinder
and inclinometer. It is determined the relative
frequency by species.
The measurements of the trees were classified at
intervals established by the methodology
recommended in [12], that is: < 2.5 cm, > 2.5 cm and <
10.0 cm and > 10.0 cm, according to the diameter at
breast height (DBH), around 1.3 m from the ground,
with the aid of a measuring tape.
Once the field data was available, for each
sampling plot the parameters of mean forest height
(H
mean in m), maximum height (Hmax in m), mean
diameter at breast height (DBH
mean in cm), basal area
(BA in m
2
.ha
-1
), relative density (RD in %) of each
species and relative contribution in basal area of living
trunks with DBH higher than 10.0 cm (RBA
DBH > 10
cm in %).
2.3 Rise of Mean Sea Level
The assessment of mean sea level rise for Port of
Santos shows a reliable consistence in comparison
with several international recommendations and with
similar trends in other two locations of São Paulo
State Coast, giving confidence to its use for estimative
impacts due to the maritime consequences of climate
changes. The estimative from 1940 to 2100 is 1.1 m,
following from 1940 to 2014 a rate of 0.33 cm/year,
which should increase in the next decades in a similar
trend of UK recommendations (Table 1) for a
moderate scenario [1].
Table 1. UK recommended net sea level rise rates and
cumulative amounts, relative to 1990 [6].
_______________________________________________
Time period Moderate rate (mm/yr.)/cumulative SLR
since 1990 (m) at end of period
_______________________________________________
1990-2025 3.5/0.12
2025-2055 8.0/0.36
2055-2085 11.5/0.71
2085-2115 14.5/1.14
_______________________________________________
3 RESULTS
3.1 Characterization of the Flooding and Levels
The following data were obtained in the field:
− Low water: - 0.069 m (CD)
− High water: 1.561 m (CD)
− Level of beginning of the mangrove vegetation:
0.771 m. Practically mean sea level
− Flooding area between low water and the
beginning of the mangrove vegetation: 426,299 m
2
− Flooding area of the mangrove up to high water:
466,668 m
2
− Total flooding area: 892,967 m
2
Calculated slopes:
− Maximum in the mangrove plain: 0.0056
− Lowest landfill of the railroad: 0.0215 and 0.0305
According to [7], in 1985 mean sea level was 80 cm
(CD), with mean higher high water 145 cm (CD).
3.2 Mangrove Structure
A total of 108 trees were measured (Figure 4), 79 of
which were Avicennia schaueriana, 23 trees of
Rhizophora mangle and 6 trees of Laguncularia
racemosa. From these data, the respective basal areas
(m
2
) were calculated, presenting the main structural
parameters of the plots (Table 2).
Table 2. Structural parameters of the plots.
_______________________________________________
Parameter Plot I Plot II Plot III
_______________________________________________
Hmean (m) 6,9 7.8 5.7
H
max (m) 14.0 12.0 12.0
DBH
mean (cm) 8.68 11.28 8.08
BA (m
2
.ha
-1
) 7.9 18.8 16.4
RDAvicennia (%) 48.0 100 68.0
RD
Rhizophora (%) 32.0 0 30.0
RD
Laguncularia (%) 20.0 0 2.0
RBA
DBH>10 cm (%) 56.9 93.23 75.47
_______________________________________________
680
Figure 4. Photo of the mangrove aspects in the three plots
during the survey.
3.2.1 Plot I
This area had 23 adult individuals (absolute basal
area = 0.176736 m
2
), most of which are individuals
with DBH > 10.0 cm (absolute basal area = 0.10222013
m
2
). The DBHmean = 8.68 cm. The plot showed 2 young
individuals (absolute basal area = 0.00079577 m
2
), one
of the species Rhizophora mangle and one of Avicennia
schaueriana. H
mean = 6.9 m and Hmax = 14.0 m. In terms
of distribution of individuals by species, Avicennia
schaueriana was represented in greater number
(48.0%), with 12 (absolute basal area = 0.07366193 m
2
),
followed by Rhizophora mangle (32.0%) with 8
(absolute basal area = 0.083856 m
2
) and Laguncularia
racemosa (20.0%) with 5 (absolute basal area = 0.020014
m
2
). The total BA = 7.9 m
2
.ha
-1
and the RBADBH>10 cm =
56.9%.
3.2.2 Plot II
The plot was composed only by the species
Avicennia shaueriana, which had 31 adult individuals
(absolute basal area = 0.422748 m
2
) and 2 young
individuals (absolute basal area = 0.00059476 m
2
).
H
mean = 7.8 m and Hmax = 12.0 m, with the total BA =
18.8 m
2
.ha
-1
and the RBADBH>10 cm = 93.23%.
3.2.3 Plot III
The region offered the largest number of adult
sampled with 48 individuals (total basal area =
0.366860 m
2
). It also presented only 2 young
individuals (total basal area = 0.00091944 m
2
) of the
species Rhizophora mangle and Avicennia schaueriana.
In terms of individuals distribution by species, the
plot presented 34 (68.0%) belonging to Avicennia
schaueriana (total basal area = 0.3024443 m
2
), 15 (30.0%)
to Rhizophora mangle (total basal area = 0.06425731 m
2
),
and only one individual (2%) of Laguncularia racemosa
(total basal area = 0.025783 m
2
). The height
measurements were H
mean = 5.7 m and Hmax = 12.0,
with the total BA = 16.4 m
2
.ha
-1
and the RBADBH>10 cm =
75.47%.
3.3 Mean Sea Level Rise
The following data are estimations of mean sea level
rise in comparison of 1985, from Table 1:
− In 2055: 38 cm.
− In 2085: 73 cm.
− In 2115: 116 cm.
4 DISCUSSION
As mentioned in the Introduction, the scope of the
survey of the mangrove was a qualitative biological
survey in order to better understand the
characteristics of the habitat.
Comparing the plots of the mangrove, this is
observed that Plot II had the largest BA (with 18.8
m
2
.ha
-1
), the highest Hmean = 7.8 m and the highest
RBA
DBH>10 cm = 93.23%. Then, Plot III had a BA with
16.4 m
2.ha
-1
and RBADBH>10 cm = 75.47%, but Hmean = 5.7
m was smaller compared to Plot I (6.9 m) with BA =
7.9 m
2
.ha
-1
and RBADBH>10 cm = 56.9%.
The specie Avicennia schaueriana was dominant in
the three plots, and it is interesting to highlight that in
Plot II its frequency was 100%. The genus Avicennia is
more tolerant to environmental stress and thus can be
abundantly found in areas with human induced
disturbances [12].
In terms of relative density, Avicennia schaueriana
was present in 73.15% of the cases, followed by
Rhizophora mangle with 21.3% and Laguncularia
racemosa with only 5.55%.
Indeed, according to [14], Avicennia is a specie of
mangrove typical of low tidal and medium low tidal
zones, near mean sea level; and Rhizophora of medium
low tidal and medium high tidal zones.
Finally, it is noted that most of the sampled trunks
are in the DBH range > 2.5 cm and > 10.0 cm,
demonstrating that the plots are composed of mature
individuals. According to [12], in the stages of greater
ripening of a forest, an increase in diameter results in
the death of a much smaller number of individuals.
According to [14], in a mangrove area near Santos,
compared to an environmental protection area in
Guanabara Bay, the main structural characteristics of
plots in the mangrove forest are: DBH
mean from 3.6 to
12.3 cm, H
mean from 2.7 to 11.6 m, Hmax from 6.5 to
16.7 m, BA from 8.5 to 24.8 m
2
.ha
-1
and
RBA
DBH>10 cm = 46 %. However, most of Guanabara Bay
comprises the second largest Brazilian industrial
region, the second largest port, two refineries, oil
storage and distribution companies, naval services,
dockyards, and intense maritime and terrestrial’s
transportation activities. According to the same paper
[5], 40% of the Guanabara Bay mangrove were
decimated as the coast underwent a process of intense
urbanization. Thus, we have observed that those
mangroves were gradually destroyed and modified
by deforestation for different purposes, different types
of landfills, slum occupation and urban expansions,
rectification, course alteration and canalization of
rivers and canals, discharge of domestic sewage and
industrial effluents, refineries and port activities,
predatory fishing, predatory crab harvesting, garbage
681
from many different sources and urban garbage
deposition. Therefore, the mangroves outside the
environmental protection area mentioned have as
main structural characteristics of plots in the
mangrove forest: DBH
mean = 5.07 cm, Hmean = 4.9,
BA = 13.2 m
2
.ha
-1
and RBADBH>10 cm = 22 %.
Comparing the structural parameters of the
mangrove of Santos with those of Guanabara Bay, one
can characterize the mangrove habitat of Largo de
Santa Rita as reasonably preserved, in spite of its
separation by the railroad landfill.
The original idea of this previous survey was that
the studied mangrove area would be monitored over
time with more plots, which unfortunately did not
happen. Due to restriction of financial resources, it
was not possible to continue monitoring annually, as
would be recommended.
The real threat for the mangrove healthy now is the
mean sea level rise in stretches where the wetland
remained surrounded by obstacles, or by terrains with
higher slopes than the usual for the mangrove growth,
or the inevitable situation of limiting expansion on
islands. As an example, the study showed that the
highest slope of the wetland plain evaluated was
0.0056, yet much more mild than typical landfills (in
this case 0.0215). According to [14], mangroves plains
have slopes from 0.0033 to 0.0050. Hence, the slope
measured could be considered at the order of
magnitude of the higher limit of this vegetation. Due
to those limitations for migration upward of the
mangroves and through the assessment of the areas in
Figure 1, the expectancy of probable losses of
mangrove areas in 2085 were estimated in Figure 6,
with the following considerations:
− Around 2085, according with the estimation made
in item 3.6, mean sea level will reach the high-
water level of 1985 (see item 3.1), which was the
highest level for the roots of the mangrove in 1985
(Figure 5).
− All the mangroves located in islands without
higher lands will disappear, since it would not
have possibility to migrate upward. The high and
low mangroves in this case correspond to 2.22 km
2
.
− Considering the reality of port, industrial and
urban expansions over the mangrove region, from
1985 to the present, it is probable that more of the
original mangrove has been degraded or altered in
comparison with Figure 1. High and low
mangroves lost in those expansions from 1985 to
2020 correspond to 0.61 km
2
.
− In regions confined by roads and their landfill, the
mangrove migration would be precluded due to
the high slope of the landfills in comparison with
the maximum slopes typical of mangrove plains.
Examples of this occurrence is the big mangrove
area of Cubatão in the north western portion of
Figure 1 (limited by the railroad line) and the
mangroves crossed by the roads (in the eastern
portion of Figure 1). Computing these areas, the
loss will be of 6.79 km
2
.
Figure 5. Typical mangrove plain and the mean sea level
rise expected in 1985.
Figure 6. Expectancy mapping of the mangrove in 2085.
5 CONCLUSIONS
From a first survey it was possible to characterize the
mangrove habitat of Largo de Santa Rita as reasonably
preserved. While these mangroves are located in the
area of the Port of Santos, with visible anthropic
influence, it is worth mentioning that, only with this
previous survey, it was possible to detect that the
number of species is expressive, as well as of high
biological importance.
In 2085, from the original 25.20 km
2
healthy
mangroves will remain only 15.58 km
2
, or 62% of the
original vegetation, due to sea level rise. This figure
supposes no new port structures, industrial plants,
roads, and urban areas in regions of mangrove, also is
expected that the upward terrain where the remaining
mangrove could migrate has mild slopes to support
the new mangroves. Otherwise, the loss will be
greater. The following suggestions can be listed:
− A mandatory starting point for future research
directions would be to update the study carried
out by [9], whose survey is over 35 years old,
aiming to precisely characterize mangroves that
have degraded or amended in this period.
− A new topographic survey that allows to precisely
define the slopes of the land above the mangroves.
− To focus on studies in the mangrove areas with the
greatest chance of survival, that is, those in which
upward migration is viable.
− Application of techniques that allow the creation of
new mangrove areas.
− Public policies with the poorest people that value
the importance of mangroves, creating housing,
sanitation, and education conditions without
invasions in the areas of this biome.
− Effective application of the concept of green ports,
aimed at preserving the environment, as an
awareness that the port itself may suffer the
consequences of a predatory occupation.
682
To take measures is not only responsibility of the
port and industrial stakeholders, but also from the
government authorities. Therefore the accountability
has to come from both sides. If people are aware of
possible threats, it is easier to convince them in taking
preventive measures. The government authorities of
the involved municipalities in the Baixada Santista
must deal with that prospect. New mangrove areas
are of great importance for that people, however not
all areas are suited for mangrove growth and to make
certain areas suitable will be an expensive
undertaking.
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