Hydrobiologia 207: 123-129, 1990.
D. J. Bonin & H. L. Golterman (eds), Fluxes Between Trophic Levels and Through the Water-Sediment
© 1990 Kluwer Academic Publishers. Printed in Belgium.
Interface.
123
Oxygen profiles in intertidal sediments of Ria Formosa (S. Portugal)
Vanda Brotas,l, Ana Amorim-Ferreira, 1 Carlos Vale? Fernando Catarino 1
1 Departamento de Biologia Vegetal, Faeuldade de Cieneias de Lisboa, Campo Grande, Bloeo C2, 1700
Lisboa, Portugal; 2Instituto Naeional de Investigac;ao des Peseas, A v. Brasilia, 1400 Lisboa, Portugal
Microelectrode oxygen profiles were measured in intertidal sediments from Ria Formosa (S. Portugal),
a very productive shallow coastal lagoon. Four intertidal sampling sites were selected according to
different sediment characteristics. Individual profiles revealed a high degree of lateral variability on a
centimeter spatial scale. Nevertheless, consistent differences were observed between oxygen profiles
measured in atmosphere-exposed and inundated intertidal sediments: in organically poor sand oxygenpenetration depth varied from 3 mm in inundated cores to more than 7 mm in exposed ones, while in
organically rich muddy sand and mud it remained between 0.5-2.0 mm. The oxygen input from inundated
to exposed conditions was estimated for each sampling site. Semi-diurnal tidal fluctuation, leading to
periodical atmospheric exposure of sediments plays a major role in the oxygenation process of intertidal
zones of Ria Formosa.
A thin oxygenated layer is commonly observed on
the sediment surface of aquatic ecosystems
(Revsbech et al., 1980a; Lindeboom et al., 1984;
Baillie, 1986; Andersen & Helder, 1987;
Silverberg et al., 1987). At the sediment surface,
oxygen is rapidly consumed, either by aerobic
degradation of organic matter and respiration of
benthic communities, or by reduced compounds
produced in anaerobic layers and transported
into the oxygenated zone. The oxygen required by
all these reactions can be supplied by the water
column, by the atmosphere and may also be produced by photosynthesis within the uppermost
sediment layer.
In recent years, membrane-covered polarographic oxygen sensors have been developed to
assay dissolved oxygen gradients near the sediment surface with a sub-millimeter spatial resolution (Revsbech et ai., 1980b; Helder & Bakker,
1985). Measurements performed in deep coastal
sediments generally show an oxygen penetration
of a few millimeters (Revsbech et al., 1980a;
Silverberg et al., 1987). Mechanisms other than
diffusion seem to dominate the oxygen supply
across the sediment-water interface (Silverberg
et ai., 1987). In intertidal sediments, however,
several sources of oxygen production, oxygen
uptake and oxygen transport are present, resulting in large variations in the oxygen profiles.
Several studies have demonstrated that microalgae photosynthesis increases oxygen concentration in the fIrst sediment millimeter (Revsbech
et al., 1981, Revsbech & Jorgensen, 1983). More
recently Baillie (1986), has emphasized the impor-
tant effect of infusion associated with the tidal.
fluctuation on the oxygenation of intertidal sediments.
The present study reports oxygen profiles
measured in intertidal sediments with different
characteristics in air-exposed and inundated conditions. The results obtained point out the effect
of tidal height fluctuations associated with periodical atmospheric exposure on the oxygenation
processes of intertidal areas.
The study area was located in Ria Formosa, a
shallow mesotidal coastal .lagoon (mean depth
3.5 m), on the south coast of Portugal, permanently connected to the sea through several narrow channels and with no relevant freshwater
input (Fig. 1). Salinity values average 36 % all
year round. The exposed intertidal area may reach
50 km2 (ca. 1/3 of the total lagoon area) at low
water during spring tides and almost 20 % of this
area is occupied by growth-banks of the clam
Ruditapes decussatus L.. Large areas of the lagoon
are covered by macrophytes (Zostera spp and
macro algae). To evaluate the spatial variability of
sediment oxygenation in the intertidal zone, four
sites were selected (Fig. 1): station 1 a sandy clam
growth bank, station 2 a muddy sand clam
growth bank, stations 3 and 4 both located on
mudflats not used for aquaculture, but station 4
had a higher carbon content. These sites emerge
for about two hours during spring tides. At sampling time (always between 10 and 12 a.m.) sediment temperature ranged from 17 °C in winter to
28°C in summer.
From January 1988 to February 1989, sediment samples were collected three times during
the winter period and once every month in the
summer period. All samples were drawn while the
sediment was atmosphere-exposed at low tide. In
January 1990 both atmosphere-exposed and inundated sediments were collected at the same
sites, at low and high tide. Samples were taken by
hand with plexiglass tubes (15-20 cm length and
8 cm i.d.) which were immediately stoppered at
the bottom with an hermetic lid and brought to the
ATLANTIC
OCEAN
laboratory. Inundated cores were transported in
a basin containing water from the sampling
locality several centimeters in depth. In January
1988, sediments from stations 1 and 2 were collected undisturbed and put inside transparent
plexiglass box cores (40 x 40 x 30 cm). These
were subsequently covered with water from the
sampling locality (ca. 20 cm depth) and transported to the field laboratory where they were kept
outdoors for two weeks under shade, with continuous stirring and aeration to maintain oxygen
in the overlying water at saturation levels.
For each sampling station sediment was analyzed for grain size, organic carbon, chlorophyll a,
phaeopigments and water content. The sand
fraction was determined by sieving and the other
fractions by means of a sedimentometer. Organic
carbon were estimated by subtracting the inorganic carbon from total carbon content measured separately in a CHN-analyzer. Concentrations of chlorophyll a and phaeopigments in the
upper centimeter were determined spectrophotometrically according to Lorenzen (1967). Water
content was calculated by weighing the sediments
before and after drying to constant weight at
105 C. Porosity was expressed as the ratio of
water volume to water volume plus volume of
sediment sample.
The vertical distribution of oxygen was studied
in the laboratory at room temperature (20- 24 C)
and room light intensity (10 p.E m - 2 S - I ), with a
polarographic needle sensor (2.5 cm length,
0:7 mm o.d.). Oxygen needle electrodes are indicated for coarse-grained sediments providing a
good spatial resolution (0.25 mm), (Helder &
Bakker, 1985). In situ measurements
were
attempted without success. The pore water movements created by the operator standing on the
mud- and sand-flats made it impossible to make
valuable measurements. The needle electrode
(produt n° 760), the chemical microsensor
(n 1201) and Agj AgCl reference electrode
(n 334) used in this study, were purchased from
Diamond
Electro-Tech
Inc., Ann Arbour,
ichigan, USA.
Before each experiment the electrode was
allowed to stabilize at - 0.7 Vat the measurement
temperature in filtered water from the lagoon for
at least 2 hours. The electrode was calibrated in
oxygen saturated water from the lagoon (21 % O2)
and in deep anoxic sediment (0 % O2), The reference electrode was always introduced in the
calibration media as well as the measurement
media (sediment or water). Stabilization was
always confirmed with a strip chart recorder.
When measuring the oxygen profiles the needle
electrode was lowered into the sediment at
0.25 mm intervals using a manual micromanipulator. In inundated cores the overlying
water above the sediment was 1-2 cm deep.
The oxygen profiles were measured on field
samples in two situations: (i) undisturbed sediment cores one hour after collection (both atmosphere-exposed and inundated); (ii) atmosphere
exposed sediment cores, which were subsequently
immersed with water (oxygen saturated) from the
sampling locality and allowed to stand with no
stirring for 10 hours. Measurements were also
performed in sediments collected from the box
cores previously described, 14 days after field
sampling.
0
0
0
0
Oxygen concentration was measured in coarseand fine-grained sediments containing low- and
high-organic matter. Sediment characteristics
within the upper 5 cm of the cores collected at the
four sampling sites are presented in Table 1.
In order to evaluate the effect of time lag
between collection and measurement of oxygen
profiles in air-exposed cores, sequential profiles
were measured in the laboratory at 10, 30, 60
and 120 minutes after collection (Fig. 2). During
the first 60 minutes slight differences were
recorded in the oxygen penetration depth which
may be attributed to small-scale spatial heterogeneity. However, after this period, penetration
depth decreased and surface value increased,
defining a different oxygen profile. Therefore, we
assume that only measurements within one hour
were reliable.
Profiles of dissolved oxygen measured in air-
Table 1. Sediment characteristics of the sampled cores within the upper 5 em; chlorophyll a and phaeopigment content represent
annual average values of the upper sediment centimeter; water content was determined for inundated and air exposed sediments,
there were no significant differences between these two conditions.
St2
~~(t~
Sand 0 > 60 Ilm (%)
Silt 211m < 0 < 60 Ilm (%)
Clay 0 < 21lm (/0)
Water content
Porosity
Organic carbon (%)
Chlorophyll a I1gg- I dw
Phaeopigments J1gg - I dw
St 3
St4
23.2
67.3
9.5
0.49
0.72
1.68
14.0
76.6
9.4
~~0
97.3
2.4
0.2
0.18
0.34
0.20
4.39
4.42
exposed sediments from stations 1 to 4, at two
different sampling periods, are presented in
Fig. 3. In spite of variations in the same core and
between sampling periods, there were consistent
differences between the stations in both the oxygen penetration depth and in the shape of the
profiles. Oxygen penetrated down to 0.5-1.0 mm
in mud (stations 3 and 4), to 1.0-1.8 mm in
muddy sand (station 2) and to 6 mm or more in
sand (station 1). Oxygen variation with depth was
quite irregular at station 1, while exponential to
almost linear distributions were found for the
other stations. The oxygen gradients at stations 2,
3 and 4 varied from 105 to 395 jiM mm - 1 while
at station 1 they were always lower than 90 jiM
81.2
16.0
2.8
0.36
0.56
1.46
7.42
6.19
18.86
13.77
0.61
0.80
2.80
2.96
12.69
(Fig. 4). At stations 2, 3 and 4, oxygen penetration depths in exposed and inundated sediments
were quite similar. A different picture was found
at station 1: the erratic variation with depth
observed in most exposed cores was not found in
mm-I.
Oxygen profiles were also measured in cores
collected from the atmosphere-exposed
and
inundated intertidal areas of the same stations
.c:
Q.
g
0,0
0 ..0
0,5
E
E
.c:
Q.
Q)
1.0
1.5
2.0
0
2.5
3.0
a
100
200
11M 02
Fig. 2. Study of time-lag effect. Oxygen profiles in an
exposed and muddy sand area (March 1988): 10 min (+ ),
30 min (*), 60 min (D) and 120 min (x).
Fig. 3. Vertical oxygen profiles measured in air-exposed
sediment cores at two different sampling periods (solid and
dashed lines). Two replicate profiles are presented for each
period. In spite of seasonal and small-scale spatial variations,
consistent differences between cores in both depth penetration and shape of the profiles are evident.
The amount of oxygen (Q) present in the sediment, from the sediment-water interface (pO) to
the maximum depth of oxygen penetration (pmax)
may be calculated by unit area as follows:
0.0
0.0
0.5
2.0
1.0
4.0
Q
1.5
StatIon 2
Station 1
6.0
J C(p)
w
dp
[pO, pmax]
2.0
E
E
0
100
0
200
100
200
.c
0.
0.0
Q)
Cl
=
oo~
I
0.5
1.0
0.5
1.0
1.5
Station 3
1.5
Station 4
2.0
2.0
0
100
200
0
100
200
UM 02
4. Oxygen profiles in air-exposed (solid line) and
inundated (dotted line) sediments collected in January 1990
at low and high tide, respectively. In the inundated cores
oxygen concentration in the bulk of the overlying water was
180 JlM for stations I and 2, 138 JlM for station 3 and
170 JlM for station 4. Oxygen saturation value at measuring
temperature was 234 JlM.
Fig.
inundated conditions and penetration depth decreased substantially. At stations 2, 3 and 4 surface levels of oxygen in air-exposed sediments
were higher than in inundated ones. For all sampling stations, oxygen gradients were 2-3 fold
steeper in exposed cores than in inundated ones:
13 versus 55 f.LM mm - I at station 1, 125 versus
220 f.LM mm - I at station 2, 50 versus 180 f.LM
mm - I at station 3, 130 versus 230 f.LM mm - I at
station 4.
According to the Fick's law of diffusion,
J = 0 Ds !J.Cj!J.x, the oxygen flux (J) at each core
(for constant values of porosity, 0, and sediment
diffusion coefficient, Ds) increases as the
gradient,!J.Cj!J.x is higher. This means that oxygen
fluxes across the sediment-water interface are
higher in the exposed cores than in the inundated
ones.
where w is the water content and C the oxygen
concentration at the oxygen penetration depth, p.
Based on oxygen profiles (Fig. 4) and water
content measurements (Table 1), one may calculate the amount of oxygen at exposed (Qe) and
inundated cores (Qi) of each site.
Assuming that oxygen profiles obtained are
representative of the conditions encountered in
these locations, the difference Qe - Qi, at each
station, represents the oxygen input from inundated to exposed periods. The result of
Qe - Qi was positive for all the stations: from 16
to 2 nmol cm - 2 at station 1, 0.8 nmol cm - 2 at
station 2, 2.7 nmol cm - 2 at station 3, and
1.5 nmol cm - 2 at station 4. Except for station 1,
where the irregularity of replicate profiles
accounted for a large range of Qe - Qi, the other
sites presented a comparable oxygen input.
Oxygen profiles measured in sediment cores
from stations 1 and 2 artificially immersed in the
laboratory for 10 hours are presented in Fig. 5. A
diffusive boundary layer above the sediment surface was formed. Jorgensen & Revsbech (1985)
relate the thickness of this layer to the rate of
~
-4
E
-=-
-2
.c
0. a
Q)
Cl
UM 02
Fig. 5. Oxygen profiles measured
in sediment cores from
station 1 (+) and station 2 (x) artificially immersed in the
laboratory for 10 hours. Oxygen saturation value at measuring temperature was 206 JlM.
oxygen uptake of sediment surface: when steep
gradients are established, the boundary layer may
represent a major resistance to the transport of
oxygen; whereas in sediments with a low respiration rate, diffusion through this layer does not
playa significant role for oxygen uptake. Figure 5
indicates a higher rate of oxygen uptake for sediment of station 2, as the thickness of the diffusive
boundary layer was 3 mm compared to 0.5 mm in
the sediment from station 1. Furthermore, the
sediment oxygenated layer recorded at station 2
was thinner (1.3 mm) than at station 1 (1.8 mm).
In comparison with inundated cores (Fig. 4), the
penetration depth was particularly reduced at
station 1. This represents a reduction of oxygen
flux from 20 nmol cm - 2 h - 1 in __exposed to
13 nmol cm - 2 h - 1 in inundated sediments
(Ds = 2 x 1O-scm2s-1,for20
°C, Andersen &
Helder, 1987).
Oxygen measurements in 14-day incubated
sediments from stations 1 and 2, revealed almost
linear profiles, oxygen dropping to 2.0-2.8 mm
(Fig. 6). With stirring and continuous air-supply,
oxygen penetration in sediment from station 1 is
comparable to that recorded in inundated conditions, but in sediments from station 2, oxygen
went 1.5 mm deeper (compare Figs. 4 and 6).
Similarly, the comparison between 14-day and
lO-hour incubations shows that at station 2 oxygen went 1 mm deeper, as oxygen is supplied to
the overlying water, whereas at station 1 penetration depth remained roughly the same. The higher
carbon content in sediments from station 2
accounts for the oxygenation differences between
200
0
IJM 02
Fig. 6. Oxygen profiles measured in cores collected from
sediment incubated with continuous stirring and aeration for
14 days. Three replicates are indicated for each core.
the experiments; Lindeboom et al. (1984) drew
the same conclusion, from sediments with low
and high POC content, in a shallow brackish lake.
Oxygen fluxes into the sediment were 35 and
59 nmol cm - 2 S - 1 for stations 1 and 2, respectively, in the 14-day experiment; whereas in the
lO-hour experiment they were 22 nmol em - 2 S - 1
for station 1 and 34 nmol O2 em - 2 S - 1 for
station 2 (see Figs. 5 and 6). Oxygen fluxes calculated in this study are comparable to those
reported for shallow nearshore marine sediments
(Silverberg et al., 1987; Andersen & Helder,
1987).
The results obtained in this study point out the
contrasting oxygenation of sand/mud-exposed/
inundated intertidal surface sediments of Ria
Formosa. While in mud and muddy sand the
oxygenated layer was thin (0.5-2.0 mm) under all
the experimental conditions, a deeper and variable layer was found in sandflats (from 3 to more
than 7 mm).
Exposure of sand to the atmosphere seems to
be the major cause for the high oxygen penetration. The irregularities found in most oxygen
profiles in sand, may be due to the presence of air
bubbles which favor the diffusional exc,hange of
. oxygen with the deeper interstitial water retained
around particle grains (Revsbech & Jorgensen,
1983). Revsbech etal.,(1981)
demonstrated that
oxygen levels in surface sediment may increase
due to micro algal photosynthesis.
In our
measurements,
autochthonous
production of
oxygen in exposed sand cannot alone justify mudsand oxygen penetration differences. In fact,
values of chlorophyll a were comparable in all the
sampling locations and experiments were performed under similar light conditions.
Reduction in the thickness of the oxygenated
layer in inundated sand probably results from the
interruption of atmospheric oxygen supply to
sand interstices with the rise of water level. Riedl
& Machan (1972) studied the dynamics of interstitial water in intertidal sand and demonstrated
the existence of a filling-draining process driven
by the tidal height fluctuation. This pulsing
mechanism may lead to the transport of oxygen
from the atmosphere as well as the transport of
reduced substances from deeper anaerobic layers
(Fenchel, 1978). In mud, this pulsing mechanism
tends to be less effective. Penetration depth was
comparable in inundated and exposed conditions,
but oxygen increased considerably in the surface
of exposed mud.
The discrepancy of oxygen profiles at stations 1
and 2, both with a high percentage of sand (97 %
and 81 %, respectively), may be related to the
different organic matter content. Anderson &
Helder (1987) showed that large differences in
oxygen depth penetration may be due to both
organic matter content and wave exposure. Infiltration of water between sand grains results in
a faster turnover rate of easily degradable organic
carbon at the exposed sites. In our study, station
1 is situated near the inlet channel of the lagoon,
whereas station 2 is located in an inner area (see
Fig. 1). Consequently sediments of station 2 have
a minor wave exposure, which may result in a
poorer renewal of interstitial water. This leads to
a progressive accumulation of organic matter in
the sediments.
The differences in oxygen penetration in sediments from stations 1 and 2 were drastically
reduced in the 14-day incubation experiment.
When tide-driven renewal of interstitial water is
absent, the oxygenation of the sediment surface
due to diffusional processes and net production of
oxygen, is similar in both sediments.
Baillie (1986) pointed out the role of the tide
and micro algae photosynthesis in the oxygenation
process of sediment surface of intertidal areas.
The experiments conducted in this study have
simply emphasized the question of how the oxygenated layer varies from exposed to inundated
conditions in sediments of Ria Formosa. Studies
from intertidal (Riedl & Machan, 1972; Baillie,
1986) to coastal sediments (Silverberg et ai.,
1987) concluded that mechanisms other than
molecular diffusion must dominate the transport
of oxygen across the sediment-water interface. In
our study, the largest differences were observed in
organically poor sands between atmosphereexposed and inundated periods. These are mainly
attributed to transport processes through pore
water movement, which are driven by the large
fluctuation of tidal height. Evapotransport,
coupled with respiration, deplecting the oxygen in
organically rich sediments, accounts for the thin
oxygenated layer. These processes may be particularly intense in summer due to the high temperatures of the water lagoon. However, seasonal
changes were not observed, probably because of
the small-scale temporal variations of the oxygenation process. Further studies should therefore be conducted to understand the role of
bottom primary producers coupled with the semidiurnal effect of tide on the formation and
maintenance of the oxygenated surface layer on
the sediments of intertidal zones.
We thank M. Falcao for all help and facilities
provided during this study, and Prof. S. Prates for
the granulometry analysis. This work was supported by the Portuguese Junta Nacional de Investiga<;ao Cientifica e Tecnol6gica (JNICT) by
contract n° 869.86.141.
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