Indian Journal of Marine Sciences
Vol. 35(3), September 2006, pp. 227-234
Sediment oxygen consumption in a developed coastal lagoon of
the Mexican Caribbean
*David S. Valdes-Lozano, Marcela Chumacero & Elizabeth Real
Centro de Investigación y de Estudios Avanzados,
Km. 6 Antigua Carretera a Progreso, C.P. 97310, Mérida, Yucatán, México
*(E-mail: [email protected] )
Received 6 June 2005; revised 10 April 2006
Nichupte is estuarine lagoon connected to the Caribbean Sea by two channels with very high levels of organic carbon
in sediments, which ranged from 1.56 to 9.29 %, with the maximum concentration at the northeastern zone, known as
Bojorquez, a section with high greater anthropogenic impacts. The rate of sediment oxygen consumption (measured in the
laboratory) during summer (July) was 292 mmol O2 m-2 d-1 at Bojorquez, with a mean value of 70 mmol O2 m-2 d-1. During
winter (January), at the same site, the rate increased to 309 mmol O2 m-2 d-1 (average 125 mmol O2 m-2 d-1). The total
average for both seasons (summer and winter) was 97 mmol O2 m-2 d-1. For the whole lagoon system (48.3 km2), the total
oxygen consumption by sediments was 4.6 × 106 mol O2 d-1. During summer, the net input of oxygen from the sea to the
lagoon was 1.6 × 105 mol O2 d-1, decreasing towards winter (1.5 × 105 mol O2 d-1). The oxygen input from sea to lagoon,
represented only 3 % of the oxygen consumed by sediments, indicating that, if some of the oxygen sources like
photosynthesis or wind re-aeration are interrupted or diminished, hypoxia might occur, particularly at Bojorquez zone.
[Key words: Sediment, Nichupte, Bojorquez, oxygen consumption, BOD, lagoon, Mexican-Caribbean coast]
Introduction
Sediment oxygen consumption is a very important
process that gives a base for oxygen demand over
which other more dynamic oxygen demands (like
water biochemical oxygen demand) overcome 1. In
addition to that, the sediment oxygen consumption
together with the nutrient and organic matter content
is an indicator of the decomposition rate in sediments;
this characteristic makes it an excellent indicator of
organic matter pollution2. Furthermore, sediment
oxygen consumption is important to develop
mathematical models and thus, predict water quality 3.
Since oxygen is the most important oxidant for
respiration process of organic matter degradation, its
concentration and distribution along sediments is
critical for organic carbon decomposition and
recycling of nitrogen and phosphorus4.
The Nichupte lagoon is a shallow estuarine system
from the Mexican Caribbean Coast. Nichupte was a
pristine lagoon and gradually become a totally
modified ecosystem during last 25 years5,6. This
estuary is surrounded by urban developments that
produce a great amount of organic wastes7,8. Nichupte
_______________
*Fax: +52 (999) 9812334
Tel.: +52 (999) 1242170
lagoon is located at the oriental coast of the Peninsula
of Yucatan, Mexico (Fig. 1), is an estuary with a
barrier formed by organic and marine sediments9. The
regional weather is tropical, semi humid and the tidal
range is of 0.3 m. The lagoon has an area of
approximately 48 km2 and a depth range of 1 and
1.5 m, excepting the channel where the main access
was dredged up to 3 m of depth. During the last
decade, tourist and urban developments have
increased along the shore. These anthropogenic
activities have brought in nutrients and organic matter
to the lagoon in a direct or indirect way. The objective
of the present study is to determine how much
sediments contributes to the oxygen consumption in
Nichupte and if the lagoon is importing or exporting
oxygen to the sea.
Materials and Methods
Sediment oxygen consumption was measured
taking sediment cores and incubating them in the
laboratory. At the same time, the water flow in and
out was measured at the two channels that connect
Nichupte lagoon to the Caribbean, measuring
dissolved oxygen concentration to quantify the net
exchange between the lagoon and the sea, during both
sampling seasons (summer and winter).
228
INDIAN J. MAR. SCI., VOL. 35, No. 3, SEPTEMBER 2006
Fig. 1⎯Nichupte System Map. Sampling sites for sediments
located within the system and the two channels can be observed.
The 24 h cycles were carried out the two channels during July
2002 and January 2003.
The sampling was carried out during July 2002 and
January 2003. A hand corer (45 mm inner diameter
core, 500 mm height, Wild Co. CA USA) was used to
obtain surface sediment samples at 10 sites distributed
within the lagoon (Fig. 1). Sediment cores were
collected in two replicates and transported immediately
in acrylic tubes from the same hand corer (they were
placed at darkness and a temperature of 10 °C). These
acrylic tubes were used for incubating the samples. The
samples were incubated in the next 72 hrs. Ambient
water samples were collected (1 m depth) and kept in
darkness at 4 °C for later analyses.
A 24 hrs cycle was carried out in summer 2002 and
winter 2003, where water sample was collected every
two hours interval, for estimating the oxygen
exchange between the sea and the lagoon system, the
flow in and out of seawater was measured using the
drifting body technique10 within two channels
connecting them (Fig. 1). The dissolved oxygen and
other parameters (salinity, pH, suspended solids,
ammonium, nitrite, nitrate, phosphate and silicate)
detailed further on, were quantified in these samples.
The oxygen consumption within the sediments was
measured by incubation at the laboratory according to
Nakamura 2 using a model 55 YSI oxygen meter. This
method analyzes the oxygen concentration of the
supernatant water of the cores kept on a moderated
agitation during 2 hours, in the darkness and at 25 °C.
At the same time, a control was incubated without
sediment to estimate the oxygen consumption and the
difference was considered as the net consumption by
sediments. In all the experiments the water was
saturated with O2 at the beginning, by bubbling air for
15 min. before the incubation.
The physical and chemical characteristics of the
top 5 cm of sediments were analyzed as follow:
organic carbon content by wet oxidation with
dichromate/H2SO4 and back titration of the excess
with Fe SO411, grain-size analysis (sand, silt and clay)
was carried out with sieves and the pipette method for
fine fractions10, porosity was estimated11 by weight
loss after drying at 90 °C; pH and redox potential with
a potentiometer and glass and platinum electrodes,
respectively12, total nitrogen and phosphorus by wet
oxidation with potassium persulfate on basic and
acidic conditions, respectively13-15. In the interstitial
water extracted under nitrogen environment16, the
total sulfide was determined by oxidation with iodine
in excess17.
Dissolved oxygen concentration of water samples of
lagoon and channels were estimated with the Winkler
titration method18; oxygen saturation using Garcia &
Gordon19 equations; salinity with an induction
salinometer Kahlsico RS-9, pH with an electrode and a
potentiometer; suspended solids using a gravimetric
method where 1 L of the sample is filtered through a
micro fiber glass filter15. In addition, the concentration
of ammonium, nitrite, nitrate, phosphates and silicates
were also determined18. The biochemical oxygen
demand was measured from all the lagoon samples by
incubating for 5 days and titrating the oxygen with the
method mentioned above. The relationships between
the sediment oxygen consumption and the other
parameters were evaluated with correlation analyses.
To test differences between July/2002 and
January/2003 samplings and among the north, center
and south regions, different types of variance analysis
were run using the software Statistica20.
Results
Table 1 shows the values for the seawater
parameters at the 10 sampled sites in both seasons.
The suspended solids increased in winter, however,
VALDES-LOZANO et al.: SEDIMENT OXYGEN CONSUMPTION
their concentration were low, i.e. less than 5 mg l-1.
Part of the reason for the suspended solid behavior
was the stabilization of sediments by aquatic
vegetation. Nutrient concentrations were very low,
except for silicates.
The characteristics of the sediments sampled
during summer 2002 and winter 2003 are summarized
in Table 2. The grain-size analysis indicated that sand
was the main fraction for all the regions (north, center
and south), followed by clay and silt. The chemical
analysis indicated high organic carbon content in the
surface sediments (the mean value was 3.7 ± 2.0 %).
Very anoxic conditions (mean redox potential –320 ±
111 mV), pH values < 7 and high sulfide concentrations with an average of 2.0 ± 0.8 mM in the
interstitial water of lagoon sediments, were found as a
result of the oxidation of organic matter. The sediment
oxygen consumption values varied in different regions
of the lagoon (0-309 mmol O2 m-2 d-1) but did not
229
differ greatly between the first and the second
sampling seasons (Table 2). The rate of oxygen
consumption
by
sediments
reached
292
mmol O2 m-2 d-1 at station No. 3 in the summer, while
the mean value in the lagoon in this season was 70 ±
85 mmol O2 m-2 d-1. In the winter season the
consumption raised to 309 mmol O2 m-2 d-1 again in
station No. 3, while the total average for the whole
lagoon system was 125 ± 108 mmol O2 m-2 d-1. The
average rate for both seasons was 97 ± 100
mmol O2 m-2 d-1. Very high variance in the oxygen
consumption in the lagoon was found, meaning that the
sediment conditions are very different from one place
to another. For the total area of the whole lagoon
system (48.3 km2), the total oxygen consumption by
sediments was estimated in 4.6 × 106 mol O2 d-1.
During both sampling periods, the oxygen flux in
the lagoon channels was of similar magnitude. In
summer the importation was 1.6 × 105 mol O2 d-1,
Table 1⎯Physical and chemical characteristics of the water in the Nichupte lagoon, Quintana Roo, Mexico
Suspended Solids
Sample Temp. Salinity pH Dissolved Oxygen
BOD Ammo- Nitrite Nitrate Phos- Silicate Total Organic Inorgnium
phate
oxygen saturation
anic
(°C)
July 2002
1
32.5
2
33.3
3
32.9
4
32.9
5
33.2
6
33.7
7
32.8
8
32.6
9
33.2
10
33.2
Average 33.0
S.D.
±0.3
January 2003
1
23.6
2
24.7
3
23.0
4
22.8
5
23.1
6
22.9
7
22.7
8
23.9
9
23.3
10
23.4
Average 23.3
S.D.
±0.6
(PSU)
(μmol l -1)
(%)
(ml l-1)
(μM)
(μM)
(μM)
(μM)
(μM)
(mg l-1) (mg l-1) (mg l-1)
33.35
30.40
31.06
32.32
27.22
30.93
25.01
23.86
32.31
35.83
30.23
±3.58
7.88
7.89
7.75
7.79
7.87
7.70
7.87
7.89
7.83
7.81
7.82
±0.06
208
166
142
157
186
121
165
216
180
177
172
±27
110
88
75
83
97
64
84
109
96
96
90
±14
0.34
0.34
0.51
0.28
0.45
0.23
0.23
0.23
0.11
0.17
0.29
±0.12
2.97
8.07
9.66
3.11
3.50
3.09
4.46
1.69
1.98
1.48
4.00
±2.60
0.14
0.22
0.14
0.06
0.22
0.14
0.16
0.12
0.05
0.02
0.13
±0.06
1.28
1.15
0.45
0.46
1.46
0.76
1.60
0.61
0.18
0.20
0.82
±0.49
0.07
0.04
0.02
0.04
0.05
0.04
0.07
0.07
0.05
0.08
0.05
±0.02
8.39
15.38
11.84
16.52
16.22
16.74
18.67
13.64
13.37
4.55
13.53
±4.08
1.1
0.8
2.2
1.8
1.0
1.6
1.6
1.0
2.5
2.1
1.6
±0.5
0.7
0.7
1.4
0.9
0.7
0.9
0.9
0.8
1.1
0.9
0.9
±0.2
0.4
0.1
0.8
0.9
0.3
0.6
0.7
0.2
1.3
1.2
0.7
±0.4
27.29
29.30
27.78
26.99
24.62
25.71
24.90
28.10
26.99
27.98
26.97
±1.41
7.21
7.45
7.69
7.99
8.15
8.01
8.10
8.25
8.17
8.17
7.92
±0.33
177
182
185
189
201
193
191
253
191
233
200
±23
78
83
81
82
87
83
82
113
84
103
88
±11
0.73
1.01
1.41
1.01
0.51
0.96
0.56
0.51
0.79
0.73
0.82
±0.27
42.13
2.37
7.95
4.57
4.64
3.68
11.10
2.53
2.73
2.89
8.46
±11.53
0.81
0.18
0.37
0.23
0.42
0.29
0.45
0.12
0.11
0.04
0.30
±0.21
1.90
0.94
1.32
1.14
2.85
1.55
2.70
0.70
0.57
0.51
1.42
±0.79
0.02
0.02
0.01
0.01
0.02
0.02
0.02
0.03
0.00
0.01
0.02
±0.01
16.40
8.15
7.64
11.00
10.68
11.74
12.36
9.90
7.84
7.56
10.33
±2.64
7.3
2.3
7.9
1.7
2.2
1.4
8.4
3.5
1.7
1.5
3.8
±2.7
3.4
1.6
3.3
0.9
1.6
1.2
2.4
2.3
1.4
1.1
1.9
±0.8
3.9
0.7
4.6
0.8
0.6
0.2
6.0
1.2
0.3
0.4
1.9
±2.0
INDIAN J. MAR. SCI., VOL. 35, No. 3, SEPTEMBER 2006
230
while in winter 1.5 × 105 mol O2 d-1. The oxygen
exchange was greater at “North (Cancun) channel”
which is the deepest and with the more direct
connection to the sea. In this channel, the oxygen input
was 1.5 × 105 mol O2 d-1 in July 2002 (summer) and
1.8 × 105 mol O2 d-1 in January 2003 (winter). While at
the “South (Nizuc) channel”, the balance in July was
the input of only 0.1 × 105 mol O2 d-1 and for January
the output of 0.3 × 105 mol O2 d-1 (this channel is
shallow, long and sinuous).
Discussion
During the sampling period 2002-2003, the water
characteristics were very similar to those reported
previously for this lagoon system7,8. As expected,
temperature and salinity decreased between the
sampling carried out in July and the one in January
because of more sunlight exposure and less
precipitation during summer. Nutrient levels could be
classified as “normal concentrations” for coastal
lagoons located at this region 21-24; dissolved inorganic
nitrogen varied between 3 and 10 µMolar (μM), being
the ammonium fraction the most important (> 90 %);
phosphates concentrations were very low (< 0.1 μM)
caused mainly by a strong ion adsorption by the
calcareous sediments 25; and silicates were present at
concentrations greater than 10 μM, due to inputs of
ions from water springs 26. The oxygen saturation was
below 100 % at both seasons, suggesting that the
production of O2 gas by photosynthesis is less than
consumption. The organic matter, estimated as
Biochemical Oxygen Demand, was less than 1 ml l-1
at the majority of the sites. Although the average
value was greater on the second sampling, still they
were acceptable as “very clean water”15.
Physical and chemical characteristics of sediments
Sediments in the lagoon are mostly composed by
sand and clay, mixture with very low permeability
Table 2⎯Physical, chemical characteristics of sediments and sediment oxygen consumption (SOC) in the Nichupte lagoon, Quintana
Roo, Mexico
Sample
Total
sulfide
(mM)
Organic
carbon
(%)
Total
phosphorus
(μmol g-1)
Total
nitrogen
(μmol g-1)
Redox
potential
(mV)
pH
Sand
Silt
Clay
SOC
(%)
(%)
(%)
(mmol O2 m-2 d-1)
1.7
2.1
1.2
1.9
1.3
1.3
1.5
1.7
1.6
1.7
1.6
±0.3
1.56
3.24
7.39
2.68
2.86
3.17
3.42
1.56
2.37
3.17
3.14
±1.55
2.9
1.8
4.2
1.8
1.3
1.6
1.6
0.6
1.3
4.3
2.1
±1.2
127.4
128.2
250.6
191.1
204.5
153.6
230.3
92.7
157.9
144.3
168.1
±47.3
-310
-299
-320
-308
-318
-329
-322
-307
-318
-792
-362
±144
6.16
6.29
6.12
6.47
6.32
6.33
6.37
6.55
6.33
6.25
6.32
±0.12
82
76
74
76
60
79
60
80
72
74
73
±7
9
11
10
10
13
9
12
10
12
11
11
±1
9
13
16
14
27
11
28
10
16
15
16
±6
55
26
292
0
136
55
5
0
34
100
70
±85
1.5
2.0
2.8
1.9
3.4
1.3
2.6
3.7
1.4
4.3
2.5
±1.0
5.29
4.00
7.50
1.85
9.29
2.81
3.22
3.29
1.92
2.74
4.19
±2.34
1.8
4.8
3.5
1.6
2.4
1.1
1.5
2.1
1.1
1.7
2.2
±1.1
75.1
96.9
111.3
64.6
85.4
82.9
49.2
102.5
62.3
60.6
79.1
±19.2
-222
-284
-285
-275
-307
-262
-299
-297
-259
-296
-278.5
±24.0
7.30
7.27
7.23
7.35
7.06
7.19
7.21
7.06
7.28
7.07
7.2
±0.1
57
61
69
66
57
45
44
60
65
62
58.6
±7.9
13
18
15
14
15
19
24
17
15
18
16.8
±3.0
30
21
16
20
28
36
32
23
20
20
24.6
±6.2
0
268
309
200
62
26
64
0
104
215
125
±108
July, 2002
1
2
3
4
5
6
7
8
9
10
Average
S. D.
January, 2003
1
2
3
4
5
6
7
8
9
10
Average
S. D.
VALDES-LOZANO et al.: SEDIMENT OXYGEN CONSUMPTION
that decreases rates of reactions and processes12.
Organic carbon content in the surface sediments was
greater than any concentration reported for pristine
lagoons from the same area23, 24, 27, although less than
those reported for other tropical coastal lagoons with
mangroves28. This high content of biogenic material
was reflected also by the high total phosphorus and
total nitrogen concentration, particularly total
nitrogen. Similar concentrations have been reported
for other regional systems affected by municipal
wastes like Chelem and Ria Lagartos lagoons in the
north coast of the Yucatan Peninsula23,29. The highest
231
concentrations of organic matter, nitrogen and
phosphorus, were measured towards northeast sites
(i.e. Bojorquez, site 3) with intense tourist activity,
even though the organic matter peak corresponded to
site 5 toward the west (next to Cancun City).
Very high concentrations of organic matter in the
sediments has led to very anoxic conditions resulting
in negative redox potential, acid pH and high sulfide
concentrations in the interstitial water of lagoon
sediments, a usual characteristic in systems with
submerged vegetation and /or mangroves28,30. The
lowest pH value (6.12) and a very negative redox
'
86° 47' 30" W
'
86° 45' W
CANCUN
- 21° 07' 30" N
- 21° 05' N
21° 05' N -
CARIBBEAN SEA
Fig. 2⎯Sediment oxygen consumption (in mmol O2 m-2 d-1) isolines at Nichupte lagoon. Average for both sampling time periods.
232
INDIAN J. MAR. SCI., VOL. 35, No. 3, SEPTEMBER 2006
Fig. 3⎯The significantly correlation A) between biochemical
oxygen demand (BOD-5) in the water and the sediment oxygen
consumption at Nichupte lagoon (r = 0.63, p < 0.05), B) between
total phosphorous in sediment and the sediment oxygen
consumption at Nichupte lagoon (r = 0.62, p < 0.05).
Table 3⎯Oxygen consumption by the sediments at different areas
Area / reference
Consumption
(mmol O2 m-2 d-1)
22-84
South River, North Carolina, USA33
Neuse River, North Carolina, USA33
22-59
34-159
Texas coast, USA34
San Francisco Bay, CA, USA35
13-22
0-106
Fourleague Bay, LA, USA36
25-72
Patuxent River, MD, USA37
69-234
Cadiz Bay, Spain38
Willamette River, Portland, Oregon, USA39
41-128
0.0-28
Chesapeake Bay, USA40
Odense Fjord, Denmark41
31-153
3-41
Mobile Bay, Alabama, USA3
31-169
Rouge River, Michigan, USA42
3-28
Port Phillip Bay, Australia4
13-241
Thames River Estuary, UK32
50-53
Upper Klamath Lake, OR, USA43
Mid-Atlantic Bight, NJ, USA44
16±6
34-72
Seto Sea, Japan2
Cauca River, Colombia45
44-228
Nichupte, Mexico (this work)
6-309
potential (-320 mV) were recorded for site 3 (at
Bojorquez zone), as a consequence of the oxidation of
organic matter, even with the presence of
macrophytes which have been reported to raise the
redox potential31.
The sediment oxygen consumption in Nichupte
lagoon had a large spatial, but low temporal variation;
Fig. 2 shows the lagoon map with isolines of the
average oxygen consumption rates. The isolines
clearly demonstrate the Bojorquez region (station No.
3, northeast of the lagoon system) as the zone with
highest oxygen consumption. As mentioned above,
this region is surrounded by tourist developments
(hotels, restaurants and golf fields) and the water
circulation is very poor (previous studies reported a
water residence time of 2.7 years7, 8). This long water
residence time causes accumulation of organic matter
in the sediments which increase the demand of
oxygen for its degradation. The rates of sediment
oxygen consumption were positively and significantly
correlated with both, water BOD (r = 0.63, p < 0.05)
and total phosphorus concentration of sediments (r =
0.62, p < 0.05), suggesting an intrinsic connection
between the processes occurring in water and
sediments (Fig. 3 A and B). Table 3 shows the oxygen
consumption by sediments at different studied sites,
the findings of this study are quite similar to those
reported previously. However, at the north region of
Nichupte (Bojorquez), the oxygen consumption was
greater than the majority of the early results (Table 3).
The peak value reported was 241 mmol O2 m-2 d-1
corresponding to the River Tamesis estuary 32. This
result reinforces the conclusion of that, in the
Bojorquez region, there is a great demand of oxygen
consumption by the sediments.
The flux estimations of dissolved oxygen in the
two channels that connect the lagoon with the sea
indicate a net input of this gas from the sea towards
the lagoon, during both summer and winter, which
means that Nichupte Lagoon is an oxygen importer
system. According to the estimated amount of oxygen
imported from the sea, and the oxygen consumption
by the sediments of the whole lagoon, the first one
represents only a 3 % of the oxygen consumed by
sediments. This clearly indicates that the lagoon is in
a critical condition and that, if some of the major
oxygen sources (photosynthesis and wind re-aeration)
might be interrupted or diminished, dissolved oxygen
will decrease and hypoxia might cause adverse
ecological consequences, particularly at Bojorquez
VALDES-LOZANO et al.: SEDIMENT OXYGEN CONSUMPTION
zone, where the sediment oxygen consumption was
greater than at the other sites.
19
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