dissolved methane and oxygen depletion in the two coastal...

Indian Journal of Geo Marine Sciences

Vol. 46 (07), July 2017, pp. 1287-1297

Dissolved methane and oxygen depletion in the two coastal lagoons,

Red Sea

Mohammed I. Orif, Yasar N Kavil*, Rasiq Kelassanthodi, Radwan Al-Farawati, & Mosa I. Al Zobidi

Marine Chemistry Department, Faculty of Marine Sciences, King Abdulaziz University, P.O. Box 80207, Jeddah 21589, Saudi

Arabia

* [E-mail: [email protected]]

Received 12 January 2017 ; revised 27 February 2017

The emission of methane (CH4), a major greenhouse gas, from polluted lagoons is of scientific interest. Polluted basins are conducive

to CH4 production through microbial activity. This study presents a baseline dataset on dissolved CH4 from two Red Sea coastal

lagoons. These lagoons are extremely polluted, primarily due to extensive sewage dumping. Both the lagoons were experienced

severe oxygen depletion. Nitrate deficit value was negative and most of the nitrate were lost through denitrification pathway. The

observed ammonia concentration was also high. Methane concentrations in both lagoons were measured, and maximum

concentrations were observed at the bottom waters of the lagoons and minimums at the surface. Average surface methane

concentrations were 0.16 and 4.09 µM in the Al-Shabab and Al-Arbaeen lagoons, respectively, and those of the bottom were 3.11

and 13.2 µM, respectively. The diffusive flux of methane from the bottom to surface waters in the Al-Arbaeen lagoon was notably

high, and significant hydrogen sulfide production was also observed. Methanotrophic bacterial activity occurred in the oxic

environment of the water column. Organic matter decomposition led to an oxygen-depleted system which will enhance the different

nitrogen transformation processes.

[Keywords: methane, oxygen depletion, Al-Shabab and Al-Arbaeen lagoons, sewage, methanogenesis, Red Sea coast]

Introduction

Atmospheric methane (CH4) is a potent

greenhouse gas. CH4’s contribution to the global

warming effect is 25 times more per molecule

than that of CO21. Over the last several decades,

the CH4 concentration in the atmosphere has

increased drastically, primarily owing to

anthropogenic activities. Specifically, the average

atmospheric concentration of CH4 has risen to

156% of the pre-industrial level of 1789 ppb1,

although its accumulation rate has declined in

recent years2. Atmospheric levels of CH4 have

been extraordinary in at least the last 650 kyr3.

Direct measurements of CH4 over the last 25 years

show that although the abundance of CH4 has

increased by approximately 30% in that time, its

growth rate increased by 1%/year in the late 1970s

and early 1980s4. The reasons for the slow growth

rate and the implications for future changes in CH4

concentration are not yet clear5, but this

phenomenon is believed to be related to the

imbalance between CH4 sources and sinks. The

removal of methane from the atmosphere is

possible through its reaction with hydroxyl

radicals6 but this reaction may reduce the

tropospheric oxidizing capacity. Other minor CH4

removal methods include the reaction of CH4with

chlorine 7, 8

and its destruction in the stratosphere

and soil sinks 9

. The main sources of methane in

marine systems include agriculture, especially

paddy fields; land erosion; and wastewater

treatment. The in-situ production of methane is

favored by methanogenic bacteria, which are

active in water with low oxygen content. Since

microbial production of CH4 cannot take place in

oxic environments 10

, CH4is believed form

primarily within the reducing interiors of particles.

mailto:[email protected]

INDIAN J. MAR. SCI., VOL. 46, NO. 07, JULY 2017

Evidence for such a source has been provided by

Owens et al. (1991)11

, Karl and Tilbrook (1994)12

,

and Marty et al. (1997)13

through incubation

experiments. The concentration of CH4 in

sediment depends mainly on organic matter

deposition. The biogeochemical cycling of

methane is purely dependent on the redox state of

the environment, which is related to ambient

dissolved oxygen (DO) concentration. Oxygen

depletion in aquatic systems can be due to natural

and anthropogenic activities. However, all natural

O2-depleted zones have arguably been affected by

human activities in a comparable manner Lagoons

are among the most common near-shore coastal

environments, occupying 13% of the world’s

coastline14

. Several studies have addressed the

CH4 efflux to the atmosphere from shallow or/and

intertidal lagoon sediments15, 16, 17

.

Over the past several decades, the number of

coastal hypoxic zones due to human activities has

increased steadily18

(Diaz and Rosenberg, 2008).

The CH4 contribution of coastal waters is

basically from marshland sources19, 20, 6, 21

. High

loads of nutrients and organic matter are the main

contributors to this ubiquitous oxygen depletion in

polluted lagoons. The main sources of these

matters are terrestrial via waste disposal and the

input of drainage systems. Bulk build-up of CH4

in the open ocean does not occur by

methanogenesis, as microbial production of

methane from CO2 or acetate is restricted by other

electron acceptors, such as oxygen and sulfate.

Materials and Methods

The eastern coast of the Red Sea is approximately

1930 km long, and 90% of the coastline belongs

to the Kingdom of Saudi Arabia22, 23

. Jeddah is

one of the major economic urban cities along the

Red Sea coast, lying in the central part of the Red

Sea. According to a survey by the Jeddah

Municipality, in 2006 the population of Jeddah

surpassed 3 million and is increasing drastically24

.

Such a large population and industrialization with

oil refineries and chemical industries have caused

excessive waste production, with waste disposal

becoming a major issue for the city. Treated as

well as non-treated wastes were being dumped

into nearby areas, such as the southern Corniche

area25, 26, 27

, Al-Arbaeen lagoon28

and Al-Shabab

lagoon29

. Compared with the Al-Arbaeen lagoon,

the Al-

Shabab lagoon is relatively small with a surface

area of approximately 145×103 m

2 and a volume

of approximately 1.1×106 m

3, while the Al-

Arbaeen lagoon has a surface area of 254×103 and

a volume of 1×106 m

3. The Al-Shabab lagoon has

an elongated basin spread in the northeast

direction, offering enhanced connectivity with the

Red Sea; thus, permanent water exchange with

open water takes place in the upper 2-msurface

layer30

. Conversely, the Al-Arbaeen lagoon has a

more complicated “T” shape comprising two

loops extended in an almost north-south direction

and a channel connecting the inside basins.

Renewal of bottom water rarely occurs during

storms and rough weather conditions in both

lagoons. The level of oxygen is declining

drastically, and there is a great risk of H2S

production in those areas. During spring, the

preferential consumption of NH4+ and the

development of NO3-are observed along the Red

Sea coast31

. These are likely indicators of

nitrification, while towards the lagoon,

denitrification is suspected as dominant pathway.

The latter condition is favorable for the

production of methane in this area. As we

described previously, the effects of methane may

include changes in weather conditions over the

Jeddah coast.

The data were collected in the present study

during June 2015. Sampling locations of both

lagoons are shown in Fig.1. Water was sampled

using a peristaltic pump. Sub-samples for DO,

hydrogen sulfide and methane measurement were

collected carefully without any bubbles. DO

samples were analyzed using Winkler’s method

on the same day of collection. Temperature,

salinity and pH of samples were measured using a

YSI Multi-parameter Sonde (U.S.A.). Salinity

samples were standardized by an MS-310 Micro-

salinometer, and pH samples were standardized

using a pH meter. Nutrient samples were

preserved at 4°C and measured using the

Grasshoff method with a spectrophotometer

(Shimadzu UV-2450). Samples for hydrogen

sulfide measurement were analyzed

spectrophotometrically using the methods of Cline

(1969)32

. Water samples for CH4 measurement

were collected in 60-ml reagent bottles

immediately following sampling for DO. Samples

were injected with saturated mercuric chloride

(0.3ml/60ml), halting microbial activity.

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ORIF et al.: DISSOLVED METHANE AND OXYGEN DEPLETION, RED SEA

Figure 1: Map showing the study locations at Al-Shabab and

Al-Arbaeen Lagoons

Dissolved CH4levels in the samples were

determined using the multiple-phase equilibration

technique33

. Briefly, 25ml of the sample was

equilibrated successively with an equal volume of

ultrapure helium in a gas-tight syringe via

vigorous shaking of the syringe at room

temperature (25°C) for 5 minutes. After

equilibrium was reached, the headspace was

injected through a 5-ml sampling loop into a gas

chromatograph (GC, Shimadzu-2010) equipped

with a flame ionization detector. Separation was

achieved over a stainless-steel column that was

1.8m in length with an inner diameter of 2mm and

packed with a molecular sieve (5A 80/100-mesh,

Toshvin) maintained at a temperature of 60°C.

Detector was calibrated at regular intervals using

a standard gas mixture of CH4 (MED gas), and the

gases were calibrated against 4.3ppm of CH4

standard reference material. For confirmation of

the linearity of the detector with the gases, one

laboratory standard was analyzed after each 10

samples. Ambient concentration of CH4was

measured through the collection of atmospheric

gas samples in a 60-ml gas-tight syringe and

direct injection of the samples into the GC in the

same manner as that of the sample headspace.

Results and Discussion

The basic chemical and physical changes along

the two lagoons were dissimilar. Temperature and

salinity varied from 29.9°C to 31.56°C and from

33.53 to 38.15 ppt, respectively, along the Al-

Shabab lagoon (Fig. 2a and 2b).

Figure 2 : Contour plots showing the vertical distribution of,

(a) Temperature, (b) Salinity and (c) Dissolved Oxygen at

Al-Shabab Lagoon

There was not much vertical and horizontal

variation in temperature, but salinity did vary

greatly in both directions. As mentioned

previously by El Sayed et al. (2011)34

, the first

mixing layer has high temperature and low

salinity. The average salinity of the Red Sea has

been reported as 39ppt23

, while the coastal water

of the Red Sea at Jeddah has a comparatively

higher salinity, averaging 39.4 ppt35, 36

. As we

mentioned in section 2, the Al-Shabab lagoon has

greater connectivity with the open water of the

Red Sea than the Al-Arbaeen lagoon. Salinity and

temperature along the Al-Arbaeen lagoon varied

from 26.74 to 34.48 psu and 29.2°C to 30.27°C,

respectively (Fig. 3a and 3b).

Figure 3: Contour plots showing the vertical distribution of,

(a) temperature and (b) Salinity at Al-Arbaeen Lagoon

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The obtained distribution of salinity along both

lagoons demonstrates the greater connectivity of

Al-Shabab lagoon with the open sea. Table 1

(Supplementary Table) shows the pH data from

the Al-Shabab and Al-Arbaeen lagoons. The pH

value of Al-Shabab lagoon exhibited decreasing

trend with depth and showing the average value of

8.20 at the bottom of the lagoon, which is in the

range with the average pH of Red Seawater.

Range of pH along the Al-Shabab lagoon were

8.10-8.72 and that of Al-Arbaeen lagoon were

7.60-8.71. In the Al-Arbaeen lagoon, the pattern

was slightly more interesting. pH was less than

that of the Al-Shabab lagoon, and the average

value at the bottom is 7.81. This trend may reflect

sewage disposal, which, in narrowed aquatic

environments, results in reduced pH owing to

organic matter decomposition and the release of

CO237

.

Continuous dumping of approximately 100,000

m3 of treated and raw sewage into the lagoons for

several years led to the depletion of DO.

According to the official source of Jeddah

Municipality, since 1996, municipal sewage and

wastewater disposal into the lagoons has ceased.

However, studies by Turki (2002, 2007)29, 38

and El

Sayed (2002a)26

have reported that the

environmental conditions are worsening due to

the dumping of waste, which will continue to be a

problem into the future. In the present study, we

noticed an unpleasant smell from the lagoons.

Recent extension of the Jeddah Islamic Port has

restricted the circulation of water. The extension

has also created an external lagoon with a

combination of seawater and wastewater that is

discharged into the two lagoons. External water

body of the lagoons is receiving wastewater with

an enormous organic load from the city’s fish

market. Such conditions contribute to oxygen

depletion along both lagoons. Spatial and vertical

distributions of DO along both lagoons are shown

in Fig. 2c and 4a.

We observed that DO is being depleted vertically

in both lagoons. Renewal of bottom water takes

place only rarely, during storms and in rough

weather. This vertical depletion leads to the

accumulation of organic matter and the

development of anoxic conditions in the bottom

layer34

. In the Al-Shabab lagoon, we observed

hypoxia at the bottom depths of station1 with a

value of 63.07µM. There were decreasing trends

of DO with increasing depth in the rest of the

stations.

Figure 4 : Contour plots showing the vertical distribution of,

(a) Dissolved Oxygen and (b) Hydrogen sulfide with stations

in Al-Arbaeen Lagoon

The DO trends along the Al-Arbaeen lagoon were

more interesting. We noted anoxia at stations 2

and 3 at the bottom of the lagoon, as well as

corresponding hydrogen sulfide production. The

most interesting finding was at station 4, where

the entire water column was anoxic, with

H2Sproduction throughout the depth up to 0.5

meter. Maximum H2S concentration observed at

station 4 was 55.54µM at the bottom. The

observed H2S concentrations along the Al-

Arbaeen lagoon are shown in Fig. 4b.

Dumping of sewage appeared to be the primary

source of nitrogen and phosphorous to the surface

waters of the study area37

. The concentrations of

N and P in the Al-Shabab and Al-Arbaeen

lagoons were greater than those in the Red Sea39

.

Distributions of nitrite, nitrate, phosphate and

ammonia along the Al-Shabab and Al-Arbaeen

lagoons are shown in Figs. 5 and 6, respectively.

The results of analysis show that the

concentrations of oxidized forms of N2were

relatively low. Their average concentrations in the

Total Inorganic Nitrogen (TIN) varied from

1290


47.73% along the surface to 17.65% along the

bottom of the Al-Shabab lagoon.

Corresponding average concentrations in the Al-

Arbaeen lagoon were 39.17% at the surface and

1.49% at the bottom. The nitrite and nitrate

distribution pattern of both lagoons showed the

decreasing trend with increasing depth.

Figure 5 : Contour plots showing the vertical distribution of

nutrients in Al- Shabab Lagoon


nutrients in Al- Arbaeen Lagoon

The nitrate deficit (N*) values were estimated as

explained by Deutsch et al,200140

. The observed

range of N* values in the Al-Shabab lagoon were

-1.38─ -78.96 and that of Al-Arbaeen lagoon

were -4.76─ -125.35.

The recorded negative N* as mostly caused by the

loss of nitrate through the denitrification process.

By this explanation, the loss of nitrate by the

denitrification process could be extensive even in

the oxic regions41

.

The concentration of ammonia was quite high at

the bottom of the Al-Arbaeen lagoon. Turki et al.

(2002)29

demonstrated gains of NO2- and NO3

-

with the loss of NH4+. Negative correlation

between NH4+ andNO2

++NO3

- and distribution of

ammonia and nitrite+nitrate with depth at the Al-

Arbaeen lagoon supportsN2-transformation (Fig.

7).

Figure 7: The vertical distribution of Nitrite+Nitrate and

Ammonia Al- Arbaeen Lagoon

The observed NO2- + NO3

- values are higher at the

surface and values are getting depleted while

going towards the bottom depth at Al-Shabab

lagoon. The values were in the range of 0.23 to

47.88µM. Maximum concentration was noted at

the inner surface of the sampling location (S1).

While moving towards the mouth of the station

(S4) the values were getting lowered at both

surface and bottom length of the water column.

The concentration of phosphate was in the range

of 0.34-7.44µM. Highest concentration was

obtained at the surface of the mouth. Profile of

ammonia was showing an increasing trend

towards the mid depth and away from S1 to S4

too. The values were in the range of 0.21-

50.52µM. The trend of NO2- + NO3

- at Al-

Arbaeen lagoon follows as Al-Shabab, but the

values were comparatively little higher and was in

the range of 1.55-50.55µM. The values are getting

depleted sharply towards the bottom depth. Level

1291



Methane (a) Al-Shabab Lagoon and (b) Al-Arbaeen Lagoon

of phosphate was quite higher than that of Shabab

water column. Maximum concentration observed

at the bottom depth of the station 3. Values were

in the range of 5.04-12.10µM. The observed range

of ammonium ion was 26.98-279.39µM. Values were comparatively far higher than that of

Al-Shabab water. But the trend was almost similar

to the Shabab water. The highest concentration

was noted at the bottom depth of station 4. From

the distribution patterns of nutrients along both

lagoons, we confirmed that phosphate was the

limiting nutrient.

The spatial and vertical distribution of methane in

the surface waters of the lagoons resulted from the

balance between transport, outgassing to the

atmosphere, production and oxidation in waters

and sediments of the lagoons. When the water

column is anaerobic, it can support significant

methanogenesis42

. Methanogens are strictly

anaerobic microorganisms that produce CH4, and

their most common habitats are freshwater and

saline sediments43

. Considering the influence of

pore water on the water column through gas

diffusion44

, we would expect higher CH4

concentrations in the overlying layer of the

sediments, which should also be an oxygen-

depleted zone. Both of these factors favor the

production of methane in the bottom layer45

. The

distribution of methane along the Al-Shabab and

Al-Arbaeen lagoons is shown in Fig. 8a and 8b,

respectively.

Figure 9 : The relationship pattern of Dissolved Oxygen and

Methane with depth in Al-Shabab Lagoon

Compared with the Al-Shabab lagoon, the Al-

Arbaeen lagoon’s methane production was quite

high. Both lagoons exhibited an increasing trend

of methane concentrations with depth. The formed

methane has a tendency to diffuse toward the

surface. The most likely mechanism behind the

transport of methane to surface water is

bubbling46

. The fraction of methane bubbles that

passes through the water column and reaches the

atmosphere primarily depends on water depth and

bubble size.

However, if the upper layer of the water

column is oxic, a considerable amount of CH4 will

be oxidized by methanotrophic bacterial activity47

.

The bacteria produced methane in anoxic

sediments/water column that was oxidized in

aerobic zones48, 49

. Hence, anoxia is the primary

factor in methane production and the sustenance

of methanogens. In our study, whenever the water

column was oxic, the formed methane in the

bottom layer was oxidized while it diffused into

the upper layer.

The production rate of methane increased with

depth12

, along with a high rate of loss in the

surface

1292


Figure 10 : The relationship pattern of Dissolved Oxygen and

Methane with depth in Al-Arbaeen Lagoon

.

Figure 11 : The vertical distribution of NO2-+NO3

- and

methane in Al-Shabab Lagoon

layer which could overpower the higher potential

rate at shallower depths50

. Average surface

concentrations of methane along the Al-Shabab

and Al-Arbaeen lagoons were 0.16 and 4.09µM,

respectively, and those along the bottom of the

lagoons were 3.11and 13.2µM, respectively. This

trend of methane is clearly correlated with that of

DO. The correlation patterns of dissolved oxygen

and methane are shown in Figs. 9 and 10,

respectively. The correlation values between DO

and CH4 in the Al-Arbaeen lagoons were -0.907, -

0.686 and -0.669 at the stations, S1, S2, and S3

respectively. Due to the complete anoxia at S4

water column, the correlation factor was not

significant. The corresponding correlation factor

values of S1, S2, S3, and S4 at Al-Shabab lagoon

were -0.892, -0.627, -0.987, and -0.816

respectively. A negative correlation was found in

the upper-layer oxidation of methane and was

more pronounced in the Al-Shabab lagoon than

the Al-Arbaeen lagoon.

Figure 12 : The vertical distribution of NO2-+NO3

- and

methane in Al-Arbaeen Lagoon

The rate of gas exchange between the surface

water and the atmosphere is a function of wind

speed, and wind speed triggers the mechanism of

methane flux to the atmosphere51

. An increase in

1293


wind speed to 15 m/s or more52

can lead to a six-

to-seven times higher flux between water and air.

The analyzed lagoons have experienced

occasional dust storms, which might have

increased the flux of methane into the atmosphere

considerably. An enormous flux of methane was

noted at station 4, where we obtained the

maximum H2S concentration in the water column.

The flux in the Al-Arbaeen lagoon was fairly

large compared to that of the Al-Shabab lagoon.

To the best of our knowledge, there has not yet

been a study regarding the flux of methane from

these lagoons and the Red Sea coast. In both

lagoons, nitrate and nitrite were being consumed,

and the production of methane occurred with the

decrease in DO, as shown in Fig.11 and Fig. 12.

These systems reflect simultaneous

methanogenesis and nitrogen transformation. For

the Al-Shabab lagoon, our data suggested that

most of the methane produced at the bottom was

oxidized when it diffused into the surface layer

owing to methanotrophs53, 54

.

Conclusions

The production and consumption of methane in

both lagoons primarily depend on the depletion of

oxygen. This environmental dilemma affects both

physical and chemical aspects of the system.

Quality of water is notably poor, and from the

assessment of nutrient contents, both systems

were observed to have experienced N-

transformation processes. Interpreted negative

nitrate deficit as mostly caused by the loss of

nitrate through the denitrification process.

Ammonium is the major contributor to nitrogen

nutrients. Apart from those findings, the Al-

Arbaeen lagoon showed relatively high H2S

concentrations, especially in location 4, reflecting

plummeting water quality. The conditions in both

lagoons favor the high production of methane.

Acknowledgments

This project was funded by the Deanship of

Scientific Research (DSR) at King Abdulaziz

University, Jeddah, under grant no. 440/150/1436.

Authors, Yasar N Kavil and Rasiq Kelassanthodi

are grateful to the Deanship of Graduate Studies,

King Abdulaziz University, for providing a Ph.D.

fellowship.

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Appendix

Table 1 : pH of Al- Arbaeen Lagoon (Supplementary Table)

Al-Arbaeen lagoon Al –Shabab Lagoon

Station Depth(m) pH Depth (m) pH

1

0 7.6 0 8.44

0.5 7.87 0.5 8.35

1 7.93 1 8.47

2.6 7.84 2 8.34

3.2 8.10

2

0 8.71 0 8.47

1 8..01 0.5 8.60

2.2 7.86 1 8.40

2 8.30

3.1 8.10

3

0 8.60 0 8.47

1 8.10 0.5 8.60

2.3 7.80 1 8.40

2 8.30

3.1 8.10

4

0 7.97 0 8.62

1 7..88 0.5 8.72

3.2 7.74 1 8.42

2 8.37

3.4 8.26

1297

dissolved methane and oxygen depletion in the two coastal...

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