Scholarly article on topic 'Methane flux from sediment into near-bottom water in the coastal area of the Puck Bay (Southern Baltic)'

Methane flux from sediment into near-bottom water in the coastal area of the Puck Bay (Southern Baltic) Academic research paper on "Environmental engineering"

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Similar topics of scientific paper in Environmental engineering , author of scholarly article — Andrzej Reindl, Jerzy Bolałek

Academic research paper on topic "Methane flux from sediment into near-bottom water in the coastal area of the Puck Bay (Southern Baltic)"

Oceanological and Hydrobiological Studies

International Journal of Oceanography and Hy drobiology

Volume 41, Issue 3

ISSN 1730-413X elSSN 1897-3191

(40-47) 2012

VERS ITA

DOI: 10.2478/s13545-012-0026-y Original research paper

Received: Accepted:

April 10, 2011 November 14, 2011

Methane flux from sediment into near-bottom water in the coastal area of the Puck Bay (Southern Baltic)

Andrzej R. Reindl*, Jerzy Bolalek

of Marine Chemistry and Environmental Protection, Faculty of Oceanography and Geography, University of Gdansk, Al. Mars%atka Pitsudskiego 48, 81 -378 Gdynia, Poland

Key words: Baltic Sea, methane emission, coastal area, sediment, benthic chamber

Abstract

This work presents the results of the study on methane emission from the sea bottom in the coastal zone of Puck Bay. The investigations were conducted from June through September 2010 at seven sampling sites located along the Hel Peninsula. The research results indicate that the methane flux rates vary periodically. Methane emission from seabed into near-bottom water in the coastal zone of Puck Bay along the Hel Peninsula ranged from 0.91 mmol m-2 d-1 to 49.15 mmol m-2 d-1.

' Corresponding author: andrzej.reindl@ug.edu.pl

Introduction

The seas and oceans play a significant role in biogeochemical circulation of various gases, which influence the climate. This has a strong impact on the atmosphere and qualitative aspects of climate, mainly due to long half-life of methane in the troposphere. Consequently methane is one of the most important greenhouse gases (IPCC 2007, Wegener 2008). The presence of methane in marine ecosystems has been confirmed by numerous studies of near-bottom waters and benthic sediments (Bange et al. 1994, Christian and Cranston 1997, Galchenko et al. 2004, Tortell 2005). The presence of this greenhouse gas in the Baltic Sea was also confirmed, based on the investigations into benthic sediments in the Beltsee and Langeland water basins (Skagerrak and Kattegat area) and the Arkona Basin (Jörgensen et al. 1990, Jensen and Fossing 2005, Mathys et al. 2005). The acoustic and chemical analyses conducted in Eckenförde Bay have demonstrated the presence of methane in benthic sediments and near-bottom water (Wilkens and Richardson 1998, Martens et al. 1999, Schlüter and Sauter 2004, Judd and Hovland 2007). Moreover, the harbor sediments sampled in the area of Howaldtswerke (Kiel, Germany) contained methane (Schmaljohan 1996). Methane emission from sediment into near-bottom water was detected in the Gotland Deep, the coastal zone of the islands Rügen and Hiddensee, and in the estuaries located in the Bothnian Bay (Piker et al. 1998, Heyer and Berger 2000, Liikanen et al. 2009).

The topic of methane and its emission sources into the atmosphere is related, inter alia, to human impact on the environment. This impact is responsible for the presence of gases with long halflife time in the troposphere, such as methane (IPCC, 2007). At the same time, it has been hypothesized that microbial decomposition of organic matter in a

natural habitat under anoxic conditions is the largest emission source of methane in the world (Wegener 2008). Wetlands, swamps, tundra and the marine ecosystems are specific natural niches where the amount of methane is reduced before reaching the atmosphere via consumption processes (Edlung 2007, Wegener 2008). The climate change and in consequence Arctic ice sheet reduction stimulates researchers to pursue studies in the field of greenhouse gases emission into the atmosphere (McGuire et al. 2006, Serreze et al. 2007).

The mechanisms that determine the chemistry of the Puck Bay waters have been discussed in previous publications (Bolalek et al. 1993, Geringer d'Oedenberg 2000). Data used to assess the existing state and trends in the deep water basins, as well as information on the content of nutrients, organic matter, and geochemical and bacteriological conditions in the dredged areas have also been presented elsewhere (Bolalek 1993, Bolalek et al. 1996, Graca 2004, Graca and Dudkowiak 2007, Graca 2009). Although there is some information on the gas phase in sediment from the Baltic Sea (Schmaljohan 1996, Thiessen et al. 2006, Judd and Hovland 2007), there is no information on the methane content in gas bubbles. Furthermore, there is no data on the methane emission from sediment into near-bottom seawater in the polish coastal area of the Baltic Sea.

The aim of the experimental work was to determine the variability of methane concentration in near-bottom water after a specific time of exposure to the methane flux from the sediment. In addition, we were degassing the areas where methane in the gas phase was detected to determine the methane concentration in gas bubbles found in sediment.

Materials And Methods

Study area and experimental setup

The research was carried out in the coastal area of Puck Bay along the Hel Peninsula. All of the sampling sites were located in the shallow area of the bay. The maximum water temperature at the coastal site on the top of the peninsula was measured for the period from June to August (Nowacki 1993). The results of the previous research showed that coastal sediments of Puck Bay along the Hel Peninsula contain from 1% to 2% of organic matter (Jankowska and L^czynski 1993). During the study period, we observed the organic fraction in some

parts of the study area. This matter was covered with sand as a result of waves. At some sites that contain this fraction, we detected free gas bubbles. The sediment in most of the sites was sandy. In some areas we detected the organic fraction in the benthic sediment or under the sand (e.g. stations: P4, P6). We also observed a thin layer of cake in surface sediment (site P7). In addition, the research area is supplied with the dissolved organic fraction, nitrate, phosphate and other pollutants from Rivers effecting and waste water treatment plants located on the peninsula (Bolalek et al. 1993).

The study area was located in the coastal waters of Puck Bay. Samples were taken during three sampling series in 2010, i.e. 14 — 15 June, 24 — 26 July and 4 — 5 September. The measurements of methane fluxes from benthic sediments into the water column were taken at seven sampling sites where chamber experiments were performed. Sampling sites were situated in the coastal zone of Puck Bay extending from Jurata to Wladyslawowo. Free gas bubbles were detected at three sites from which sediment was mechanically degassed to obtain the gas phase (Fig. 1).

All sampling sites were located at the distance not longer than 10 m from the shoreline. Benthic chambers were placed on the seabed at the depth ranging from 40 to 50 cm below the sea surface.

Research methodology

Methane flux determination

Determinations of the methane flux emitted from the bottom to the water column are commonly done by using an air-tight chamber. According to that technique, the analyzed part of the sediment is being enclosed in order to assess the concentration of methane in near-bottom water after exposure to flux from sediment. In order to investigate the methane fluxes, plexiglass benthic chambers were used. The chamber base was a square with the side length of 8.9 cm. Such dimensions allowed for enclosing 79.21 cm2 of the sediment surface area. The chamber used in the experimental work was 12.5 cm high and a closable sampling port for drawing a sample of near-bottom water was located ca. 1 cm below the top edge of the chamber. With much care taken to avoid the possible degassing of the sediment, benthic chambers were pushed ca. 3 cm into the sediment.

Samples of near-bottom water were taken twice: before and ca. 24 hrs after the exposure. The six cm3

60° N

55° N

Fig. 1. Research area, location of sampling stations and free gas bubbles in benthic sediment.

J PS ■OS'y

Puck Bay №

Sampling points

• Pi - Jurats

A Gdynia ^H • P2 - Jastarnia

• P3 - Kuznica (1) • P4 - Kuznica (2) • PS - Chalupy (1) • P6 - Chalupy (2) • P7 - Wtadystawowo

of near-bottom water was collected with a syringe, and transferred into the previously prepared nontransparent chromatographic vials. Non-translucent glass vials were used in order to eliminate photodegradation of the gases present in the analyzed near-bottom water sample. 20 cm3 vials were prepared in the lab by placing 2 g of sodium chloride in each vial and closing them under atmospheric air with stoppers made of butyl rubber. Before exposure, a control sample of near-bottom water was collected at each sampling point and for each sampling period. Control samples were treated the same way as the samples of water exposed to the flux of gases emitted from the sediment. In order to characterize the diel changes occurring in the exchange zone between the sediment and near-bottom water, the exposure time of ca. 24 hrs was used.

Determination of the dissolved gases

The methodology for determination of the methane content in near-bottom water was based on the change of saturation conditions of this gas in response to salinity. Methane escapes into the gaseous phase from the sampled water when solubility conditions change and the sample is subjected to mechanical degassing via shaking (Schmaljohann 1996, Piker et al. 1998, Sommer et al. 2006). The near-bottom water samples were collected from chambers into the chromatographic vials by

using a syringe, and immediately closed with the rubber stoppers. The proper placement of each stopper was secured with an aluminum cap. Such samples were shaken at 230 rpm for a minimum of 1 hour on a VWR Digital Vortex Mixer. Because the methane saturation in water depends on salinity (Amouroux et al. 2002), the technique based on bringing the sample salinity to the maximum value results in the transformation of methane dissolved in water into the gaseous phase. The methodology used in this study predicts that 99.5% of the gases dissolved in the analyzed water sample will end up in the gaseous phase (Schmaljohann 1996).

Gas bubbles sampling

Gas contained in benthic sediment was sampled in situ by mechanical degassing. The obtained gas was collected into non-transparent 20 cm3 vials. Because the vial diameter was rather small and therefore it would have made the mechanical degassing impossible, a pyramid-shaped funnel was used during sampling. A sampling apparatus of similar design was previously used by Chanton et al. (1989) for sediment degassing. In this study the original apparatus has been modified by removing the flow ducts, and gas was collected directly into the vials. The purpose of the sampling apparatus was to increase the surface area of the degassed sediment and at the same time, to minimize the volume of gas sample vials. After sediment degassing, the vials filled with gas were

closed under water with butyl rubber stoppers. After bringing vials to the surface, the stoppers were additionally secured with aluminum caps. Such samples were immediately transported to the laboratory to determine the content of methane in the collected gas.

Quantitative analysis of methane

Determinations of the methane content were performed on the samples of gases salted out from near-bottom water collected in the vials prepared beforehand. The recommended gas chromatography with a flame ionization detector (GC-FID) method was applied to obtain the methane measurements (DeLaune et al. 1983, Martens et al. 1999, Liikanen et al. 2009) using a Perkin Elmer Autosytem XL gas chromatograph equipped with a standard non-polar HP-5 capillary column (30 mm x 0.32 mm outer diameter; 0.25 mm inner diameter). Helium was used as a carrier gas at the flow rate of 3 cm3 min-1. Chromatographic conditions allowed methane detection at the concentration level of 1 mg kg-1 (0.0001%) per sample.

Calculations

The chromatographic result of the percentage methane concentration was recalculated into molar concentration of methane by using the defined volume of the gaseous phase in a chromatographic vial (HeadSpace) and applying Avogadro's law. The measurement of methane in near-bottom water before exposure was always below the detection limit. This indicates no methane concentration in the prepared vials and no methane concentration in near-bottom water before exposure. The flux calculations were obtained using the following formula (Bolalek 1993):

j=- H AC

where:

J — flux of dissolved compounds emitted from

sediments into water (mol m-2 day-1) H — chamber height (m)

AC — change in concentration of the dissolved gas

due to exchange between the sediment and near-bottom water (mol dm-3 day-1) At — exposure time (day).

Results

Based on the obtained data, it was established that the methane flux from the Puck Bay seabed into the water column tended to increase from June until September (Fig. 2). In June, the mean methane flux from the sediment into near-bottom water was 3.29 mmol m-2 d-1 (SD 2.59 mmol m-2 d-1), while in July and September, it was 10.12 mmol m-2 d-1 (SD 10.01 mmol m-2 d-1) and 20.55 mmol m-2 d-1 (SD 15.00 mmol m-2 d-1), respectively. In July, the mean flux increased almost three times as compared to that in June. On the other hand, about twofold increase of the flux was observed in September as compared to the value obtained in July.

I» w □ M ^BVptiHnlwrf

= I -J Jl I fl

Fig. 2. Variability of methane flux in the coastal area of the Puck Bay.

The highest methane flux in the research area was measured at site P7 (Wladyslawowo area). Methane emission from sediment into near-bottom water at this site increased during the study period. In June it was 6.89 mmol m-2 d-1 but in July the measured value was about four times higher as compared to that in June. In September, the emission value was about eight times higher than the value measured in June, and it was the highest methane emission measured in the research area. Relatively high values of methane flux in September were observed at site P1 (Jurata area) and P2 (Jastarnia area) where the flux from the sediment into near-bottom water reached the values of 28.62 mmol m-2 d-1 and 31.50 mmol m-2 d-1, respectively. During the investigations conducted in June, methane was not detected in measurable amounts in near-bottom water at the following sites: P1 (Jurata area), P2 (Jastarnia area), P3 (Kuznica area) and P5 (Chalupy area).

In July, methane was not present in near-bottom water only at the sites P1 and P5. In September,

however, methane was detected at all sampling sites. During the entire study duration the methane flux from the sediment into near-bottom water in the coastal area of Puck Bay was determined at only three sites, i.e. P4, P6 and P7.

During the chamber experiments, a significant variability of methane flux from benthic sediments into near-bottom water was observed in the coastal waters along the Hel Peninsula. In September, the methane flux at sampling site P3 (Kuznica area) increased about seven times as compared to the value obtained in July at the same location. At site P4 (Kuznica area) the methane flux increased about five times from June to September, while at site P6 (Chalupy area) the methane flux was about ten times higher in September than in June.

The presence of free gas bubbles in sediment was observed during scientific investigations conducted in the coastal area of Puck Bay. Methane can be a component of the gas formed during organic matter decomposition. Therefore chemical testing for the presence of methane in sediment gases had to be conducted. Based on the obtained test results, it has been demonstrated that methane was one of the many components of the gas contained in sediment (Table 1).

Methane concentration in gas bubbles released from sediment at site P4 (Kuznica area) reached its maximum value in July; similar concentration values were measured in June. During that period, the presence of decomposing plant matter was observed in benthic sediments. Due to wave action, this area was covered with sand, which could have resulted in the reduced diffusion of oxygen from the atmosphere. Such conditions could create the local anoxic environment. The lowest methane concentration in samples of gas collected from sediment was measured in September when most of the plant matter under the sand layer had already decomposed. Only insignificant traces of plant matter were found in sediment cores. Based on the above presented facts, it can be concluded that methane concentration in gas bubbles at this particular site was closely connected with the organic matter content in sediment.

At site P6 (Chalupy area) the maximum concentration of methane was measured in September, while slightly lower concentration values were observed in July. In June, free gas bubbles were not detected in sediment, which coincided with the occurrence of an intensive algal bloom. Only after the bloom had diminished and the sinking organic

Table 1

Methane concentration in gas bubbles.

sampling date methane concentration in gas bubbles (% in gas phase)

P4 (Kuznica 3) P6 (Chatupy 2) P7 (Wtadystawowo)

15.06.2010 3.6500 n.d. 3.0100

26.07.2010 3.7551 3.1695 3.5319

04.09.2011 0.0035 3.4476 6.9386

n.d. - gas phase not detected.

matter replenished the seabed, new gas bubbles appeared again in sediment.

Throughout the entire study duration, the methane content in gas bubbles was measured at site P7 (Wladyslawowo area). Methane concentration values at site P7 in September were two times higher as compared to those measured in June.

Discussion

Benthic chambers are used for measuring the emission of greenhouse gases from wetlands, and for assessing the content of those gases in near-bottom water (DeLaune et al. 1983, Happell and Chanton 1995, Urban et al. 1997, Liikanen et al. 2009). Studies on the estimation of the exchange rate of nutrients between the sediment and the near-bottom water layer by employing the chamber experiments have already been performed in Puck Bay (Bolalek 1993, Geringer d'Oedenberg 2000). In this study, we demonstrate the methane emission from sediment into near-bottom water, applying the chamber techniques. The experiment was performed in Puck Bay along the Hel Peninsula.

The estuaries of Lumijoki and Temmesjoki rivers, located in the north-eastern part of the Bothnian Bay, are scientifically confirmed sources of methane emission from the sediment into the water column in the Baltic coastal zone. Methane emission in the Lumijoki area was estimated at 15.09 mmol m-2 d-1, while in the Temmesjoki estuary the emission was assessed at the lower level of 10.78 mmol m-2 d-1 (Liikanen et al. 2009). The coastal zone of Rügen and Hiddensee islands is also the source of methane emitted into the near-bottom water. In this ecosystem the maximum emission was determined between June and July. At the same time, the content of this greenhouse gas is strongly dependent on the content of organic matter that accumulates in the benthic zone of the above mentioned area. The range of methane emission in the coastal ecosystem of Rügen and Hiddensee islands was estimated between 0.048 mmol m-2 d-1 and 363.6 mmol m-2 d-1 (Heyer and Berger 2000).

Based on the research conducted in the costal ecosystem of Puck Bay, it has been established that in 2010, methane fluxes from sediment into near-bottom water varied from 0.91 mmol m-2 d-1 in June to 49.15 mmol m-2 d-1 in September. The increasing trend was noted during the sampling period with maximum values in September. We observed the presence of the decomposing organic fraction in the benthic sediment during the study period.

Seasonal changes in the coastal area of Puck Bay are driven by wave action, which results in the accumulation of the decomposing plant fraction under the sand layer. Such spatial distribution of organic matter along the profile of coastal sediments in the bay caused methane production and its emission from sediments into near-bottom water. The maximum value of the methane flux from the seabed into near-bottom water in the coastal zone of Puck Bay was observed in September. Only at three sites, i.e. P4, P6 and P7 methane gas was observed in near-bottom water during the entire study duration. At this site gas bubbles occur in the sediment. This gaseous phase contains methane.

This study was conducted in coastal waters which, due to shallow depth, are characterized by relatively easy transpiration of oxygen from the atmosphere into the surface sediment layer. Therefore local oxygen deficits or anaerobic conditions may occur in sediments, and in the consequence, the reduction reactions will also take place in such environment. Local oxygen deficits and anaerobic conditions present in small areas of sediment that occur beside the prevailing well-oxygenated sediments or the water column have already been described in the Baltic ecosystem (Edlund 2007). In the case of the coastal waters in Puck Bay, we deal with such a phenomenon, i.e. the occurrence of localized anaerobic conditions in benthic sediment spreading under the well-oxygenated water column (Witkowski 1993).

Free gas bubbles were found in the Baltic Sea sediment (Schmaljohan 1996, Thiessen et al. 2006, Judd and Hovland 2007), but there is no information on methane concentration in them. We find free gas bubbles in the coastal area of Puck Bay. To obtain this gas phase, we were mechanically degassing the sediment. The outcome of chemical analysis indicated the presence of methane in the gas produced in the benthic sediment at sampling sites P4 (Kuznica area), P6 (Chalupy area) and P7 (Wladyslawowo area). Methane concentrations ranged between 0.0035 and 6.9386 percent in the gas

phase. A decrease in the gas content in sediment and lower methane concentration in this gas was observed at one site only. Such scenario resulted from a visible disappearance of detritus from sediment. This also clearly shows the efficiency of decomposition of organic polymers in the natural environment. The observations made at site P6 (Chalupy area) confirm the latter conclusion. In this particular coastal area of Puck Bay, the intensive growth of algae was noted. It started in June and was declining in July, while in September the suspended organic matter completely disappeared from the water column. A successive replenishment of the benthic sediment with organic matter resulted in the presence of gas in this sediment. Therefore, the intensive algal bloom, triggered off by eutrophication, resulted in anaerobic decomposition of organic matter accompanied by the production of one of the most important greenhouse gases, i.e. methane. The environmental conditions in the coastal zone of Puck Bay allow the carbon cycle in the sea to be completed with the organic matter decomposition to i.a. methane as a final product.

Microbiological methane production as a result of anaerobic decomposition of organic matter is dependent on water temperature. This relationship is the same as in enzymatic reactions, which are increasing with the increasing temperature (Zeikus and Winfrey 1976, DelSontro et al. 2010). Water temperature in the research area, measured at the top of the peninsula, had a maximal the maximum value between June and August (Nowacki 1993). Besides the temperature limiting effect on the methanogenic activity, methane concentration in the analyzed ecosystems are strongly dependent on the organic fraction in the benthic sediment. This has been shown during the observation on the algae fraction and methane concentration in gas bubbles at site P6 (Chalupy area).

Summary

Based on the conducted research, it was established that the coastal zone of Puck Bay constitutes a compartment of the marine environment where the methane flux occurs in the direction from the seabed into near-bottom water. As a result of this study methane content in gas bubbles are shown too. It should be assumed that the sediments in the coastal area of Puck Bay are the confirmed source of methane emission into the water column.

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