Spatial Distribution of Organic Matter in the Surface ...

[Pages:9]Philippine Journal of Science 142 (2): 133-141, December 2013 ISSN 0031 - 7683 Date Received: ?? Feb 20??

Spatial Distribution of Organic Matter in the Surface Sediments of Calape Bay, Bohol, Central Philippines

Francis Albert T. Argente*,1,2, Herminie P. Palla1,3, Charina I. Narido1,4, Milagros A. Celedonio1,5, and Danilo T. Dy1

1Marine Biology Section, Biology Department, University of San Carlos, Cebu City, 6000 Philippines 2Department of Fisheries Science, Pangasinan State University ? Binmaley Campus,

Binmaley, Pangasinan, 2417 Philippines 3College of Fisheries and Aquatic Science, Western Philippines University ? Puerto Princesa Campus,

Puerto Princesa City, Palawan, 5300 Philippines 4Mathematics and Science Department, Holy Name University,

Tagbilaran City, Bohol, 6300 Philippines 5Natural Science Department, Dr. Emilio B. Espinosa Sr. Memorial State College of Agriculture

and Technology, Mandaon, Masbate, 5411 Philippines

Calape Bay is a semi-enclosed marine system located in Bohol, Central Philippines. Despite the relatively restricted water circulation and dispersion of particulate organic matter, mariculture activities have not decreased in Calape Bay. Organic matter (OM) content of the surface sediment was quantified to serve as indicator of the current status of the benthic system after almost 30 years since mariculture was launched. Surface sediments were mainly composed of sand and gravel. Highest sediment OM (15.3%) was observed at the middle portion of the bay where the mariculture facilities are located. Mean OM at the mariculture facilities (11.4%) was higher than at the mouth of the bay (5.7%) and near the river mouth (9.4%). Finer sediments contained more OM. The high sediment OM in the bay will bring changes in the benthic environment in the years to come if mariculture activities are not regulated. Hence, monitoring of sediment OM is recommended.

Key Words: Coastal pollution, embayment, eutrophication, Gradistat, mariculture

INTRODUCTION

Sediments perform major functions in the marine environment by serving as natural trap for various substances including toxicants (Nasr et al. 2006; Hu et al. 2009; Knies & Martinez 2009). Bottom sediments are a good source of nutrients which supports primary production in the water column, as shown in benthic pelagic coupling (Navarrete et al.2005; Arndt & Regnier 2007; Ubertini et al. 2012). Sediments are also capable

*Corresponding author: faargente@

of storing considerable amount of organic matter (OM) which may influence the bottom water oxygen concentration (Werner et al.2003; Koho et al. 2013). Hence sea floor is a site of intense biogeochemical and mineral-dissolution-precipitation reactions (Zhu et al. 2006). Marine sediments are composed of various complex organic substances from marine and terrestrial sources (Mayer et al. 2007; Knies & Martinez 2009). External inputs of OM to coastal waters originate from domestic sewage, agricultural fertilizers, largely via rivers, aquaculture, and airborne loading (Bonsdorff et al. 1997). These are ultimately caused by the dramatic growth of

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human populations near coastal zones (Nixon 1995).The amount of OM in sediment influences the functioning of the seafloor ecosystem (van Nugteren et al.2009; Krumins et al. 2013). The distribution and concentration of OM are important factors in understanding the role of sediment OM in the marine benthic environment.

Calape Bay, located in Bohol, Central Philippines, is a semi-enclosed marine system, characterized by a large opening at the southwest facing two islets (Sandingan and Calibao) and a very narrow passage that cut the causeway on the north. It is a small embayment surrounded by the municipalities of Loon and Calape in the southern and eastern boundaries and Pangangan Island in the northern and western boundaries (Corrales 2005). The basin is relatively shallow, with a mean depth of 6.09 ?1.09 m SE. Water depth increases sharply near the outer bay. The bottom substrate is characterized by corals and seagrasses along its periphery and sandy-gravelly deposits in the center of the bay.

Various natural and anthropogenic sources of organic load are found within the embayment. It is a mariculture site, having several fish pens and cages along the bay. In 2002, there were 44 operational fish cages in the bay (Corrales 2005). To date, the municipal government has registered a total of 49 fish pens and 55 fish cages operating in Calape Bay. The bay is surrounded by mangroves. Two rivers drain into the bay, the Tultugan and San Isidro Rivers. The latter is near the town's floating market and business establishments. There are poultry and piggery production facilities operating near the bay area (Green et al. 2000). After a decade, the 2010 population in the coastal town of Calape increased by 8.03% to 30,146 residents (NSO 2010).

Mariculture activities in Calape Bay started in 1982 when the Bureau of Fisheries and Aquatic Resources (BFAR) launched the National Bangus Breeding Project (Corrales 2005). University of the Philippines-Marine Science Institute & University of the Philippines Visayas-Cebu College (2002) reported lower dissolved oxygen (DO) (4.5-5.5 mg L-1) and higher phosphates (PO4)(0.3-0.4 ?M) and ammonia (NH3) (0.5-0.7 ?M) in the surface water near the mariculture sites. In other areas of the bay, DO (5.58.0 mg L-1) was higher while PO4 (0.2-0.3 ?M) and NH3 (0.3-0.4 ?M) were lower. Furthermore, Silapan & Maeda (2007) indicated that growth of microbial colonies on the sediments in Calape Bay was due to high sediment OM (4.9%) in the mariculture sites. Corrales (2005) attributed the high OM in these areas to the waste food and fish feces produced by mariculture activities.

The physiography of the bay suggests that water circulation and dispersion of particulate organic matter are restricted. UP-MSI & UPV-CC (2002) reported

slow water movement in the bay as indicated by an average residence time of 15 days. Because of the slow circulation, the numerous anthropogenic activities have a high potential to easily modify the benthic environment of Calape Bay. Changes in the chemical profile of bottom sediments can influence the benthic ecosystem, including the survival of its inhabitants (Rosenberg et al. 2001).On the other hand, sedimentation of particulate OM leads to its burial within the sediments, thus, the sediments provide a better "record" of past activities in the water column. In this study, we quantified OM content and particle size of the surface sediment, the objective was to determine the current OM depositionary status of the bay after 30 years of mariculture activities. The OM content of the sediment is a parameter that can be determined easily in situations where laboratory equipment and chemicals are limited.

MATERIALS AND METHODS

Surface sediments were collected from 48 sampling sites (SS) (Figure 1) using a van Veen grab (0.03 m2) on February 18, 2013. Depending on the type of sediments, the van Veen grab can extract down to 5 cm of the sediment surface. One hundred g of sediment samples were taken from each site and stored in pre-labeled plastic containers for transport to the laboratory. Samples were immediately air-dried and kept for subsequent analyses. The geographic coordinates of each point were recorded using a MagellanTM global positioning system.

The loss-on-ignition (LOI) technique (Luczak et al. 1997) was used to determine OM content of the substrate. Before ashing, crucibles were oven-dried (80?C) to constant dry weight. Two two-gram subsamples (dried for 24h at 110?C) from each sediment samples were placed in each crucible and burned inside a ThermolyneTM 1400 furnace at 500?C for 6 hours. Afterwards, cooling of the crucibles and their contents took place inside a desiccator. Upon cooling, the crucibles and their contents were weighed. OM contents of the sediments were determined by the difference in the weight of crucible and its contents before and after ashing. Results were expressed as percentage weight loss on ignition (%), the data gridded and plotted using a contour mapping program.

Grain size composition of the sediment samples was determined through dry sieving after an overnight drying in oven at 80?C. Dried sediments were homogenized and classified according to the Wentworth (1922) scale grade by passing through a stack of sieves with mesh apertures of 4 mm (granules), 2 mm (very coarse sand), 1 mm (coarse sand), 0.50 mm (medium sand), 0.25 mm (fine sand), and 0.063 mm (silt). Weight (?0.0001 g) of sediment fractions were used to determine the general composition of the

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Figure 1. Map of Calape Bay, Bohol, Central Philippines. Sampling sites are shown with different markers and sediment texture groupings. Inset map: A is Bohol, B is Philippines, C is Southeast Asia.

Philippine Journal of Science Vol. 142 No. 2, December 2013

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Figure 2. Spatial distribution of surface sediment OM content (%) in Calape Bay, Bohol, Central Philippines. Inset map: A is Bohol, B is Philippines, C is Southeast Asia.

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sediments. The Gradistat v 4.0 software (Blott & Pye 2001) was used to analyze the grain size distribution of sediments. Moreover, the grain size data for all samples were reclassified into gravel, sand, and mud and presented in a ternary diagram using the same software. The mean grain size and sediment OM content were fitted using a power function model to explain the effect of sediment texture on OM accumulation.

RESULTS

Surface sediment OM content in Calape Bay (Figure 2) ranged from 1.2 to 15.3%. Highest OM content was observed at the middle portion of the bay where the mariculture facilities are located. Another high sediment OM content (13.6%) was recorded near Tultugan River (SS 10). Sediment OM decreased radially from the

mariculture site. The mean OM content at the mouth of the bay (SS 1-5) and in areas least influenced by river runoffs (SS 6-9, 33-43) were 5.7% and 5.5%, respectively.

Analysis of sediment grain size showed that Calape Bay surface sediment (Figure 3) can be categorized into seven textural groups, namely, "gravelly mud" (GM), slightly gravelly muddy sand" (SGMS), "gravelly muddy sand" (GMS), "slightly gravelly sand" (SGS), "gravelly sand" (GS), "sandy gravel" (SG), and "sand" (S). Majority of the sediment samples (48%) belonged to the GS category. It was observed that GS, SG (2%) and S (8%) samples were collected near mangrove and coral reef areas. The SGS (31%), SGMS (2%), GMS (7%), and GM (2%) sediments were recorded near the mariculture site, mouth, intertidal and mid portion of the bay.

A fairly good correlation (r2 = 0.55) was observed between the mean sediment grain size and OM contents fitted using

Figure 3. Ternary diagram showing the proportion of mud, sand, and gravel of the sediment samples collected at the sampling sites.

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Figure 4. Mean sediment grain size and OM contents fitted with a power function model.

a power function model (Figure 4). It appears that the finer texture sediments contain more OM.

DISCUSSION

Our study revealed a mean sediment OM content of 7.4% (?4.04%SD) in Calape Bay. Silapan & Maeda (2007) reported a mean sediment OM content of 3.0% when there were relatively less mariculture activities in the area. Historically, sediment organic matter in Calape Bay increased with the increasing mariculture activities in the area.

The high sediment OM in Calape Bay may be attributed to slow water movement and shallow depth profile. Long water residence time (15 days) was recorded in Calape Bay (UP-MSI & UPV-CC 2002).If the settling velocities of OM components (0.24 to 2.32 m d-1) as reported by Burns & Rosa (1980) were to be followed together with the long residence time of the bay, then there would definitely be

a net accumulation of OM in the sediments.

Increased amount of OM will lead to the decline of species richness, abundance, and biomass of the benthic community due to hypoxia and buildup of toxic byproducts such as ammonia and hydrogen sulphide (Nilsson & Rosenberg 2000; Gray et al. 2002; Belley et al. 2010). High organic content of sediment can be associated with the increase in chemical contaminants, this is especially the case for finer-grained substrates which provide greater surface area for sorption of organic and other chemically-induced pollutants (Oren et al. 2006; Elias et al. 2007; Mirza et al. 2012). Organic enrichment leading to hypoxia can have major effects on benthic fauna (Gray et al. 2002) and as such, can also alter the biologically mediated geochemical cycles of the aquatic environment (Rosenberg et al. 2001).

The sediments in Calape Bay are mainly composed of sand and gravel. Variation in sediment grain size composition is dependent on the long-term current and wave action in the marine environment (Malvarez et al. 2001). Moreover,

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the bathymetry and the sedimentary sources are also factors that may influence the grain size composition of the surface sediment (Roman & Achab 1999; Riegl et al. 2007). The 15day water residence time in Calape Bay (UP-MSI & UPVCC 2002) suggests low water movement. It is plausible that in the past the surface sediments within the bay, being of coralline nature, were moderately homogenous. If organic deposition at the water surface is minimal, the gravelly-sandy nature of the surface sediments could have experienced aeration to some degree due to the effects of tidal currents. Seasonal river run-offs as well as mariculture activities are expected to contribute to the deposition of clay- and silt-sized particles, and could possibly lead to future eutrophication events during periods of low water movement. If grain size distribution is associated with the content and distribution of OM (Bergamaschi et al. 1997), then our study showed that finer sediment texture had more OM content, confirming the findings of Premuzic et al. (1982) and Bergamaschi et al. (1997).

Fine-grained sediments is important in the preservation of OM. OM tend to form aggregates with clay minerals (Yu et al. 2009).Organic components and fine-grained particles have similar settling velocities (Drake et al. 2002). Therefore, where clay is abundant OM can be enriched. High particulate OM content lodged in the fine sediments may also lead to low diffusibility of DO to the deeper part of the sediments.

Currently, there are two potential sources of organic load in Calape Bay: (1) the mariculture site and (2) the river and sewage run-offs. Higher concentration of sedimentary OM was observed at the mid portion of the bay, especially in areas near the mariculture sites. In the Philippines, mariculture activities have been reported as potential source of organic load in marine sediments (UP-MSI & UPV-CC 2004; Becira 2006). In Pangasinan, NW Philippines, higher OM was detected in area with aggregated rather than with dispersed mariculture activities (Nacorda et al. 2012). Too much OM from these mariculture facilities will lead to depletion of dissolved oxygen and thus significant changes in the composition of the macrobenthic communities (Yokoyama 2002). Tomassetti et al. (2009) reported a reduction in macrobenthic diversity and dominance of opportunistic species underneath a mariculture site. In such scenario, the endobenthic macrofauna community will be the first to deteriorate, then the epibenthos and finally the nearbottom neritic fauna (Toupoint et al. 2008). This leads to instability of the faunal structure and may lead to the degradation of the benthic ecosystem.

The present study provides baseline information on the potential effect of mariculture activities on the sediment organic load of Calape Bay. However, there is need to determine the major components (C, N, or P) of OM

in the sediments to better understand the origin of the OM in the sediment of Calape Bay. Future studies using macrobenthic fauna as biological indicators should further confirm the effect of maricultures in Calape Bay.

ACKNOWLEDGMENT

The authors are grateful to the assistance provided by the LGU personnel of Calape, Bohol and Bureau of Fisheries and Aquatic Resources-VII during the preparation and implementation of the study. We thank the University of San Carlos, Marine Biology Section for the use of its facilities. The map made by Department of Science and Technology-VII staff is likewise appreciated. FATA, CIN and HPP thank the scholarship grant provided by Philippine Council for Agriculture, Aquatic and Natural Resources Research and Development-Department of Science and Technology. This is a marine science contribution of the University of San Carlos, Cebu City, Philippines.

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