Estuaries and Coasts (2012) 35:1060–1068 DOI 10.1007/s12237-012-9487-x Sources and Exchange of Particulate Organic Matter in an Estuarine Mangrove Ecosystem of Xuan Thuy National Park, Vietnam Nguyen Tai Tue & Tran Dang Quy & Hideki Hamaoka & Mai Trong Nhuan & Koji Omori Received: 20 June 2011 / Revised: February 2012 / Accepted: 11 February 2012 / Published online: 29 February 2012 # Coastal and Estuarine Research Federation 2012 Abstract The spatio-temporal variations in stable isotope signatures (δ13C and δ15N) and C/N ratios of particulate organic matter (POM), and physicochemical parameters in a creek water column were examined in an estuarine mangrove ecosystem of Xuan Thuy National Park, Vietnam The objective was to examine the factors influencing creek water properties, and the sources and exchange of POM in this important mangrove ecosystem The diel and seasonal variations in water temperature, flow velocity, pH, dissolved oxygen, and salinity demonstrated that tidal level, season, and biological factors affected the creek water properties Mangroves had relatively low δ15N and very low δ13C values, with respective average values of 1.5±0.9‰ and −28.1±1.4‰ The low mangrove leaf δ15N indicated minor anthropogenic nitrogen loading to the mangrove forests A significant positive correlation between POM–δ13C and salinity along the axis of Ba Lat Estuary, Red River, indicated that marine phytoplankton (δ13C value, −21.4±0.5‰) was the predominant source of POM at the estuary mouth Based Electronic supplementary material The online version of this article (doi:10.1007/s12237-012-9487-x) contains supplementary material, which is available to authorized users N T Tue Graduate School of Science and Engineering, Ehime University, 2-5 Bunkyo-cho, Matsuyama, Japan N T Tue (*) : H Hamaoka : K Omori Center for Marine Environmental Studies, Ehime University, 2-5 Bunkyo-cho, 790-8577, Matsuyama, Japan e-mail: tuenguyentai@gmail.com T D Quy : M T Nhuan Faculty of Geology, Hanoi University of Science, 334 Nguyen Trai, Thanh Xuan, Hanoi, Vietnam on the co-variation of δ13C and C/N ratios, marine phytoplankton and mangrove detritus were predominant in POM of major creeks and small creeks, respectively During the diurnal tidal cycle, the dynamics of POM were affected by sources of organic matter, tidal energy, and seasonal factors The contribution of mangrove detritus to POM reached a maximum at the low tide and was enhanced during the rainy season, whereas marine phytoplankton contribution was highest at high tide Keywords Particulate organic matter Mangrove ecosystem Stable isotopes Xuan Thuy National Park Vietnam Introduction Mangrove ecosystems are highly productive coastal systems, forming both a critical boundary between the sea and land and providing essential habitats for plants and animals They are traditionally considered to play a critical role in the biogeochemical carbon cycle in (sub)trophic regions, being an important source of particulate organic matter (POM) and dissolved organic carbon in coastal waters (Jennerjahn and Ittekkot 2002) The export of mangrove detritus is correlated with total litter production (Mfilinge et al 2005), particularly during the rainy season (Dittmar and Lara 2001a; Alongi 2009) Mangrove-derived POM has been recognized as a major food source for a variety of invertebrate species (Camilleri 1992) and contributes up to 84% of the diet of commercially important juvenile prawns (Chong et al 2001) Therefore, the dynamics of POM are likely to influence the whole food web in the mangrove ecosystem (Chong et al 2001; Bouillon et al 2002) However, the factors influencing the sources and Estuaries and Coasts (2012) 35:1060–1068 exchange of POM have not yet been fully understood in mangrove ecosystems, particularly in developing countries such as Vietnam Mangrove creeks act as a major route for transport of mangrove detritus to adjacent environments (Alongi 2009) and likewise the import of phytoplankton from coastal waters (Romigh et al 2006; Bouillon et al 2007) Thus, the creek systems play a vital role in maintaining the physical and biological structure of the mangrove ecosystem, as well as the out-welling of nutrients and organic matter from mangrove forests In addition, POM, nutrients and other physicochemical properties of creek water are highly influenced by tidal level (Bouillon et al 2007), season (Romigh et al 2006), and biogeochemical processes in sediments (Kristensen 2008) The POM pool often originates from variable sources (e.g., mangrove detritus, marine phytoplankton, benthic microalgae, and terrestrially derived organic matter) (Rezende et al 1990; Bouillon et al 2007) Therefore, to understand the dynamics of POM in mangrove ecosystems, it is important to examine the contributions of mangrove detritus under different tidal, spatial, and seasonal scales In the present study, we test the hypothesis that the physicochemical properties of water column and the sources Fig Sampling sites within mangrove ecosystem of Xuan Thuy National Park in northern Vietnam Map numbers are based on the order number of spatial POM samples MS denotes monitoring site, and E1 to E6 denote sampling sites from the Ba Lat Estuary Site E7 (15 km from the estuary mouth) is not shown on the map 1061 and dynamics of POM in an estuarine mangrove ecosystem are controlled by the changes in tidal level, seasonal, and biological factors We investigate the spatio-temporal observations of stable isotopes (δ13C and δ15N) and C/N ratios of POM and the physicochemical parameters (water level, flow velocity, water temperature, dissolved oxygen (DO), pH, salinity, and total suspended matter (TSM)) of creek water for (1) examining the factors influencing the physicochemical properties of the creek water and (2) determining the spatio-temporal variations in POM constituents in the estuarine mangrove ecosystem Materials and Methods Study Area The present study was conducted in an estuarine mangrove ecosystem of Xuan Thuy National Park (XTP) in Northern Vietnam The XTP is located along the south entrance of Ba Lat Estuary (BLE), Red River (Fig 1) A detailed description of the XTP is shown in the Electronic supplementary material The XTP has two major creek systems (Tra and Vop Creeks) that meander through the dense mangrove 1062 forests Tra Creek is the largest and is one of the major connections between mangrove forests and the sea Therefore, Tra Creek likely plays a vital role in the physical and biological processes underlying the development and sustenance of the mangrove ecosystem Field Sampling Field sampling was carried out in both rainy (from 10 to 16 September, 2009) and dry seasons (from to 26 March, 2010) During the rainy season, a monitoring site (MS, 20° 14′ 15.2″ N, 106° 33′ 56.7″ E) was established in the middle of Tra Creek for collecting water samples for POM and physicochemical measurements at 1-h intervals during a diurnal tidal cycle from 14:25, 11th to 14:25, 12th September, 2009 To assure no site contamination by anthropogenic sources and the examination of a natural system, the MS location was chosen within the most developed mangroves and far from human activity and settlements During the dry season, water samples were likewise collected during a diurnal tidal cycle from 10:40, 11th to 10:40, 12th March, 2010 for determination of POM and physicochemical parameters, following the same method and location (MS) as during the rainy season To examine the potential marine POM end-member, estuarine water samples were collected along a salinity gradient of the BLE during flood tide, and the sampling sites were assigned from E-1 to E-7 (Fig 1) In addition, creek water samples were spatially collected during flood tide in order to examine the sources of POM in the geographical distribution (Fig 1) Water samples were all taken at 30 cm below the surface by a Van Dorn bottle POM samples were collected by filtering 0.5 L of water through pre-weighed and pre-combusted (at 550°C) 47 mm Whatman GF/F glass fiber filters After collection, POM samples were lightly rinsed with distilled–deionized water to remove salt residue, immediately placed on ice, and transported to the field laboratory where the samples were dried at 60°C for 24 h At the MS station, flow velocity and water temperature were simultaneously recorded by a velocity–temperature compact instrument (Compact–EM, model AEM–HR, Alec electronics Co.ltd) during a diurnal tidal cycle from both the rainy and dry season The basic parameters of pH, salinity, and DO were measured on-site using HORIBA portable instruments, which consisted of a pH electrode (model 9621–10D) with an accuracy of ±1%, DO electrode (model OM–51) with an accuracy of ±1%, and salinity electrode (model 9382–10D) with an accuracy of ±0.1%, respectively The dominant sources of terrestrial organic matter in the XTP are mangroves Fresh mangrove leaves were Estuaries and Coasts (2012) 35:1060–1068 collected by hand from three dominant mangrove species, consisting of Sonneratia caseolaris, Kandelia obovata, and Aegiceras corniculatum The leaves were carefully rinsed with distilled water after collection to remove any potential extraneous material and stored on ice and transported to the field laboratory for drying at 60°C for 36 h Sample Preparation and Analysis TSM was obtained after re-drying at 60°C until a constant weight The POM samples were first decarbonated by fumigating with HCl (12 N) within a contained desiccator for 12 h The POM samples were then dried at 60°C for 24 h to evaporate the HCl and were subsequently ground to a fine powder using an agate mortar and pestle for analysis of stable isotopes (δ13C and δ15N) and C/N ratios Stable isotope signatures (δ13C and δ15N) and C/N ratios of POM, and mangrove leaves were analyzed using a gas chromatograph combustion isotope ratio mass spectrometry (PDZ Europa Ltd., ANCA-SL) Stable isotope signatures were expressed in permil (‰) deviations from the standard value by the following Eq 1: ð1Þ where R0 13C/12C or 15N/14N, Rsample is the isotope ratio of the sample, and Rstandard is the isotope ratio of a standard referenced to Pee Dee Belemnite limestone carbonate for 13C/12C and to atmospheric nitrogen for 15 N/14N During analysis processes, L-histidine was used as certified reference material The precision of analytical methods were ±0.1‰ and ±0.2‰ for δ13C and δ15N, respectively Statistical Analyses Pearson’s correlation was used to determine relationships between various physicochemical parameters and stable isotope signatures (δ13C and δ15N) and C/N ratios of POM from the BLE and during the diurnal tidal cycles of the mangrove creek The significance level of the Pearson’s correlation was 0.05 (p