TY - JOUR
T1 - Changes in dissolved organic matter composition and dynamics in a subtropical mangrove river driven by rainfall
AU - Kida, Morimaru
AU - Tanabe, Mai
AU - Tomotsune, Mitsutoshi
AU - Yoshitake, Shinpei
AU - Kinjo, Kazutoshi
AU - Ohtsuka, Toshiyuki
AU - Fujitake, Nobuhide
N1 - Funding Information:
We are grateful to two anonymous reviewers for detailed and constructive comments on the original manuscript. This study was supported by JSPS KAKENHI Grant Number JP15H05240 and Grant-in-Aid for JSPS Research Fellow ( 17J00808 ).
Funding Information:
The behavior of riverine DOM during estuarine mixing has mainly been studied in the context of changes in DOC concentration along salinity gradients. The dynamics of DOM in wetland-influenced coastal rivers can be more complex than the simple mixing of riverine DOM with seawater because of additional DOM supply from wetlands (Cawley et al., 2014; Dittmar et al., 2006; Maie et al., 2012). If we assumed a simple two-endmember mixing model for the DOM fractions and PARAFAC components, all data should be on a straight line (conservative mixing line) connecting headwater and seawater data points. However, the positive deviations of the quantitative parameters along the salinity gradient in Fig. 3 indicate significant non-conservative mixing in most cases (Table 3). Although seagrasses and aquatic macrophytes and macroalgae have been suggested as a source of DOM to mangrove rivers (Maie et al., 2012, 2006, 2005), they were not observed within the Fukido mangrove forest, suggesting that the mangroves were the major additional source of DOM to the Fukido River. The increase in the HPO concentrations at S = 5–15 under the baseflow conditions indicates that precipitation (or flocculation) of HPO in the river water was very small and/or the HPO inputs from the mangrove forest were larger than the removal of the riverine HPO, although it is known that humic acids precipitate under the interaction with cations in the seawater even at low salinity range (Sholkovitz, 1976). The HPO inputs from the mangrove forest support the inputs of colored DOM from mangrove forests (Cawley et al., 2014; Dittmar et al., 2006; Maie et al., 2012). We presumed that the HPO supplied from the mangrove forest was dominantly composed of fulvic acids. For example, Fox (1983) reported that, in seven coastal plain estuaries located in the mid-Atlantic US, humic acids were 100% removed during estuarine mixing. In contrast, fulvic acids hardly precipitate (<10%) even in the seawater (Kida et al., 2016). In any case, the supplies of both HPO and HPI with different biogeochemical and ecological roles from the mangrove forest support its importance as a heterogeneous DOM source (Cawley et al., 2014; Dittmar et al., 2006, 2001; Maie et al., 2012). The HPO/HPI fractionation reported here was quantitative, i.e., 100% of DOM was tracked in terms of carbon, in contrast to colored or fluorescent DOM, which represents only a fraction of the total DOC pool. Future studies are encouraged to link DOM composition and primary productivity and/or heterotrophic activity to separately assess the impact of HPO and HPI on coastal ecosystems.We are grateful to two anonymous reviewers for detailed and constructive comments on the original manuscript. This study was supported by JSPS KAKENHI Grant Number JP15H05240 and Grant-in-Aid for JSPS Research Fellow (17J00808).
Publisher Copyright:
© 2019 Elsevier Ltd
PY - 2019/7/31
Y1 - 2019/7/31
N2 - Dissolved organic matter (DOM) plays an important role in sustaining ecosystem services of mangrove forests through well-described biogeochemical and ecological functions. This study was conducted in the Fukido River (Ishigaki Island, Japan) to better understand the seasonal and episodic changes in DOM concentration and composition in a subtropical mangrove system. Water samples were collected seasonally along a headwater–mangrove–sea transect on 10 occasions from September 2014 through June 2016. DOM was fractionated based on hydrophobicity into two fractions (hydrophobic and hydrophilic) and also analyzed by excitation-emission matrix spectroscopy combined with parallel factor analysis (PARAFAC). Although seasonal changes in DOM concentration and composition were not observed, both hydrophobic and hydrophilic DOM concentrations and levels of the identified three PARAFAC components clearly increased during a typhoon event. It is suggested that episodic increases in freshwater input due to a typhoon caused enhanced leaching of DOM from mangrove litter and dissolution of mangrove soil organic matter (SOM), which was otherwise retained in the mangrove soil by salinity-induced aggregation. The aggregation–dissolution properties of SOM are crucial in determining the magnitude of DOM outwelling and possibly SOM accumulation rate by enhancing advective DOM exchanges. Future studies are needed to evaluate the size of the carbon pool and outwelling of DOM after classifying mangrove forests based on the hydrological regime that influences biogeochemical conditions in the forests. Regional Index Terms: Japan, Okinawa, Ishigaki, Fukido.
AB - Dissolved organic matter (DOM) plays an important role in sustaining ecosystem services of mangrove forests through well-described biogeochemical and ecological functions. This study was conducted in the Fukido River (Ishigaki Island, Japan) to better understand the seasonal and episodic changes in DOM concentration and composition in a subtropical mangrove system. Water samples were collected seasonally along a headwater–mangrove–sea transect on 10 occasions from September 2014 through June 2016. DOM was fractionated based on hydrophobicity into two fractions (hydrophobic and hydrophilic) and also analyzed by excitation-emission matrix spectroscopy combined with parallel factor analysis (PARAFAC). Although seasonal changes in DOM concentration and composition were not observed, both hydrophobic and hydrophilic DOM concentrations and levels of the identified three PARAFAC components clearly increased during a typhoon event. It is suggested that episodic increases in freshwater input due to a typhoon caused enhanced leaching of DOM from mangrove litter and dissolution of mangrove soil organic matter (SOM), which was otherwise retained in the mangrove soil by salinity-induced aggregation. The aggregation–dissolution properties of SOM are crucial in determining the magnitude of DOM outwelling and possibly SOM accumulation rate by enhancing advective DOM exchanges. Future studies are needed to evaluate the size of the carbon pool and outwelling of DOM after classifying mangrove forests based on the hydrological regime that influences biogeochemical conditions in the forests. Regional Index Terms: Japan, Okinawa, Ishigaki, Fukido.
KW - Aggregation
KW - Blue carbon
KW - DOM
KW - Dissolution
KW - Hydrological regime
KW - SOM
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U2 - 10.1016/j.ecss.2019.04.029
DO - 10.1016/j.ecss.2019.04.029
M3 - Article
AN - SCOPUS:85064627546
SN - 0272-7714
VL - 223
SP - 6
EP - 17
JO - Estuarine, Coastal and Shelf Science
JF - Estuarine, Coastal and Shelf Science
ER -
