The decomposition rate of the organic carbon content of suspended particulate matter in the tropical seagrass meadows
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Abstract: In terms of downward transport, suspended particulate matter (SPM) from marine or terrigenous sources is an essential contributor to the carbon cycle. Within mesoscale environments such as seagrass ecosystems, SPM flux is an essential part of the total carbon budget that is transported within the ecosystem. By assessing the total SPM transport from water column to sediment, potential carbon burial can be estimated. However, SPM may decompose or reforming aggregate during transport, so estimating the vertical flux without knowing the decomposition rate will lead to over- or underestimation of the total carbon budget. Here this paper presents the potential decomposition rate of the SPM in seagrass ecosystems in an attempt to elucidate the carbon dynamics of SPM. SPM was collected from the seagrass ecosystems located at Sikka and Sorong in Indonesia. In situ experiments using SPM traps were conducted to assess the vertical downward flux and decomposition rate of SPM. The isotopic profile of SPM was measured together with organic carbon and total nitrogen content. The results show that SPM was transported to the bottom of the seagrass ecosystem at a rate of up to (129.45±53.79) mg/(m2·h) (according to carbon). Considering the whole period of inundation of seagrass meadows, SPM downward flux reached a maximum of 3 096 mg/(m2·d) (according to carbon). The decomposition rate was estimated at from 5.9 µg/(mg·d) (according to carbon) to 26.6 µg/(mg·d) (according to carbon). Considering the total downward flux of SPM in the study site, the maximum decomposed SPM was estimated 39.9 mg/(m2·d) (according to carbon) and 82.6 mg/(m2·d) (according to carbon) for study site at Sorong and Sikka, respectively. The decomposed SPM can be 0.6%–2.7% of the total SPM flux, indicating that it is a small proportion of the total flux. The seagrass ecosystems of Sorong and Sikka SPM show an autochthonous tendency with the primary composition of marine-end materials.
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Key words:
- carbon dynamic /
- biogeochemistry /
- coastal ecosystem /
- particulate matter
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Figure 1. Study site at Maumere (Sikka, East Nusa Tenggara), Sop Island, and Nana Island (Sorong, West Papua).
Figure 2. SPM profile according to C/N ratio and isotopic profile of δ13C and δ15N. C/N ratio threshold for marine- and terrigenous-end (6.6 and 20, respectively) follows Martiny et al. (2014) and Gilhooly et al. (2008), respectively. Alternative range of the end-member (shaded area) refers to Lamb et al. (2006). Recorded range of δ13C for seagrasses is from −15.5 to −5.6 (Andrews and Abel, 1979).
Figure 3. Bi-plot of carbon and nitrogen stable isotopes of SPM to determine the correlation of SPM potential sources. The potential sources refer to Wahyudi and Afdal (2019), namely periphyton, Chlorophyta, C3 and C4 plants, soils, estuary SPM (eSPM), bay SPM (bSPM), river sediment (river-sed), Enhalus acoroides (above and below ground: EA-abg & EA-blg), Thalassia hemprichii (TH), particulate organic matter (POM), and marine SPM (mSPM).
Figure 4. The proportion of sources in the SPM derived from Bayesian mixing model (MixSIAR). The potential sources refer to Wahyudi and Afdal (2019), namely periphyton, Chlorophyta, C3 and C4 plants, soils, estuary SPM (eSPM), bay SPM (bSPM), river sediment (river-sed), Enhalus acoroides (above and below ground: EA-abg & EA-blg), Thalassia hemprichii (Th), particulate organic matter (POM), and marine SPM (mSPM of four locations).
Figure 5. The decomposition rate of SPM from seagrass ecosystems as expressed on organic cabon (Corg) and total nitrogen (Ntot). Black dashed line (and ӿ symbol) shows the linear regression of decomposition rate of SPM’s organic carbon for the 48 h; blue line (and blue filled circle) shows the polynomial regression of organic carbon decomposition rate for the whole experiment periods; red dashed line (and red filled triangle) shows decomposition rate of SPM’s total nitrogen. Shaded area shows the standard deviation.
Table 1. SPM isotopic profile and vertical flux of seagrass ecosystems in the Sikka and Sorong notated in average ± Std Dev
Location δ15N
/‰δ13C
/‰C:N ratio Ntot flux
/(µg·h−1)*Corg flux
/(µg·h−1)*Ntot flux
/(mg·m−2·h−1)
(according to nitrogen)Corg flux
/(mg·m−2·h−1)
(according to carbon)Sikka 4.10±1.56 −16.76±3.36 15.47±3.53 2.96±0.72 38.48±10.10 4.80±1.17 62.52±16.41 Sorong 4.01±1.36 −7.22±3.36 22.50±6.89 5.89±3.41 79.67±33.10 9.56±5.54 129.45±53.79 Note: * Hourly flux of Corg and Ntot trapped in the SPM trap (mouth area: 6.154 4 cm2). 
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