## 4 Discussion  
### Sediment-Dissolved Oxygen Fluxes  
The respiration rates in sediments of the Nanilu mangrove forest are within the range found in similar Asian and Australian mangrove settings [28; 55; 56; 58; 59; 60; 61; 62; 63; 64; 65], but are lower than in the mangrove sediments of Central America [66; 67]. Oxygen uptake rates are particularly low compared to mangroves of the Jiulongjiang Estuary in China, for which [PERSON] _et al._[68] assumed that rapid sediment accumulation supplies reactive organic matter supporting high respiration rate. Respiration rates observed in the present study may also be low because of lacking bioturbation (e.g., crab burrows) in sampled sediment cores. Oxygen decrease in overlying water has been shown to be higher in areas with burrows compared to undisturbed sediments in the same mangrove [66]. Burrows and roots, furthermore, promote respiration during desiccation [56]. Though differences between inundated and desiccated respiration cannot be generalized [56; 58; 64; 69], the diel rates shown in Figure 5b should be considered minimum fluxes. Rates  
\begin{table}
\begin{tabular}{c c c c c c} \hline  & \multicolumn{3}{c}{**Hourly flux [\(\mu\)mol m\({}^{-2}\) h\({}^{-1}\)]**} & \multicolumn{3}{c}{**Daily flux [\(\mu\)mol m\({}^{-2}\) day\({}^{-1}\)]**} \\ \cline{2-6}  & \multicolumn{1}{c}{**Spring**} & \multicolumn{1}{c}{**Summer**} & \multicolumn{1}{c}{**Spring**} & \multicolumn{1}{c}{**Summer**} \\ \cline{2-6}
**a** & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & & & \\ \hline Respiration & \(-\)499.4 & \(-\)669.3 & \(-\)11985.1 & \(-\)16062.2 \\ NPP & \(-\)270.5 & \(-\)420.8 & \(-\)9238.4 & \(-\)12583.2 \\ GPP & 228.9 & 248.5 & 2746.7 & 3478.9 \\ CO\({}_{2}\) efflux & 499.4 & 669.3 & 11985.1 & 16062.2 \\ CO\({}_{2}\) influx & \(-\)228.9 & \(-\)248.5 & \(-\)2746.7 & \(-\)3478.9 \\ CO\({}_{2}\) net flux & 384.9 & 524.3 & 9238.4 & 12583.2 \\ N demand & 22.9 & 24.8 & 274.7 & 347.9 \\ \hline
**b** & & & & & \\ Measured & Light & Dark & Light & Dark & Diel & Diel \\ \hline NO\({}_{3}\)\({}^{-}\) & \(-\)279.1 & \(-\)339.6 & \(-\)16.3 & \(-\)83.9 & \(-\)1856.1 & \(-\)266.7 \\ NO\({}_{2}\)\({}^{-}\) & 37.0 & 13.1 & 4.5 & \(-\)17.1 & 150.4 & \(-\)27.1 \\ NH\({}_{4}\)\({}^{+}\) & 28.4 & 85.9 & \(-\)104.3 & \(-\)251.8 of gross primary production (GPP) are also lower than in other mangrove systems [59, 61]. This may be due to low microhydrobethos biomass, as GPP and the P:R ratio are similar to those in mangrove systems with low benthic microalgal standing stocks [70]. Stronger O\({}_{2}\) uptake occurred in March (Figure 5). Generally, increased sediment O\({}_{2}\) dynamics and gas fluxes during are expected during warmer seasons [52, 60, 62, 68, 70, 71, 72, 73, 74], which favor the activity of benthic microorganisms [75, 76, 77]. Higher GPP in October indicates that microbytobenthos productivity indeed did increase. Decreased aerobic respiration is likely due to sediment disturbance by the tropical storm Nesat a few days before the experiment. Sediment mobilization by extreme storms ventilates the pore space [78, 79], reducing the demand for oxidizing agents. Together with increased GPP, this led to  
Figure 6: Total sediment fluxes of dissolved nutrients; (**a**) hourly [\(\mathrm{\SIUnitSymbolMicro m^{-2}\cdot\mathrm{h}^{-1}}\)]; and (**b**) diel [\(\mathrm{\SIUnitSymbolMicro m^{-2}\cdot\mathrm{day}^{-1}}\)]. Dissolved inorganic nitrogen (DIN) flux is calculated as the sum of nitrate (NO\({}_{3}\)-), nitrite (NO\({}_{2}\)-), and ammonium (NH\({}_{4}\)+) fluxes. Diel fluxes are based on an inundation time of 6 h per day. lower O\({}_{2}\) influx on October. The decreased reductive state of the post-storm sediments is also reflected in nutrient dynamics (Section 4.2). Constant net uptake of dissolved oxygen in the incubation experiment (Figure 5) shows that the benthic system of these mangroves is always heterotrophic, as is common in mangrove sediments [56, 60, 70, 72, 80], which are consequently a net source of carbon dioxide (CO\({}_{2}\)) to the overlying bottom water [72].