## 4 Discussion
### Organic Carbon and Nutrient Fluxes  
Dissolved organic carbon (DOC) exhibits the strongest benthic fluxes measured in the Naniu mangrove forest (Figure 6), but rates are low compared to other mangroves, estuaries, and shallow coastal areas [82, 83]. The observed temporal variability in benthic DOC flux magnitude and direction is common in mangroves (e.g., [47, 60, 62]). Despite high pore water concentrations, efflux may cease or even reverse direction due to DOC uptake by bacteria at the sediment surface [70, 84, 85]. Benthic-dissolved nitrogen fluxes in the Naniu mangrove forest are within the range measured in other mangrove areas, and show comparatively low temporal variability between March and October [28, 55, 58, 61, 62, 63, 64, 68, 86]. Both the incubation experiment (Figure 6) and pore water profiles (Figure 3) show that the sediments are a sink for dissolved nitrogen. Mangrove sediments are generally DIN sinks, mostly due to the uptake of NO\({}_{3}\)\({}^{-}\), while NH\({}_{4}\)\({}^{+}\) often shows efflux [60, 61, 62, 87, 88, 89, 90, 91]. Several benthic processes cause the removal of NO\({}_{3}\)\({}^{-}\) from overlying water. While assimilation by primary producers removes NO\({}_{3}\)\({}^{-}\)[92], it is probably no major NO\({}_{3}\)\({}^{-}\) sink as daytime nitrogen removal strongly exceeded GPP-associated nitrogen demand (Table 2), and NO\({}_{3}\)\({}^{-}\) uptake was higher in the dark (Figure 6). Pore water profiles from the Naniu mangrove forest indicate the removal of NO\({}_{3}\)\({}^{-}\) by denitrification (compare e.g., [26]), an important sink for NO\({}_{3}\)\({}^{-}\) in mangroves (e.g., [93, 94]), during which NO\({}_{3}\)\({}^{-}\) retains oxidizing conditions when oxygen availability is limited [39]. This reducing process can increase with NO\({}_{3}\)\({}^{-}\) availability [91, 95, 96], but see [72, 97]. Since NO\({}_{3}\)\({}^{-}\) is the most common form of anthropogenic nitrogen input [98, 99, 100, 101, 102], denitrification is particularly important in heavily impacted systems [93] such as the studied mangroves, which receive high loads of NO\({}_{3}\)\({}^{-}\) from the Naniu River [50]. Denitrification can decrease by downward expansion of the oxic layer by photosynthetic oxygen production in light [36], explaining the lower NO\({}_{3}\)\({}^{-}\) influx in the light (Figure 6). NO\({}_{3}\)\({}^{-}\) can also be utilized in the reoxidation of sulfide [103], produced during SO\({}_{4}\)\({}^{2-}\) reduction (Figure 3), or reoxidation of NH\({}_{4}\)\({}^{+}\) at the sediment surface, which introduces NO\({}_{3}\)\({}^{-}\) into bottom water. Oxidation with O\({}_{2}\) produced during primary production and interception by benthic phytoplankton [36] may be reasons for lower NH\({}_{4}\)\({}^{+}\) fluxes during light. The strong increase of NH\({}_{4}\)\({}^{+}\) with sediment depth (Figure 3a,b) and its efflux in March suggest NH\({}_{4}\)\({}^{+}\) production concurrent with NO\({}_{3}\)\({}^{-}\) removal. Dissimilatory nitrate reduction to ammonium (DNRA) has been proposed as an important and often underestimated NO\({}_{3}\)\({}^{-}\) sink [104, 105, 106], especially in tropical estuaries with low benthic DO flux rates [107]. In anthropogenically impacted reduced sediments, DNRA may be more important than denitrification [52,108]. The production of NH\({}_{4}\)\({}^{+}\) by DNRA can exceed that by ammonification of organic matter by a factor of 10 [105]. The occurrence of DNRA has important implications for the mangrove because contrary to denitrification it does not remove reactive N as gaseous N\({}_{2}\) but keeps it available in the system as NH\({}^{+}\) (e.g., [40]). Both denitrification and DNRA yield NO\({}^{-}\) as an intermediate product [40,109, 110, 111]. Benthic NO\({}^{-}\) fluxes and concentration \(>\) 3 \(\upmu\)M in the incubated water column, as well as measurable pore water concentrations (Figure 3), reflect active DIN conversion processes in the sediment, with NO\({}_{2}\)\({}^{-}\) as intermediate product.