## 4 Discussion
### Mangrove System Nutrient Dynamics  
High variability of nutrient concentrations at low salinity and particularly during neap tide shows that at low water levels nutrient composition in estuarine mangrove channels is influenced by effluents and pore water seepage from the mangrove forest. Near-conservative behavior at higher salinity shows that with rising water levels the hydraulic gradient reverses and there is no influx into the channel. Varying concentrations in the estuarine channel and in mangrove effluents during low tide suggest that the influence from the mangrove is temporally variable (Figure 2). During neap tide, salinity was higher in mangrove effluents than inundating water. Increased salinity may be due to evapo-transpiration in the mangrove (see e.g., [47]). This is not evident during spring tides, rather indicating a delayed release of saline water delivered into the mangrove during previous high tides with stronger marine influence (see e.g., [48]). Deviations of effluent concentrations from those expected from mixing of marine and fresh waters reflect nutrient addition to (positive deviation) or removal from (negative deviation) inundating water during residence time in the mangrove. Since concentrations of all nutrients are higher in riverine than marine water, freshwater influence cannot explain the decrease of NO3-, NO2-, and PO43- in effluents relative to marine inundating water (Figure 2). Extremely low NO3 concentrations in effluents reflect the uptake by sediments. Nevertheless, NO3 concentrations during low tide are similar to those found in the river [50], and display lowest relative variability among nutrient species, indicating that estuarine NO3- dynamics are not significantly influenced by mangroves but are dominated by high riverine inputs. Similarly, DOC concentrations during low tide and in effluents are similar to riverine values (235.3 to 339.6 and 282.9 to 2345.6 in March and September 2011, respectively; [PERSON], unpublished [124]), indicating that despite the benthic uptake of DOC in the mangrove, the influence from the river dominates over mangrove dynamics. Low tide concentrations of NO2 in the estuarine mangrove channel are high compared to effluents and pore water, and are also higher than in Naniu River. This increase in concentrations despite the dilution by effluents and pore water drainage suggests high rates of _in situ_ nitrogen conversion in the mangrove channel, particularly in March. The dependence of _in situ_ processes, rather than external inputs, was also suggested for NO2 concentrations throughout the estuary [50]. Effluent and low tide Si and NH\({}_{4}\)+ exceed river concentrations [50] and thus cannot solely be sourced from riverine input, but indicate sources within the mangrove which influence estuarine concentrations via effluent runoff and pore water drainage. Increased NH\({}_{4}\)+ values in effluents are due to benthic effluxes (Figures 3a,b and 6) and lateral belowground transport of interstitial water and solutes [47, 69]. Lateral transport also increased Si concentrations in effluents (Figure 2) despite measured sediment uptake (Figure 6). An additional possible source of Si to effluents is leaching from macrophytes as well as their epiphytes and debris [125, 126, 127, 128]. Deep pore water drainage would also introduce PO\({}_{4}\)3-, but concentrations are low in effluents compared to channel concentrations (Figure 2). Pore water PO\({}^{3-}\) concentrations only increase strongly below 5 cm sediment depth, while Si concentrations in the upper 5 cm are high enough to support high effluent concentrations even during early desiccation and drainage of surface sediment pores. This, together with the shallow depth of small creeks, suggests that lateral drainage into effluents is restricted to the upper sediment layers. Shallow pore water may also be released during sediment resuspension by tidal currents [123].