In order to understand and assess the role of mangrove sediments in the overall nitrogen budget of a tropical mangrove ecosystem, the following was investigated: ( 1) granulometry and pore-water chemistry; (2) main nitrogen transformational processes - organic nitrogen mineralization (regeneration and assimilation rates), nitrification (both potential and actual) and (3) sediment - water column nutrient fluxes, for sediments underlying stands of the two most predominant species of mangrove vegetation (i.e. Rhizophora mucronala and Ceriops tagal) in Gazi bay. These processes were investigated and related to both; possible sources of the remineralized sedimentary organic material and the concentrations of dissolved inorganic nitrogenous compounds found in the overlying water column. Though senescent leaves of both, Rhizophora mucronala (Rm) and Ceriops tagal (Ct) species were found to have a C/N atom ratio of about 200, the sedimentary organic matter indicated a C/N atom ratio of between 19 and 30 for the upper 6 cm depth. This tremendous reduction of the C/N ratio in mangrove sediments in comparison to the mangrove leaf litter (expected to be the main source of organic matter) indicates a high possibility of another (perhaps more important) external supply of organic nitrogen into the sediments. Though this could come from nitrogen fixation, the maximum possible nitrogen addition into both Rm and Ct sediments through N2 fixation and mangrove litterfall is relatively low when compared to the observed remineralization rates implying that without a constant external supply of organic nitrogen, the system would not be self-sustaining. Using a simple conservative mass balance model, this study has demonstrated the likelihood of organic material in mangrove sediments to be composed of two main parts; ( 1) a pool of highly refractory organic material from mangrove vegetation which is very poor in organic nitrogen and; (2) a pool of labile organic nitrogen, most likely marine POM of phytoplankton origin. However, when comparing the actual fluxed CO., (Middelburg et al., in press) and the observed nitrogen remineralization rates in the sediments, the remineralized organic matter is observed to have a C/N atom ratio of between 3 and 8. This implies that another source of organic matter whose C/N is < 6.6 is actively involved in the mineralization process. Since bacterial biomass has a C/N atom ratio of between 3 and 7, it is very likely that this biomass could be the major source of the regenerated NH4+ in mangrove sediments. The marine PON therefore possibly acts as the easily available source of the labile organic nitrogen pool for bacterial utilization supporting the high bacterial productivities observed in most mangrove sediments. Upon death, these bacterial cells are then remineralized becoming the main source of the ammonium in mangrove sediments. It is furthermore observed that between 44 % and 60 % of the ammonium mineralized is again taken up during bacterial growth. The remaining fraction of ammonium produced is then available for ( 1) nitrification; (2) outflux to the overlaying water column and (3) uptake by roots. When excluding the effect of CX, supplied by roots, nitrification appears essentially limited to the first cm of the sediment column and represents only between 25 % (Rm sediment) and 3 % (Ct sediment) of the net ammonification rate found at the upper 1 cm of the sediment. These low nitrification rates are ascribed to low oxygen availability for Rm sediments and toxic conditions found in Ct sediments as a result of relatively high ¦ ----\ temperatures and salinities coupled with extreme-acidic conditions lowering the pH of the pore water out of the limiting range (6.0 - 9.5) of most nitrifying bacteria. The possible strength of the mangrove sediments as a source of dissolved inorganic nitrogen to the overlying water column was also investigated, both by measuring field epibenthic fluxes and by calculating fluxes based on measured concentration gradients across the sediment - water column interface. Nitrate (+ nitrite) fluxes were generally much lower and amounted only to about 10 to 15 % of the observed ammonium outflux. Ammonium outflux represents at most 0.7 % (Rm sediments) and 1.1% (Ct sediments) of the ammonium regenerated stressing the minor role of epibenthic fluxes in the sedimentary nitrogen budget of these mangrove ecosystems. It is estimated that periodic tidal resuspension of the upper 1.5 mm of sediment is necessary in order to add the required extra ? N (NH4+ + NO,' + NO,') necessary to support the observed primary productivity in Gazi mangrove creeks. From these results, bacterial ammonium production and uptake appear to be the main processes affecting the organic nitrogen pool while nitrification, denitrification and nitrogen fixation represent relatively minor processes. Between 35 % (Rm) and 55 % (Ct) of the regenerated ammonium is found to be taken up by the trees themselves. It thus appears that nitrogen introduced into the mangrove sedimentary compartment is mainly left for (I) recycling through an active bacterial production system; (II) uptake by the mangrove root system and (III) accumulation as refractory nitrogen. These different observations indicate that mangrove ecosystems, at least those similar to the system present in Gazi bay, are probably not important exporters of organic nitrogen and dissolved inorganic nutrients to the coastal ecosystem. Instead, they function as rather efficient traps and appear to efficiently recycle nitrogen in order to primarily satisfy nitrogen requirements of the mangrove trees themselves. |
