Preliminary study of benthic metabolism and sulfate reduction in a mangrove swamp of the Indus Delta, Pakistan

Benthic metabolism (sediment O2 uptake, CO2 production and sulfate reduction) and nitrogen dynam~cs were studied in a mangrove swamp of the Indus Delta, Pakistan during fall 1990. The mangrove, whlch is characterized by large salinity fluctuations, has hlghly variable concentrations of C 1 and S o d 2 in the sediment porewater. Vertical profiles of these const~tuents reflect the historical events of salinity changes in the overlying water. Rates of O2 uptake (both measured d~rectly and estimated from 0, profiles) at intertidal creek banks were generally low and about 2 tlmes higher in airexposed than in water-covered sediment. The high rates found during air exposure was caused by a reduction in the thickness of the diffusive boundary layer and an increased area of oxic-anoxic interfaces due to drainage of water from large sediment interstices (burrows and cracks in the surface). The frequency and duration of water cover were important determinants for microbial respiration in the sediment. Highest rates during water cover were measured subtidally in creek sediment and lowest rates at intertidal creek bank sites. Desiccation may reduce overall microbial respiration at the creek banks. About half of the CO2 production measured during water cover could be accounted for by sulfate reduction in intertidal sediments, whereas only 18 "/o of the CO2 production in subtidal creek sedi m e n t ~ could be ascribed to sulfate reduction. The remainder may be produced by other respiration processes. The low metabolic activity in the mangrove sediment was partly caused by the refractory nature of sediment detritus (mostly remains from tree leaves and roots). The generally low fluxes of dissolved inorganic nitrogen combined with the low nitrogen content in sediment detritus (C:N = 20) also indicated that nitrogen was a limiting factor for microbial activity in this mangrove swamp.