Rates and pathways of benthic mineralization in extensive shrimp ponds of the Mekong delta, Vietnam

Benthic mineralization rates and pathways were measured in two extensive shrimp ponds of the Mekong delta, Vietnam. Sediments of both ponds were unconsolidated, oxic to suboxic silt-clays of neutral pH (6.8-7.3). Free sulfides and methane were not detected in the porewater, which was dominated by NH4+ (up to 400 mu M); other interstitial solutes were very variable among replicate cores, ponds, and sediment depths. Particulate C and N concentrations ranged from 2-8% and 0.15-0.36% by sediment dry weight (DW) with few depth differences between ponds. Pyrite was abundant (0.3-5.6% of sediment DW) in both ponds. Total carbon oxidation rates were not significantly different between the pond located separate from mangroves (separate pond) and the pond located within a mangrove forest (mixed pond). Fluxes of O-2 and CO2 (= total carbon oxidation, T-COX) were highly variable, with slow rates of CO2 release (range: 7.7-30.5 mmol m(-2) day(-1)) but higher rates of O-2 consumption (range: 9.8-135.9 mmol m(-2) day(-1)), especially in the separate pond. A budget of the contribution of the various diagenetic pathways to total carbon oxidation indicates that aerobic respiration accounted for 41-60% of T-COX, with active manganese and iron reduction in the mixed and separate ponds, respectively. No denitrification or methane flux was detected from sediments of either pond. Rates of sulfate reduction were slow (range: 0.94-2.73 mmol S m(-2) day(-1)) and highly variable, accounting for 13-26% of T-COX. Rates of solute flux across the sediment-water interface were dominated by DOC, NO2- NO3-, and Mn. There was no measurable NH4+ flux. Most light-dark bottle O-2 fluxes indicated no net benthic primary productivity. The slow rates of benthic decomposition and the dominance of oxic and suboxic pathways reflect the slow rates of organic matter input, and phytoplankton and shrimp production in these extensive ponds. (C) 1999 Elsevier Science B.V. All rights reserved.