DENITRIFICATION AND OXYGEN-CONSUMPTION IN SEDIMENTS OF 2 SOUTH TEXAS ESTUARIES

Spatial and temporal variations in rates of denitrification and oxygen consumption were measured in sediments of the Nueces and Guddalupe Estuaries in southern Texas, USA. Denitrification rates varied from 4.0 to 71.1 mumol N2 m-2 h-1 in the Nueces Estuary and from 4.6 to 34.7 mumol N2 m-2 h-1 in the Guadalupe Estuary. Denitrification accounted for 29 to 80 % of total benthic N flux in the study areas. Oxygen consumption rates ranged from 176 to 818 mumol O2 m-2 h-1 in Nueces Estuary and from 208 to 550 mumol O2 m-2 h-1 in Guadalupe Estuary. In both estuaries denitrification and oxygen consumption rates were generally higher in the upper estuaries where the porosity and organic matter contents of sediments were typically highest. Among 3 measurements in different seasons, the lowest denitrification and oxygen consumption rates were usually observed during winter. Chemical oxygen consumption, as measured using formalin-killed controls, was about 50 % of the total oxygen consumption. A considerable portion (ca 50 %) of biological oxygen consumption was attributed to nitrification in sediments. We estimated that carbon mineralization by denitrifiers was as much as 40 to 179 % of that by aerobic heterotrophs. In the Guadalupe Estuary, denitrification removed about 38 % of the measured inputs of organic and inorganic nitrogen. In the Nueces Estuary, the amount of nitrogen removed by denitrification was ca 2 times greater than the measured nitrogen inputs from the Nueces River and precipitation, suggesting that unmeasured anthropogenic inputs along the perimeter of the estuary were very important for maintaining nitrogen balance.