Denitrification in Sediments of a Lake Erie Coastal Wetland (Old Woman Creek, Huron, Ohio, USA)

Abstract

Denitrification in Old Woman Creek estuary (Lake Erie) sediments was measured by an in vitro N2-flux method with intact cores and by an in situ chamber method. In both methods, nitrogen gas, the end product of denitrification, was measured directly by gas chromatography. The in situ approach allowed measurement of denitrification directly over short time intervals but its use was limited to shallow depths. Denitrification rates measured with in situ chambers agreed well with those from in vitro intact cores when temperatures in the estuary remained constant. However, the two methods could not be accurately compared during the spring when temperature increased rapidly, because of the 4-day pre-incubation time needed for sparging for the in vitro method. In vitro denitrification rates ranged from ca 40 to 135 μmole N2 m−2 h−1 in October 1993 and from 66 to 428 μmole N2 m−2 h−1 in May and July 1994. Oxygen consumption rates in these experiments ranged from 0.71 to 3.0 mmole O2 m−2 h−1. Denitrification rates tended to decrease along the flow axis but differences among stations were usually not significant. In situ N2 accumulation rates ranged from 45 μmole N2 m−2 h−1 in dark chambers during October 1993 up to apparent values of 2,100 μmole N2 m−2 h−1 in May 1994, immediately after the water temperature had rapidly increased to 27°C. These calculated values included gas-solubility corrections due to the water-temperature increases. In situ measurements of denitrification rates in transparent chambers were 76–79% higher than rates measured in a similar dark chamber. The results suggest that denitrification is an important sink for nitrogen in Old Woman Creek estuary and that environmental conditions such as temperature, light, and available substrate affect denitrification rates.