Seasonal Variability of Crude Oil Respiration Potential in Salt and Fresh Marshes

Abstract

Seasonal variations of the potential of Gulf coast marshes to degrade crude oil naturally or under nutrient-enhanced conditions is essentially unknown. Seasonal variations in crude oil mineralization were determined under ambient and enhanced nutrient conditions for both a fresh- and a salt-water marsh in Louisiana's Barataria basin over a 1-yr period. Mineralization of ( 14 C) phenanthrene and hexadecane was measured by radiorespirometry in marsh soil slurries dosed with crude oil under ambient and enhanced nutrient conditions. Background nutrient conditions in these marsh systems, as well as numbers of hydrocarbon-utilizing microorganisms, were also monitored. Seasonal variations were found in both marsh systems, although they were greater in the salt marsh. Seasonal trends in mineralization rates were different for phenanthrene and hexadecane as well as for each marsh. Hexadecane mineralization rates ranged from 0.2 to 2.4%/d (ambient nutrient conditions) and 3.1 to 10.4%/d (enhanced nutrient conditions) for the salt marsh and 0.81 to 3.1%/d (ambient nutrient conditions) and 2.1 to 7.3%/d (enhanced nutrient conditions) for the fresh marsh. Phenanthrene mineralization rates ranged from 0.5 to 4.5%/d (ambient nutrient conditions) and 1.9 to 12.1%/d (enhanced nutrient conditions) for the salt marsh and 2.8 to 5.5%/d (ambient nutrient conditions) and 2.4 to 5.6%/d (enhanced nutrient conditions) for the fresh marsh. Nutrient enhancement reduced the lag time of hexadecane mineralization in both salt and fresh marsh soils but had little effect on phenanthrene mineralization lag time. The population of hydrocarbon utilizers was correlated to phenanthrene mineralization rates under ambient and enhanced nutrient conditions in the fresh marsh, and to hexadecane mineralization rates under ambient and enhanced nutrient conditions for the salt marsh. Mineralization rates of phenanthrene and hexadecane appear to be uncorrelated for both marshes. Both the fresh marsh and salt marsh soils examined had a substantial capacity to degrade representative crude oil components under ambient conditions. Nutrient enhancement had the greatest benefit in the salt marsh studies, and seasonal differences in the rates of hexadecane and phenanthrene mineralization under both ambient and enhanced nutrient conditions were significant.