SEASONAL VARIABILITY OF CRUDE-OIL RESPIRATION POTENTIAL IN SALT AND FRESH MARSHES

Seasonal variations of the potential of Gulf coast marshes to degrade crude oil naturally or under nutrient-enhanced conditions is essential ly unknown. Seasonal variations in crude oil mineralization were deter mined under ambient and enhanced nutrient conditions for both a fresh- and a salt-water marsh in Louisiana's Barataria basin over a 1-yr per iod. Mineralization of (C-14) phenanthrene and hexadecane was measured by radiorespirometry in marsh soil slurries dosed with crude oil unde r ambient and enhanced nutrient conditions. Background nutrient condit ions in these marsh systems, as well as numbers of hydrocarbon-utilizi ng microorganisms, were also monitored. Seasonal variations were found in both marsh systems, although they were greater in salt marsh. Seas onal trends in mineralization rates were different for phenanthrene an d hexadecane as well as for each marsh. Hexadecane mineralization rate s ranged form 0.2 to 2.4%/d (ambient nutrient conditions) and 3.1 to 1 0.4%/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 nutrie nt 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 b ut had little effect on phenanthrene mineralization lag time. The popu lation of hydrocarbon utilizers was correlated to phenanthrene mineral ization rates under ambient and enhanced nutrient conditions in the fr esh marsh, and to hexadecane mineralization rates under ambient and en hanced nutrient conditions for the salt marsh. Mineralization rates of phenanthrene and hexadecane appear to be uncorrelated for both marshe s. Both the fresh marsh and salt marsh soils examined had a substantia l capacity to degrade representative crude oil components under ambien t conditions. Nutrient enhancement had the greatest benefit in the sal t marsh studies, and seasonal differences in the rates of hexadecane a nd phenanthrene mineralization under both ambient and enhanced nutrien t conditions were significant.