Benthic communities from a freshwater marsh were exposed to a No. 2 Fuel Oil, a Nigerian Crude Oil, and used crankcase oil in microcosm experiments in which natural conditions were simulated and hydrocarbon concentrations and exposure conditions were monitored. Growths ofVaucheriasp. were destroyed to varying degrees depending on oil type and exposure. Blue‐green algal development was fostered. Community metabolism shifted to a heterotrophic mode as a result of effects on algae and reduced photosynthesis but community respiration did not increase to a statistically significant extent on oil exposure. No. 2 Fuel Oil was the most damaging, used crankcase oil was least toxic and Nigerian Crude Oil was intermediate in degree of effect on metabolism.