Oxygen concentrations in estuarine water columns result from the balance of autotrophic and heterotrophic processes. Contamination by petroleum hydrocarbons is common in anthropogenically impacted coastal waterways but the effects of petroleum hydrocarbons on water column oxygen are difficult to predict because the hydrocarbons can be used as a carbon source by heterotrophic bacteria, but also may cause toxicity. Oil concentration and temperature are also likely to influence the outcome. In many aquatic environments, autotrophy and heterotrophic decomposition of oil are both nitrogen limited. However, N-limitation of either process is unlikely in the Hudson, given extremely high anthropogenic N-loading. To examine the effects of crude oil on the balance of autotrophy and heterotrophy in a nutrient-rich environment, experiments were conducted using water collected from the Hudson River Estuary from winter to summer. An emulsion of light sweet crude oil was added to oxygen bottles at a range of concentrations (0-250 ppm), that were incubated at ambient temperatures for ~2 d. The change in oxygen levels in light and dark bottles during incubation was used to quantify the impacts of the oil additions on heterotrophy and autotrophy. The fresh crude oil emulsion was toxic to autotrophy at much lower concentrations than heterotrophy. However, the heterotrophic response may have been more sensitive to temperature. These results will help predict how contamination by petroleum hydrocarbons will impact estuarine oxygen dynamics.