Satellite view of the Gulf oil spill as of 12 June 2010 (partial excerpt). Image credit:SkyTruth, via NASA.
So far, I have read only unsupported speculation about the Gulf 'Dead Zone' potentially expanding as a result of increased biological oxygen demand (BOD) from oil and natural gas. It is plausible - assuming phytoplankton blooms are not reduced somehow by the presences of so much oil and gas - but it would be pretty hard to model how oil, gas, and plankton interact in the water column without pilot scale testing. Marine scientists lack data on the long term BOD and other characteristics of those specific hydrocarbons; and by the time they did get that data the zone will be underway. So, it really is an open question. That said, the pounds of hydrocarbon BOD in the Gulf is a huge and growing number. A summer tropical storm in the Gulf could raise the average temperature and speed up the decay...so there is room for concern, in spite of the following caveat.
If you are reading this far down, you probably realize that nitrogen loading from the Mississippi River watershed is the factor to which northern Gulf of Mexico plankton blooms, and hence the dead zone extent, is most directly correlated. The dead zone size is not as closely correlated with soil particles, for example. Adding hydrocarbons to the mix is a big unknown for modelers. Here is a graph of Gulf dead zone trends as currently predicted.
The measured size, the modeled size (Model 1), and the predicted the size of the
hypoxic zone for 2009 (Model 1 and 2). Model 1 is the oldest model that uses data for nitrate-N loading (USGS estimates) to the Gulf of Mexico (GOM) from 1980 to present. Model 2 is the new model that uses estimates of nitrate+nitrite N loading in the Mississippi River at Baton Rouge, LA, from 1997 to present. Image and caption credit:GulfHypoxia.net.