Effect on foundation of the food chain in the Gulf? The short answer is thay we don't know.

Ed Overton, Louisiana State Univ.- "We don't know whether it's affecting wildlife or not. We're right in the middle of this. We really won't know for a while yet."

Samantha Joye, University of Georgia, researcher and cruise leader tracking underwater oil plumes-
"Dispersants are a complicated topic. No one that I have spoken to about this has a full understanding of what the full range of dispersant effects might be. How do dispersants influence microorganisms and microbially-mediated processes? I don't know. How do they impact fish, larvae, phytoplankton, shrimp? I don't know the answer to that either."
She saysi dispersants break oil into smaller particles that keeps oil off the beaches but "I am not convinced this is a good thing because there are so many potential unknown effects of dispersants."
.........by keeping the oil in the water instead of at the surface, other organisms suffer."

Mitchelmore, Univ. Maryland- When you add dispersant, organisms are exposed to oil that wouldn't have been. Dissolved oil can go directly across organisms' membranes...... it can stick to gills."

George Crozier, Dauphin Island Sea Lab, Alabama- "A lot of organisms that can swim are probably saying this doesn't smell good or taste good and leaving, but the plankton that forms the base of the food chain doesn't have that option."

Andy Nyman, Louisiana State Univ.-
"We found that working with South Louisiana crude and COREXIT 9500, the dispersed oil was more toxic than the undispersed oil initially and even six months later," he said.
The plankton and a tiny worm- the major food source for shrimp- were the most sensitive.
In the Gulf, Nyman said, "I would expect the dispersed oil to be more toxic and for the effects to last longer unless I saw data otherwise."
Another concern with dispersants is that by keeping oil in the water column where microbes can degrade it, oxygen levels in the water can drop to potentially dangerous levels as the microbes feed on the oil and consume oxygen.

In other words, scientists working on the Gulf don't know, but are worried.

More later. Above extracted from Discovery News
http://news.discovery.com/earth/oil-dispersants-wildlife.html