HOUSTON—Scientists studying the environmental impact of the Deepwater Horizon oil spill in the Gulf of Mexico are raising fresh concerns about the effect of the leaked crude on a range of sea life, from tiny animal plankton to dolphins.

So far, studies have not uncovered the ecological apocalypse that some feared after the Deepwater Horizon drilling rig exploded two years ago this month, unleashing the biggest offshore oil spill in U.S. history. But hopes that the Gulf would be relatively unaffected are dimming.

"The death and destruction that many predicted hasn't come through for a lot of reasons," said Robert Haddad, head of the National Oceanic and Atmospheric Administration's assessment and restoration effort. "But everywhere we look throughout the Gulf things are just a little bit out of kilter."

Zooplankton—microscopic organisms that are a source of food for many fish—were found to have ingested hazardous components of the specific oil from the spill, according to a study released last month by researchers at East Carolina University and other colleges and funded by the National Science Foundation. The study didn't speculate on whether the oil may have harmed the zooplankton nor did it say what the effect could be on larger organisms.

A large coral formation on the sea floor several miles from the well site appears to be dying because of a coating of oil from the spill, according to a study by Pennsylvania State University, Haverford College and other institutions, also funded by a National Science Foundation grant.