The Chesapeake Bay's lush wildlife is threatened by spreading pollution, including "alarming" amounts of toxic chemicals, the Evironmental Protection Agency says in a report to be released tomorrow.

The document -- the first comprehensive report on a $27 million, six-year federal research study -- describes widespread accumulations of potentially suffocating phosphorus and nitrogen, extensive buildups of toxic metals and other hazardous substances, and severe declines in submerged bay grasses, a prime index of the Chesapeake's health. A copy of the 635-page report was obtained this week by The Washington Post.

The study largely stops short of spelling out in detail how pollutants in the bay may affect its fish, shellfish, waterfowl and aquatic life. Nor does it recommend steps to halt environmental damage. These issues are expected to be examined in later reports, scheduled to be released by January.

"In certain areas, present levels of toxic substances could threaten the health of organisms," the first report says. "Tests performed on effluent from industrial plants around the bay area revealed that up to half of the effluents sampled killed test fish and invertebrates.

"Present abundance of bay grasses is at its lowest level in recorded history," the study says. "A marked decline has occurred throughout the estuary since the mid-1960s." The report suggests that the increased levels phosphorus and nitrogen may be the chief cause of the dwindling of grasses, which are considered essential as food and habitat for other bay wildlife.

The EPA report, based on research by more than 100 government and university scientists, is being published as Congress takes steps to extend the Chesapeake Bay research program beyond January, when current federal allocations end.

The House has approved an amendment, sponsored by Rep. Roy Dyson (D-Md.), to appropriate $900,000 to continue bay monitoring and other research efforts next year. Sen. Charles McC. Mathias Jr. (R-Md.), long a bay advocate, has urged Senate assent to the amendment. A conference committee was expected to consider the proposal last night. In addition, a separate $250,000 allotment for bay monitoring studies was approved by a Senate committee last week.

The new EPA report provides an extraordinary assortment of scientific data, viewed by researchers as vital for further bay studies and probably useful for understanding other estuaries. These are partly enclosed inlets where fresh and salt water mix. The Chesapeake is regarded as America's most productive estuary.

The sometimes strongly worded document also represents the Reagan administration's first official statement about the bay's outlook. Some bay advocates previously feared the government might seek to soft-pedal the findings.Many of the report's central conclusions have already been were already made public unofficially through bay activists' newsletters and newspaper accounts, including a Post article in June.

The report indicates that substantial accumulations of phosphorous, nitrogen, toxic metals and other hazardous substances have occurred in the northern part of the bay, mainly above the Chesapeake Bay Bridge. Other buildups were found in heavily industrial areas, such as Baltimore Harbor. The Susquehanna River is described as a significant source of pollutants, a finding that already has stirred objections from Pennsylvania.