Low levels of dissolved oxygen in the canal's waters predate World War I. The minimum level of oxygen required to support healthy marine life is estimated at 4 parts per million. As early as 1909, it was reported that the canal had no oxygen at all. In 1975 a severe lack of oxygen was still observed, indicating the water was incapable of sustaining plant life or fish. In 1999, just before the flushing tunnel was reactivated, "The Environmental Magazine" reported that oxygen levels in the Gowanus Canal measured about 1.5 parts per million; this number continued to be quoted fourteen years later. However, by 2008, nine years after the flushing tunnel was reopened to provide oxygenation of the water, biologist Kathleen Nolan and students from St. Francis College sampled the water and reported that levels of dissolved oxygen had substantially increased. In 2014, a NYSDEC representative stated that dissolved oxygen levels were reported to be in the range of , or approximately 9–12 parts per million. With respect to the underlying layers of residue in the canal, the EPA and other organizations have performed detailed analyses of the composition and distribution of black mayonnaise throughout it. The 2012 Superfund Proposed Plan also includes detailed assessments of risk related to the types of pollutants in the layers of sediment, the water, and the surrounding area. The Gowanus Canal's pollution has also spread to Gowanus Creek, at the mouth of the canal. In 1982, USACE released the results of a report on navigation in the creek. It found that there were nonexistent levels of oxygen; high concentrations of fecal coliforms; and significant clusters of oil and grease. Originally, the marshland and freshwater springs that drained into the Atlantic Ocean in Upper New York Bay were capable of supporting massive oyster beds. As late as 1911, people reported fishing in the Gowanus Canal and treading for clams. By 1927, the last of New York's oyster beds had closed as a result of habitat destruction, over-harvesting, and pollution. Attempts have been made to reintroduce oysters and other shellfish to the canal, because they can filter out toxins and help clean the water. One oyster can process as much as of water a day. The NY/NJ Baykeeper environmental group gives oysters to volunteers who then monitor their health and growth in local waterways. They have helped Katie Mosher-Smith and The Gowanus Community Oyster Garden's Stewards to partner with teachers, students and the Gowanus Dredgers Canoe Club to install and monitor oyster cages in the canal.