One common path of entry by contaminants to the sea are rivers. The Hudson in New York State and the Raritan in New Jersey , which empty at the northern and southern ends of Staten Island , are a source of mercury contamination of zooplankton ( copepods ) in the open ocean. The highest concentration in the filter-feeding copepods is not at the mouths of these rivers but 70 miles south, nearer Atlantic City , because water flows close to the coast. It takes a few days before toxins are taken up by the plankton.

Many particles combine chemically in a manner highly depletive of oxygen , causing estuaries to become anoxic