I live in the Chesapeake Bay watershed, and what has happened in the bay and waterways nearby is depressing. In essence, too many nutrients, typically nitrogen and phosphorus, are finding their way into those waters, causing algae to grow, then die and in the decomposition process, use up all the oxygen. Once the oxygen gets too low in the bay the sea life dies as well, and we get dead zones. In some cases the algae creates toxins, as we saw in Lake Erie near Toledo, Ohio, last summer.
According to the National Oceanic and Atmospheric Administration’s National Ocean Service (NOS):