There is a horizontal salinity gradient at depth and strong vertical haloclines. Fresh water may also be withdrawn upstream for other uses. A good example is the Chesapeake Bay. This anoxia kills bottom-dwelling organisms like oysters, blue crabs, and others. The anoxia has increased in the last 40 years, presumably due to increased nutrient input from human activities.
Point sources are large localized sources like sewage treatment plants and industrial facilities.
Pollution from these is increasingly well controlled. Non-point sources are many small dispersed sources like agricultural runoff, runoff from suburban lawns and gardens, atmospheric deposition in rain and fallout, and sometimes storm water runoff. These sources have become increasingly common and difficult to control. They are intermittently submerged by ocean water, and they usually have low-oxygen mud and as well as peat deposits. They can also remove pollutants from the water, especially inorganic nitrogen and metals from polluted groundwater.