According to Dr. Sibyl Seitzinger and Renee M. Styles of Rutgers, nitrogen enters estuaries and coastal systems from a variety of point and non-point sources . The situation that results from excessive nutrients creating negative effects is known as eutrophication in which increased algal growth decreases water clarity, changes food available to other organisms and depletes dissolved oxygen in the water column.  Not all nitrogen molecules are the same.  There is the inorganic nitrogen in the form of ammonia and nitrate as well as organic molecules that contain nitrogen.  It had been generally thought that the more readily available inorganic nitrogen inputs were responsible for algal blooms and eutrophication. Recent studies, however, demonstrate that nitrogen entering in the form of organic molecules can contribute to eutrophication.  Studies at Rutgers address cow, horse, pig farms contribution as well as miscellaneous particles from atmospheric precipitation. The percent of the nitrogen that is inorganic versus organic varies among sources.  Before organic nitrogen can be used by algae for growth, it generally has to be broken down by microorganisms.
Typical Ranges of organic nitrogen