Historical evolvement trends of DIN and DIP in waters of Bohai bay from 2004 to 2010

Water samples were collected in 17 stations in the Bohai Bay of china to analyze the distribution of dissolved nutrients and assess the situation of pollution status. Monitoring results of 2010 showed that high-value points of dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) were concentrated in the 7 station (near the Xingang port). The DIN concentration in Bohai bay was rising from 2005 to 2008 while a significant improvement occurred in 2009. It suggested that land-based DIN pollution can be effectively controlled. DIP nutrient in Bohai bay has a downward trend from 2004 to 2010. DIP did not reflect a strong terrestrial input characteristics and DIP pollution around the Bohai bay sea area did not have a strong threat to seawater quality in recent years. Coastal water of Tianjin Bohai bay has always been in a “phosphorus restricted” status from 2004 to 2010. That is to say, N/P ratio was greater than 16 through these years. The single-factor analyses to evaluate the pollution status showed that the DIN pollution is not optimistic while DIP pollution has been effectively curbed in the past years. It implied that the nitrogen loading into this semi-enclosed bay is significantly high than phosphorus and the major nitrogen sources are domestic sewage, industrial wastewater and the pollutants generated from the nearby anchoring ships.