Distributions And Changes Of Nutrient Concentration In The East China Sea In Late 50 Years

Since the mid-20th century, the nutrient levels and productivity have undergone significant changes in waters of estuaries and the continental shelf in many countries in the world. Plenty of studies were on the nutrients in the East China Sea. But, so far, due to the limitations of the scope of investigation, special study in depth in a larger scale of time and space is still scare. Based on above, this paper analyzed the distribution, seasonal variation, the average annual concentration and changes in nutrient structure of DIN, PO₄-P and SiO₃-Si in the year 1959,1984,1998 and 2007. Concentration distribution and changes in a long term of the nutrients in Changjiang River Estuary and adjacent waters were studied. The main conclusions are as follows:(1) The nutrient concentrations in the surface of East China Sea showed the characterization that the value was higher near the western shore while low in the open sea. From 1959 to 2009, high value area of DIN in the entire area was increasing while high-value region of PO₄-P and SiO₃-Si continued to decrease. The annual concentrations of DIN, PO₄-P and SiO₃-Si has also undergone significant changes, that is DIN concentration increased rapidly from 3.5μmol/L in the 60's to 4.1μmol/L in 80's, and increased from 4.8μmol/L in 98's to the current 5.5μmol/L, while PO₄-Pconcentration decreased steadily, from 0.4μmol/L in 60th to 0.2μmol/L in 98's, then to the current 0.15μmol/L, and SiO₃-Si concentration was significantly decreased from 19.8μmol/L in the 60's down to 8μmol/L in the 84's , then 9.7μmol/L in the 98's,and then the current 7.6μmol/L. The rise of the DIN concentrations and fall of the SiO₃-Si concentrations were both the same as their trend flux in the East China Sea while the decrease in the PO₄-P concentration was contrary to the flux in the East China Sea. N/P ratio continued to rise, Si/N ratio continued to declined, and Si/P ratio decreased first and then increased. Currently, N/P, Si/N and Si/P ratios were 37,1.4 and 52, seperatedly.PO₄-P in the East China Sea water was the factor that limited the growth of phytoplankton. (2) The average annual of DIN in the waters nearby the Changjiang River Estuary increase during the last 50 years. The nutrient concentrations in the Changjiang River Estuary and adjacent coastal waters decreased from the nearby shore, and existed a wide range of the high value area from the Changjiang River mouth to Zhoushan Islands area all year round. In the past 50 years, the distribution of nutrients showed the characterization that high nearby the shore while low off the shore. The isoline based the Changjiang Estuary and Hangzhou Bay as the center, showing distribution of fan-shaped expansion of the open sea. However, the content of the expansion of different nutrient changed significantly . The high concentration isoline of the DIN expanded significantly, while high concentration isoline of SiO₃-Si contracted largely in the coastal. The average annual concentration of DIN was about 4μmol/L in the the early 60's and 80's of 20th century, and increased to 12.5μmol/L during the mid-80's to the mid 90's, and then fluctuated around 22μmol/L from the late 90's to the current. On the whole, annual average concentration of PO₄-P showed the trend that first decrease, then increase, then decrease, then increase, then decrease during, of which the overall annual average concentration was around 0.5μmol/L in the early 80's to the mid-90's, and fluctuated around 0.6μmol/L in the early 21century. Average annual concentration of SiO₃-Si showed a declining trend from 36μmol/L in the late 50's in the 20th century to current 12.5μmol/L .Since the 60's to the early 80s in the 20th century, the N/P ratio in the East China Sea fluctuated around 12, which was lower than the Redfield ratio, and fluctuated around 20 in the mid-80's to the early 90's, and then increased significantly almost linearly, and up to about 54 now. In the recent years the N/P ratio fluctuated around 40. The annual Si/N ratio was in a decline trend which decreased from 10 in the late 50's in the 20th century to about the current 0.6. And the annual Si/ P average ratio was declining with time, from about 60 in the late 50's in the 20th century to the current 22.