Temporal-spatial variations of chlorophyll-a in the adjacent sea area of Yangtze river estuary

Abstract

As one of the indicators of the essential ecological characteristics of the sea area, the temporal and spatial variations of ocean chlorophyll-a is an important index to measure the distribution of phytoplankton, marine primary productivity and eutrophication status. Accurately detecting the temporal-spatial distributions of ocean chlorophyll-a with the remote sensing technology has significant importance for the study of marine ecological, carbon cycle between the ocean-atmospheric system and global change. Taking the Yangtze river estuary as the study area, the present paper aims to obtain the spatial and temporal distributions of chlorophyll-a, based on the chlorophyll-a data remote sensed by MODIS from July 2002 to December 2009. To obtain the spatial distribution of chlorophyll-a, the study area was firstly divided into 10 buffer zones from the coastline to the open sea every 40 nautical miles distance. Then the yearly and monthly average values of chlorophyll-a were calculated in every buffer zone. The yearly analysis results showed that annual average chlorophyll-a concentration declined as power function of the distances from coastline to open sea. The coefficient of determination (R2) is 0.9981. The first derivative of annual averages showed that the chlorophyll-a within 120 nautical miles away from the shoreline drop sharply, while the decline without 120 nautical miles turned to moderate. In the area without 120 nautical miles away from the shoreline, the annual variation trends of chlorophyll-a were relatively consistent. There was an obvious pear in spring which appeared in April and the minimum value occurred in June of summer. The chlorophyll-a had no obvious changes in autumn and winter. The annual trends within 120 nautical miles were disordered and unconspicuous. A sinusoidal curve model was established to fit the time series in these buffer zones, to simulate the change trends, variation periods and fluctuation ranges of chlorophyll-a concentration. The results showed that the variations of these regions could be well simulated by the model between 80 to 400 nautical miles, and the values of R2 were more then 0.55. However, the simulated results were not satisfied from the shoreline to 80 nautical miles. The spatial variation of chlorophyll-a was that the less distance away from coastline, the less stability chlorophyll-a concentration is. The important factor to effect the distribution of chlorophyll-a was the humane activities in the coastline.