Spatial And Seasonal Variation Of Chlorophyll And Primary Productivity And Their Controlling Factors In The Northern Yellow Sea

Chlorophyll is the main pigment in phytoplankton that carries out the process of photosynthesis and a good estimator of algal biomass in the ocean. Primary productivity indicates the capacity of producing organic compounds by phytoplankton. It plays an important role in the global carbon cycle leading to a strong influence on global climate change. The investigation of chlorophyll and primary productivity in the northern Yellow Sea can improve the understanding of maintenance of biological resources and can provide an aid for estimating the quality of ocean environment in order to improve aquaculture of this sea area.Chlorophyll and primary productivity were measured during four expeditions to the northern Yellow Sea during the summer and winter of 2006, and the spring and autumn of 2007 in order to assess their spatial and seasonal variability as well as controlling factors. The comparison of chlorophyll and primary productivity is made among the continental shelf seas of China. The difference between two models for estimating primary productivity is analyzed by comparing the results of these two models with the measured values. A modified model for pursuing higher precision of primary productivity estimation is built up the first time for the northern Yellow Sea.The average values of surface chlorophyll concentration were (1.026±0.907) mg·m^-3 in spring, (0.779±0.640) mg·m^-3 in summer, (1.849±1.584) mg·m^-3 in autumn and (0.652±0.508) mg·m^-3 in winter. The highest average value of autumn is around three times of the value in winter which is the lowest among the whole year. The concentration of sea surface chlorophyll decreases from the near shore area to the central part, and from western narrow area to eastern open area. This distribution pattern is mainly influenced by the underwater terrain, the hydrodynamic features and the import of territorial nutrients into the northern Yellow Sea.The water column integrated chlorophyll in spring, summer, autumn and winter were (51.27±38.77) mg·m^-2, (30.75±19.01) mg·m^-2, (54.40±34.76) mg·m^-2, and (18.72±11.98) mg·m^-2, respectively. The highest seasonal average value occurred in autumn whereas the lowest occurred in winter. Due to the influence of the euphotic layer thickness and transparence, the high values of integrated chlorophyll were found in the deep sea area which is different from the distribution of surface chlorophyll.The vertical distribution of chlorophyll concentration is mainly controlled by physical properties of sea water. Due to intense water stratification in summer, the maximum chlorophyll concentration appears at the subsurface layer which differs from the even chlorophyll concentration in the more uniformly mixed water column in winter.While both nitrate and phosphate are likely to be the controlling factors of phytoplankton growth in winter, only phosphate can be considered as a controlling factor in summer. Further the sea surface temperature and the chlorophyll concentration show negative correlations in all four seasons during the year with R2 values of -0.37 (P = 0.01, n = 70) in spring, -0.66 (P = 0.01, n = 73) in summer, -0.37 (P = 0.01, n = 80) in autumn and -0.64 (P = 0.01, n = 67) in winter. It appears that the phytoplankton growth depends on temperature regarding to the temperature optimum for primary productivity.The model based on water column integrated chlorophyll for estimating primary productivity shows a better correlation to the measured values than the Cadée & Hegeman's model which is based on surface chlorophyll concentration. In the northern Yellow Sea the subsurface chlorophyll maximum occurs for almost seven months during a year which supports the use of the integrated chlorophyll model.The estimated primary productivity in spring, summer, autumn and winter were (2137.7±1612.7) mgC·m^-2·d^-1, (637.4±394.1) mgC·m^-2·d^-1, (996.8±636.9)mgC·m^-2·d^-1, and (414.8±265.4) mgC·m^-2·d^-1 on average, respectively. The horizontal distribution of primary productivity in winter had less variation than other seasons and the seasonal average value is only one fifth of the value in spring.The values in summer show that the surface chlorophyll concentration has the strongest correlation with primary productivity. Zooplankton, ammonia nitrogen and phosphate also present significant correlations while phytoplankton abundance has nearly no correlation with primary productivity. Further regression analysis indicates that factors, which control primary productivity in summer, with decreasing importance are phosphate, surface chlorophyll concentration and nitrate (negative correlation). In conclusion, the summer primary productivity is limited by phosphate but surface chlorophyll and nitrate can contribute to the distribution of summer primary productivity as well.