Distributions of Size-fractions of Chlorophyll A and Its Controlling Factors in Summer in the Southern South China Sea

Chlorophyll a(Chl a) of micro(20-200 μm),nano(2-20 μm) and picoplankton(2 μm) were measured in different layers of the Nansha Islands in the South China Sea(SCS) to explore the marine environment in June,2009.The spatial distribution and contribution of chlorophyll of micro,nano,and picoplankton to phytoplankton were studied.Chl-a contents were varied from below detection limit to 0.51 μg / L with an average of 0.10 ± 0.09 μg / L,and decrease gradually below the depth of 50 m with the depth,reaching the level of zero in the layer of 200 m.Picoplankton contents were ranged from 0.02 to 0.04 μg / L with a mean value of 0.10 ± 0.07 μg / L.The concentrations of chlorophyll of micro and nanoplankton were ranged from 0.001-0.05 μg / L(0.006 ± 0.009 μg/L) and 0.004-0.12 μg/L(0.02 ±0.02 μg/L),respectively.Picoplankton was the most important contributor to primary production accounting for 81.7% ± 8.9% of total phytoplankton Chl a,whereas micro and nano plankton accounted for only 5.1% ± 3.7% and 13.2% ± 6.2%,respectively.According to the results,temperature,dissolved oxygen concentration,nutrient distribution and limitation,hydrological conditions,euphoric were the main factors restricting the phytoplankton growth and their distributions.