Production and export of copepods fecal pellets in an eutrophic coastal sea: The Changjiang (Yangtze River) estuary

Abstract

Copepods fecal pellets (FPs) are important contributors to carbon export in the ocean, but their role in the carbon export is poorly understood in the eutrophic coastal sea, where copepods biomass is always high. Two cruises were carried out in one of the most eutrophic coastal seas in the world — the Changjiang (Yangtze River) estuary during spring and summer in 2011, and copepods fecal pellets carbon (FPC) concentration, their production rates and sinking rates were measured, and a potential export flux of copepods FPs was also calculated. The copepods FPC concentration ranged from 0.21 to 1.25 mg C m−3 (mean = 0.58 ± 0.24 mg C m−3) in spring and 0.46–1.53 mg C m−3 (mean = 0.76 ± 0.25 mg C m−3) in summer, which was significantly lower than that of phytoplankton cells, indicating that copepods FPs only accounted for a minor part of the POC pool in this area. Production rates of copepods FPs fell in the range of the reported values, and the high values always appeared at stations with high phytoplankton biomass. Sinking rates of copepods FPs determined in the lab ranged from 17.82 to 223.30 m d−1 (mean = 73.46 ± 47.56 m d−1) in spring and 34.62–262.33 m d−1 (mean = 92.81 ± 56.80 m d−1) in summer, and they were significantly positive correlated with the FPs volume. The density of copepods FPs was also calculated with the determined sinking rate, and it was higher in summer than that in spring due to the variation of phytoplankton diets from spring to summer. The potential copepods FPs export flux ranged from 17.72 to 60.20 mg C m−2 d−1 (mean = 37.13 ± 16.21 mg C m−2 d−1) in spring and 23.80–166.10 mg C m−2 d−1 (mean = 79.44 ± 48.86 mg C m−2 d−1) in summer, and it was at a comparable level with the export flux of phytoplankton cells during both cruises. This study indicated that copepods FPs comprised a substantial fraction of exported carbon in the Changjiang (Yangtze River) estuary, and they should be taken into account when we study the biological pump in other eutrophic coastal seas around the world.