Phytoplankton-driven dark plankton respiration in the hypoxic zone off the Changjiang Estuary, revealed by in vitro incubations

Hypoxia in near-bottom waters has been increasing globally. Dark plankton respiration is a key aspect of hypoxia studies. In situations where the general background eutrophication level is high, more blooms are found in estuaries and adjacent coastal zones, suggesting an increase in respiration from phytoplankton and heterotrophs. An assessment of the phytoplankton biomass-specific rate of dark plankton respiration is therefore of considerable value in terms of environmental assessments and modeling. During the summer of 2011 a series of concentrated in vitro incubation experiments were conducted on board a ship off the Changjiang Estuary and in the adjacent coastal zone, to simulate phytoplankton-driven dark plankton respiration under elevated phytoplankton biomass (i.e. high Chlorophyll a concentration) conditions and to further quantify the relationship between dark plankton respiration and phytoplankton biomass (measured as Chlorophyll a). A power function was used to elucidate the relationship for the concentrated incubation system. Based on our results we determined that the value for this constant was 0.67, which is similar to a previous value derived from other estuaries. Given the strong allochthonous (i.e. terrestrial) material input and the specific incubation condition, an empirical formula is suggested, which applies to conditions in which a high chlorophyll a concentration prevails and in situations where diatoms are the dominant phytoplankton.