Bulk and amino acid nitrogen specific isotope data from particulate organic matter and mesozooplankton (1000-2000 µm) from the Mekong River plume and southern South China Sea

The mean trophic position (TP) of mesozooplankton largely determines how much mass and energy is available for higher trophic levels like fish.  Unfortunately, the ratio of herbivores to carnivores in mesozooplankton is difficult to identify in field samples.  Here we investigated changes in the mean TP of mesozooplankton in a highly dynamic environment encompassing four distinct habitats in the southern South China Sea: the Mekong River plume, coastal upwelling region, shelf waters, and offshore oceanic waters.  We used a set of parameters derived from bulk and amino acid nitrogen stable isotopes from particulate organic matter (POM) and four mesozooplankton size fractions to identify changes in the nitrogen source and structure of the planktonic food web across these habitats.  We found clear indications of a shift in N sources for biological production from nitrate in near-coastal waters towards an increase in diazotroph-N inputs in oceanic waters where diazotrophs shaped the phytopl...