Impact of diazotroph nitrogen on zooplankton nutrition in the ocean
Final Report Abstract
The Amazon River plume inhabited three different phytoplankton communities including two different diazotroph communities. The impact and transfer efficiency (TE) of nitrogen from either community into mesozooplankton were quantified and factors influencing the TE were elucidated. The estuarine community comprised different non-N2 fixing species including diatoms, the oceanic community comprised N2-fixing Trichodesmium, and the mesohaline community was dominated by Diatom-Diazotroph Associations (DDAs). DDAs incorporated nitrogen from N2-fixation as well as from advected subthermocline nitrate, while the nitrate influence on PN at Trichodesmium dominated stations was generally low. Nitrate was the principal N source for the estuarine community. Despite high mixed layer ammonium concentrations at some stations, ammonium had no large impact of the δ15N of PN, while mixed layer depth, sea surface salinity, the depth of the nitracline, and sea surface phosphate concentrations directly affected the δ15N of PN and consequently the incorporation of diazotroph nitrogen into PN. The transfer efficiency of nitrogen from particulate nitrogen (PN) into the mesozooplankton food web as well as its transfer among different zooplankton size fractions differed largely among estuarine and oceanic sites as well as among zooplankton size fractions. Factors that may influence the transfer efficiency among the different planktonic compartments are numerous. According to this data set they most likely included the temporal decoupling between N incorporation into PN and into zooplankton (e.g. at St. 2), the complexity of the microzooplankton community that connects the microbial with the mesozooplankton food web (lower in estuarine than in oceanic waters), the amount of herbivores in the different size fractions in estuarine and oceanic waters (more large herbivores in estuarine waters), different nitrogen demands in copepods compared to gelatinous mesozooplankton taxa (higher N content in copepods compared e.g. to Chaetognaths), stoichiometric differences among consumers and their diet (DDAs may be more suitable as food for mesozooplankton than Trichodesmium), as well as differently developed intracellular nitrogen recycling strategies to retain amino acid nitrogen in the cells rather than to excrete it as ammonium. I found a high degree of plasticity in the amino acid pools of tropical zooplankton that reflects strong shifts between catabolic and anabolic processes on a daily basis. This is in sharp contrast to the observed invariant amino acid composition of mid-latitude animals, and will be investigated in the next three years within my DFG-funded project “Zooplankton Energy Turnover in a Changing Environment-ZET Change”.
