Dissolved organic nitrogen bioavailability indicated by amino acids during a diatom to dinoflagellate bloom succession in the Changjiang River estuary and its adjacent shelf

This study investigates the bioavailability and potential nutritional role of dissolved organic nitrogen (DON) in the development and progression of a large-scale dinoflagellate bloom that occurred in the Changjiang River estuary and its adjacent shelf (CJREAS) in the East China Sea (ECS). Three cruises were conducted during the late spring and early summer following the bloom progression, i.e., the termination phase of the diatom bloom, and the initiation/growth and peak phases of the dinoflagellate bloom. Concentrations of dissolved organic carbon (DOC), DON, dissolved inorganic nitrogen (DIN), dissolved free amino acids (DFAA) and dissolved combined amino acids (DCAA) were measured. The degradation index (DI), based on the relative abundance of total dissolved amino acids (TDAA) and amino acids yields was applied to indicate the diagenetic alteration of dissolved organic matter (DOM). During the succession from a diatom to dinoflagellate bloom, a decrease in DON co-occurred with a decrease in DIN. Concentrations of DFAA and DCAA also clearly decreased. The yields of DFAA and the DCAA and DI values decreased concomitantly with an increase in the DOC/DON, indicating that bioavailability of DON was relatively higher during the end stage of the diatom bloom than during the peak of the dinoflagellate bloom. These results indicated that DON may provide a significant portion of the total N demand throughout the duration of the large-scale dinoflagellate bloom under conditions of low DIN availability in the CIRFAS. (C) 2015 Elsevier B.V. All rights reserved.