Dynamics and molecular composition of dissolved organic material during experimental phytoplankton blooms

This study examined the formation and degradation of dissolved organic carbon (DOC), combined neutral sugars (DCNS) and amino acids (DCAA) during phytoplankton blooms in two mesocosms differing in nutrient levels and phytoplankton species composition. Concentrations of DOC increased by 16% and 50% in unenriched and nutrient-enriched mesocosms, respectively. The accumulated DOC was eventually degraded in the unenriched mesocosm. In contrast, 32% of the bloom-derived DOC in the nutrient-enriched mesocosm was still present after 7 weeks. Concentrations of DCNS in the mesocosms increased by 127% and 203% in the unenriched and enriched mesocosm, respectively. The increase in DCNS was caused by the enhanced release of fucose-, galactose-, and glucose-rich polymers by the bloom-forming diatom species. The introduced sugar polymers appeared to be labile since concentrations and the molecular composition of the DCNS pool quickly returned to the initial conditions after the bloom ended. Concentrations of DCAA in the nutrient-enriched mesocosm increased by 81%, but DCAA did not change in the unenriched mesocosm. Our observations suggest that phytoplankton blooms differing in biomass levels, phytoplankton species composition and nutrient availability can lead to different short- and long-term variations in the concentrations of DOC, DCNS and DCAA. However, the blooms had little long-term impact on DCAA and DCNS composition, suggesting that degradation pathways rather than production processes were more important in determining the molecular composition of these compound classes and perhaps of the entire DOC pool.