Phytoplankton community succession and biogeochemistry in a bloom simulation experiment at an estuary-ocean interface

Abstract. Phytoplankton blooms, especially diatom blooms, account for a large fraction of marine carbon fixation. Species succession and biogeochemical parameters change rapidly over a bloom, and determine the resulting biological productivity. This study implemented daily sampling of a 24–L microcosm bloom simulation experiment to assess changes in assemblage and biogeochemical processes while excluding changes due to advection. ¹⁵NO₃^- and H¹³CO₃^- tracer incubations were performed alongside pigment and DNA sampling to compare temporal trends in community composition and primary productivity (nitrogen (N) and carbon (C) transport rates). Rapid drawdown of nutrients and maximum C and N transport rates corresponded with peak chlorophyll–a and fucoxanthin pigment concentrations. Fucoxanthin, typically associated with diatoms, was the dominant diagnostic pigment, with very low peridinin (dinoflagellate) and zeaxanthin (cyanobacteria) concentrations, indicating a diatom bloom. 18S rRNA gene analysis showed clear community succession throughout the duration of the bloom and multiple species of diatoms co–occurred, including during the bloom peak. The presence of metazoan 18S, high carbon–to–chlorophyll ratios, and a model analysis provide evidence of grazing in the latter half of the bloom. A traditional bloom framework suggests that species succession occurs as the bloom progresses and that phytoplankton diversity reaches a minimum of just one or two dominant species when phytoplankton productivity is at its maximum. However, this study produced a negatively monotonic productivity–diversity relationship with relatively high minimum diversity values. This 18S–based analysis therefore presents a more complex relationship between bloom progression and phytoplankton diversity.