Microbial dynamics during the decline of a spring diatom bloom in the Northeast Atlantic

Abstract

The microbial dynamics during a spring diatom bloom decline was monitored in the Northeast Atlantic during a 5-day Lagrangian study (8-12 April 2002). Phytoplankton abundance, composition and health status were related to viral and bacterial abundance, zooplankton abundance and grazing rates, as well as bacterial production. Phytoplankton reached maximum concentration on Day 3 (Chl a >5 μg L-1) and declined on Day 5 (Chl a ∼2 μg L -1) and was dominated (70% of Chl a) by diatoms. Bacterial production increased substantially to >20 μg C L-1 day-1 on Day 3 and concomitantly large viruses decreased in number by half to <10 × 103 mL-1. This was followed by a 5-fold increase in large viruses on Day 5, indicating infection and subsequent lysis on Days 3 and 5, respectively. Micro- and mesozooplankton grazing were not the principal cause for the decline of the bloom and pheophorbide-a showing little variation in concentration from Days 1-4 (∼100 ng L-1) although doubled on Day 5. The poor physiological status of the diatoms, indicated by the high chlorophyllide-a concentrations (50-480 ng L-1), likely promoted a series of closely interrelated events involving bacteria and viruses leading to the demise of the diatom bloom. © The Author 2007. Published by Oxford University Press. All rights reserved.