Production of the CopepodPseudodiaptomus forbesiIs Not Enhanced by Ingestion of the DiatomAulacoseira granulataDuring a Bloom

In 2016, a massive bloom of the chain-forming diatomAulacoseira granulataoccurred in the upper San Francisco Estuary, California, with chlorophyll concentrations up to 75 mu g Chl L-1. In this study, quantitative PCR was used to investigate consumption of the bloom organism by the numerically dominant zooplankterPseudodiaptomus forbesi(Copepoda: Calanoida) and to estimate the contribution of the bloom to egg production. Copepods were collected on four transects during May and June 2016; egg production rates were somewhat elevated above previous rates measured in the estuary. Ingestion ofA. granulatawas highest on the first sampling day, just after the peak of the bloom, ranging from 175 to 945 cells copepod(-1)day(-1). One month later ingestion rates dropped to 0-130 cells copepod(-1)day(-1), despite continued dominance ofA. granulatain the plankton. Ingestion ofA. granulataprovided from 0 to 21% (median 1%) of the estimated daily carbon required for growth and reproduction ofP. forbesi. Although the copepods probably obtained nutrition from a microbial food web stimulated by the bloom, monitoring data showed little demographic response to this bloom. Thus, a massive diatom bloom in an unproductive estuary provided only a minor stimulus through an abundant consumer to the pelagic food web.

Automated summary

The study investigated the consumption of a massive bloom of Aulacoseira granulata by the dominant zooplankton Pseudodiaptomus forbesi in the upper San Francisco Estuary. Results showed that ingestion rates decreased over time, providing only a minor contribution to the growth and reproduction of P. forbesi.