In situ ingestion of Microcystis is negatively related to copepod abundance in the upper San Francisco Estuary

Abstract

AbstractAlthough zooplankton grazing on cyanobacteria is a key process regulating the structure and function of bloom‐dominated waters, few studies have measured in situ ingestion and none have linked it to zooplankton abundance. This study measured in situ zooplankton grazing rate on Microcystis via quantitative polymerase chain reaction (qPCR) amplification of prey DNA from the guts of the copepod Pseudodiaptomus forbesi, an abundant and important prey species for endangered fish in the upper San Francisco Estuary (SFE). We calibrated the qPCR assay in laboratory grazing experiments with Eurytemora affinis, a similar sized copepod abundant in the SFE during winter, due to higher ingestion of Microcystis by this copepod compared to Pseudodiaptomus forbesi. In the field, Pseudodiaptomus forbesi ingested Microcystis during blooms, including strains capable of producing microcystin toxins. Although the Microcystis content in Pseudodiaptomus forbesi guts increased with bloom intensity, estimated ingestion rates were low and suggested copepods avoided this cyanobacterium. Moreover, Pseudodiaptomus forbesi abundance was inversely related to its gut Microcystis content, but not related to other factors including temperature, salinity, turbidity, and gut fluorescence of chlorophyll a. Results demonstrate that the gut Microcystis content of zooplankton may be a key factor explaining their abundance in nature. Our results also highlight the utility of molecular methods for understanding in situ trophic interactions in bloom‐dominated waters. Increased blooms under future climate scenarios (e.g., extreme drought and higher temperatures) may decrease the abundance of key zooplankton like Pseudodiaptomus forbesi through increased ingestion of Microcystis, with potentially negative effects on endangered fishes in the SFE.