AME 32:39-46 (2003)  -  doi:10.3354/ame032039

ABSTRACT: We compared the effects of the presence of the appendicularian Oikopleura dioica and the copepods Acartia clausii and Calanus helgolandicus on the coastal bacterioplankton community off Plymouth. Mesozooplankton were added to water samples and bacterioplankton growth was monitored by flow cytometry. Phylogenetic composition of bacterioplankton was analysed using fluorescence in situ hybridisation (FISH) with rRNA-targeted oligonucleotide probes. The bacterioplankton composition did not change in the presence of either appendicularians or copepods, and generally the same proportions of bacterioplankton groups were determined. In late spring, 15 ± 2% of cells hybridised with a probe specific to the Kingdom Archaea. The majority of cells (88 ± 2%) belonged to the Kingdom Bacteria, and 86% of cells were identified using group-specific probes. The Cytophage-Flavobacterium cluster dominated the community, comprising 64 ± 0.5% of cells. The γ-proteobacteria were the second abundant group, comprising 11 ± 0.5% of cells, and the SAR86 cluster of γ-proteobacteria accounted for 6 ± 5%. The a-proteobacteria comprised 10 ± 5% of bacterioplankton, and the Roseobacteria related cluster represented 9 ± 3% of cells. The reduction of bacterioplankton growth caused by appendicularian bacterivory was 0.4 to 14% ind.^-1 l^-1, and the total appendicularian population could reduce bacterial growth in coastal waters in late spring-summer by up to 9%. In contrast to the appendicularians the copepods stimulated bacterial growth, and in summer the bacterioplankton growth may be increased by up to 13% by the combined effect of dominant copepod populations. Thus, the appendicularians and copepods had an opposite but moderate effect on the bacterioplankton growth and no effect on the bacterioplankton composition.

KEY WORDS: Community structure · Zooplankton · FISH · Flow cytometry · Bacterial production · Nutrient bioavailability