Trophic interactions in the microbial food web at a coastal station in the Alboran Sea in winter Microplankton grazing impact on nanoplankton and free bacteria
Bautista, B.; Rodriguez, V.; Jimenez Gomez, F.
Scientia Marina 58(1-2): 143-152
ISSN/ISBN: 0214-8358 Accession: 034082778
Microplankton grazing rates were estimated in natural communities at a coastal station in the Alboran sea (Western Mediterranean) in March 1993. A dilution technique and a combination of different methods (fluorimetry, flow cytometry, electronic particle counter, epifluorescence microscopy and image analysis) were used to characterise and quantify the different prey and predator components of the microplanktonic food web. The water column, at this time of the year, was characterised by very low pigment concentration and the dominance of small eukaryotic picoplankton and nanoplankton cells, mostly smaller than 5 mu-m equivalent spherical diameter (ESD). Cyanobacteria and large cells were scarce. No significant grazing rates were obtained in any of the autotrophic particles estimated by flow cytometry, which on the contrary increased in number during the 24 h incubation. Nevertheless. when total particles estimated by Coulter Counter were taken into account as prey, grazing rate accounted for 1.18 d-1. This indicated that grazing pressure was effectively taking place but on non-autotrophic particles. Thus, when heterotrophic bacterioplankton was considered as prey, grazing rate accounted for 2.64 d-1, showing that most of the grazing impact was due to feeding on bacteria by heterotrophic and mixotrophic nanoflagellates. These results suggest a predominance of the microbial heterotrophic pathway in the microplankton fraction of coastal waters at this time of the year.