Heterotrophic nanoflagellate and ciliate bacterivorous activity and growth in the northeast Atlantic Ocean: a seasonal mesoscale study

Abstract

The grazing effect of heterotrophic nanoflagellates (HNF) and ciliates on bacterial pro- duction (BP), as well as their growth rates, was studied in winter, spring and autumn 2001 during the French research project Programme Ocean Multidisciplinaire Meso-Echelle (POMME) in the north- east Atlantic Ocean (38 to 45° N, 16 to 22° W). The variability of different parameters studied appears to be largely controlled by the seasonal and latitudinal gradients of primary production rather than the strong eddy activity at the mesoscale level in the area. Heterotrophic microbial abundance, bio- mass and protistan grazing varied temporally, presenting highest values during the phytoplankton bloom, during the spring period and following the northward propagation of the bloom. HNF biomass integrated over the upper 100 m was highest in spring (270 to 850 mg C m -2 ). Ciliate integrated bio- mass was generally ≤160 mg C m -2 except in a Tintinnus sp. bloom in a northern anticyclonic eddy (A1) in spring when it reached 637 mg C m -2 . HNF and ciliate growth rates varied from 0.2 to 0.7 d -1 and 0.2 to 1.4 d -1 , respectively. The fraction of BP consumed by ciliates was generally <10% except in the anticyclonic eddy A1 in spring during a tintinnid bloom when it reached 37% of BP. In conclu- sion, our data revealed that HNF can remove a large fraction of bacterial production in the northeast Atlantic Ocean (83 ± 27%, average of all sampling sites and seasons). Ciliates transferred less carbon to higher trophic levels than did HNF; however, episodic high occurrence of large bacterivorous cili- ates, primarily tintinnids, increased the role of theses organisms as C-links in the microbial food web.