Microzooplankton dynamics during the development of the spring bloom in the north-east Atlantic E.S. Fileman* P and R.J.G. Leakey O *Plymouth Marine Laboratory, Prospect Place, The Hoe, Plymouth, PL1 3DH. O Scottish Association for Marine Science, Dunsta¡nage Marine Laboratory, Oban, Argyll, PA37 1QA. P Corresponding author, e-mail: ese@pml.ac.uk. Microzooplankton community composition, abundance, biomass and grazing impact were assessed, along with measurements of ciliate growth and mortality, during the onset of the spring bloom in the north-east Atlantic. The study was undertaken as part of the UK Biogeochemical Ocean Flux Study during 1 May to 15 June 1990. The microzooplankton community was composed of protozoans and metazoan developmental stages with respective mixed-layer depth integrated biomass values ranging from 127 to 638 and 74 to 394 mg C m 72 . High numbers of aloricate ciliates (up to 35,000 cellsl 71 ) domi- nated the microzooplankton community during early May prior to the onset of the spring bloom. Ciliate abundance then declined rapidly during mid-May with community growth rates ranging from 70.71 to 0.23 d 71 . High abundances of metazoplankton (up to 400 l 71 ) were also recorded at this time and may have contributed to the decline in ciliate numbers. In late May and early June the protozoan community comprised a more even mix of dino£agellates, tintinnids and aloricate ciliates. Phytoplankton mortality rates, measured using a dilution technique, ranged from 0.2 to 0.5 d 71 . The microzooplankton consumed 8 to 44 mgCl 71 d 71 , equivalent to between 16 and 40% of the chlorophyll biomass and 38 and 154% of primary production. These high rates of herbivory re£ect the predominance of small (55 mm in length) phytoplankton cells present throughout the ¢rst half of the study and support previous studies demon- strating the microzooplankton to be the main grazers of phytoplankton in the north-east Atlantic. However, there is also evidence that a disparity between predator and prey may have prevented a response by the microzooplankton to rapid increases in phytoplankton biomass and production during the spring bloom. INTRODUCTION It is now well established that the microzooplankton form a signi¢cant proportion of the total zooplankton biomass in both coastal and oceanic environments and play an important role in carbon and energy £ow through pelagic ecosystems (Calbet & Landry, 2004). In particular, microzooplankton have been shown to consume a large fraction of primary production in the sea and they often exert a higher grazing pressure on phyto- plankton than the mesozooplankton (e.g. Paranjape et al., 1987; Landry et al., 1997; Fileman & Burkill, 2001; Strom et al., 2001; Verity et al., 2002). The microzooplankton, as de¢ned by Dussart (1965), comprise protozoa and metazoa 5200 mm in length. This is a useful practical de¢nition in so far as it relates to pre-screening protocols often used in experimental techniques such as dilution assays (Landry & Hassett, 1982). However, it is the protozoan component of the microzooplankton, mainly composed of ciliates and dino£agellates (and referred to hereafter as the protozoo- plankton), which is responsible for most herbivorous activity. The small size and high growth rates of these herbivorous protozooplankton enable them to respond rapidly to changes in pico- and nanophytoplankton, thereby maintaining close coupling between production and consumption in the euphotic zone. In addition to their role as herbivores, these protozoa may feed on heterotrophic cells (Jeong, 1999), are a source of food for organisms in higher trophic levels (Stoecker & Capuzzo, 1990; Fessenden & Cowles, 1994) and are important re- mineralizers of organic material and nutrients (Goldman & Caron, 1985; Goldman et al., 1987). The North Atlantic Ocean is characterized by large seasonal variations in phytoplankton biomass, the most prominent of these seasonal events being the spring bloom. These blooms have been witnessed by satellite imagery (Esaias et al., 1986) and have been described as ‘the largest biological signal on the planet’ (Lewis, 1989). It has been suggested that a large part of the phyto- plankton may sink directly out of surface waters at the end of the spring bloom (Billet et al., 1983); however, the magnitude of this vertical £ux of biological material is thought to depend upon the composition of the phyto- plankton assemblage (Michaels & Silver, 1988). The bloom has traditionally been described as being numeri- cally dominated by large diatoms (Robinson, 1965; Colebrook, 1982) although more recent evidence suggests that the bloom can sometimes be dominated by small cells in the pico- and nanoplankton size-classes (Murphy & Haugen, 1985; Sieracki et al., 1993). Microzooplankton grazing is often important when these small phyto- plankton dominate productivity resulting in the loss of ¢xed carbon through respiration and excretion; signi¢cant vertical £ux of carbon out of the euphotic zone is therefore unlikely under these conditions (Longhurst & Harrison, 1989). J. Mar. Biol. Ass. U.K. (2005), 85, 741^753 Printed in the United Kingdom Journal of the Marine Biological Association of the United Kingdom (2005)