Nanoplankton of the equatorial Pacific with emphasis on the heterotrophic protists

In the equatorial Pacific during the boreal spring of 1992, nanoplankton, represented by chrysophytes, dinoflagellates, amoebae, choanoflagellates, naked flagellates and ciliates, dominated the heterotrophic protistan biomass. This component contributed 11-60% of the combined phytoplankton and heterotrophic protistan biomass. The heterotrophs, other than the dinoflagellates and ciliates, were represented by amoebae and flagellates at a mean (±SD) density and biomass of 5.85 ± 2.53 × 105 cells l−1 and 1.76 ± 1.37 μgC l−1. Fifty-two species of lobose amoebae, apusomonads, bicosoecids, cercomonads, choanoflagellates, chrysomonads, euglenids, jakobids, kathablepharids, kinetoplastids, pedinellids and a number of taxa of uncertain position (incertaesedis taxa) were identified. The heterotrophs represent five different trophic types of organisms, defined by habitat and prey. Fifty per cent of the species identified in this study are principally associated with detritus (marine snow). The majority of free-living suspension feeders we identify in this study are choanoflagellates. Other suspension feeding planktonic taxa may not be detected using the protocols we employ. Only 42% of the species identified are obligately bacterivorous and 12% are not bacterivores at all but graze principally on algae. The remainder of heterotrophic species prey on an array of DOM, bacteria, other protists and detritus.