be mineralized to ammonium. Poorvin et al. (2004) have shown that virally induced
mortality in coastal waters results in enough release of iron to support as much as
90% of primary production. Viral lysis constitutes an internal recycling of bacterial
carbon and other constituent elements within the microbial food web, while
zooplankton feeding on bacterivorous protists moves carbon and other elements
toward higher trophic levels.
Herbivorous Microzooplankton
(^) Larger planktonic protists, 20–200 μm in diameter, are primarily herbivores
(consuming phytoplankton). These are mostly spirotrichous ciliates (choreotrichs and
oligotrichs) and larger dinoflagellates (see Plate 5.2). Choreotrichs are ovoid or
conical, and have a full or partial circlet of cilia surrounding the anterior end of the
cell. Ingestion by drawing prey into food vacuoles occurs within this circlet. Genera
are readily recognized, but species distinctions are difficult, particularly when using
the standard Lugol’s iodine preservation which renders them opaque. Tintinnids are a
subgroup of choreotrichs, particularly common in neritic waters. These form an
external lorica, often covering that with sediment particles. Shapes and decoration
patterns of their loricae make tintinnids amenable to very detailed taxonomic division,
and, thus, distributional study. In the southwest Atlantic Ocean, along the coast of
Argentina, tintinnid species showed a very strong biogeographical pattern of five
distinct latitudinal bands between 34° and 58°S (Thompson et al. 1999).
(^) Protists that graze picoplankton consume cyanobacteria and picoeukaryotes, while
most protists larger than 5 μm consume prey larger than bacteria, mainly
phytoplankton. Methods for determination of microzooplankton grazing rates are
discussed in Chapter 7. We cover the importance of protist grazers on larger
phytoplankton in Chapter 9.