Biological Oceanography

(^) Strom et al. (2001) used dilution experiments to show that very large fractions of
phytoplankton production are consumed by protists, even in coastal waters and even
for relatively large phytoplankton including diatoms. Over a large series of
experiments, the ratio g/μ = grazing rate/growth rate was 80% for cells smaller than 8
mm and 42% for cells larger than 8 mm, with an overall average of 64%. They found
that large ciliates and heterotrophic dinoflagellates tend to increase as stocks of large
phytoplankton increase, consuming larger relative fractions of production at high
phytoplankton standing stocks. This is possible in part because many heterotrophic
protists, particularly dinoflagellates, can digest phytoplankton externally, inserting a
feeding tube into prey cells or surrounding them with a pallial sac.
(^) While all protists are small, relative to adult copepods or appendicularians, they
vary over a wide size range and can occupy multiple successive trophic levels. Very
large ciliates like Strombidinopsis (to 150 μm) can capture and eat heterotrophic
dinoflagellates like Oxyrrhis or Oblea (20–25 μm) that are in turn eating small to
large algal cells (Jeong et al. 2004). Feeding by individuals at each level follows
functional response patterns to varying food availability like those of
mesozooplankters (Fig. 7.10).
Fig. 7.10 (a–c) Water-clearance (“filtration”) rates, ingestion rates and cell growth
rates of the pallium-feeding dinoflagellate Oblea rotunda as a function of food
availability. Foods were the diatom Ditylum ( ) or the chlorophyte Dunaliella ().
(^) (After Strom & Buskey 1993.)
(d & e) Ingestion and growth rates of the 149 × 70 μm ciliate Strombidinopsis jeokjo
feeding on the dinoflagellate Gyrodinium dominans. Note carefully the different
ingestion-rate units
(After Jeong et al. 2004.)