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Vol. 43 No. 2, 2004
The Importance of Small Planktonic Copepods and Their Roles
in Pelagic Marine Food Webs
Jefferson T. Turner
School
for Marine Science and Technology, University of Massachusetts
Dartmouth, 706 South Rodney French Boulevard, New Bedford, MA 02744, USA
Jefferson T. Turner (2004)
Small planktonic marine copepods (< 1 mm in length) are the most
abundant metazoans on Earth. Included are adults and copepodites of
calanoid genera such as Paracalanus, Clausocalanus, and Acartia; cyclopoid genera such as Oithona, Oncaea, and Corycaeus; planktonic harpacticoids of the genus Microsetella;
and nauplii of almost all copepod species. Despite the abundance of
small copepods, they have historically been undersampled due to the use
of nets with meshes > 200- 333 µm. Recent studies have shown,
however, that when appropriate net meshes of 100 µm or less are used,
small copepods vastly exceed the abundance and sometimes the biomass of
larger ones. Failure to adequately account for small copepods may cause
serious underestimations of zooplankton abundance and biomass, the
copepod grazing impact on phytoplankton primary production,
zooplankton-mediated fluxes of chemicals and materials, and trophic
interactions in the sea. The feeding ecology of small copepods is less
well-known than that of adults of larger copepod species, such as
members of the genus Calanus. Further, most feeding information for small copepods is for coastal genera such as Acartia,
rather than for offshore taxa. Although it is generally assumed that
small copepods, including nauplii, feed primarily upon small-sized
phytoplankton cells, most such information comes from rearing or
feeding studies on limited laboratory diets. There have been few
examinations of actual copepod feeding on mixed diets of natural
phytoplankton and microzooplankton found in the sea, but some of those
have produced surprises. For instance, some species of Oithona and Paracalanus and even nauplii of Arctic Calanus
spp. may feed primarily as predators upon heterotrophic protists,
rather than as grazers of phytoplankton. Also, nauplii of various
tropical copepod species have been shown to feed upon bacterioplankton.
Thus, numerous basic questions remain as to the feeding ecology and
grazing/predation impact of small copepods in the sea. Despite limited
knowledge of what small copepods eat, it is clear that many
higher-trophic-level consumers eat them. Numerous studies have shown
that copepod nauplii, Oithona
spp., and other small copepods are important prey of fish larvae and
other planktivores. Small copepods exhibit a variety of reproductive
strategies to compensate for losses to their populations due to
predation. These include having high fecundity and growth rates, when
not limited by insufficient food; having high reproduction and growth
rates at warmer temperatures; having limited motion and low respiration
rates, allowing the investment of more energy in reproduction; and
having extended longevity to maximize lifetime reproductive output.
Thus, small copepods are important links in marine food webs, serving
as major grazers of phytoplankton, as components of the microbial loop,
and as prey for ichthyoplankton and other larger pelagic carnivores.
Our present inadequate understanding of the true abundance, biomass,
trophic ecology, and role of small copepods in biogenic fluxes
precludes proper understanding of the ecology of the sea.
Key words: Plankton, Copepods, Marine ecosystems.
*Correspondence: Fax: 508-999-8197. E-mail: jturner@umassd.edu
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