Biological Oceanography

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Chapter 2


The phycology of phytoplankton


Phytoplankon are the assemblage of photoautotrophic microorganisms making up the
first trophic level of pelagic food chains. All belong to the botanical categories
grouped as algae and studied by “phycologists”. Compared to the roughly quarter
million species of plants in terrestrial habitats, we find very few species of algae in the
plankton, about 5000 (Tett & Barton 1995). Similar comparisons hold for zooplankton
and fish, and an explanation will be sought below in the treatment of pelagic
biogeography. Planktonic algae are classified into both ecological and botanical
groups, sets associated functionally or taxonomically. We will mix those groupings in
our study of what these algae are like. Representatives of most of the major algal
divisions live as plankton. Divisions, the largest units of algal and plant classification
and equivalent to the zoologist’s phyla, are defined on both morphological and
biochemical criteria. We will focus on a few ecologically dominant groups, some of
them exactly parallel to the botanical ones, others not (Box 2.1).


Box 2.1 Ecologically distinct major groups of


phytoplankton


(^) GROUP PHYCOLOGICAL TERMINOLOGY
Picoplankton Photoautotrophs <2 μm
Cyanobacteria
Photosynthetic prokaryotes, size <2 μm, from division Cyanophyta (lately
“cyanobacteria”)

Synechococcus and Prochlorococcus
Eukaryotic
picoplankton Very small, but structurally advanced forms
Microflagellates An ecological assemblage of several divisions and classes: Cryptophyta, Haptophyta,Prasinophyceae, Prymnesiophyceae
Diatoms Bacillariophyceae from division Heterokontophyta
Dinoflagellates Dinophyceae from division Dinophyta
The relative importance of phytoplankton groups varies with the ecological
situation. The importance of the smaller phytoplankton has only been recognized
recently, and, because of their size, some were not discovered until after 1979. We
will examine each of these groups in considerable biological detail because of their
large role in oceanic ecology. Often biological oceanographers must take off their

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