The phytoplanktonic ways of life
Authors: FOGG G. E. 
Published: April 2006

summary

The adaptations of phytoplankton to life in suspension in water are considered with particular reference to hydromechanical factors ranging from molecular motion to ocean currents. The smallest phytoplankton, the picoplankton (0.2–2.0 μm), are the best adapted to the physico‐chemical environment of the open waters of seas and lakes and, other things being equal, can out‐compete the larger forms. The picophytoplankton are an autotrophic component in a microbial community, the ultraplankton, which also includes bacteria and flagellates up to about 20 μm in linear dimensions. This community is a highly dynamic and self‐contained equilibrium system operating within a domain dominated by molecular diffusion. Within the photic zone it is limited, not by nutrient supply, but by its internal predator‐prey relationships. It appears to be cosmopolitan, both in the sea and in freshwaters, to vary little either in time or space in species composition or in biomass concentration, and to contribute a minimum of organic carbon to higher trophic levels. There appears to be a fundamental divide in form and function between this and the microplankton, composed of organisms larger than about 20 μm. This community can only develop when nutrient levels are in excess of the concentrations required by the picoplankton. It is opportunistic, non‐equilibrium in its dynamics, and highly variable in floristic composition and biomass concentration. Its life processes are dominated by turbulence. Nutrient supply is largely determined by turbulent eddy diffusion and movement of the organisms relative to the water mass. The microphytoplankton falls broadly into two types, one of which, exemplified by the diatoms, depends on turbulence to maintain it in the photic zone and the other, exemplified by the dinoflagellates and colony‐forming cyanobacteria, relies on motility or buoyancy control to position it in a relatively stable water column so as to have best access to light and nutrients. The waxing and waning of microplankton populations is largely determined by hydrography and their floristic compositions by the interactions of the daily and seasonal rhythms of the organisms with the periodicities in the environment. In contrast to the ultraplankton microplankton species show distinct differences in biogeographical distribution. Throughout the discussion attention is drawn to the intimate relationships between the activities of phytoplankton and those of viruses, bacteria and zooplankton and the impossibility of getting a proper understanding of the physiology of the phytoplankton if they are considered in isolation.

CONTENTS

Summary
191

I.
Introduction
192

II.
Some reminders about the aqueous medium
193

III.
The ideal phytoplankton organism
194

IV.
Some aspects of the physiological ecology of the picoplankton
195

V.
The ultraplanktonic community
197

VI.
A divide in the phytoplankton
201

VII.
Some aspects of the physiological ecology of the microphytoplankton
203

VIII.
Small scale turbulence in the environment of phytoplankton
205

IX.
Intermediate scale turbulence in the environment of phytoplankton
207

X.
The effects of large scale rotational stirring
220

XI.
Phytoplankton as part of a community
221

XII.
Conclusions
222

References
223