26
Stoichiometric Analysis of Pelagic
Ecosystems: The Biogeochemistry
of Planktonic Food Webs
James J. Elser
Introduction
For a pelagic ecosystem scientist in the field,
whether in a 40-m oceanographic research vessel
cruising the central Pacific or in an inflatable
dinghy bobbing in a Minnesota kettle lake, the objects of interest are underneath and out of sight. In
this way it might be said that pelagic ecology has
more in common with soil ecology than soil ecology has with aboveground ecology. Whether or
not this is true might be judged by comparison of
various contributions to this volume. In any case,
because direct observation is infrequent for both
pelagic and soil ecologists, the major players and
the processes that connect them are sometimes
largely abstractions in ecosystem approaches to
pelagic and soil systems. Perhaps because pelagic
systems enforce abstraction, many of ecology's
most influential concepts and theories, such as the
trophic dynamics concept (Lindeman 1942), resource ratio competition theory (Tilman 1982), the
trophic cascade (Carpenter et al. 1985), and the
multidimensional niche (Hutchinson 1961), have
their roots in the water. Indeed, a dominant concept in ecosystem science is that of the watershed,
often with a lake at its terminus (Likens 1985).
Our current understanding of the general patterns
in the distribution and dynamics of organisms and
of physical and chemical conditions in the subsurface pelagic world has been hard-won and obtained via a combination of hard work and technical advancement.
In this chapter, I describe some new aspects of
the interrelationships between physical, chemical,
and biological factors influencing pelagic ecosystem function, with a primary emphasis on food web
dynamics and nutrient cycling. This involves a central focus on individual organisms in pelagic ecosystems, the microscopic and potentially enigmatic
plankton, and how the interplay between ecosystem
conditions and the physiological requirements of
the plankton influences ecological dynamics. My
emphasis is on a synthesis of a suite of new observations regarding the complex interactions among
these players, observations that have arisen by application of a new way of thinking about ecological
dynamics, "ecological stoichiometry" (sensu
Reiners 1986; Elser et al. 1996; Hessen 1997). It is
my hope that this presentation will document the
connections between the pelagic food web and the
hydrodynamic and geochemical environment in
which it is imbedded. I assume that the reader has
familiarity with well-known aspects of pelagic ecosystems, such as the biological components of pelagic food webs (bacteria, protozoans, phytoplankton, metazoan zooplankton); patterns of thermal
stratification, light penetration, and photosynthetic
response; the role of nutrients in eutrophication;
and analysis of trophic interactions from energetic
perspectives. By describing new findings obtained
via stoichiometric approaches I hope to show that
integration of these findings with our existing, but
potentially incomplete, frameworks may permit us
to understand pelagic ecosystems comprehensively
from wind to water to the food web itself. Thus,
my primary goal is not to describe specific methods
of measurement or manipulation in pelagic ecosystems; many of the specific approaches useful in
ecological stoichiometry have already been described in previous chapters that are aimed at the
389
Stoichiometric Analysis of Pelagic
Ecosystems: The Biogeochemistry
of Planktonic Food Webs
James J. Elser
Introduction
For a pelagic ecosystem scientist in the field,
whether in a 40-m oceanographic research vessel
cruising the central Pacific or in an inflatable
dinghy bobbing in a Minnesota kettle lake, the objects of interest are underneath and out of sight. In
this way it might be said that pelagic ecology has
more in common with soil ecology than soil ecology has with aboveground ecology. Whether or
not this is true might be judged by comparison of
various contributions to this volume. In any case,
because direct observation is infrequent for both
pelagic and soil ecologists, the major players and
the processes that connect them are sometimes
largely abstractions in ecosystem approaches to
pelagic and soil systems. Perhaps because pelagic
systems enforce abstraction, many of ecology's
most influential concepts and theories, such as the
trophic dynamics concept (Lindeman 1942), resource ratio competition theory (Tilman 1982), the
trophic cascade (Carpenter et al. 1985), and the
multidimensional niche (Hutchinson 1961), have
their roots in the water. Indeed, a dominant concept in ecosystem science is that of the watershed,
often with a lake at its terminus (Likens 1985).
Our current understanding of the general patterns
in the distribution and dynamics of organisms and
of physical and chemical conditions in the subsurface pelagic world has been hard-won and obtained via a combination of hard work and technical advancement.
In this chapter, I describe some new aspects of
the interrelationships between physical, chemical,
and biological factors influencing pelagic ecosystem function, with a primary emphasis on food web
dynamics and nutrient cycling. This involves a central focus on individual organisms in pelagic ecosystems, the microscopic and potentially enigmatic
plankton, and how the interplay between ecosystem
conditions and the physiological requirements of
the plankton influences ecological dynamics. My
emphasis is on a synthesis of a suite of new observations regarding the complex interactions among
these players, observations that have arisen by application of a new way of thinking about ecological
dynamics, "ecological stoichiometry" (sensu
Reiners 1986; Elser et al. 1996; Hessen 1997). It is
my hope that this presentation will document the
connections between the pelagic food web and the
hydrodynamic and geochemical environment in
which it is imbedded. I assume that the reader has
familiarity with well-known aspects of pelagic ecosystems, such as the biological components of pelagic food webs (bacteria, protozoans, phytoplankton, metazoan zooplankton); patterns of thermal
stratification, light penetration, and photosynthetic
response; the role of nutrients in eutrophication;
and analysis of trophic interactions from energetic
perspectives. By describing new findings obtained
via stoichiometric approaches I hope to show that
integration of these findings with our existing, but
potentially incomplete, frameworks may permit us
to understand pelagic ecosystems comprehensively
from wind to water to the food web itself. Thus,
my primary goal is not to describe specific methods
of measurement or manipulation in pelagic ecosystems; many of the specific approaches useful in
ecological stoichiometry have already been described in previous chapters that are aimed at the
389