5
The Measurement of Primary Production
in Aquatic Ecosystems
Robert W. Howarth and Anthony F. Michaels
Introduction
All biological systems exist as a result of continual
inputs of energy to maintain structure and order.
At the scale of the ecosystem, most of this energy
comes from sunlight, which is converted into the
energy of organic matter in living biomass through
the process of primary production, and from imports of organic matter from adjacent ecosystems.
This import of energy in organic matter is, of
course, dependent upon primary production in the
"upstream" ecosystem. Thus, a critical aspect of
understanding the functioning of an ecosystem is
an accurate estimate of its rate of primary
production.
Primary production in aquatic ecosystems is
also of great interest for practical concerns. Eutrophication, or the excess primary production caused
by accelerated nutrient inputs, is one of the greatest environmental problems facing both freshwater
and coastal marine ecosystems (NRC 1993; Vitousek et al. 1997; Carpenter et al. 1998). On the positive side, primary production in oceans makes up
30 to 60% of global primary production (Sakshaug
et al. 1997), and the balance between primary production and respiration is important in regulating
carbon dioxide uptake by the oceans, and therefore
climate change.
In aquatic ecosystems, both phytoplankton and
benthic autotrophs, such as vascular plants, macroalgae, and mats of photosynthetic bacteria, can
contribute to primary production. In this chapter,
we concentrate on approaches to measuring primary production by phytoplankton (Table 5.1).
Methods for measuring primary production by
72
benthic macroalgae and vascular plants in aquatic
ecosystems often have more in common with
methods used to measure production in terrestrial
ecosystems (see Sand-Jensen and Krause-Jensen
1997; see also Chapter 2 and 4). Among the phytoplankton community, primary production is performed by both microscopic algae and photosynthetic bacteria (particularly cyanobacteria).
Gross primary production (GPP) is defined as
the total rate of production of new organic matter
by phytoplankton through photosynthesis. Net primary production (NPP) is GPP minus the organic
matter respired by the phytoplankton themselves,
that is, the rate of accumulation of new phytoplankton biomass. Net ecosystem production
(NEP; also referred to as net community production, or NCP) is the difference between NPP and
the respiration of all of the heterotrophs in the ecosystem (equivalent to the difference between GPP
and all respiration by both autotrophs and heterotrophs). All three of these rates are of ecological
interest, and we will discuss all in this chapter.
Primary production can be simply illustrated as
the following reaction:
light
CO2 + H20 -+ CH20 + O2
(5.1)
where CH 2 0 represents generic organic matter. In
theory, one could estimate primary production by
measuring the rate of change in any of the products or reactants, or even other biomass constituents that are stoichiometrically related to the production rate (i.e., nutrient uptake). In practice,
however, almost all measurements of primary production in planktonic systems are based on mea-
The Measurement of Primary Production
in Aquatic Ecosystems
Robert W. Howarth and Anthony F. Michaels
Introduction
All biological systems exist as a result of continual
inputs of energy to maintain structure and order.
At the scale of the ecosystem, most of this energy
comes from sunlight, which is converted into the
energy of organic matter in living biomass through
the process of primary production, and from imports of organic matter from adjacent ecosystems.
This import of energy in organic matter is, of
course, dependent upon primary production in the
"upstream" ecosystem. Thus, a critical aspect of
understanding the functioning of an ecosystem is
an accurate estimate of its rate of primary
production.
Primary production in aquatic ecosystems is
also of great interest for practical concerns. Eutrophication, or the excess primary production caused
by accelerated nutrient inputs, is one of the greatest environmental problems facing both freshwater
and coastal marine ecosystems (NRC 1993; Vitousek et al. 1997; Carpenter et al. 1998). On the positive side, primary production in oceans makes up
30 to 60% of global primary production (Sakshaug
et al. 1997), and the balance between primary production and respiration is important in regulating
carbon dioxide uptake by the oceans, and therefore
climate change.
In aquatic ecosystems, both phytoplankton and
benthic autotrophs, such as vascular plants, macroalgae, and mats of photosynthetic bacteria, can
contribute to primary production. In this chapter,
we concentrate on approaches to measuring primary production by phytoplankton (Table 5.1).
Methods for measuring primary production by
72
benthic macroalgae and vascular plants in aquatic
ecosystems often have more in common with
methods used to measure production in terrestrial
ecosystems (see Sand-Jensen and Krause-Jensen
1997; see also Chapter 2 and 4). Among the phytoplankton community, primary production is performed by both microscopic algae and photosynthetic bacteria (particularly cyanobacteria).
Gross primary production (GPP) is defined as
the total rate of production of new organic matter
by phytoplankton through photosynthesis. Net primary production (NPP) is GPP minus the organic
matter respired by the phytoplankton themselves,
that is, the rate of accumulation of new phytoplankton biomass. Net ecosystem production
(NEP; also referred to as net community production, or NCP) is the difference between NPP and
the respiration of all of the heterotrophs in the ecosystem (equivalent to the difference between GPP
and all respiration by both autotrophs and heterotrophs). All three of these rates are of ecological
interest, and we will discuss all in this chapter.
Primary production can be simply illustrated as
the following reaction:
light
CO2 + H20 -+ CH20 + O2
(5.1)
where CH 2 0 represents generic organic matter. In
theory, one could estimate primary production by
measuring the rate of change in any of the products or reactants, or even other biomass constituents that are stoichiometrically related to the production rate (i.e., nutrient uptake). In practice,
however, almost all measurements of primary production in planktonic systems are based on mea-