Download “primary production” in aquatic ecology

COMMENT
Limnol.
Oceanogr., 33(5), 1988, 1215-1216
0 1988, by the American Society of Limnology
and Oceanography,
Inc.
The concept of “primary production” in aquatic ecology
“Primary production”
is a term that appears frequently in the literature, being used
loosely to describe production at the base
of a conceptual food chain; it is difficult to
find an explicit definition (see Odum 197 1;
UNESCO 1973; Barnes and Mann 1980;
Fogg 1980; Parsons et al. 1984). Terrestrial
ecologists use “primary production”
to describe an increase in biomass of higher plants
and, by analogy, aquatic ecologists have used
it to describe micro- as well as macrophytic
algal production. During this decade something of a revolution has occurred in our
perception of marine ecology with the recognition of the importance of marine bacteria and the “microbial loop” (Azam et al.
1983; Hobbie and Williams 1984). It would
appear to be an appropriate time to question
the meaning and use of the term “primary
production”
in aquatic ecology.
In higher plants only a few percent of cells
are actually photosynthetic.
These cells release dissolved organic C (DOC) into the
phloem which is then used by heterotrophic
cells elsewhere in the plant. When higher
plant production is estimated, the biomass
of the entire plant and not just of the photosynthetic tissue is considered. By strict
analogy, when considering microalgal primary production we should include production by heterotrophs using DOC leaked
or released by algae. Before the ramifications of the microbial loop were apparent,
UNESCO (1973, p. 9) defined primary
production as “the formation of organic
particulate material at the primary stages of
the food chain.” Such a definition allows
inclusion of heterotrophic
production but
colloquially primary production is considered to be limited to autotrophy or C fixation.
The inclusion of heterotrophic microbial
activity has the advantage that measure-
ments of 14C0, fixation could be considered
a better estimate of potential primary production because the significant proportion
released or leaked by the algae as DOC
(Lancelot and Billen 1985) and subsequently used by heterotrophs would still be included in the estimate of biomass production. Another advantage is that, because
marine organisms appear often to feed on
particles within certain size ranges and
hence, for example, picophytoplankton
and
bacteria could be preyed upon by the same
organism (Goldman and Caron 1985; Fogg
1986), the flow of energy would still be from
“primary producers” (in this instance organisms using dissolved matter to produce
living particulate
matter) to “secondary
producers.”
There are, however, some important
problems to weigh against these arguments.
DOC is released by activities other than excess photosynthesis (such as grazing and lysis at death: Cadee 1986) and it may be
unlikely that bacteria, or our methods of
analysis, will differentiate between “new”
and “old” DOC of similar chemistry. Secondly, the conflict between phototrophic and
heterotrophic activities, performed by organisms which may compete for common
nutrients such as N and P (Currie and Kalff
1984; Laws et al. 1985) may be considered
too great to allow inclusion of both within
the same trophic level.
The concept of primary production that
has been borrowed from terrestrial applications appears unsuitable for aquatic use
and may be undesirable because it compels
us to view aquatic ecosystems in discrete,
artificial, trophic levels (Pomeroy 1974). The
term has no advantage over more accurate
words such as “autotrophic,”
“phototrophic,” or “heterotrophic”
production which
should be substituted. There seems to be a
1215
Comment
1216
compulsion for aquatic scientists to refer to
CO2 fixation as “primary production.” What
is wrong with the term “CO2 fixation”? It
is accurate, concise, and descriptive for the
methods most often used. The importance
of COZ fixation is in the conversion of radiant to chemical energy, and, as indicators
of energy input 14C02 fixation estimates are
of undoubted value. But, 14C02 fixation gives
an estimate of photosynthesis, not of production (Talling 1984); production has units
of Abiomass/space/time
(Parsons et al.
1984). Lancelot et al. (1986) and Lohrenz
and Taylor ( 1987) described more appropriate methods of estimating production
based on determining the rate of protein
synthesis, which is at least a direct function
of biomass synthesis.
Statements claiming “nutrient limitation
of primary production”
should invariably
read “nutrient
limitation
of phototrophic
production.”
Frequently,
however, such
claims should be directed toward limitation
of photosynthetic
activity because rates of
14C02 fixation and not increase in biomass
have been measured. Either way production
of predators need not have been limited by
such an occurrence provided that there is
sufficient particulate
material (POM) of
suitable size as food.
It is essential that scientists define terms
simply and rigidly in order to foster a questioning attitude. Whereas a paper titled “Nitrogen limitation
of 14C0, fixation”
may
immediately prompt questions about methodology in the mind of the reader, a title of
“Nitrogen
limitation
of primary production” is much more vague and may not.
“Primary production” offers little other than
historical convenience. It is used with little
thought to mean different things, even within single editions of journals or books. The
time has come either to define it properly
or to discard it.
Kevin J. Flynn
School of Biological Sciences
University College Swansea
Swansea SA2 8PP
South Wales, U.K.
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