146
o
C\I
« -1
U
a.. -2
-3
-2
Sa
Pa
Co
+
Su
Ch
-1
o
peA 1
FIGURE 9.4. Results of a principle components analysis
(CPA) of extracellular enzyme activity for the water column bacteria in a series of riverine and estuarine sites.
Sites include the Child's River (Ch; Massachusetts); the
Columbia River (Co); the Susquehanna River (Su); the
Parker River (Pa, Massachusetts); and the Satilla River
(Sa; Georgia). The principal component scores and their
standard deviations in both dimensions are plotted. (Data
are redrawn from Hopkinson et al. (1998) and data on
the rivers and methods used can be found there.)
niques may offer a powerful way to examine how
different ecosystems process complex molecules.
Conclusions
While it is clear that C cycling in pelagic ecosystems is often dominated by bacterial processes a
number of intriguing unresolved questions remain.
First, we have no widely accepted unifying theory
of bacterial production or respiration, although
there have been some noble attempts (Thingstad
and Lignell 1997; Valino et al. 1997). That is, we
can crudely predict these processes across environments but we do not have a simple, physiologically
based model for either BP or BR (Valino et al.
1997). Phytoplankton primary production, in contrast, is much better understood from a theoretical
and physiological perspective than is bacterial production (Pace and Cole 1994). Second, there is usually a very large pool of DOC relative to instantaneous bacterial demand (see Fig. 9.1). In soils,
where there is also a large organic C pool, moisture
clearly limits microbial activity in most environJonathan J. Cole
ments. What, then, limits BP and BR in aquatic
systems? While a number of studies show significant stimulation of BP by adding inorganic nutrients (Elser et al. 1995; Waiser and Robarts 1995;
Pace and Funke 1991; Toolan et al. 1991) many
others show stimulation of BP by the addition of
labile C sources (Coveney et al. 1995; Wetzel et al.
1995). We know less about planktonic BR. Are bacteria nutrient-limited and competing with phytoplankton for inorganic nitrogen and phosphorus
(see Currie 1990)? Does the addition of labile C
sources (e.g., from new phytoplankton production)
allow bacteria to co-metabolize the more refractory
allochthonous sources? Further, in many environments, BR appears to exceed the input of C from
autotrophic primary production (del Giorgio et al.
1997a). This heterotrophy in excess of primary production ("net heterotrophy") (Cole et al. 1994; Howarth et al. 1996) implies that allochthonous substrates are being used by bacteria. On the other
hand, several broad comparative studies have
shown that BP varies systematically with primary
production among diverse environments (Cole et
al. 1988; White et al. 1991). If allochthonous inputs
are important to bacterial production and respiration, why do these processes co-vary with the primary production (Caraco et al. 1992)? Bacteria are
capable of metabolizing a large amount of DOC.
At a middling value ofBP of lO)lg C liter- 1 day-l
and a growth efficiency of 30%, bacteria utilize (BP
+ BR) about 12 mg C liter - 1 yr - '. Typical values
for DOC in fresh waters are between 1 and 10
mg C liter - 1. With a water residence time on the
order of 1 year (a good value for many lakes), bacteria should tum over the entire DOC pool once or
more each year. In what ways, then, do aquatic bacteria modify the amount or composition of DOC,
and what consequences might these modifications
have for the ecosystem? We tended to look at bacterial processes in response to alterations in eutrophication (Hobbie and Cole 1984): primary production (White et al. 1991; Cole et al. 1988),
grazing pressure from protozoa and zooplankton
(Porter et al. 1985; Sanders et al. 1992), and topdown changes in food web structure (del Giorgio
et al. 1996a; Vaque and Pace 1992). What are the
important feedbacks by which the microbial food
web regulates or influences other components of
the ecosystem?
o
C\I
« -1
U
a.. -2
-3
-2
Sa
Pa
Co
+
Su
Ch
-1
o
peA 1
FIGURE 9.4. Results of a principle components analysis
(CPA) of extracellular enzyme activity for the water column bacteria in a series of riverine and estuarine sites.
Sites include the Child's River (Ch; Massachusetts); the
Columbia River (Co); the Susquehanna River (Su); the
Parker River (Pa, Massachusetts); and the Satilla River
(Sa; Georgia). The principal component scores and their
standard deviations in both dimensions are plotted. (Data
are redrawn from Hopkinson et al. (1998) and data on
the rivers and methods used can be found there.)
niques may offer a powerful way to examine how
different ecosystems process complex molecules.
Conclusions
While it is clear that C cycling in pelagic ecosystems is often dominated by bacterial processes a
number of intriguing unresolved questions remain.
First, we have no widely accepted unifying theory
of bacterial production or respiration, although
there have been some noble attempts (Thingstad
and Lignell 1997; Valino et al. 1997). That is, we
can crudely predict these processes across environments but we do not have a simple, physiologically
based model for either BP or BR (Valino et al.
1997). Phytoplankton primary production, in contrast, is much better understood from a theoretical
and physiological perspective than is bacterial production (Pace and Cole 1994). Second, there is usually a very large pool of DOC relative to instantaneous bacterial demand (see Fig. 9.1). In soils,
where there is also a large organic C pool, moisture
clearly limits microbial activity in most environJonathan J. Cole
ments. What, then, limits BP and BR in aquatic
systems? While a number of studies show significant stimulation of BP by adding inorganic nutrients (Elser et al. 1995; Waiser and Robarts 1995;
Pace and Funke 1991; Toolan et al. 1991) many
others show stimulation of BP by the addition of
labile C sources (Coveney et al. 1995; Wetzel et al.
1995). We know less about planktonic BR. Are bacteria nutrient-limited and competing with phytoplankton for inorganic nitrogen and phosphorus
(see Currie 1990)? Does the addition of labile C
sources (e.g., from new phytoplankton production)
allow bacteria to co-metabolize the more refractory
allochthonous sources? Further, in many environments, BR appears to exceed the input of C from
autotrophic primary production (del Giorgio et al.
1997a). This heterotrophy in excess of primary production ("net heterotrophy") (Cole et al. 1994; Howarth et al. 1996) implies that allochthonous substrates are being used by bacteria. On the other
hand, several broad comparative studies have
shown that BP varies systematically with primary
production among diverse environments (Cole et
al. 1988; White et al. 1991). If allochthonous inputs
are important to bacterial production and respiration, why do these processes co-vary with the primary production (Caraco et al. 1992)? Bacteria are
capable of metabolizing a large amount of DOC.
At a middling value ofBP of lO)lg C liter- 1 day-l
and a growth efficiency of 30%, bacteria utilize (BP
+ BR) about 12 mg C liter - 1 yr - '. Typical values
for DOC in fresh waters are between 1 and 10
mg C liter - 1. With a water residence time on the
order of 1 year (a good value for many lakes), bacteria should tum over the entire DOC pool once or
more each year. In what ways, then, do aquatic bacteria modify the amount or composition of DOC,
and what consequences might these modifications
have for the ecosystem? We tended to look at bacterial processes in response to alterations in eutrophication (Hobbie and Cole 1984): primary production (White et al. 1991; Cole et al. 1988),
grazing pressure from protozoa and zooplankton
(Porter et al. 1985; Sanders et al. 1992), and topdown changes in food web structure (del Giorgio
et al. 1996a; Vaque and Pace 1992). What are the
important feedbacks by which the microbial food
web regulates or influences other components of
the ecosystem?