Principles of terrestrial ecosystem ecology.pdf

234 10. Aquatic Carbon and Nutrient Cycling
Herbivory (g C m -2 yr -1 )
[log scale]
10 4
10 2
1
10
5 10
-2
Aquatic
Terrestrial
10 2
10 3
Net primary production (g C m -2 yr -1 )
[log scale]
Figure 10.8. Comparative productivity and herbivory
rates between aquatic and terrestrial ecosystems.
(Redrawn with permission from Nature; Cyr
and Pace 1993.)
digestible due to their lack of structural support
tissue. The resulting high rate of herbivory by
zooplankton in pelagic ecosystems transfers a
large proportion of primary producer carbon
from plants to animals. Herbivory is strongly
correlated with NPP, so the secondary productivity
of marine fisheries and other components
of secondary production depend strongly on
NPP (see Chapter 11). Food webs in the
three-dimensional pelagic environment are frequently
longer and more complex than those
in the two-dimensional benthic environment
(Thurman 1991). Because predation is strongly
size dependent, the wide range of sizes of
pelagic plankton (0.1 to 2000mm) also contributes
to long food chains and complex webs
in pelagic ecosystems.
Decomposition within the euphotic zone
recycles nutrients and contributes energy to
higher trophic levels. Phytoplankton release
about 10% (5 to 60%) of their production as
exudates into the water column (Valiela 1995),
a proportion of NPP similar to that which
terrestrial plants transfer to the soil as root
exudates and to support mycorrhizal fungi.
Zooplankton spill phytoplankton cytoplasm
into the water, as they eat, and excrete their
own waste products. Pelagic bacteria break
down the resulting organic compounds and
mineralize the associated nutrients, which
are then available to primary producers.
This decomposition occurs relatively quickly
because the carbon substrates are mostly labile
organic compounds of low molecular weight
with a low C:N ratio (Fenchel 1994). This
contrasts with the structurally complex,
carbon-rich compounds (cellulose,lignin,phenols,
tannins) that dominate terrestrial detritus.
Viruses play an important role in planktonic
food webs, lysing bacteria and algae. Viral lysis
may account for 5 to 25% of bacterial mortality
in pelagic ecosystems (Valiela 1995). Pelagic
bacteria and viruses are grazed by small
(nanoplankton) flagellate protozoans, which
in turn are eaten by larger zooplankton. The
detritus-based food web (see Chapter 11) is
therefore tightly integrated with the plantbased
trophic system in pelagic food webs and
contributes substantially to the energy and
nutrients that support marine fisheries. This
microbial loop in pelagic ecosystems recycles
most of the carbon and nutrients within
the euphotic zone, so nutrients are recycled
through food webs multiple times before being
lost to depth (Fig. 10.9).
Pelagic carbon cycling pumps carbon and
nutrients from the ocean surface to depth (Fig.
10.9). Although most of the planktonic carbon
acquired through photosynthesis returns to the
environment in respiration, just as in terrestrial
ecosystems, marine pelagic ecosystems also
transport 5 to 20% of the carbon fixed in the
euphotic zone into the deeper ocean (Valiela
1995). This process is called the biological
pump. The carbon flux to depth correlates
closely with primary production, so the environmental
controls over NPP largely determine
the rate of carbon export to the deep ocean.
This carbon export consists of particulate dead
organic matter (feces and dead cells) and the
carbonate exoskeletons that provide structural
rigidity to many marine organisms. Carbonate
accounts for about 25% of the biotically fixed
carbon that rains out of the euphotic zone
(Houghton et al. 1996). The carbonates redissolve
under pressure as they sink to depth.
Over decades to centuries, some of this carbon
in deep waters recirculates to the surface
through upwelling and mixing. This long-term
circulation pattern will cause the effects of
the current increase in atmospheric CO2 to
influence marine biogeochemistry for centuries
after its impacts are felt in terrestrial ecosys-