Principles of terrestrial ecosystem ecology.pdf

232 10. Aquatic Carbon and Nutrient Cycling
Frequency
15
10
5
0
15
10
5
0
Marine phytoplankton
Freshwater phytoplankton
10 100
N:P (by atoms) [log scale]
caused phytoplankton blooms large enough to
be seen from satellites, indicating that iron
limits the capacity of phytoplankton to use
nitrogen and phosphorus. During glacial
periods there may have been 10-fold greater
input of iron- and phosphorus-bearing dust to
the oceans, thus stimulating ocean productivity
and in turn lowering atmospheric CO2 concentrations
(Falkowski et al. 1999). The key role of
iron in regulating production in some sectors of
the open ocean has led to the suggestion that
large-scale iron fertilization might stimulate
ocean production sufficiently to scavange large
amounts of CO2 from the atmosphere and
sequester it in deep oceans in the form of dead
organic matter. The iron-addition experiments,
however, show that this stimulation of production
is relatively short-lived, presumably
because other elements quickly become limiting
to production, as soon as the iron demands
of phytoplankton are met. Grazing is another
factor that contributes to low phytoplankton
biomass and productivity in portions of the
open ocean. In some HNLC zones of the ocean
there is simply not enough phytoplankton
biomass to use the nutrients that are available
(Valiela 1995).
N limited
P limited
No data available
1 1000
Figure 10.7. Frequency distribution
of the N : P ratio in marine and
fresh-water phytoplankton. Nutrient
addition experiments (shaded
bars) indicate that the high N:P
ratios in lakes reflect phosphorus
limitation of algal growth, whereas
the generally low N:P ratios of
marine phytoplankton commonly
reflect nitrogen limitation. (Modified
with permission from Springer-
Verlag; Valiela 1995.)
The strong nutrient limitation and lack of
CO2 limitation of productivity in most of the
world’s oceans make it unlikely that marine
productivity will respond directly to increasing
atmospheric CO2. Nutrient limitation of marine
production, however, makes these ecosystems
potentially vulnerable to anthropogenic
nutrient inputs. Estuaries, for example, have
been substantially altered by the large nutrient
inputs from agricultural lands. Runoff and
sewage effluents have substantially altered
coastal ecosystems near heavily populated
or agricultural areas (Howarth et al. 1996).
Oxygen depletion by decomposers in the water
column of the Mississippi Delta, for example,
has created a large dead zone in the Gulf of
Mexico. Although the impacts of nutrients on
the open ocean may be more subtle and difficult
to detect because most pollution sources
are remote, they could, over the long term, be
important because of the high degree of nutrient
limitation of pelagic ecosystems and their
large aerial extent.
Ocean productivity is ultimately limited by
the rate of nutrient supply from the land or
deep ocean waters. For this reason, productivity
is greater in coastal waters than in the