Principles of terrestrial ecosystem ecology.pdf

228 10. Aquatic Carbon and Nutrient Cycling
shallow-water ecosystems, including coral reefs,
seagrass beds, and kelp forests. In ecosystems
where physical attachment is possible, phytoplankton,
benthic algal mats, multicellular algae
such as sea lettuce (e.g., Ulva) and kelp (e.g.,
Laminaria), and vascular plants like eelgrass
(Zostera) occur in various combinations. The
relative abundance of different types of
primary producers depends on many factors,
including nutrient availability, water depth, the
stability of substrates, and disturbance regime.
Nutrient addition, for example, favors rapidly
growing phytoplankton, which reduce the light
available to multicellular algae at depth. Intermediate
levels of disturbance and presence of
stable substrates favor multicellular primary
producers (Sousa 1985).
Oceans
The large area and low productivity per unit
area of ocean cancel out, so the ocean contributes
nearly half (about 40%) of global NPP.
MARINE PRODUCERS
Corals
Kelp and rockweeds
Salt marsh grasses
Sea grasses
Mangroves
Benthic microalgae
Coastal phytoplankton
Open sea phytoplankton
FRESH-WATER PRODUCERS
Macrophytes
Phytoplankton (nutrient rich)
Phytoplankton (nutrient poor)
TERRESTRIAL PRODUCERS
Tropical wet forests
Temperate forests
Grasslands
Deserts and tundra
0 1 2 3 4
Production (kg C m -2 yr -1 )
Although oceans cover 70% of Earth’s surface,
the average NPP per unit area is only 20% of
that on land (Table 10.2). Aquatic productivity,
however, is highly variable, just as on land.
The most productive aquatic ecosystems, such
as coral reefs, kelp forests, and eutrophic lakes,
can be at least as productive as the most productive
terrestrial ecosystems (Fig. 10.3). NPP
in the open ocean, which accounts for 90% of
the ocean area, however, is similar to that of
terrestrial deserts and tundra. Because of its
large area, the open ocean accounts for 60% of
marine production, with picoplankton accounting
for about 90% of this production (Valiela
1995).
Carbon and Light Availability
Photosynthesis is seldom carbon limited in
the ocean. In marine pelagic ecosystems, for
example, only 1% of the carbon in a given
water volume is involved in primary production,
whereas the nitrogen in this water may
cycle through primary production 10 times a
Figure 10.3. NPP of selected marine,
fresh-water, and terrestrial ecosystems.
Marine and fresh-water ecosystems exhibit
the same range of NPP that occurs on land,
but unproductive marine ecosystems (the
open ocean) are much more widespread.
(Redrawn with permission from Springer-
Verlag; Valiela 1995.)