Principles of terrestrial ecosystem ecology.pdf

230 10. Aquatic Carbon and Nutrient Cycling
Depth (m)
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0.1 1 10 100 0.1 1 10 100
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tively low light intensity, from 5 to 25% of
full sun, depending on the algal group (Valiela
1995). Photosynthesis declines at higher light
levels, so maximum photosynthesis usually
occurs at about 10m depth on sunny days. One
reason that phytoplankton function as shade
plants is that they mix vertically through the
water column, so an individual cell spends relatively
little time near the surface. Photosynthetic
acclimation to high light requires about
12h, which is probably longer than the time that
most phytoplankton would be exposed to high
light. In the ocean and clear lakes, UV-B radiation
may also contribute to low photosynthetic
rates in surface waters, raising questions about
the consequences of ozone holes and increased
UV-B at high latitudes. Light appears to limit
ocean and lake production at the water surface,
primarily beneath ice cover or during winter at
high latitudes when the sun angle is low. At
depth, light limits production in all aquatic
habitats.
Primary producers exhibit a greater diversity
of pigment types in the ocean than on land,
presumably related to the complex light
environment of water. Red algae, which are
abundant at depth in tropical oceans, and
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W
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Oceanic water Coastal water Forest
Figure 10.5. Light quality at different depths in
ocean and coastal waters and in forests (Chazdon
and Fetcher 1984a). R, red; Y, yellow; G, green; B,
Irradiance (% of incident light) [log scale]
blue; W, white. (Modified with permission from
Springer-Verlag; Valiela 1995.)
brown algae like kelp, which are abundant at
depth in cool temperate oceans, have pigments
that absorb the blue light that is
available to them. Surface algae tend to have
green pigments. These differences in algal pigments
may also be adaptations to light quantity
per se.
Reduced sulfur compounds provide the
energy for carbohydrate synthesis from CO2
in some anaerobic aquatic habitats. Although
most organisms depend on carbon fixed
through light-dependent photosynthesis, some
bacteria use the energy of reduced sulfur compounds
to reduce CO2 to form organic compounds.
Entire ecosystems are built on such a
base near hydrothermal vents in zones of seafloor
spreading in midocean regions. Although
hydrothermal vents account for only a tiny fraction
of total ocean production, they support
unique communities and complex food webs
that are completely independent of energy
input from the sun (Karl et al. 1980). Similar
chemosynthesis occurs in anaerobic sediments,
but these sulfur-dependent oxidation–reduction
reactions usually account for only a small
fraction of the total carbon budget of these
environments.
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