Principles of terrestrial ecosystem ecology.pdf

226 10. Aquatic Carbon and Nutrient Cycling
Speed of travel (m s -1 )
10 3
10 0
10 -3
10 -6
10 -6
10 -3
Pure viscosity
effect
10 0
Reynolds number
Effect of
both factors
Figure 10.2. Range of Reynolds numbers for organisms
of different lengths and speeds. Small organisms
like phytoplankton have small Reynolds numbers
and derive their nutrition by diffusion. As size and
energetically expensive, so diffusion is the
major process that moves nutrients to the cell
surface, just as with fine roots growing in the
soil solution.
The small size and lack of nonphotosynthetic
support structures in marine phytoplankton
mean that marine primary producers require
relatively little biomass to support a given photosynthetic
capacity. The average primary producer
biomass per unit area on land, for
example, is 660-fold greater than in the ocean,
although the average net primary production
(NPP) per unit area on land is only 5-fold greater
than in the ocean (Table 10.2) (Cohen 1994).
Phytoplankton biomass of oceans and lakes
turns over 20 to 40 times per year, or even daily
under conditions that are favorable for growth,
whereas turnover for terrestrial plant biomass
often requires years to decades (Valiela 1995).
The air that surrounds terrestrial organisms
delivers oxygen and other gases orders of
magnitude more rapidly than occurs in water.
The surface ocean water, for example, has an
oxygen concentration 30-fold lower than in air
(Table 10.1), and aquatic sediments are much
more likely to be anaerobic than are terrestrial
soils. Aquatic organisms therefore exhibit a
variety of adaptations to acquire oxygen and
10 3
10 6
Pure inertial
effect
Reynolds number increase, nutrition based on
movement (filter feeding and swimming) become
progressively more important. (Redrawn with permission
from Halsted Press; Schwoerbel 1987.)
withstand anaerobic conditions, whereas on
land the acquisition of water and the avoidance
or tolerance of desiccation are more common
evolutionary themes.
The buoyancy of water produces an environment
that is rich in small particles, including
algal cells and suspended particles of detritus.
Filter feeders are organisms that feed on suspended
particles through use of a diverse array
of tools, including hairy appendages on legs or
mouth parts, sticky secretions, and silken nets.
Filter feeders have no counterpart in terrestrial
Table 10.2. Characteristics of oceans and
continents.
Unit Oceans Continents
Surface area (% of Earth) 71 29
Volume of life zone (% of Earth) 99.5 0.5
Living biomass (10 15 gC) 2 560
Living biomass (gm -2 ) 5.6 3700
Dead organic matter 5.5 10
(10 3 gm -2 )
Net primary production 69 330
(gC m -2 yr -1 )
Residence time of C in living 0.08 11.2
biomass (yr)
Data from Cohen (1994).