Environmental Engineering Reference
In-Depth Information
Autotrophs synthesize organic matter de novo by definition but require a variety of
inorganic elements to do so. These elements are described as either macronutrients, which
constitute greater than 0.1% of organism wet weight, or micronutrients, which constitute
less than 0.1%. The macronutrients of organic matter are C, N, H, O, P, S, K, Mg, Na, and
Ca (plus Si, for diatoms and some land plants). Micronutrients include Fe, Mn, Zn, Cu, B,
Mo, Cl, V, and Co, which are used mainly as enzyme cofactors. Some autotrophs have
more specialized requirements including various trace metals and vitamins (e.g., vitamin
B
12
). Relative uptake and loss of macro- and micronutrients influence the stoichiometric
composition of autotrophs. For example, limitation of primary production by nitrogen
and phosphorus often results in autotrophs with high carbon-to-nitrogen and carbon-to-
phosphorus ratios, and such ratios can constrain other ecosystem processes like herbivory
and decomposition (
Sterner and Elser 2002
; Figure 3.7).
The ratio of the key elements carbon, nitrogen, and phosphorus can be quite variable in
autotrophs. Terrestrial plant leaves have an enormous range of C:N and C:P ratios that
reflect investment in structural carbon as documented by
Sterner and Elser (2002)
, who
summarized a large compilation of data from a variety of sources and sites. Mean foliar
C:N and C:P ratios (expressed in moles) were 36 and 970, respectively (
Sterner and Elser
2002
). These high ratios contrast with marine seston (particulate matter collected on filters)
from the ocean, which is dominated by phytoplankton cells with lower mean C:N and C:P
ratios of 7.7 and 143, respectively. Freshwater seston ratios were intermediate with mean
C:N and C:P of 30 and 307, respectively, probably reflecting the mixture of terrestrial detri-
tus and phytoplankton cells suspended in the water as well as the chemical composition
of phytoplankton, which is often depleted in P. Importantly, the ranges in ratios observed
in marine seston are relatively small, larger in freshwater, and huge for terrestrial leaves
(
Figure 2.4
; note the log scale). Animals maintain a more rigid C:N:P composition and
must compensate when feeding on nitrogen- or phosphorus-poor autotrophic organic
matter (
Sterner and Elser 2002
).
To illustrate nutrient limitation of primary production consider the results of several
experimental additions of nutrients to ecosystems (
Figure 2.5
). An example of a single
nutrient addition comes from the Southern Ocean, which is the oceanic region bordering
the Antarctic continent. Iron was added to a 1000 km
2
area that was also marked with an
inert tracer so that the labeled patch could be identified and followed as it diluted with
the surrounding waters (
Boyd et al. 2000
). In the iron-addition patch NPP (measured with
14
C) increased approximately five-fold relative to NPP measured outside the patch
(
Figure 2.5a
). Similar results were obtained in another addition experiment when nitrogen
and phosphorus were added to a lake (
Carpenter et al. 2005
). Primary production mea-
sured as GPP using continuous oxygen measurements increased two-fold (
Figure 2.5b
).
Terrestrial ecosystems also respond to nutrient enrichment. For example, nitrogen and
phosphorus additions enhanced forest primary production as measured by tree incre-
ments (
Figure 2.5c
). This forest fertilization study was conducted on different islands in
the Hawaiian chain that vary in age of the geological substrate (
Vitousek 2004
). Nutrient
limitation of production changed from primarily nitrogen on younger substrate to primar-
ily phosphorus on older substrate (
Figure 2.5c
).
The increase of production in the lake example (
Figure 2.5b
) illustrates a common
response to nutrient additions in ecosystems. Phosphorus is often the primary limiting
