Environmental Engineering Reference
In-Depth Information
trient upwelling or flux from rivers, average 10
4
-10
5
cells/L, and their standing phytomass is 100-1500 mg/
m
3
. In contrast, the oligotrophic regions of the central
Pacific or Atlantic have as few as 100 cells/L. Because
of short cell life the annual phytomass turnover rates are
300-400 in nutrient-rich waters and 40-50 even in the
nutrient-poor open ocean.
Benthic autotrophs are mixtures of algae and a limited
variety (only about 60 species) of vascular plants. Rocky
coastal zones often support dense stands of macroalgae,
above all, kelp and fucoid rockweeds. Reef-building cor-
als live symbiotically with photosynthesizing dinoflagel-
lates. Standing stock of intertidal phytomass is lowest in
the tropics, highest on the boreal shores: measurements
in the Sea of Japan show up to 6.5-7 kg/m
2
in grassy
beds (Menzies, George, and Rowe 1973). The average
density of algal beds and reefs is most likely no greater
than 2 kg/m
2
. Estuarine waters may have the same order
of phytomass densities, whereas the mean for upwelling
regions is 2 OM smaller, and for open ocean 3 OM
smaller.
While we cannot reconstruct global phytomass totals
that were characteristic of past geological eras, we have
some revealing numbers regarding the historic trend,
and increasingly accurate indications of recent changes.
Bazilevich, Rodin, and Rozov (1971) reconstructed the
world's potential vegetation cover (the extent of natural
biomes of the preagricultural era) and came up with a
continental phytomass of 2.4 Tt, or almost 1.1 Tt C.
The terrestrial phytomass total of 500-600 Gt C in the
year 2000 would be roughly half of the preagricultural
storage, and about half of this loss took place during the
last three centuries. The Earth's standing phytomass has
declined by about 25% since 1700, and during the twen-
tieth century the net loss of global plant mass amounted
to about 15% of the 1900 total, with deforestation being
the main reason.
In addition to deforestation there has been major phy-
tomass loss in temperate grasslands. Deep and fertile soils,
like chernozems, that gave rise to this extensive biome
have also been the main cause of its enormous retreat be-
cause of the conversion to cropland of most of the U.S.
Great Plains, Canadian Prairies, Ukrainian, Russian, and
Kazakh steppes, Argentinian pampas, and South Africa's
veld. Crops replacing the grasses may have similar NPP,
but their phytomass is generally much lower and of short
duration. The third major category of phytomass loss has
been the destruction and drainage of wetlands. Recon-
structions of global land use changes (Ramankutty and
Foley 1999; Goldewijk 2001) offer the best available esti-
mates of the historic progression of these processes, and
they indicate cropland expansion from about 265 Mha
in 1700 to nearly 1.5 Gha by the year 2000.
DeFries et al. (1999), Houghton and Hackler (2002),
and R. A. Houghton (2005) expressed these losses in
terms of declining phytomass: a cumulative phytomass
decline of about 50 Gt C before 1850 and, depending
on land use data and phytomass density used, losses of
125-200 Gt C by the year 2000, with more than 85%
from deforestation and the rest from the conversion of
temperate grasslands (fig. 3.10). This massive phytomass
loss has been partially counterbalanced by post-WW II
expansion of forests in Europe and the United States,
massive post-1980 afforestation in China, and higher
NPP due to inadvertent atmospheric deposition of nitro-
gen and better management of forests. Most notably, the
standing phytomass of European forests has increased
by more than 40% since 1950, and these forests have be-
come a substantial (140 Mt C/a) carbon sink (Nabuurs
et al. 2003).
