Environmental Engineering Reference
In-Depth Information
lower bacterial production as well as lower decomposition rates of organic matter in low-
P systems.
There is some evidence that grazers (e.g., crustacean zooplankton, benthic invertebrates)
in aquatic systems can also be P-limited. Lower growth rates and lower reproductive rates
can result when their food has very low P content compared to the P content of the
consumer (see Box 3.1 in Chapter 3). Phytoplankton P content varies substantially with the
severity of P limitation; thus, the low P content in inland waters may lead to lower grazer
abundance both due to lower overall primary production and lower P content of this
production.
The Importance of Phosphorus in Marine Systems
In many ways, the importance of P in marine systems echoes its role in freshwater
systems. While there remains active debate about whether estuaries are limited by P, nitro-
gen, or colimited by both nutrients, P clearly plays an important role in stimulating
primary production in these systems. Not only is P important for biological productivity
of the oceans, but it controls the long-term carbon cycle via its role as a limiting nutrient.
The main P source in the ocean is runoff from rivers, which delivers annually about 1.5 Mt
of dissolved P and more than 20 Mt of suspended P into the ocean.
THE GLOBAL PHOSPHORUS CYCLE
The P cycle is one of the slowest biogeochemical cycles on Earth. While P can cycle rela-
tively quickly through plants and animals, its movement from rock through soils to the
oceans is extremely slow (500 million years;
Figure 8.2
). Unlike many other biogeochemical
cycles, the atmosphere does not play a significant role in the movement of P, because
P-based compounds are usually solids at the typical ranges of temperature and pressure
found on Earth. (See
Box 8.1
for definitions of the key terms in this section.)
The global P cycle follows the new inputs of P into ecosystems from weathering of rocks,
through burial in the ocean, to rock formation from pressure and heat, and eventual uplift
to terrestrial environments. Weathering of primary minerals is the major source of P to
terrestrial ecosystems and is controlled by both biological and geochemical processes.
Weathering involves both chemical and physical breakdown of P-bearing minerals,
primarily apatite, to release phosphates. The rate of P release due to weathering is affected
by mineral type, topography, climate, and biota (
Cross and Schlesinger 1995
). Weathering
rates tend to increase with increasing temperature, precipitation, and slope. Acids pro-
duced from reactions of rainwater and soil CO
2
and organic acids produced by plants and
microbes increase the dissolution of apatite through chemical weathering. The weathering
of primary minerals is nonreversible.
Once released from rock, P can be adsorbed, or chemically adhered, to soil or sediment
particles, moving with erosion or other processes that cause movement of soils. This pro-
cess varies with soil type: Each soil has a P “fixation” capacity determined by the number
of positively charged sites that can bind phosphate ions. Adsorption reactions are usually
