Phosphorus cycle

The principal stocks of phosphorus occur in the water of the soil, rivers, lakes and oceans and in rocks and ocean sediments. The phosphorus

Precipitation gaseous and aerosol uptake

Terestrial communities

Internal cycling

Biotic uptake

Runoff

Weathering

Weathering

Atmosphere

Atmosphere

Land clearance, forestry, agriculture

Increased emission to atmosphere

Increased concentrations in water

Human activities

Increased concentrations in water

Harvesting

Human activities

Respiration, gaseous and aerosol emission

Harvesting

Aquatic communities

Internal cycling

Stream flow

Biotic uptake

Geological uplift creating new land

Sedimentation ►

plants live between two counterflowing movements of water major nutrient compartments and fluxes in global biogeochemical cycles phosphorus derives mainly from the weathering of rocks

Figure 18.20 The major global pathways of nutrients between the abiotic 'reservoirs' of atmosphere, water (hydrosphere) and rock and sediments (lithosphere), and the biotic 'reservoirs' constituted by terrestrial and aquatic communities. Human activities (in color) affect nutrient fluxes through the terrestrial and aquatic communities both directly and indirectly, via their effects on global biogeochemical cycling through the release of extra nutrients into the atmosphere and water.

Figure 18.21 The main pathways of nutrient flux (black) and the perturbations caused by human activities (color) for four important nutrient elements: (a) phosphorus, (b) nitrogen, (c) sulfur (DMS, dimethylsufide), and (d) carbon. Insignificant compartments and fluxes are represented by dashed lines. (Based on the model illustrated in Figure 18.10, where further details can be found.)

(a) Phosphorus cycle

Sewage

Atmosphere

Terestrial communities

Fertilizers

_ Fishing

\ soil! rivers, lakes land oceans^

Rock

Sewage

Deforestation in>lik

Human activities

Aquatic communities

Aquatic communities

(c) Sulfur cycle

Volcanic activity

-> Atmosphere

-> Atmosphere

Terestrial CJ communities

Water _

S02 from burning of fossil fuels

Human activities

Aquatic communities

Aquatic communities

(b) Nitrogen cycle

(b) Nitrogen cycle

(d) Carbon cycle

CO, dissolves

uptake in photosynthesis

Combustion of fossil fuels

Combustion of fossil fuels

Terestrial ("j) communities

Respiration

uptake in photosynthesis

Terestrial ("j) communities

Respiration

cycle may be described as an 'open' cycle because of the general tendency for mineral phosphorus to be carried from the land inexorably to the oceans, mainly in rivers, but also to smaller extents in groundwater, or via volcanic activity and atmospheric fallout, or through abrasion of coastal land. The cycle may alternatively be termed a 'sedimentary cycle' because ultimately phosphorus becomes incorporated in ocean sediments (Figure 18.21a). We can unravel an intriguing story that starts in a terrestrial catchment area. A typical phosphorus atom, released from the rock by chemical weathering, may enter and cycle within the terrestrial community for years, decades or centuries before it is carried via groundwater into a stream, where it takes part in the nutrient spiraling described in Section 18.3.1. Within a short time of entering the stream (weeks, months or years), the atom is carried to the ocean. It then makes, on average, about 100 round trips between the surface and deep waters, each lasting perhaps 1000 years. During each trip, it is taken up by organisms that live at the ocean surface, before eventually settling into the deep again.

On average, on its 100th descent (after 10 million years in the ocean) it fails to be released as soluble phosphorus, but instead enters the bottom sediment in particulate form. Perhaps 100 million years later, the ocean floor is lifted up by geological activity to become dry land. Thus, our phosphorus atom will eventually find its way back via a river to the sea, and to its existence of cycle (biotic uptake and decomposition) within cycle (ocean mixing) within cycle (continental uplift and erosion).

Human activities affect the phosphorus cycle in a number of ways. Marine fishing transfers about 50 Tg (1 teragram = 1012 g) of phosphorus from the ocean to the land each year. Since the total oceanic pool of phosphorus is around 120 Pg (1 petagram = 1015 g), this reverse flow has negligible consequences for the ocean compartment. However, phosphorus from the fish catch will eventually move back through the rivers to the sea and, thus, fishing contributes indirectly to increased concentrations in inland waters. More than 13 Tg of phosphorus are dispersed annually over agricultural land as fertilizer (some derived from the marine fish catch) and a further 2 or 3 Tg as an additive to domestic detergents. Much of the former reaches the aquatic system as agricultural runoff, whereas the latter arrives in domestic sewage. In addition, deforestation and many forms of land cultivation increase erosion in catchment areas and contribute to artificially high amounts of phosphorus in runoff water. All told, human activities have almost doubled the inflow of phosphorus to the oceans above that which occurs naturally (Savenko, 2001).

An increase to phosphorus input to the oceans on this scale is likely to have increased productivity to some extent, but as the more concentrated water passes through rivers, estuaries, coastal waters and particularly lakes, its influence can be particularly profound. This is because phosphorus is often the nutrient whose supply limits aquatic plant growth. In many lakes worldwide, the input of large quantities of phosphorus from agricultural runoff and sewage and also of nitrogen (mainly as runoff from agricultural land) produces ideal conditions for high phytoplankton productivity. In such cases of cultural eutrophication (enrichment), the lake water becomes turbid because of dense populations of phytoplankton (often the blue-green species), and large aquatic plants are out-competed and disappear along with their associated invertebrate populations. Moreover, decomposition of the large biomass of phytoplankton cells may lead to low oxygen concentrations, which kill fish and invertebrates. The outcome is a productive community, but one with low biodiversity and low esthetic appeal. The remedy is to reduce nutrient input; for example, by altering agricultural practices and by diverting sewage, or by chemically 'stripping' phosphorus from treated sewage before it is discharged. Where phosphate loading has been reduced in deep lakes, such as Lake Washington in North America, a reversal of the trends described above may occur within a few years (Edmonson, 1970). In shallow lakes, however, phosphorus stored in the sediment may continue to be released and the physical removal of some of the sediment may be called for (Moss et al., 1988).

The effects of agricultural runoff and sewage discharge are localized, in the sense that only those waters that drain the catchment area concerned are affected. But the problem is pervasive and worldwide.

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