Community Structure

Lecture 39: Nutrient Cycles

Reading: Economy of Nature, pp. 148-165.

Biological Accumulation and Magnification

Energy is dispersed in food chains but matter is often concentrated.

Radioisotopes such as strontium-90, which behaves like calcium, is accumulated by organisms and passed along a food chain magnifying with each link in the food chain. Consequently, top carnivores can accumulate very high concentrations of this radioactive isotope, even if only very low concentrations are released to the environment.

The natural phenomenon of magnification makes the accidental (or intentional) release of radioisotopes a very serious problem. For example, the Fernald, OH Department of Energy site discharged 3970 kg of uranium to the Great Miami River between 1989 and 1993. Despite the fact that annual uranium discharges have decreased during that period of years (down to 474 kg in 1993) and there is massive dilution of each liter of discharged effluent (2,100 liter of river water per liter of effluent on average), fishes sampled in the discharge area contained 1.6x to 6.3x the concentration of uranium as fishes upstream. Dilution and quantity of discharge cannot accurately describe risk when biological magnification is possible.

This problem is compounded by the fact that many radioisotopes have extremely long (geological time scales) half-lives (the time it takes for one-half of the atoms to release their energy to yield a stable non-radioactive isotope). The table below shows the half-live for some components of high-level radioactive waste.

Radioactive Isotope

Half-Life (years)

Uranium U²³³

165,000

Uranium U²³⁵

710,000,000

Uranium U²³⁸

4,510,000,000

Plutonium Pu²³⁹

24,400

Given the long period of time during which these elements remain radioactive, waste disposal will require isolation and monitoring for incredibly long periods of time. The continuing debate about the suitability of Yucca Mountain, NV for high-level radioactive waste storage ultimately concerns the risk of radioisotope releases (in geological time) and the inevitable magnification of those radioisotopes by aquatic or terrestrial food chains.

Heavy Metals

Many metals are subject to biological magnification: mercury, cadmium, lead, arsenic, copper, nickel, zinc

Mercury

Minamata Disease

Minamata Bay, Japan, industrial dumping of methyl mercury to the marine waters of the bay resulted in biological magnification of mercury. Human consumption of fishes and crustacea high on the food chain resulted in mercury poisoning of as many as 12,000 victims (=Minamata Disease).

Lead

Lead was an additive to gasoline (tetraethykkead) in the United States from the 1940’s until it was banned in the 1980’s. The combustion of leaded gasoline released lead to the environment at very low concentrations. Soil lead concentrations typically decreased with distance from highways but could be as high as 1200 ppm at the edge of a high traffic volume road. Lead is taken-up by plants and passed along to herbivores, and then to higher consumers resulting in biological magnification. The figure below shows the lead concentration in plants and three types of insects. Both sucking and chewing insects are herbivores but chewing insects appear to have a greater exposure to lead in their foods (data from Boggess and Wixson, 1977).

Toxic substances like dioxin and polychlorinated biphenyl are also subject to biological magnification.

The longer the food chain, the greater the biological magnification.

Biogeochemical Cycles

Matter is recycled in exchanges between the living and non-living components of an ecosystem.

Energy flows are unidirectional, open system, requiring constant inputs.
Nutrient flows are true cycles between autotrophs, heterotrophs, decomposers, and the environment.

Nutrient Pool (Reservoir Pool)

Large, slow moving, generally abiotic repository for a nutrient

Exchange Pool (Cycling Pool)

Smaller, more active pool, exchange between organisms and immediate environment (does not apply to all matter cycles).

Two types of matter cycles: Atmospheric and Sedimentary

Atmospheric: reservoir pool is the atmosphere or hydrosphere

water, oxygen, nitrogen

Sedimentary: reservoir pool is the earth’s crust, sedimentary rocks

carbon, phosphorus, sulfur, many trace nutrients

Hydrological Cycle

Reservoir is the oceans.

Water goes from liquid to gas phase by evaporation and transpiration.

Water returns to earth’s surface as precipitation

Movement back to the ocean:

runoff to streams, rivers, lakes

infiltration to ground water

Hydrological cycle is shown with pool and transfer values in parentheses (10¹⁸g and 10¹⁸g per year) (Ricklefs, 1996, p 152, Fig. 7.4).

Nitrogen Cycle

Reservoir for nitrogen is the atmosphere (N₂ gas).

Nitrogen Fixation: N₂ >>>> NH₄⁺ (requires energy input)

Root nodule bacteria, Rhizobium
Cyanobacteria
Free-living soil bacteria
Abiotic photochemical fixation
Abiotic electrochemical fixation (lightning)

Nitrification: NH₄⁺ >>>>>NO₂^- >>>>>NO₃^- (energy releasing)

Soil and water bacteria, chemosynthetic bacteria
Anthropogenic reactive nitrogen: agricultural runoff, internal combustion engine exhaust (NO_x), and fertilizer production
Nitrogen oxides (NO_x) produced by internal combustion engines return as nitric acid (HNO₃) in acid precipitation.

Plant Uptake and Utilization: NO₃^- >>> NH₄⁺ >>>Organic N

(energy requiring)

All plants except legumes with root nodules

Heterotrophic Consumption: Organic N >>>>>>>Organic N

Decompostion and Elimination of Nitrogen

Ammonification: Organic N >>>>>>> NH₄⁺ (energy releasing)

Denitrification: NO₂^- >>> N₂ (energy requiring)

Anaerobic soil bacteria

The relationship between the nitrogen transformation reactions in the nitrogen cycle are shown on a scale of potential energy (oxidation state) (Ricklefs, 1996, p 159, Fig. 7.9).