Summary of Chapter Concepts

The hydrologic cycle moves many elements through ecosystems. Water evaporates from water bodies, soil, and plants and moves as water vapor into the atmosphere. This water vapor condenses into clouds and eventually falls back to Earth as precipitation. This precipitation can be taken up by plants, run across the surface of the ground, or infiltrate into the groundwater. This water moves to streams and lakes and ultimately makes its way back to the oceans. Humans can alter this cycle by reducing infiltration due to the construction of impermeable surfaces, by logging trees and thereby increasing runoff, and by causing global warming, which increases the rate of evaporation.

The carbon cycle is closely tied to the movement of energy. Carbon exists in the atmosphere in the form of CO₂. This CO₂ can be used by terrestrial producers and, after dissolving into the water, can be used by aquatic producers. The producers, consumers, scavengers, detritivores, and decomposers in terrestrial and aquatic ecosystems can produce CO₂ when they respire. Carbon can also leave the water by sedimentation and be buried both in the water and on land. Buried carbon can be extracted in the form of fossil fuels. The combustion of fossil fuels and the combustion of organic matter during fires release CO₂ into the atmosphere. Humans can alter this cycle primarily by affecting the extraction and combustion of carbon.

Nitrogen cycles through ecosystems in many different forms. Nitrogen gas in the atmosphere can be converted into ammonia and nitrates by lightning, nitrogen-fixing bacteria, and the manufacture of fertilizers. Producers can take up these forms of nitrogen and assimilate them. This nitrogen is then transferred through terrestrial and aquatic food webs. During decomposition, nitrogen in organisms and their wastes can be converted to ammonia by the process of mineralization. Ammonia can be converted into nitrites and nitrates through the process of nitrification and nitrates can be converted to nitrous oxide and nitrogen gas by the process of denitrification. Humans alter this cycle primarily by manufacturing and applying large amounts of fertilizer and combusting fossil fuels, which produces nitric oxide in the air that later mixes with precipitation and falls to the ground as nitrates. These activities alter the fertility of terrestrial and aquatic environments.

The phosphorus cycle moves between land and water. Most phosphorus is released by weathering from rocks. This phosphorus is taken up by terrestrial and aquatic producers, which pass it to consumers, scavengers, detritivores, and decomposers. The phosphorus from excretions and decomposed organisms dissolves in the water of soil or in the water of streams, rivers, lakes, and oceans. In the ocean, phosphorus precipitates into sediments that are slowly converted into rocks. Humans affect the phosphorus cycle primarily by mining rocks for fertilizer. This phosphorus-rich fertilizer can alter the fertility of terrestrial and aquatic habitats and lead to algal blooms.

In terrestrial ecosystems, most nutrients regenerate in the soil. The nutrients in terrestrial ecosystems are primarily regenerated in soils. Some nutrients such as phosphorus are regenerated by weathering of rocks and all nutrients are regenerated by the decomposition of dead organic matter. Because decomposition rates are faster under warm temperatures and high precipitation, tropical ecosystems have high decomposition rates and low amounts of dead organic matter. Boreal and other cold ecosystems have low rates of decomposition and large amounts of dead organic matter.

In aquatic ecosystems, most nutrients regenerate in the sediments. In many streams and some wetlands, allochthonous inputs of leaves from the surrounding terrestrial environment are the major source of nutrients. The rate of leaf decomposition is determined primarily by the temperatures of the water and the lignin content of the leaves. In rivers, lakes, and streams, much of the organic matter can settle out of the water and onto the bottom sediments where they decompose. When lakes and oceans stratify, the movement of nutrients that have been released by decomposition are hindered from moving to the more productive surface waters. Although nitrogen and phosphorus are the most common limiting nutrients in aquatic ecosystems, some regions of the ocean are limited by the availability of other nutrients, including silicon and iron.