Both aerobic and anaerobic biodegradation get rid of biomass and return carbon dioxide to the atmosphere. The latter reaction is the major source of atmospheric methane. Nondegraded remains of these processes constitute organic matter in aquatic sediments and in soil, which has an important influence on the characteristics of these solids. carbon that was originally fixed photosynthetically forms the basis of all fossil fuels in the geosphere.There is a strong interconnection between the biosphere and the atmosphere. Humans depend upon the biosphere for food, fuel, and raw materials. Human influence on the biosphere continues to change it drastically. Fertilizer, pesticides, and cultivation practices have vastly increased yields in some cases even before they are discovered. Bioengineering of organism with recombinant DNA technology and older techniques of selection and hybridization are causing great changes in the characteristics of organisms and promise to result in even more striking alterations in the future. It is the responsibility of humankind to make such changes intelligently and to protect and nurture the biosphere.EcologyEcology is the science that deals with the relationships between living organisms with their physical environment and with each other. An ecosystem consists of an assembly of mutually interacting organism (a community) and their environment in which materials are interchanged in a largely cyclical manner. An ecosystem has physical, chemical biological components along with energy sources and pathways of energy and materials interchange. The environment in which a particular organism lives is called its habitat. The role of an organism in a habitat is called its niche.For the study of ecology it is often convenient to divide the environment into four broad categories. The terrestrial environment is based on land and consists of biomes, such as grassland, savannas, deserts, or one of several kind of forest. The fresh water environment can be further subdivided between standing-water habitats (lakes, reservoir) and running-water habitats (streams, rivers). The oceanic marine environment is characterized by salt water and may be divided broadly into shallow waters of the continental shelf composing the neritic zone and deeper waters of the ocean that constitute the oceanic region an environment in which two or more kinds of organisms exist together to their mutual benefit is termed a symbiotic environmentA particularly important factor in describing ecosystem is that of populations consisting of numbers of a specific species occupying a specific habitat. Populations may be stable, or they may grow exponentially as a population explosion. A population explosion that is unchecked results in resource depletion, waste accumulation and predation, culminating in an abrupt decline called a population crash. Behavior in areas such as hierarchies, territoriality, social stress, and feeding patterns plays a strong role in determining the fates of population

EcotoxicologyAs discussed in detail in Chapters 22 and 23, toxicology refers to the detrimental effects of substances of organism. Substances with such effect are called toxic substances, toxicants, or poisons. Whether a substances is toxic depends upon the amount to which an organism is exposed and the manner of exposure. Some substances are harmless or even beneficial at low levels are toxic are higher levels of exposure.Toxic substances have a strong influence ecosystems and the organisms in ecosystems, and the interaction between ecology and toxicology are very important. These interaction may be complex and involve a number of organisms. They may involve food chains and complex food webs. For example persistent organohalide compounds may become more highly concentrated through food chains and exert their most adverse effect on organisms such as birds of prey at the top of the food chain. The combination of ecology and toxicology-the study of the effects of toxic substances upon ecosystem has come to be known as ecotoxicology, which has developed into an important discipline in environmental science.Ecotoxicology effects may be considered through several different organizational levels, as sound in figure 1.2. the first step consists of the introduction of the toxicant or pollutant into the system. This may result biochemical changes at the molecular level. As a result psicologycal changes may occur in tissues and organs. These may result in detrimental alterations of organisms. Thus, the organisms affected may undergo population changes, such as occurred in the 1950s and 1960s with decreased populations of hawks exposed to DDT. Such changes can alter communities; for example, decreased numbers of hawks may allow increase numbers of rodents accompanied by greater destruction of grains crops. Finally, whole ecosystems may be altered significantly. Whereas toxicology usually deals with the effects of toxic substances individual, ecotoxicology emphasizes population. large numbers of individuals may be poisoned by toxic, whereas the population may survive. Dealing with populations as it does, ecotoxicology is significantly more complex than toxicology. In reducing risk, emphasis is usually placed upon the protection of species composition since automatically protects ecosystem processes.ENERGY AND CYCLES OF ENERGYBiogeochemical cycles and virtually all other process on earth are driven by energy for the sun. the sun acts as a so called blackbody radiator with an effective surface temperature 5780 K (absolute temperature each unit is the same as celcius degree, but with zero taken as absolute zero). It transmits energy to earth as electromagnetic. Radiation (see below) with maximum energy flux at about 500 nm, which in the visible region of the spectrum. A 1m2 area perpendicular to the line of solar flux at the top of the atmosphere receives energy as a rate of 1340 W, sufficient, for example, to power an electric iron. This is called the solar flux (see Figure 9.3).Light and Electromagnetic RadiationElectromagnetic radiation, particularly light, is of utmost importance in considering energy in environmental system. Therefore, the following important points related to electromagnetic radiation should be noted :Energy can be carried through space at the speed of light , 3.00 x 108 m/sec in a vacuum, by electromagnetic radiation, which includes visible light, ultraviolet radiation, infrared radiation, microwaves, radio waves, gamma rays, and x-rays.Electromagnetic radiation has a wave character. The waves move at the speed of light and have characteristics of wavelength (), amplitude, and frequency (v), as illustrated below