The ecosystems illustrated in figure 2-1 are contrasting types of sun powered ecosystems, and thus emphasize basic similarities and differences. A terrestrial ecosystem (illustrated by the field shown on the left) and an open – water aquatic system (illustrated by a lake or the sea as shown on the right) are populated by entirely different kinds of organisms, with the possible exceptions of a few kinds of bacteria that may be able to live permanently in either situation. Yet the same basic ecological components are present and function in much the same manner in both of ecosystem. On land, the autotrophy are usually rooted plants ranging in size from grasses and other herbs that occupy dry or recently denuded lands to very large forest trees adapted to moist lands. In deep water systems the autotrophs are microscopic suspended plants called phytoplankton ( phyto = plant; plankton = floating), which be long to several different classes of algae. The include: (1) the diatoms, tiny plants with silicon shells; (2) green flagellates that move about propelled by rapidly beating ship like flagella; (3)the green algae which may occur as single cells, colonies, or filaments of cells; and (4)the blue – green algae, some of which have gelatinous capsules and thrive on organic pollution, thus clogging public water supplies and creating nuisances in recreational lakes. As would be expected, shallow water ecosystems are occupied by mixtures of macroscopic plants and microscopic algae.

Because of size differences in plants, the bimass, or standing crop, of terrestrial and aquatic ecosystems may be widely different. Plant biomass in terms of grams of dry matter per square meter may be 10000 or more in a forest in contrast to less than 5 in a pond, lake, or ocean. Despite the size discrepancy, 5 g of tiny plants are capable of manufacturing as much food in a given period of time as are 10,000 g of large plants given the same quantity of light, minerals, and energy subsidies. This is because the rate of metabolism of small organisms is very much greater per unit of weight than that large organisms. Furthermore, large land plants are mostly composed of woody tissues that are relatively inactive; only the leaves are active in photosynthesis, and in forest leaves comprise only about 1 to 5 persent of the total plant biomass.

This is a good place to introduce the concept of turnover as a first step in relating structure to function in an ecosystem. We can think of turnover as the ratio of the standing state (that is, amount present) of biotic or abiotic components to the rate of replacement of the standing state. For example, if the biomass of a forest is 20,000grams per square meter (g/m) and the annual growth increment is 1000 g, then the ratio 20/1 can be expressed as a turnover time or replacement time of 20 wears. The reciprocal, that is, 1/20 = 0.05, is the turnover rate. In a pond the turnover time for phytoplankton would be measured in days rather than years.

It is particularly important to have information on turnover rates between biotic compartments when it comes to evaluating the impact of mineral nutrients or other chemical components in an ecosystem. It is more important to know how fast materials are moving along the pathways between organisms and environment than it is to know the total amount present. This, a soil might contain a large amount of phosphorus, but if it is not available to organisms, perhaps because it is in an insoluble form, then it might as well not be there. We have already made of man’s tendency to extract materials from the environment and return them to the environment in poisonous forms; man also, often inadvertently, returns them in unusable form. Then it is like the marooned in the middle of the sea – there is ‘’water, water everywhere, but not a drop to drink.’’