The animals living on and in the sediments and large plants of lakes are usually highly diverse. Much emphasis in the study of benthic fauna has been given to the immature stages of insects that often make up the dominant part of the total animal biomass of these habitats. Nearly all insect orders are represented. Some orders of insects are entirely aquatic; others inhabit fresh waters only during certain life stages. Segmented worms (oligochaetes and leeches), microcrustacea (ostracods), and macrocrus-tacea (mysids, isopods, decapods, and amphi-pods) often form major components of benthic fauna of fresh waters.
The sediment composition and characteristics of the water adjacent to the sediments are also highly variable. Attached microflora (bacteria, fungi, and algae) may occur in great abundance on sediments. These organisms and associated detrital organic matter often provide the predominant energy sources for the benthic fauna. Consequently, a variety of types of feeding and of reproductive dynamics occur among the benthic fauna. It is, therefore, essential in limnological analyses to obtain reliable quantitative estimates of the sizes and distributions of populations constituting these communities.
Quantitative estimations of benthic fauna require effective sampling procedures, separation of the organisms fjrom the substratum, identification, and evaluation of biomass of species and of their life history stages. Methods to accomplish these tasks are not totally satisfactory, and the taxonomy of many immature stages of benthic invertebrates is difficult and incompletely understood. Nonetheless, in spite of these problems, often it is important to analyze the response of population growth and survival and physiological characteristics (e.g., respiration, excretion, and assimilation) of the benthic fauna to environmental variations, such as temperature, oxygen concentrations, and food quantity and quality.
The living biomass (g/m2) of a population of animals at an instant in time and in a given habitat represents the net result of reproduction and growth and the opposing processes of loss (respiration, predation, mortality, emigration, and so on). Although the measurement of biomass gives an estimate of the extent of population development, from these data alone nothing can be said about the growth and reproduction of the organisms. More information is needed to evaluate production rates (g/m2/time). This information can be obtained only by detailed analyses of reproductive and physiological characteristics or survivorship and growth.
There are some general direct correlations between the overall productivity of fresh waters and benthic animal productivity [cf., summary of Wetzel (1999)]. Moreover, some general insight into benthic faunal composition and distribution in relation to lake characteristics can be obtained from simple quantitative analyses. The following exercise is directed toward the understanding of problems of sampling, sorting, and quantitative estimates of population size. Obviously, much more detailed investigations would be needed to determine secondary productivity and its control in research efforts. The monographs of Edmond-son and Winberg (1971), Winberg (1971), Brinkhurst (1974), Waters (1977),Benke (1984), and Rigler and Downing (1984) are excellent critical compilations on methods and are highly recommended sources for further study.
KeywordsBiomass Sugar Formaldehyde Respiration Shrinkage
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