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Energy transfer in an autotrophic-based ecosystem : with special reference to effects of fertilizer nitrogen additions

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  • Three aspects of lotic ecology have been analyzed utilizing data collected from totally autotrophic-based streams maintained by the Weyerhaeuser Company: energy transfer, effects of nitrogen addition, and simulation modeling. All three streams had very similar periphyton, grazer, collector, invertebrate predator, and vertebrate predator (fish) communities during the study period (1972-1974). Calorific determinations were made on the most important taxa of each lotic system. Indirect methods of converting data into forms usable for energy investigations appeared to work satisfactorily. Approximately 9000 kcal/m² of gross primary production occurred in each stream during 1973. Losses of this input averaged: mortality and decomposition (eventually respiration), 35 percent; direct periphyton, invertebrate and fish respiration, 30 percent; export, 29 percent, FPOM processing, 4.5 percent; and insect emergence, 0.5 percent. Accumulation figures were: periphyton, 0.5 percent; and fish yield, 0.5 percent. Primary and community P/R ratios averaged 4.2 and 1.5 respectively. In its most realistically estimated form, energy output exceeded input by 1.2 percent. Effects of fertilizer nitrogen on aquatic plants, invertebrates. and especially fish were tested for about two years. In one stream, the input was constant; in the other, nitrogen entered only during periods of precipitation. The third stream was maintained as a control. Gains in periphyton biomass and primary production were temporary after nitrogen input, and lasted about six months. Periphyton biomass variations between streams were regulated primarily by invertebrate consumption. Invertebrate standing crop and production, in turn, was influenced most by differences in drift rates. Trout fed almost entirely on invertebrates in the water column. Fish production appeared most closely correlated to the portion of the drift containing larger invertebrates. In analyzing all trophic levels, no direct or indirect nitrogen influence on fish production could be found. Utilizing input data from the streams, computer-based lotic ecosystem modeling has been accomplished for over four months. Primary objectives of this portion of the study have been. (1) to accurately simulate trophic dynamics in an autotrophic-based stream and (2) upon stabilization of the system, manipulate relationships between trophic levels in order to understand how aquatic ecosystems function. Though a reference simulation has yet to be produced, one should be forthcoming by the end of the year. Much progress has been made in improving the analysis capability of the model.
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