Hidrológiai Közlöny 1967 (47. évfolyam)
7. szám - A „Szervesanyag meghatározási problémák édesvizekben” című 1966. szeptember 25–28. között Tihanyben rendezett Szimpózium előadásai - Wetzel, Róbert G.: Oldott organikus vegyületek két indiánai tóban
Hidrológiai Közlöny 1967. 7. sz. 298 Dissolved Organic Compounds and Their Utilization in Two Mari Lakes* RÓBERT G. WETZEL" Abstraet A substrate dilution technique, using C 1 4 labelled organio compounds and based on Michaelis-Menten enzyme kinetics, was employed to assay the in situ bacterial metabolism of glueoSe, fructoSe, galactose, and acetate. Mathematical and graphieal analyses of the culture data permitted an eStimation of (1) natural substrate concentrations (fig 1(2) the maximum velocity of substrate uptake (mg l1 hr1), and (3) the turnover time of substrate regeneration (hrs). Algal utilization of the organie substrates assayed was extremely low and followed simple diffusion kinetics. Effective heterotrophic utilization by the algae was not evident. SeaSonal and verticai variations in the distribution and utilization rates of these compounds were followed for six months in two calcareous lakes of northeastern Indiana as a portion of investigations on the photosynthetic rates and nutrient factors regulating growth. Natural substrate concentrations of glueoSe and acetate exhibited a progressive decreaSe from mid-Summer to winter; those of fructoSe and galactose increased during the same period. Heterotrophic growth capacity, as indicated by substrate uptake velocities, changed little from Summer to winter for acetate, fruetose, and galactose; bacterial utilization of glueoSe deereaSed in the colder season. Substrate turnover times of the carbohydrates were large and increased towards winter; those of acetate decreased during this interval. During thermal stratification, levels of glucose, galactose, and acetate increased markedly in or slightly below the metalimnion, and decreased in the upper hypolimnion. Glucose levels increased in the lower hypolimnion near the sediments. Velocity of bacterial uptake of glucose and acetate commonly increased in the metalimnion, decreased in the hypolimnion, and increased sharply in the lower hypolimnion. Turnover times generally were inversely correlated to velocity determinations with depth. Regeneration times of acetate decreased markedly in the lower hypolimnion during thermal stratification. All parameters were distributed uniformly during vernal overturn. Introduction Highly calcareous lakes of the mid-continental region of the United States are characterized by moderate to low productivity. Particularly at he primary productivity le vei, an apparent disparity exists between the in situ rates of photosynthesis and the potential productivity of these waters. Initial investigations into the causal relationships involved have indicated that the low growth rates are intimately associated with the existing inorganic and organic biogeochemical cycles (Wetzel, 1965, 1966a, 1966b, 1966c, in preparation). Absolute deficiencies of nutrients within the calcareous, mari lakes and surrounding watersheds do not generally occur and are not the causal factors limiting photosynthetic metebolism. Rather, the lakes exhibit condiditons of physiological unavailability of nutrient components, particularly essential metals such as iron and manganese (c/. * Contribution No. 000, W K.. Kellogg Biological Station, Michigan State University and Contribution No. 000, Department of Zoology, Indiana University. ** W. K. Kellogg Biological Station, Michigan State University. Hickory Corners, Michigan 49060. Schelske, 1962; Wetzel, 1966b, 1966c). Dissolved organic compounds can function directly, e. g. as biotics, growth substances, and in chemo-organotrophy, or indirectly, e.g. in chelation or other complexing processes that affect the availability of metals and influence monovalent to divalent cation ratios. The experimentál results of the effects of synthetic and natural chelating and other complexing agents on metabolism of natural phytoplankton populations of several lakes of northern Indiana demonstrated that dissolved organic compounds can function, among other ways, in increasing the availability of inorganic nutrients. In the course of these investigations a large difference was found between the in situ annual rates of photosynthesis in interconnected lakes of similar chemical composition (Wetzel, 1966b). Evidence indicated that the causal factors were, in part, associated with differences in dissolved organic matter. At the time Drs. J. Hobbié and R. Wright, then of the Institute of Limnology, University of Uppsala, were developing metliods based on enzyme kinetics for analyzing quantitatively in situ substrate levels of simple organic compounds and their rates of utilization by algal and bacterial populations. Their techniques were employed by the author for a six-month period in two interconnected lakes to determine the quantitative differences of several organic compounds. Although the analyses represented only a small portion of the experimentál investigations and data are few, significant patterns of distribution of organic compounds did emerge from this investigation. More detailed studies are underway to firmly substantiate such relationships. Methods Techniques employed for the measurement of dissolved organic substrates and chemo-organotrophic utilization by plankton have developed rapidly in the past few years. Stimulated by the work of Parsons and Strickland (1962), Wright and Hobbié improved greatly the bioanalytical methods for the determination of heterotrophic utilization of simple compounds based on Michaelis-Menten enzyme kinetics. Their techniques are presented in detail (1966) and were essentially those employed in this work, except for minor modifications. A brief résumé of the methods used follows: Samples of laké water containing natural plankton populations were collected with a nonmetallic Van Dorn sampler and retained at the temperature of collection. The samples from each depth were promptly (-< 30 minutes) distributed into a series of twelve 50 ml cultures in opaque Pyrex bottles. Serially increasing amounts of organic substrate (both non-radioactive and the
