The light response of nitrogen fixation in Lake Valencia, Venezuela’

Water samples from six depths in the upper water column of Lake Valencia, Venezuela, were incubated in situ with acetylene at several depths corresponding to different light exposures. On five dates there was sufficient nitrogen fixation to define the light-response curve for samples taken at 2 and 100 cm. The light response of nitrogen fixation was modeled successfully with an equation previously developed for the light response of photosynthesis. All parameter values of the response curves for samples originating at a given depth were relatively stable across dates, but differed between depths. Rates of increase of N fixation with irradiance in the subsaturation range (a) were much lower than is typical of photosynthesis. While the general shape of nitrogen fixation curves was similar to that of photosynthesis curves, critical parameters were more sensitive to the light history (depth of origin) of cells than would be expected for photosynthesis. Light-response curves for phytoplankton photosynthesis have been useful as a basis for modeling primary prcduction under natural conditions (e.g. Fee 1969; Bannister 1974). The parameters that describe the shapes of such curves can be interpreted physiologically and thus facilitate comparison of field populations between lakes or between seasons in a given lake (Harris 1978). Light-response curves for nitrogen fixation should offer the same advantages but have not yet been well defined for natural populations. It has been generally established that there is a correlation between light and nitrogen fixation, and that inhibition can occur at high n-radiances (e.g. Dugdale and Dugdale 1962; Goering and Neess 1964; Horne and Fogg 1970; Flett et al. 1980). A more detailed and extensive knowledge of the shapes of light response curves for N fixers in situ would be useful, however. We provide here information on the light response of nitrogen fixation in situ by heterocystous blue-green algae in Lake Valencia, Venezuela. We thank S. StadlerMorris and M. Benson for field assistance, B. Chronic for heterocyst counts, and P. Brezonik, A. Home, and T. Platt for their comments on the manuscript. 1 Supported by NSF grants DEB 78-05324 and 8003 88 3 as part of the Venezuelan-North American Lake Valencia Project. * Present address: Ecosystems Research Center, Cornell University, Ithaca, N.Y. 14853. Methods and study site Lake Valencia is described briefly by Levine and Lewis (1984) and more extensively by Lewis and Weibezahn (1976) and Lewis ( 19 8 3). Light response determinations were done on samples originating at six depths on 29 different dates between December 19 80 and December 19 8 1. Until 13 October 198 1, samples to be used in studies of nitrogen fixation were collected at the index station near the center of the lake. After this they were collected over deep water near Isla Otama, a large island midway between the index station and the south shore (map reference: Lewis 19 8 3). Because nitrogen fixation was too low to meet minimum requirements for a study of light response except for samples originating near the surface between September and December, our analysis is limited to the data taken on five dates between September and December 198 1 for samples originating at depths of 2 and 100 cm. These dates coincide with the last portion of the seasonal stratification, when nutrient depletion is most pronounced. At the beginning of each study, water was pumped from depths of 2 and 100 cm into dark bottles. Since the pump would draw water from an area of several centimeters, the samples should be considered to have originated only in the close vicinity of the nominal depth. Samples were returned to shore and held in a dark box except when