Nitrogen Turnover in Marine and Brackish Habitats II. Use of 15N in Measuring Nitrogen Fixation in the Field

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Abstract
A technique for measuring nitrogen fixation in situ by natural populations of blue-green algae is described. It involves exposing test samples of known area to a gas mixture containing molecular nitrogen enriched with 15N and measuring the rate of incorporation of the isotope over a standard 24-h exposure period. The accuracy of the method is not seriously affected by changes in pH, pCO2 and humidity which may occur during the exposure period, or by the degassing procedure used to remove air from the exposure flask prior to introduction of the isotope. Temperature and pN2 values inside the exposure flask are different from those to which natural populations are exposed outside, and corrections for these have to be made in calculating the final results. The minimum pN2 which allows optimum fixation by Calothrix scopulorum in the presence of 0.2 atm. oxygen is 0.4 atm. In an area of the supra-littoral fringe of a rocky shore and in an area of sand dune-slack over a 12-month period nitrogen fixation is high in spring and autumn and negligible in winter. On the rocky shore fixation is low in summer; in the dune-slack summer nitrogen fixation is erratic. Nitrogen-fixing efficiency in terms of nitrogen fixed per unit weight of test material is high when algal recolonization is occurring. The mean fixation rate per annum corresponds to approximately 2.5 g/m2/annum for the rocky shore. The nitrogen fixed per annum represents approximately 41 per cent and 21 per cent of the mean total nitrogen present per annum on the rocky shore and in open areas of the sand dune-slack respectively.

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