Aspects of nitrogen fixation in Lough Neagh. I. Acetylene reduction and the frequency of Aphanizomenon flos-aquae heterocysts

SUMMARY.

• 1 An investigation was made of the relationships between rates of nitrogen fixation (acetylene reduction), Aphanizomenon flosaquae heterocysts, NO₃-N concentration and other environmental factors which occurred in Lough Neagh between 1969 and 1972.
• 2 The maxima of cells differentiated as heterocysts (approximately 5% (1970), and approximately 7% (1971 and 1972) occurred when NO₃-N concentration was very low or undetectable.
• 3 Acetylene reduction was detectable between 16 May and 8 September 1972 but was not measured in the other years. Rates were greatest in the euphotic zone and it appeared that the nitrogenase activity was closely linked to a light dependent system.
• 4 Areal rates of acetylene reduction were highly correlated with Aphanizomenon heterocyst concentration (r=0.92). There was some indication that a release of phosphorus from the sediments might have stimulated Aphanizomenon nitrogen fixation on one occasion at the end of a calm period. Results show that the most likely conditions for the development of large crops of heterocystous species in Lough Neagh include a scarcity of NO₃-N and a plentiful supply of phosphorus.