Iron, Molybdenum and Phosphorus Limitation of N2 Fixation Maintains Nitrogen Deficiency of Plankton in the Great Salt Lake Drainage (Utah, USA)

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Verhandlungen der Internationalen Vereinigung für Theoretische und Angewandte Limnologie



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The factors that directly and indirectly restrict the productivity of the world's lakes, streams, and oceans have been studied for decades, but no clear picture has emerged that explains when and why particular nutrients will limit algal growth. In many lakes phosphorus often controls phytoplankton production (SCHINDLER 1977), but numerous freshwater and marine systems are frequently limited by supplies of nitrogen or micronutrients. A priori, we would not expect nitrogen to become limiting because cyanobacteria and other nitrogen fixing organisms can utilize the vast supply of N2 gas to produce ammonia (SCHINDLER 1977). However, systems chronically limited by nitrogen are frequently observed, suggesting that some factor(s) is restricting the fixation of N2.

Four hypotheses have been suggested to account for the limitation of nitrogen fixation in aquatic systems. First, STEWART et al. (1970) and SCHINDLER (1977) have demonstrated that supplies of phosphorus can limit N2 fixation in some lakes. Secondly, iron, a component of the nitrogenase enzyme system, has been indicated to limit N2 fixation in lakes (WURTSBAUGH & HORNE 1983, WURTSBAUGH et al. 1985). RUETER (1982) has suggested that N2 fixation in the oceans may also be limited by iron. Thirdly, the uptake of molybdenum, another metal necessary for nitrogenase, has been suggested to limit fixation in an estuarine system (HOWARTH & COLE 1985). Finally, PAERL et al. 1987 have provided evidence that the addition of organic matter (sugars) to natural waters promotes the formation of anoxic microzones in bacterial aggregates; this, in turn, allows oxygensensitive N2 fixation to increase.

Here I report on factors that control algal production in three contrasting lentic systems located in the Great Basin of North America. I first demonstrate that algal growth in these systems is directly limited by supplies of nitrogen, and then show that phosphorus, iron and molybdenum may all be important in regulating nitrogen fixation.

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