Biological nitrogen fixation in a nitrogen limited tropical lake, Lake Chapala, Mexico.

Abstract

The objective of my study was to quantify the rate of biological nitrogen fixation in Lake Chapala, Mexico. Lake Chapala is the largest lake in Mexico. It is an ancient, tropical lake which is shaped in an east-west culde-sac with the inflow and outflow at the eastern end. Previous work showed low available nitrogen leading to nitrogen limitation of primary production. These measurements of available nitrogen ranged from undetectable to 1.8 _ - | mg 1 on an east-west gradient of sampling stations throughout the year. Nitrogen levels were lowest at the western end of the lake. Biological nitrogen fixation rates were expected to increase as the ambient nitrogen decreased. However, biological nitrogen fixation, measured by the acetylene reduction technique, was negligible at all stations. A reverse from the expected gradient of nitrogen fixation was found in the water from Lake Chapala when mixed with known nitrogen-fixing cyanobacteria. Greater nitrogen fixation by the added cyanobacteria was found with greater available nitrogen in the water. Also, the rate of nitrogen fixation by the introduced cyanobacteria increased when the water from Lake Chapala was filtered. This increased rate i i i of nitrogen fixation by the introduced cyanobacteria was attributed to the removal of an inhibitor within the water of Lake Chapala, Mexico. The lack of nitrogen fixation in Lake Chapala was attributed to the lack of filamentous cyanobacteria. There are three factors which may have inhibited the growth or functioning of cyanobacteria: 1) The high winds mixing this shallow lake inhibited growth of chains and mat formation of cyanobacteria. 2) Wind-induced turbidity contributed to light inhibition of cyanobacteria in the water column and sediments. 3) Organism on the suspended particulates or chemical compounds adsorbed to the suspended particulates may have been an inhibiting factor.