δ15N as an indicator of N2-fixation by cyanobacterial mats in tropical marshes

Cyanobacterial mats (CBM) are important components of wetland ecosystems in limestone-based regions of the Caribbean. During two sampling periods (July 1999 and January 2000) we measured N2-fixation in samples from 23 different marshes simultaneously with measurements of relevant environmental factors. Samples were evaluated for abundance of five groups of cyanobacteria: (1) Leptolyngbya, (2) Oscillatoria, (3) Chroococcales, (4) Nostoc-& Stigonematales, and (5) dead sheaths. Differences in nitrogen fixation, expressed as nitrogenase activity in nmol C2H4 cm−2 h−1, were best explained by the proportion of heterocyst-forming cyanobacteria. The samples were analyzed for the natural abundance of δ15N. δ15N values ranged from −1.99 to 11.44 ‰ and were strongly negatively correlated with N2-fixation. With all data included, δ15N was also strongly correlated with nitrates in water. With the samples from Little Belize (high nitrate content marshes) excluded, the effect of nitrate became insignificant. N2-fixation predicted from δ15N measured on an independent data set from September 2000 was moderately accurate (r2 = 0.68, 0.52 and 0.54 for predictions based on July 1999, January 2000 and combined data sets, respectively). When individual sample sets were divided into two groups with δ15N < 2 and δ15N > 2, the two groups were always highly significantly different in terms of their N2-fixation. The presented evidence suggests that δ15N can be used as a reliable indicator of N2-fixation by CBM.