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 N-2-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 delta(15)N. delta(15)N values ranged from - 1.99 to 11.44 parts per thousand and were strongly negatively correlated with N-2-fixation. With all data included, delta(15)N 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. N-2-fixation predicted from delta(15)N measured on an independent data set from September 2000 was moderately accurate (r(2) = 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 delta(15)N < 2 and delta N-15 > 2, the two groups were always highly significantly different in terms of their N-2-fixation. The presented evidence suggests that delta(15)N can be used as a reliable indicator of N-2-fixation by CBM.