Biogeochemical Significance of Marine Nitrogen Fixation

with comprehensive data from more spatially extensive surveys, the direct estimate for open ocean planktonic N2 fixation far exceeds that in sediments and rivals terrestrial biological rates. For instance, a recent accounting of N2 fixation at 154 stations from 6 cruises in the tropical Adantic yielded a conservative annual input of at least 1.6 x 10 mol N (22 x 10 g) by Trichodesmium alone (Capone et al, 2005) (Table 4.3). 

Several reports in varied locations have now provided quantification of N input in the small size fraction of the plankton. Montoya ei a/. (2004) found highly variable but at times, extremely high, rates of N2 fixation in studies in the sub-tropical N. Pacific and along the north coast of Australia (Tables 4.4 and 4.5). Average rates (excluding one extreme value) for 10 stations on the North Pacific transect north of the Hawaiian Islands were about 520 pmol N m day. Curiously, rates were considerably and consistently lower at station ALOHA in the vicinity of the Hawaiian Islands. This was also concordant with an earlier study (Dore et al., 2002) and another recent effort that employed plankton concentrates (Falc6n et al., 2004). 

As mentioned, diazotrophic diatom associations (DDAs) can, at times, be of quantitative significance. A bloom of the endosymbiont Richelia intracellularis within 

Fixation by in Unconcentrated Water Samples of Plankton with and without 

Source Location Notes nmolL h nmolL h nmolL h Source 

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