7 Implications
While we appear to be making strides in our ability to derive global estimates for marine N2 fixation, we have a long way to go before we understand the role of diazotrophy in the context of N and C dynamics in the ocean. Because many direct estimates of global N2 fixation are based on highly spatially, temporally, and physiologically limited and variable data, and because many geochemical estimates rely on stoichiometric relationships of nutrient standing stocks without considering the imbalances between rate estimates of C and N2 fixation, we should proceed cautiously when inferring one from the other (see examples in Sect. 3 above). Based on the observed C drawdown from the atmosphere, we may be trying to find “too much” N2 fixation if we use Redfield stoichiometry versus the observed relative rates of C and N2 fixation. Further, because we know so little about the physiology of marine diazotrophs, it is difficult to model the contribution of new production from diazotrophy in the present, past, or future ocean where conditions vary in space and time.
It is also difficult to determine the effect of N2 fixation on system trophic status. In some systems Trichodesmium appears to fuel primary productivity and make the system more autotrophic (e.g., Tseng et al., 2005). In other systems dominated by Trichodesmium, heterotrophic processes appear to dominate (Fig. 2; also see Sect. 6.3 above). For example, it was observed that along the Kenyan coast, primary productivity, even during Trichodesmium blooms, could barely sustain the observed bacterial productivity (Kromkamp et al., 1997).
The preponderance of filaments versus colonial morphology can also seriously bias our understanding of trophodynamics associated with Trichodesmium and the net outcome of elemental cycling (e.g., recycling and respiratory losses versus export). Not only does colony size affect colony-specific estimates of N and C fixation, but the degree of aggregation and size of colonies also affects the degree to which Trichodesmium is colonized by other organisms and thereby recycled via bacteria, grazers, and other enzymatically-mediated processess. Free filaments often dominate populations in the Pacific (Saino and Hattori, 1978, 1980; Letelier and Karl, 1996; Tseng et al., 2005) and the Atlantic, at least seasonally (Orcutt et al., 2001) although in many systems, colonies appear to be more common (Capone et al., 1997; Carpenter et al., 2004). Better common metrics need to be employed to express rate measurements not only because Trichodesmium morphologies vary, but also because we now know there are a variety of non-Trichodesmium diazotrophs. Finally, different diazotrophic groups may have different fates and we know even less about non-Trichodesmium marine N2 fixers. Diatom/Richelia assemblages may be prone to gravitational settling while unicellular cyanobacteria may be more readily grazed. There are a variety of conflicting reports based on isotopic and geochemical tracers suggesting that diazotrophic growth fuels export production versus remineralization. The absence of robust direct estimates of both, hinder our ability to speculate regarding the fate of new N from diazotrophic growth.
Acknowledgements. Funding for this project was provided by the National Science Foundation, grants OCE-0095923 and OCE-0136367 to MRM. The author also wishes to thank M. Voss, W. Naqvi, and an anonymous reviewer for their comments on earlier versions of this manuscript, the organizers of the Significant Processes, Observations and Transformations in Oceanic Nitrogen (SPOT-ON) workshop (M. Voss, S. W. A. Naqvi, and J. P. Montoya) for the opportunity to contribute to the workshop and this special volume, and C. Dupuoy and A. Leboutellier for their invitation and support to participate in field work in New Caledonia.
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