Re-Evaluation of Nitrogen Limitation and New Production in the Sea

We now recognize that NOs is not always the dominant source of new N in the open ocean (Table 7.1). N2 fixation is more widespread than previously thought. Filamentous diazotrophs (e.g., Trichodesmium sp. Capone et al., 1997), diazotrophic endosymbionts (Carpenter et al., 1999), and unicellular cyanobacteria aU fix N2 at high rates in nature (Carpenter and Capone, this volume Montoya et al., 2004 Zehr etai, 2001). In tropical and subtropical waters, it has been calculated that N2 fixation by Trichodesmium sp. alone fuels 50% of the new production (Capone et al., 2005 Karl et al., 1997). Montoya et al. (2004) estimated that diazotrophic uniceUs may contribute up to 10% of total oceanic new production globally. 

Both reductive and oxidative pathways can produce NOa in the ocean, and until recently it was beUeved that oxidative pathways dominated. SpecificaUy, it was 

Inputs of new N into coastal systems are more diverse than inputs to the open ocean and include compounds considered regenerated N sources in the open ocean. New N can be dehvered by rivers, runoff events, and atmospheric deposition and can include a whole spectrum of N compounds including NH4 , urea, and DON (e.g., Anderson et al., 2002 Glibert et al., 2001, 2005c MuIhoUand et al., 2007). Similarly NH4 derived from natural processes and anthropogenic activities can support new production in estuarine and coastal systems (see Chapter 9 by Seitzinger and Harrison and Chapter 11 by Paerl and Piehler, this volume). 

Another consideration when calculating f-ratios is the diurnal variabUity of N versus carbon (C) metaboUsm. When relating new production to C export (sensu Eppley and Peterson, 1979), stoichiometric calculations of C export from N uptake can be in error when C and N uptake are not evaluated over appropriate timescales. Diel variability in uptake rates of various N compounds has been demonstrated (e.g., Cochlan and Harrison, 1991b Cochlan et al., 1991a Tremblay et al., 2000 Wilkerson and Grunseich, 1990) and faUure to integrate daUy uptake rates over an entire diel cycle may seriously bias estimates of f-ratios. 

Bury et al. (2000), on the other hand, found uncoupling between N and C uptake on longer timescales. Similarly, C and N2 fixation rates appear to be uncoupled or stoichiometricaUy different from particulate C N ratios (MulhoUand et al., 2007 MuUioUand and Capone, 2000 MulhoUand et al, 2006) making it difficult or impossible to estimate C export from N2 fixation. 

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