Temporal Patterns in River Export of Nitrogen

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Increases in river N export over time often correlate relatively well with increases in N inputs to watersheds. However, the response of river N export to decreases in watershed N inputs is not necessarily as rapid or well understood. In at least one case, the Patuxent River (MD, USA), point source reductions in N resulted in rapid reductions in N export (Boynton et al., 1995 Jordan et al., 2003). However, N export is generally dominated by non-point sources, not point sources (Carpenter et al., 1998 Dumont et al., 2005 Seitzinger et al., 2005b others), so substantially reducing N export often requires a reduction in non-point N inputs. Reductions of non-point nutrient inputs have been attempted in a number of systems, but response of river N export to these reductions is often much slower and less predictable than responses to reductions in point sources (Stalnacke et al., 1999). 

The response of coastal ecosystems to river nutrient inputs depends, in part, on the seasonal pattern of the N dehvery, including seasonal changes in the form of N delivered (e.g., DIN, DON, and PN). Most studies of river nutrient export only report annual rates and often only report TN export. Where seasonal patterns of N export by individual river basins have been reported, a number of patterns have been observed, and the different forms of N can exhibit different patterns of export (e.g., Alexander et ah, 1996 Eyre and Pont, 2003 Lesack et ah, 1984) (Figs. 9.9 and 9.10). Seasonal patterns are observed in both N concentration and in N export (Fig. 9.10). 

Tropical storms, typhoons, or hurricanes, depending on their strength and geographical context, often deliver torrential rains. Increased river flow during such high intensity, low frequency rainfall events can transport a major portion of annual river N load to coastal systems. Historically, there has been a paucity of data on such events due to their irregular and extreme nature and the generally manual nature of water sample collection for nutrient analyses. However, with the use of in situ automatic water samplers and nutrient analysis systems, data from such events can now more readily be captured. 

In arid regions the majority of water and nutrients in un-dammed rivers can be exported during a few major storms. In a study of 7 subtropical, Australian rivers, approximately 75% of the annual TN load was delivered in 20% of the time (two 10-day flood periods during 1996 Eyre and Pont, 2003). This contrasts with a number of temperate systems in which it takes 23—52% of the time to deliver 75% of the annual TN load (Eyre and Pont, 2003). Different N forms also exhibited different patterns of export in this study of Australian rivers, with DON typically accounting 

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