Nutrient inputs, catchments

A recent review of research on phosphorus input to surface waters from agriculture highlights the variability of particulate and dissolved phosphorus contributions to catchments. The input varies with rainfall, fertilizer application rates, the history of the application of the fertilizer, land use, soil type, and between surface and sub-surface water. The balance struck between export of nutrients from the catchment and recipient-water productivity is the primary factor which controls its quality. 

The Action plans will also include options for controlling nutrient inputs in the upstream catchments of affected stillwaters. These are likely to include reduction of nutrients from both point and diffuse sources and a range of different combinations aimed at reaching the target concentrations of nutrients required to achieve control in the receiving waters. 

In remote alpine areas where anthropogenic pollution is limited, the retention of nutrients associated with trapped particles may become apparent. Case studies in two Rocky Mountain lakes (Kootenay and Arrows) indicated a reduced biological productivity, which is not compensated - as is usually the case in the European Alps -by enhanced nutrient input due to anthropogenic activities in the catchment [5]. 

Figure 3.5 Correlation between nutrient inputs to the catchment and streams/rivers on a small regional scale. Nitrate concentrations in streams are shown to be directly related to fertilizer applications in croplands in the mid-Atlantic region of North America. (Modified from Jordan et al., 1997.)...

Different attempts have been made to estimate the nutrient inputs a century ago when the anthropogenic influence was very low. Larsson et al. (1985) have done one of the first estimates (Table 12.2). Their calculation suggests an increase in the total phosphorus loading by about eight times, and in total nitrogen load by about four times from the catchment area due to human activities. The authors used recent nutrient data from northern Swedish rivers as natural background conditions. From these data they concluded that the northern Baltic Sea is less affected by the increase in nutrient inputs compared to the rest of the Baltic Sea, which is more under anthropogenic influence. 

In all of the techniques which use artificial barriers to surface run-off of nutrients there is a need to consider the influence of land drains. If these are widespread in a catchment a reduction in nitrogen loading to the watercourses will be unlikely, because the nitrogen is predominantly dissolved and runs through the sub-soil to the drains. Phosphorus control by these barriers will be less affected by land drains because the main input of the phosphorus is in the particulate form which would be prevented from running off the surface to the watercourses. 

More generally, changing flow alters the relative proportion of input, storage, transfer, and transformation processes for organic matter and nutrients. Hence, the relative extent and the dynamics of the temporary channels within a catchment may control the capacity of a river network to produce, transform, and store nutrients and organic matter. 

Merseburger GC, Marti E, Sabater F (2005) Net changes in nutrient concentrations below a point source input in two streams draining catchments with contrasting land uses. Sci Total Environ 347 217-229... 

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