Phosphorus dynamics observed through increasing scales in a nested headwater-to-river channel study

Abstract

The hypothesis that the dynamic patterns of phosphorus (P) transport at plot scale are similar to the patterns that could be observed quasi-simultaneously (i.e., approximately at the same time) at a river basin scale, in terms of inputs and dilution of P, across a range of rainfall and runoff conditions, was tested. From this information, it was hoped to be able to make some simple inferences about the connectivity or mass flux of P transport between the different scales of observation. An intensive study using 30-m2 plots, 1-ha plots and nested river channel sites ranging in scale from 20 ha up to a maximum of 834 km2 in the River Taw basin, South West England, UK, was conducted with three campaigns under differing flow conditions: (1) a zero rainfall base flow period, (2) a 10-mm rainfall residual flow period, and (3) a 42-mm rainfall storm flow period. The mass flux of total P ranged from 49 kg during base flow to 4 tonnes during the storm period at the largest 834 km2 scale. During base flow conditions, total phosphorus (TP) concentrations from diffuse sources were low (26 microg L-1 in the upper catchment) and reactive P forms dominated the fractions filtered <0.45 microm. During storm flow, concentrations of TP increased at all scales within the drainage basin, to a maximum of 500 microg L-1 and were sufficient to override the effect of any point source inputs of P. Unreactive (i.e., mostly 'organic') forms of P dominated the fractions that were >0.45 microm during residual flows and storm flows. Spatially normalised discharge apparently decreased with increasing scale, most notably during storm flow conditions and this implies that there is some storage of water through the catchment and in part may reflect varying velocities of water in different pathways. Most attenuation and dilution of P appeared to occur at larger scales, whilst the hydrological connectivity between source areas and receiving waters was greatest at smaller scales (<20 ha), and during the highest flows. The importance of diffuse agricultural sources in contributing to P export through the basin was dominated by dynamic temporal changes in hydrological activity, with a 'piston pushing' effect of particulate associated P through the basin as it wets up in response to rainfall input. We concluded that the processes of P transfer are different at different scales. However, some uncertainties of spatial heterogeneity around the catchment underlie the difficulties in dealing with scale and thus more data and studies of this nature are required.