EXPORT OF PHOSPHORUS FROM DARTMOOR CATCHMENTS - MODEL TO EXPLAIN VARIATIONS OF PHOSPHORUS CONCENTRATIONS IN STREAM-WATER

The export of total phosphorus from the moorland streams of Dartmoor was investigated. To prepare water samples for analysis two digestion techniques were compared. Digestion by ultraviolet light give much less variable results than digestion with perchloric acid, but gave total phosphorus values 20% lower. Total phosphorus concentration in streams is strongly influenced by discharge. It increases during floods, more during ascending than descending stage, and more during summer than winter. Most of the increase during floods is due to particulate phosphorus, which varies during a flood almost exactly as suspended solids. In a variety of streams, 0.2-0.25% of the dry weight of suspended solids is phosphorus. During late descending stage and periods of base flow, soluble phosphorus predominates, its concentration being highly correlated with water colour and discharge. The temporal changes in phosphorus concentration are explained by postulating accumulation of soluble phosphorus on the stream bed during periods of base flow and resuspension of particulate phosphorus during floods as well as by postulating varying relative contributions of shallow and deep through flow to discharge as discharge changes. Deep throughflow is colourless and low in total phosphorus which is predominantly orthophosphate. Shallow throughflow is highly coloured, and contains a high concentration of phosphorus bound to humic colloids. Experimental data consistent with these hypotheses are presented. Some data are consistent with the hypothesis that a stream with a steep gradient, flowing over coarse growan, rapidly exchanges its water with that in a large subsurface reservoir. Annual export of phosphorus from a grazed granitic catchment is 19-9 mg m-2 yr-1. Although this value is within the range predicted by Dillon & Kirchner (1975) it is shown that methods used by them and others to measure export of phosphorus from this type of catchment can seriously underestimate true export. © 1979, Marine Biological Association of the United Kingdom. All rights reserved.