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MATERIALS AND METHODS
At the 5-km 2 catchment outlet, a Dr Lange Sigmatax and Phosphax suite of
instrumentation was installed to extract and analyse a water sample for TP on a
10-min cycle. The Sigmatax extracts a sample from the stream water column using
positive air pressure and pumps into a 100-mL homogenisation chamber. This ultrasound
homogenisation cycle is included to break up larger organic particles prior
to digestion. A 10-mL aliquot is delivered to a digestion chamber where the sample
is reduced by boiling using the sulphuric acid–persulphate method. Phosphorus is
determined photometrically using the molybdate antimony–ascorbic acid method
and conforms to DIN EN 38405 D11. Data are stored by internal and external
dataloggers and downloaded by telemetry. A weekly cleaning cycle is performed to
ensure that intake tubing is free from detritus. Reagents are changed every 3 months.
Concurrent with the TP monitoring, stream discharge and rainfall data are collected
by a Starflow depth–velocity recorder on a 15-min cycle, and an ARG100 tipping
bucket rain gauge set to measure events on a 5-min cycle, respectively. In this paper,
all data are averaged to a 1-h resolution for comparison.
RESULTS
Results are presented for April to October 2005 and incorporate several large storm
events and periods of baseflow (Figure 1). Three TP transfer ‘event-types’ were
observed during this period. First, large TP transfers were coincident with storm
discharges, although lower TP peaks were observed with succeeding storms and
TP depletion during storm events occurred prior to discharge peaks. This indicates
periods of P flushing where build-up from, for example, channel attenuation occurs
during smaller events. This acute diffuse transfer is likely to be from a combination
of soil P and channel sediment P sources. Second, as baseflow established over the
summer period, non-storm TP transfers became greater, indicating a concentration
effect of one or more chronic point sources. Furthermore, during periods of zero
rainfall, these chronic TP transfers had a diurnal pattern with a recovery between
23:00 and approximately 11:00 hours each day followed by a rapid increase that was
sustained for the rest of the day (Jordan et al., 2005b). The third ‘event-type’ was
characterised by sudden TP increases that did not correspond with similar increases
in stream discharge. These events were attributed to discrete pollution episodes
unrelated to storm diffuse or chronic point sources.
DISCUSSION
The identification of three TP transfer ‘event-types’ in an agricultural mini-catchment
was only possible by implementing a high resolution, time-based sampling regime, in
this case using a bank-side automated analyser. Even with the data limited to a single
summer, the results have important implications for monitoring and management
of freshwater eutrophication in rural rivers. The diffuse, storm dependent transfers
accounted for most of the TP load transferred from this grassland catchment. The
chronic and discrete TP transfers that were independent of stream discharges did,
however, present an ongoing eutrophication impact and maintained the stream
system in the hypertrophic range for the summer of 2005. This is a pattern observed
in many Northern Ireland rivers. In the absence of waste-water treatment works
and other industrial point sources, it will be important to define and mitigate these
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