Elizabeth J. Sutton, Helen P. Jarvie, and Richard J. Williams Centre for Ecology & Hydrology, Wallingford, Oxfordshire OX10 8BB. UK ejsu@ceh.ac.uk
Introduction
The links between ecological status of lowland rivers and diffuse and point-source nutrient inputs are of key scientific importance under the EU Water Framework Directive (WFD) (EU, 2000). Many previous studies on the sources and effects of phosphorus in rivers have been based on weekly, storm event or even monthly sampling. However phosphorus concentrations are sensitive to a wide range of short-term dynamics and diurnal cycles that are not captured by conventional sampling programmes. Hence phosphorus loads may be under estimated and valuable information about the variability in P concentrations and links with in-stream processes and ecology may be missed. In this study, we use in-situ ‘continuous’
(hourly) measurements of total reactive phosphorus (TRP; unfiltered molybdate-reactive P), chlorophyll, dissolved oxygen (DO), conductivity, turbidity and flow to examine dynamics in P concentrations and sources and ecological status for the River Kennet, a lowland chalk river in south east England.
Methods
TRP measurements were taken at hourly intervals and analysed by an in-situ Nutrient Probe Analyser (NPA, Systea, Italy). Dissolved oxygen, pH, conductivity, turbidity and chlorophyll measurements were also taken at hourly intervals and analysed using a YSI6600 multiparameter probe. pH was used to calculate excess carbon dioxide pressure (Neal et al.,1998), and diurnal variations in dissolved oxygen were used to estimate rate of photosynthesis and respiration, by the methods of Williams et al. (2000). These parameters were examined alongside hourly river flow measurements.
Influence of point and diffuse sources on TRP dynamics
Seasonal, diurnal and storm event patterns in TRP concentrations were observed.
Daily average TRP concentrations were typically below the ‘threshold’ concentration of 100 µg/l (delineating the boundary between ‘Moderate’ and ‘Good’ ecological status (UKTAG, 2006)) for most of the year, with daily average TRP concentrations of c. 60-80 µg/l. However daily average TRP concentrations increased to between 120-150 µg/l during the summer, and in October and November 2005. There was a close temporal linkage between the time series of daily average TRP concentrations and river flow: TRP decreased as river flow increased, as a result of dilution of point-source P inputs.
Diurnal variability of TRP was typically 30-50P/l, but reached as much as 150 µg-P/l over the summer months (Figure 1). Daily TRP maxima occurred at around 14:00h. A double diurnal pattern was seen during the summer months, with a secondary peak occurring at 02:00h. This reflects typical patterns in domestic water usage and hence the diurnal signal in TRP is most likely due to the daily variations in effluent discharged from a sewage treatment works (STW) located approximately 3km upstream of the monitoring station. Sharp increases in TRP concentrations above the diurnal concentration patterns corresponded with storm events in the flow hydrograph, as well as peaks in turbidity and either peaks or dips in conductivity.
These event-related increases in TRP concentration are attributed to diffuse sources, either from P-associated sediment transport from the land surface, and/or in-stream resuspension of sediment.
Total Reactive P (ug-P/l)
150 175 200 225 250 275 300
17/6/2005 (0h) 17/6/2005 (12h) 18/6/2005 (0h) 18/6/2005 (12h) 19/6/2005 (0h) 19/6/2005 (12h) 20/6/2005 (0h) 20/6/2005 (12h) 21/6/2005 (0h) 21/6/2005 (12h) 22/6/2005 (0h) 22/6/2005 (12h) 23/6/2005 (0h) 23/6/2005 (12h) 24/6/2005 (0h) 24/6/2005 (12h) 25/6/2005 (0h) 25/6/2005 (12h) 26/6/2005 (0h)
Figure 1. Diurnal patterns in Total Reactive Phosphorus (TRP) from hourly in-situ measurements during June 2005.
Phosphorus and ecological status
There was no clear link between periods of elevated TRP concentrations and peaks in chlorophyll concentration or photosynthesis rates. A spring phytoplankton bloom, corresponding with elevated chlorophyll concentrations occurred between March and May. Rates of photosynthesis and respiration also started to increase around March,
but remained at elevated levels until August and October respectively (well beyond the period of high chlorophyll concentrations). This indicates that in-stream
productivity during the summer months was not primarily driven by phytoplankton, but that other autotrophs such as periphyton and macrophytes play an important role in overall river primary productivity.
Conclusions and wider comments
Diurnal cycling and storm event associated variations in TRP concentrations were observed, which would have been missed by a routine weekly sampling regime.
These hourly TRP data, in conjunction with other water quality measurements, enable us to examine the sources and dynamics in TRP in the River Kennet under varying hydrological conditions. The major source of phosphorus under base flow appears to be from sewage effluent, with varying inputs from diffuse sources during storm events. The results of this monitoring indicate that TRP concentrations are not a major control on phytoplankton growth or on overall primary productivity in the River Kennet. Indeed, the data indicate that in-stream productivity in the Kennet is more closely related to light levels and that, after a spring phytoplankton bloom, the primary productivity in the Kennet is linked to growth of other aquatic plants (such as periphyton and/or macrophytes). This study shows that continuous monitoring of phosphorus and associated parameters can provide valuable information about the dynamics and sources of nutrients and general river ecological status. We highlight the need to integrate hydrochemical monitoring with wider biological monitoring, including phytoplankton, periphyton and macrophytes, when dealing with such complex and dynamic aquatic systems.
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