Elsevier

Science of The Total Environment

Volume 434, 15 September 2012, Pages 201-212
Science of The Total Environment

Nutrient and light limitation of periphyton in the River Thames: Implications for catchment management

https://doi.org/10.1016/j.scitotenv.2011.09.082Get rights and content

Abstract

Soluble reactive phosphorus (SRP) concentrations in the River Thames, south east England, have significantly decreased from an annual maximum of 2100 μg l 1 in 1997 to 344 in 2010, primarily due to the introduction of phosphorus (P) removal at sewage treatment works within the catchment. However, despite this improvement in water quality, phytoplankton biomass in the River Thames has greatly increased in recent years, with peak chlorophyll concentrations increasing from 87 μg l 1 in the period 1997 to 2002, to 328 μg l 1 in 2009. A series of within-river flume mesocosm experiments were performed to determine the effect of changing nutrient concentrations and light levels on periphyton biomass accrual. Nutrient enrichment experiments showed that phosphorus, nitrogen and silicon were not limiting or co-limiting periphyton growth in the Thames at the time of the experiment (August–September 2010). Decreasing ambient SRP concentration from 225 μg l 1 to 173 μg l 1 had no effect on periphyton biomass accrual rate or diatom assemblage. Phosphorus limitation became apparent at 83 μg SRP l 1, at which point a 25% reduction in periphyton biomass was observed. Diatom assemblage significantly changed when the SRP concentration was reduced to 30 μg l 1. Such stringent phosphorus targets are costly and difficult to achieve for the River Thames, due to the high population density and intensive agriculture within the Thames basin. Reducing light levels by shading reduced the periphyton accrual rate by 50%. Providing shading along the River Thames by planting riparian tree cover could be an effective measure to reduce the risk of excessive algal growth. If the ecology of the Thames is to reach the WFD's “good ecological status”, then both SRP concentration reductions (probably to below 100 μg l 1) and increased shading will be required.

Highlights

► Significant reductions in phosphorus concentrations since the late 1990s have not reduced phytoplankton concentrations in the River Thames. ► Periphyton growth in the Thames is not limited or co-limited by phosphorus, nitrogen or silicon. ► Soluble reactive phosphorus concentrations need to be reduced to below 100 μg l 1 before periphyton accrual rate is reduced. ► Shading of the river channel by riparian trees could reduce periphyton accrual rate in the River Thames by 50%.

Introduction

The reduction of phosphorus (P) loading to UK rivers is seen as the key measure in reducing excessive algal growth and other problems associated with freshwater eutrophication, and thereby vital in delivering the “good ecological status” that is demanded by the European Union's Water Framework Directive (WFD). Many UK rivers have seen significant reductions in P concentration, due primarily to the introduction of phosphate removal at sewage treatment works (STW) (Bowes et al., 2009, Bowes et al., 2010b, Foy, 2007, Jarvie et al., 2002b, Neal et al., 2010a). These reduced P loadings have delivered the intended improvements in ecology in some rivers (Bowes et al., 2011, Kelly et al., 2009), but many others have seen no change in either algal biomass or community structure (Kelly and Wilson, 2004, Neal et al., 2010b), because the current P concentrations are still in excess, and therefore do not limit algal growth rate.

If algal growth in a particular nutrient-enriched river is to be controlled by reducing phosphorus concentration, it is vital that catchment managers and policy makers know the threshold phosphorus concentration at which the algae become P-limited. If nutrient mitigation results in a river phosphorus concentration below this threshold value (termed the Phosphorus Limiting Concentration), algal growth will begin to decline, resulting in a potential shift in river ecology towards good ecological status (Hilton et al., 2006). If the phosphorus concentration remains above the P Limiting Concentration following mitigation, algal growth rate will continue unabated, and there is unlikely to be a change in river ecology. Previous studies have shown that Phosphorus Limiting Concentrations vary greatly from river to river, ranging from less than 20 μg l 1 SRP (Bothwell, 1985, Chambers et al., 2006, Popova et al., 2006, Welch et al., 1989) to ca. 100 μg l 1 or greater (Bowes et al., 2007, Matlock et al., 1999). Although the reduction of phosphorus loadings to rivers is seen as the main tool being used by government environmental and conservation agencies to control algal growth and improve ecological status in UK rivers, there are other parameters that can also affect algal accrual rate; in particular flow-velocity and light. Increasing river flow velocity can increase the scouring of epilithic and epiphytic biofilms from their substrates (Horner et al., 1990), thereby reducing shading of macrophyte leaves and ‘cleaning’ gravel substrates, thus increasing their utility as invertebrate habitat and potential fish spawning grounds. Increased flow velocity will also decrease residence times for phytoplankton (autotrophic organisms that are suspended within a water body), meaning that they have less time in the river to proliferate (Hilton et al., 2006). Light intensity within the river channel also affects the rate of algal growth (Hill and Fanta, 2008, Mosisch et al., 2001). Recent modelling studies on the River Swale, northern England, have suggested that reducing light levels in river headwaters by increasing riparian shading could be a more effective means of reducing phytoplankton growth than reducing phosphorus concentration (Hutchins et al., 2010).

In this paper, phosphorus and suspended chlorophyll-a concentration data for the River Thames from 1997 to 2010 are presented. These data were used to test the hypothesis that the improvements in water quality observed in other studies (Kinniburgh and Barnett, 2010, Neal et al., 2010a) have reduced phytoplankton concentration in the Thames. This paper then aimed to identify the Phosphorus Limiting Concentration of periphyton (attached biofilm) for the River Thames near Oxford, using within-river flume mesocosms. The flume mesocosms were also used to identify if the other major plant nutrients, nitrogen and silicon, were limiting or co-limiting biofilm development, and how increases in P, N and Si affect periphyton community structure. Finally, this paper aims to determine if decreased light intensity (equivalent to shading from riparian tree cover) will decrease periphyton accrual rates.

The River Thames is the largest river that is wholly in England, with a total length of 354 km and a catchment area (to the tidal limit at Teddington in south west London) of 9948 km2 (Marsh and Hannaford, 2008). The river rises at Thames Head in Gloucestershire, and flows in an easterly direction into the North Sea, east of London (Fig. 1). The Thames basin not only contains the UK's capital, London, but also many other major urban centres, including Swindon, Oxford, Slough, Maidenhead and Reading. The many STW associated with this high human population density (ca. 960 people km 2 (Merrett, 2007)) have a major impact on the water quality of the River Thames, with an estimated 50% of the soluble reactive phosphorus (SRP) load derived from STW effluent between 1997 and 1999 for the relatively rural middle reaches of the Thames at Wallingford (Bowes et al., 2010b). Tertiary treatment has been installed at the 36 largest STW (serving approximately 2.7 million people) upstream of the tidal limit since 2003, resulting in an average 85% reduction in phosphorus load from each sewage works (Kinniburgh and Barnett, 2010). This has resulted in significant reductions in SRP concentration in the River Thames and its tributaries during the 2000s (Kinniburgh and Barnett, 2010, Neal et al., 2010a). Over the coming decade, the population within the Thames basin is likely to increase further, with the planned building of an extra 375 000 homes within the basin by 2016 (Environment_Agency, 2009). This will greatly increase pressures on drinking water supplies, wastewater treatment, water quality and river ecology (Evans et al., 2003, Neal and Jarvie, 2005). These pressures are likely to be exacerbated by projected climate change scenarios that predict declining river flows and increasing water temperatures (Johnson et al., 2009).

Despite the high population density, much of the River Thames basin upstream of London is relatively rural (Environment_Agency, 2009), with ca. 45% of land area being classified as arable, 11% woodland and 34% grassland (Fuller et al., 2002). Only ca. 6% of the catchment land cover was urban or semi-urban development. Agriculture is relatively intensive, and the resulting diffuse phosphorus, nitrogen and sediment losses will impact on water quality within the basin.

The flume mesocosms experiments took place on the Seacourt Stream at Wytham, which is a small distributary/mill stream that is fed directly by the River Thames, just west of the city of Oxford (Fig. 1). The Seacourt Stream was chosen to carry out nutrient limitation and shading experiments, rather than the River Thames itself, as the Thames is too deep for fieldworkers to safely operate the flumes. The Thames is also extensively used by leisure boats, and the large flume mesocosms used in this study would pose a hazard to this boat traffic. Twelve flumes (4 sets of 3 flumes) were installed ca. 50–80 m from the River Thames, along a relatively straight, uniform flowing section of river with a negligible amount of riparian shading. Maximum average river depth and width were ca. 1 m and 5–6 m respectively. Land use at the site was grassland, with sheep and cattle grazing. Simultaneous water sampling and analysis of the Seacourt Stream and Thames (just upstream of the confluence) showed that there was no observable change in nutrient concentrations taking place within the Seacourt Stream itself, due to interactions with bed sediments or biota, and so the flume experiments were carried out on unaltered River Thames water.

The water quality and chlorophyll-a concentration of the River Thames at Wallingford (Fig. 1) has been monitored at weekly interval from 1997 to 2002, 2006 to 2007 (Neal et al., 2010a), and from February 2009 until present, as part of the Centre for Ecology and Hydrology's Thames Initiative research platform. Water quality was also monitored at weekly intervals since February 2009 for the River Thames at Swinford (ca. 2 km upstream of the Seacourt Stream study site) (Fig. 1).

Section snippets

Water quality analysis

Samples of river water were taken manually at weekly interval from the main flow of the River Thames at Wallingford (1997 to 2010) and Swinford (2009 to 2010). Subsamples were filtered immediately in the field (0.45 µm cellulose nitrate membrane filter, WCN grade; Whatman, Maidstone, UK), and analysed for nutrient concentration. Soluble reactive phosphorus was determined using the phosphomolybdenum blue colorimetry method of Murphy and Riley (1962), as modified by Neal et al. (2000). Samples

Changes in River Thames water quality

Phosphorus concentrations have decreased significantly in the River Thames at Wallingford since the late 1990s (Fig. 3). The maximum SRP concentrations observed in 1997 and 1998 were 2100 and 1788 μg l 1 respectively, with mean annual concentrations of 1320 and 789 μg l 1. The annual maximum and mean SRP concentrations declined to ca. 800 and 420 μg l 1 in each of the following 3 years (1999 to 2001). Maximum and mean total phosphorus (TP) concentrations also declined from 2792 and 1461 μg l 1 in 1997

Acknowledgements

This project was funded by the Natural Environment Research Council. The authors would like to thank FAI Farms Ltd. for allowing us to conduct the flume experiments at their Wytham farm site, and in particular, the support and assistance from Mike Gooding and David Crutchley. We would also like to thank the University of Oxford and the John Krebs Field Station for providing their laboratory facilities, especially Phil Smith. We would especially like to thank Professor Colin Neal for supplying

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