Macrophyte and periphyton dynamics in a UK Cretaceous chalk stream: the River Kennet, a tributary of the Thames

Abstract

An initial study to observe the seasonal trends and to determine the factors influencing macrophyte and periphyton growth patterns was undertaken on a representative reach of the River Kennet (UK) over a 2-year period (1998–2000). Maximum average macrophyte and average periphyton dry matter biomass recorded during the growing season were 200 and 21 g m⁻², respectively. The relationships between macrophyte and periphyton percentage cover and biomass data with physico-chemical variables were investigated. Regression analysis indicated that of the parameters measured, flow, and in the case of the dominant Ranunculus spp., solar radiation, were best able to predict macrophyte biomass and cover. The periphytic biomass within the reach was low, possibly as a result of relatively high flows and low phosphorus concentrations following the introduction of effluent treatment at the sewage works immediately upstream of the reach. Periphytic biomass was poorly correlated with the physical variables measured. This indicates that biomass is regulated by complex interactions between the physical and chemical factors, such as flow, solar radiation and phosphorus concentration. These interrelationships require further investigation.

Introduction

The importance of the species-rich Cretaceous chalk streams of southern England was acknowledged by the UK Steering Group on Biodiversity in 1995 (Department of the Environment, 1995). The fragility of the plant communities in these systems makes them susceptible to relatively small changes in the environmental factors upon which their abundance/diversity is dependent. It is important that studies of macrophyte production within a reach are assessed concomitant with measured river physico-chemical conditions. This is needed to identify relationships between habitat and abundance/diversity trends and to give possible explanations for any patterns or anomalies observed. The term ‘periphyton’ refers to the complex community of bacteria, algae, fungi, animals, inorganic matter and organic detritus covering the surface of the aquatic plant (Wetzel, 1983, van Dijk, 1993). Monitoring of periphytic biomass is valuable for rapid assessment of the river biological water quality. It is also of value for assessing whether periphytic cover is a factor affecting or limiting macrophyte growth.

Despite several studies of macrophyte growth in Cretaceous chalk streams (Dawson, 1976, Ham et al., 1981, Wright et al., 1981, Wright et al., 1982, Wright et al., 2002, Wilby et al., 1998), few studies have investigated the effects of periphyton dynamics in conjunction with river physico-chemical factors upon macrophyte population. Sampling on Cretaceous chalk streams has often been limited to occasional assessments of biomass monitoring throughout the growing season to obtain single estimates of net production (Westlake et al., 1986). More regular monitoring has been carried out on the River Lambourn (Wright et al., 1982) and on the River Itchen (Wilby et al., 1998), where a more comprehensive study was undertaken to investigate the significance of environmental and physico-chemical parameters on the spatial and temporal distribution of macrophyte cover.

Wilby et al. (1998), and other workers (Dawson, 1976, Sand-Jensen et al., 1989), found macrophyte cover to be influenced by factors such as flow, water temperature and solar radiation. Other influencing factors include nutrient concentrations, sediment type, turbidity, water depth, grazing and human disturbance (Ham et al., 1981), although the importance attributed to these factors varies. Periphytic communities also have the ability to influence the growth of macrophytes by preventing or reducing photosynthesis through shading (Sand-Jensen and Sondergaard, 1981, Sand-Jensen, 1983, Wetzel, 1983). Studies of periphyton accumulation rates have attributed varying importance to the main known factors governing periphytic growth. Flow velocity, solar radiation, water temperature, grazing, abundance of macrophyte substrata and water quality conditions are all factors that have been linked with the accumulation of periphyton on macrophytes (Hooper-Reid and Robinson, 1978, Sand-Jensen and Sondergaard, 1981, Sand-Jensen, 1983, Bulthius and Woelkerling, 1983, Kaireselo, 1983, Meulemans, 1988, Munn et al., 1989). However, the factors that control the growth of macrophytes and periphyton are site-specific and they vary both spatially and temporally.

An objective of this study was to obtain macrophyte taxa percentage cover and biomass estimates, in addition to the distribution and biomass of periphyton communities. The study provided data to be included in an instream model of phosphorus and macrophyte dynamics (Wade et al., 2002a, Wade et al., 2002c, Wade et al., 2001b) which is to be used to investigate the impacts of changing environmental conditions on macrophyte growth. Local environmental data were used to attempt to explain the growth patterns and abundance of the macrophytes and periphyton in the reach. The growth patterns and abundance of the highly valued Ranunculus spp. were of particular interest, due to the observed decline in growth that occurred in the upper River Kennet during the late 1980s and 1990s before effluent treatment at Marlborough began in 1997 (Wright et al., 2002). Ranunculus penicillatus var. calcareous (R.W. Butcher) C.D.K. Cook, the dominant macrophyte in this and other Cretaceous chalk streams, is of high ecological importance and is scheduled as a priority habitat under the EC Habitats Directive (92/43/EEC). This plant is particularly valued because of its attractive flower and the cover it offers to fish, principally brown trout (Salmo trutta). The growth of Ranunculus in the River Kennet is deemed important, as high populations reflect good water quality. This is of particular significance in a river such as the Kennet, which has a high ecological status and is a Site of Special Scientific Interest (SSSI) due to its outstanding Cretaceous chalk river plant and animal communities.

Under the requirements of the EC Urban WasteWater Treatment Directive (UWWTD, 91/271/EEC), the UK Environment Agency is using macrophyte surveys (mean trophic rank method: MTR) to assess the trophic status of rivers for possible designation of Sensitive Areas (eutrophic). The method involves surveying the presence, absence and % area covered by macrophyte flora, which subsequently leads to calculation of the MTR score (Environment Agency, 1999). The use of macrophyte/periphyton monitoring is clearly important in determining the water quality and ecological status of freshwater systems. The reduction of point sources of phosphorus is a key objective for the protection of surface waters under the UWWTD, and the present study attempts to relate macrophyte and periphyton dynamics to the physical and chemical characteristics of the aquatic environment in a reach which has recently experienced a reduction in phosphorus inputs.