Sediment-macrophyte relationships in lowland English rivers: using macrophytes for biological assessment

Abstract

A study of the relationship between submerged-rooted aquatic macrophytes and the chemical and physical characteristics of sediments in lowland British rivers is described. The focus of the research is the spatial variability of nutrient concentrations in river sediments and the preferences of particular macrophyte species for different sediment types. This information is required to further develop the use of macrophytes in the biological assessment of rivers. Sediment samples were collected from beneath macrophyte stands and unvegetated areas of the riverbed in 17 rivers. The sediment samples were analysed for total and inorganic phosphorus, total nitrogen, organic carbon, organic matter and silt-clay content. Data on water chemistry flow regimes, channel geometry and macrophyte habitat were also collected. Sediments were found to exhibit a high degree of variability both within 100m sites and between the different rivers, particularly with respect to phosphorus concentrations. There were relationships between sediment concentrations of total and inorganic phosphorus and between concentrations of total nitrogen, organic carbon and organic matter. No clear relationships between mean values for sediment parameters and either water column nutrient concentrations or flow regime were apparent. The significance of the sediment variables as a control on macrophyte community structure was investigated through the use of canonical ordination and discriminant analysis. Macrophyte species showed broad tolerances to all sediment variables and it was not possible to separate the influence of sediment nutrients from other sediment parameters or differences between rivers. Comparisons of water sediment and plant tissue nutrient concentrations at sites upstream and downstream of waste water treatment work outfalls on two rivers indicated that the discharges affected both the water and sediment concentrations but not plant tissue levels. The research suggests that the relationships between macrophytes and sediments in lowland rivers are complex and confounded by the effect of the plants themselves upon flow and sediment dynamics.