The long-term impact of urbanisation on aquatic plants: Cambridge and the River Cam.

Historical and contemporary records have been used to assess the impact of urbanisation on the aquatic plants of the River Cam and its narrow floodplain in Cambridge. Of the 62 native aquatic plant species which have been recorded in the study area since 1660, 40 (65%) were still present in the period 1985-1999 whereas 22 (35%) are apparently extinct. There is a striking relationship between the fate of species and their trophic requirements, with species of less eutrophic habitats having suffered disproportionately. Historical records demonstrate that the River Cam became grossly polluted by sewage from Cambridge in the 19th century, but the chemical and biological quality of the river improved from 1897 onwards. However, the majority of the species recorded from the river and nearby ditches persisted until after maximum incidence of sewage pollution, which may even have stimulated the growth of 'weed' in the river. Losses of aquatic plant species from two riparian commons, Coe Fen (35%) and Sheep's Green (50%), have been particularly great. The level of these and other areas of low-lying common land by the river has been systematically raised by the controlled tipping of waste in hollows, followed by levelling and resowing. The main effects of urbanisation on the flora therefore arose from the transformation of riparian pastures into suburban open spaces. Commons which are used purely for amenity purposes have lost almost all their aquatic plant species. Those which are still grazed retain more, and continuance of grazing is probably essential if a varied aquatic flora is to be maintained. The interpretation of botanical records in terms of recorded management history is likely to throw further light on the processes of urbanisation, although the number of sites with a sufficiently detailed botanical record may be limited.

Developing environment-specific water quality guidelines for suspended particulate matter.

It is generally well recognised that suspended particulate matter (SPM), from nano-scale particles to sand-sized sediments, can cause serious degradation of aquatic ecosystems. However, at present there is a poor understanding of the SPM conditions that water quality managers should aim to achieve in contrasting environments in order to support good ecological status. In this article, we analyse long-term SPM data collected from a wide range of reference-condition temperate environments in the UK (638 stream/river sites comprising 42 different ecosystem-types). One-way analysis of variance reveals that there is a statistically significant difference (p < 0.001) between the background SPM concentrations observed in contrasting ecosystems that are in reference condition (minimal anthropogenic disturbance). One of the 42 ecosystems studied had mean background concentrations of SPM in excess of the current European Union (EU) water quality guideline, despite being in reference condition. The implications of this finding are that the EU's current blanket water quality guideline (25 mg L(-1) for all environments) is inappropriate for this specific ecosystem-type which will be non-compliant with the guideline regardless of the intensity of land-use. The other 41 ecosystems studied had mean concentrations below the current EU water quality guideline. However, this does not necessarily mean that the guideline is appropriate for these ecosystems, as previous research has demonstrated that detrimental impacts can be experienced by some freshwater organisms, of all trophic levels, when exposed to concentrations below 25 mg L(-1). Therefore, it is suggested here that it is likely that some ecosystems, particularly those with mean concentrations in the 0.00-5.99 mg L(-1) range, require much lower guideline values in order to be effectively protected. We propose a model for predicting environment-specific water quality guidelines for SPM. In order to develop this model, the 638 reference condition sites were first classified into one of five mean background SPM ranges (0.00-5.99, 6.00-11.99, 12.00-17.99, 18.00-23.99 and >24.00 mg L(-1)). Stepwise Multiple Discriminant Analysis (MDA) of these ranges showed that a site's SPM range can be predicted as a function of: mean annual air temperature, mean annual precipitation, mean altitude of upstream catchment, distance from source, slope to source, channel width and depth, the percentage of catchment area comprised of clay, chalk, and hard rock solid geology, and the percentage of the catchment area comprised of blown sand as the surface (drift) material. The MDA technique, with cross-validation (Wilks-Lambda 0.358, p 0.000), can predict the correct or the next closest SPM range of a site in 90% of cases. This technique can also predict SPM range membership in a probabilistic manner, allowing for an estimate of uncertainty to be made in the allocation of a site to an environment-specific SPM range.

Changes in water quality of the River Frome (UK) from 1965 to 2009: is phosphorus mitigation finally working?

The water quality of the River Frome, Dorset, southern England, was monitored at weekly intervals from 1965 until 2009. Determinands included phosphorus, nitrogen, silicon, potassium, calcium, sodium, magnesium, pH, alkalinity and temperature. Nitrate-N concentrations increased from an annual average of 2.4 mg l⁻¹ in the mid to late 1960s to 6.0 mg l⁻¹ in 2008-2009, but the rate of increase was beginning to slow. Annual soluble reactive phosphorus (SRP) concentrations increased from 101 µg l⁻¹ in the mid 1960s to a maximum of 190 µg l⁻¹ in 1989. In 2002, there was a step reduction in SRP concentration (average=88 µg l⁻¹ in 2002-2005), with further improvement in 2007-2009 (average=49 µg l⁻¹), due to the introduction of phosphorus stripping at sewage treatment works. Phosphorus and nitrate concentrations showed clear annual cycles, related to the timing of inputs from the catchment, and within-stream bioaccumulation and release. Annual depressions in silicon concentration each spring (due to diatom proliferation) reached a maximum between 1980 and 1991, (the period of maximum SRP concentration) indicating that algal biomass had increased within the river. The timing of these silicon depressions was closely related to temperature. Excess carbon dioxide partial pressures (EpCO2) of 60 times atmospheric CO2 were also observed through the winter periods from 1980 to 1992, when phosphorus concentration was greatest, indicating very high respiration rates due to microbial decomposition of this enhanced biomass. Declining phosphorus concentrations since 2002 reduced productivity and algal biomass in the summer, and EpCO2 through the winter, indicating that sewage treatment improvements had improved riverine ecology. Algal blooms were limited by phosphorus, rather than silicon concentration. The value of long-term water quality data sets is discussed. The data from this monitoring programme are made freely available to the wider science community through the CEH data portal (http://gateway.ceh.ac.uk/).