Effect of phosphorus stripping on water chemistry and diatom ecology in an eastern lowland river -Kelly and Wilson (2004)

The Urban Wastewater Treatment Directive (UWWTD) aimed to reduce eutrophication in running waters.
It mandated phosphorus stripping at sewage treatment works (STWs) with >10,000 population equivalent (p.e.) unless it would have no effect on eutrophication levels.
The UK developed the Mean Trophic Rank (MTR) and Trophic Diatom Index (TDI) to assess eutrophication.
Initial sensitive areas were designated based on chemical data due to time constraints, requiring phosphorus stripping by 1998.
Prior efforts focused on standing waters, making this a large-scale experiment on lowland rivers.
The study examines the impact of phosphorus stripping at Ashford STW on the River Stour.
It addresses the assumption that spatial survey differences disappear with water quality improvements.

The River Stour originates from the Great Stour and East Stour, flowing through chalk downland to the English Channel.
Ashford is the first major urban area, with sewage effluent entering 20 km downstream from the source and 1.2 km from the town.
General water quality is ‘good’ but declines to ‘fair’ below Ashford.
Sampling sites: Bybrook Bridge (upstream) and Longport Bridge (downstream).
Bybrook Bridge is downstream of the urban area of Ashford.

Samples were collected from the final effluent and two sample sites and processed using standard procedures.
N:P ratios were computed as total oxidized nitrogen (TON): filtrable reactive phosphorus (FRP).
$TON = NO<em>3-N + NO</em>2-N$
$FRP = soluble reactive P = orthophosphate-P$
N:P > 15 indicates P limitation, N:P < 10 indicates N limitation.

Diatom samples were collected by scrubbing cobbles/boulders.
Samples were treated with Lugol’s iodine and hot hydrogen peroxide.
Cleaned valves were mounted on slides and identified using a Leica microscope.
At least 300 valves were counted per slide.
Primary floras and identification guides included Krammer and Lange-Bertalot, Hartley et al.
Nomenclature followed Whitton et al. and Round et al., with revisions to Achnanthes.
The term “pennate, undifferentiated” was used for uncertain taxa which never exceeded 3.2% of valve count.

Diatom-based indices: TDI, TDI-D, TI, IPS, Hill’s N2 diversity.
Motile diatoms were defined as Navicula, Nitzschia, Bacillaria, Cylindrotheca, Gyrosigma, and Surirella to assess physical substratum changes.
Detrended Correspondence Analysis (DCA) was used for an overview of the dataset.

Before nutrient removal (pre-Nov 1998), effluent phosphorus ranged from $2$ to $10 mg/l$ .
After nutrient removal, effluent phosphorus ranged from <0.5 to $1.42 mg/l$ .
Upstream FRP was $0.1-0.5 mg/l$ , downstream was up to $1.4 mg/l$ before removal, then similar to upstream.
FRP concentrations were highest in summer, TON highest in winter, and N:P ratios lowest in summer.
N is more likely to be limiting at Longport Bridge and Bybrook Bridge due to N:P values often below $10$ .

$114$ diatom species were recorded, typical of eutrophic conditions at both sites both before and after nutrient removal.
Achnanthes conspicua and Psammothidium lauenburgianum were more abundant downstream.
TDI and other trophic indices showed no significant difference between sites.
IPS mean values were lower at Longport Bridge but not statistically significant.
Percent motile valves showed no statistically significant difference.
DCA showed no major trends distinguishing sites before or after nutrient removal.
Samples from Bybrook and Longport (post-stripping) formed a cluster, with Longport (pre-stripping) slightly separated on axis 1, possibly due to organic pollution indicators.
A small cluster with high $N.$ lanceolata represented early spring assemblages.

Chemical conditions changed post-nutrient removal, but diatom flora and macrophyte assemblages remained relatively unchanged.
River Stour upstream is already eutrophic.
Environment Agency proposes SRP standards of $0.1 mg/l$ (mesotrophic) and $0.2 mg/l$ (eutrophic).
Mainstone et al. suggest $0.1 mg/l$ for chalk/sandstone rivers and $0.2 mg/l$ for clay rivers.
Upstream FRP exceeds both targets, and downstream post-stripping also exceeds limits (max $0.86 mg/l$ ).
Diatom indices indicate high eutrophication at both sites.
DCA detected a slight change missed by indices, highlighting the dangers of over-simplistic interpretation.
Community-based indices of eutrophication have an upper limit of effectiveness dependent on local conditions like other nutrient concentrations.
N:P ratios suggest P limitation is unlikely at FRP > $0.3 mg/l$ , while P limitation is more likely in winter (Fig. 3).
Seasonal N:P variations are due to hydrology and biological activity.
Ecological improvements require controlling nutrients from other sources like smaller STWs, agriculture, storm sewers, and urban runoff.
The UWWTD needs to be part of a broader management strategy.
Case studies help refine decision-making for future designations.
Statistical power analysis can strengthen the case for designation by showing significant differences between sites.

Phosphorus concentrations downstream were similar to upstream after nutrient removal.
Diatom taxa typical of eutrophic conditions dominated all samples.
Downstream flora showed a slight change after nutrient removal, with fewer taxa tolerant to organic pollution.
River may be nitrogen-limited, and phosphorus stripping does not bring phosphorus concentrations to target level for eutrophic river.
Further work is needed to improve ecological indices for monitoring eutrophication in rivers.