Background 

Over the last decade Northumbrian Water Ltd (NWL) has implemented a series of major improvements to the treatment and discharge of sewage at sites along the coast between Berwick-upon-Tweed and Saltburn (from Northumberland to Cleveland), so as to comply with the EC’s Urban Waste Water Treatment Directive. 

The impact of these directives on coastal waterbirds has raised concern as waste water discharges from outfalls may provide considerable supplies of food for bird species, either as directly edible matter or by artificially enhancing concentrations of invertebrate food through nutrient enrichment. 

This report reviews the results of and extends the investigation, commissioned by NWL and begun in 1996/97 by the University of Durham, of the impacts of the improvements to sewage discharges on waterbirds wintering on a 36 km stretch of the Northumbrian coast between the Coquet Estuary and St. Mary’s Island (Figure 1.1). The area comprises extensive areas of rocky shore which are included in the Northumbria Coast Special Protection Area (SPA), which is designated for its importance for wintering Purple Sandpipers Calidris maritima and Turnstones Arenaria interpres. Between these rocky areas are the sandy (bathing) beaches of Druridge Bay, Cambois and South Blyth. The majority of improvements to sewage discharges in the area were completed by the end of the winter of 2000/01. 

The report provides a summary of work carried out by the British Trust for Ornithology (BTO) and University of East Anglia (UEA) in the winter of 2003/04 and analyses of the changes to the invertebrate and wintering waterbird communities that have occurred since monitoring was begun by the University of Durham. Additional analyses investigate whether changes in Turnstone numbers might be explained by changes in this species’ survival rates. In addition to Purple Sandpiper and Turnstone, analyses of waterbird count data were carried out for eight other waterbird species: Eider Somateria mollissima, Oystercatcher Haematopus ostralegus, Ringed Plover Charadrius hiaticula, Knot Calidris canutus, Sanderling C. alba, Dunlin C. alpina, Curlew Numenius arquata and Redshank Tringa totanus. Analyses were carried out at two scales: firstly, for the whole coast from the Coquet Estuary to St. Mary’s Island and secondly, for the Amble-Hauxley and Newbiggin areas alone. Two of the three largest outfalls in the study area discharge in these areas; both discharges received improved treatment from late in the winter of 2000/01. 

Changes in Invertebrate Abundance Following Improvements to Sewage Treatment 

Comparison between (rocky shore and soft sediment) invertebrate data collected by the University of Durham between 1995/96 and 1997/98 and that collected in November 2003 showed no apparent declines in abundance following the improvements. However, particularly in the rocky shore data, there was considerable variability in invertebrate abundance both within and between transects and, as transects were not permanently marked, differences between years could have been the result of the heterogeneity within the sites, rather than actual changes over time. As a result of these factors and the limited size of the datasets, it was not possible to conclude that invertebrates had not been affected by the changes to sewage discharges. 

Changes in Waterbird Numbers Following Improvements to Sewage Treatment 

Across the study area as a whole, there were no clear trends in numbers across species following the completion of the majority of improvements to discharges in the winter of 2000/01. However, there was some evidence that the changes to sewage treatment affected both of the species for which the SPA is designated in winter – Purple Sandpiper and Turnstone. These species showed declines following the winter of 2000/01, having previously risen in number. No other species showed such clear declines following 2000/01. At the more local scale, after a period of high stability in numbers, there was a clear decline (of >30%) in the numbers of Turnstone at Amble-Hauxley over the three years following the improvement to the Amble discharge. Purple Sandpiper numbers also fell here in 2003/04, having risen prior to the improvement to the discharge. Sanderling likewise showed a decline in numbers at Amble-Hauxley after 2000/01 following an earlier increase. No other declines were noted at this scale that couldn’t be attributed to longer-term trends (and thus potentially reflecting factors unrelated to sewage improvements). 

Changes in Turnstone Survival Rates Following Improvements to Sewage Treatment.

Analysis of data from resightings of colour-ringed Turnstone showed that the decline in Turnstone numbers at Amble-Hauxley following the winter of 2000/01 might have been the result of reduced survival among adults. Results suggested a drop in annual survival from 77% over the four years before the improvement to the Amble discharge to 68% over the following three years, though this change was not significant. The adult survival rate for Turnstone at Amble-Hauxley for the whole period from 1997/98 to 2003/04 was estimated to be 75.9% (95% confidence limits = 70.9-80.3). 

Conclusions

There was some evidence, from analyses of changes in numbers (and survival rates) that the changes to sewage treatment affected Purple Sandpiper and Turnstone. However, it was not possible to determine whether invertebrate food supplies had been affected by the changes to sewage discharges and there were no clear indications that the improvements to sewage treatment might have affected other waterbird species, with the exception of Sanderling. 

Recommendations 

The declines of Purple Sandpiper and Turnstone in the study area – and, in particular, that of Turnstone at Amble-Hauxley – are of concern given that they began immediately following the changes to sewage treatment. It is thus recommended that waterbird counts continue for a minimum of two more winters. This would show whether numbers of Turnstone and Purple Sandpiper continue to decline across the study area as a whole and whether the decline recorded in Turnstone numbers at Amble-Hauxley continues or whether a new equilibrium has been reached.

By collecting sightings of colour-ringed Turnstone for additional winters it would also be possible to improve the accuracy of existing survival estimates and thus determine more fully the significance and duration of changes in survival following the changes to sewage treatment. These data could be collected while undertaking waterbird counts.

Supplementary work that could be of benefit includes further invertebrate sampling (particularly of rocky shores), though only if the larger dataset collected by Mark Eaton was available for comparison.

In addition, it is recommended that the stable isotope analysis undertaken by Eaton (2001) be repeated. By sampling in the same locations it would be possible to examine how the proportion of sewage-derived Particulate Organic Matter has changed in the study area following the improvements to sewage treatment.