This report provides a better understanding of the evidence for water quality change being a potential driver of declines evident in some waterbird populations. It updates a previous review by Burton et al. (2002).

1. Over the past 50 years there has been widespread improvement in water quality in many freshwater and coastal systems driven by domestic and European legislation, most recently the EU Water Framework Directive, but also including the Urban Waste Water Treatment Directive and Bathing Water Directive. Over the same period, declines have been recorded in the populations of a number of non-breeding waterbird species in the UK. It has been suggested that the change to systems with more natural nutrient levels could be one of the contributory causes of these wintering bird population declines.
2. This work seeks to provide a better understanding of the evidence for water quality change being a potential driver of bird population change at a whole site and site-network level. Five objectives have been identified by Natural England to meet this need:
   i. An update of a previous review by Burton et al. (2002) to provide a review of the global literature relating water quality change and its direct and indirect effects on bird communities;
   ii. A review of the global literature relating to water quality change and its impact on demersal and benthic communities used as food resources by non-breeding waders and wildfowl;
   iii. Identification of those species most at theoretical risk of population change from water quality improvement as a consequence of their foraging requirements;
   iv. Recommendations for further work to improve understanding of this subject area;
   v. Analysis of water quality data (if this can be sourced) and bird numbers for species identified in (iii) to see if there are any relationships.
   The present work focuses on the first and fourth of these objectives and informs assessment of the third.
3. Sewage discharged into coastal waters can provide a direct food source for bird such as gulls, scavenging on the effluent, but mostly it is the algal, plant, invertebrate and fish communities sustained by the outfalls that provide food for a variety of birds, such as dabbling ducks, waders, cormorants and grebes. As such, the presence and location of sewage discharge points has been linked to variation in numbers of Grey Plover Pluvialis squatarola and Lesser Black-backed Gull Larus fuscus, for example, and the structure and distribution of waterbird communities.
4. Nutrient enrichment of coastal waters from diffuse sources (e.g. from agricultural run-off) may also lead to increased food resources and may therefore be expected to be positively associated with waterbird abundance (especially waders and benthivorous species). In one of the only large scale studies of potential effects of diffuse pollution of coastal waters on waterbirds, the use of fertilisers in Denmark accounted for 15% of the variation in the winter population index of 15 species. Across Europe, 14 positive and 36 negative relationships were observed between fertiliser use and waterbird population indices, with indications that negative effects of fertiliser use may be reversible, over long timescales. While nutrient enrichment may increase food resources, it may also lead to the formation of macroalgal mats on mudflats, affecting the availability of invertebrate prey, and therefore the foraging strategies, abundances and distribution of waterbirds, particularly waders such as Curlew Numenius arquata, Redshank Tringa totanus and Black-tailed Godwit Limosa limosa.
5. Improvements to water quality of coastal systems, by reducing the nutrient content and organic matter available, may lead to shifts in waterbird community structure, and have been suggested as contributing factors in declines in wader abundance and the disappearance of some local diving duck populations. However, associations are correlative, or otherwise postulated, thus attributing changes in waterbird numbers solely to water treatment is unwise. Any effects of programmes to improve water quality are likely to be site- and species-specific.
6. The pathways by which nutrient loading affects waterbird communities in freshwater systems are likely to be similar to those in coastal waters, with nutrient enrichment leading to enhanced food resources and therefore positive associations with waterbird numbers (e.g. aggregations of Tufted Duck Aythya fuligula and Pochard Aythya ferina associated with sewage discharges into the Manchester Ship Canal and predicted positive effects of nutrient loading on piscivores). However, freshwater systems, and shallow lakes in particular, may be more sensitive to changes in nutrient loading than coastal systems, due to lower flushing and dilution of the system. Nutrient enrichment can reach a point where the system shifts from transparent, submerged macrophyte-dominated waters to a turbid, phytoplankton-covered state, which may impede the foraging behaviour of piscivores, and reduce food and habitat availability for dabbling ducks such as Gadwall Anas strepera and Teal Anas crecca, or benthivores such as Tufted Duck, Pochard and Goldeneye Bucephala clangula.
7. Effects of improvements to the water quality of freshwater systems on waterbirds will depend on the species (diet, foraging behaviour, etc.) and severity of nutrient loading. Programmes to reverse eutrophication of freshwater lakes have been shown to benefit Tufted Duck, Coot Fulicra atra, Mute Swan Cygnus olor, Goldeneye and Pochard, thought to be partly due to changes in the vegetation structure and water transparency.
8. Given that the impacts of nutrient enrichment of coastal or freshwater systems on waterbirds vary depending on natural water quality of the system, the severity of nutrient loading, and physical characteristics of the system, efforts to improve water quality will also have varying impacts, depending on the state of the system prior to improvement. Effects of changes in water quality are therefore likely to be both site- and species-specific and may lead to, and have been implicated in, both population increases and declines. Generally, species that are less flexible in their foraging strategy would be expected to be more vulnerable to any reductions in food resources resulting from reduced nutrient inputs. Improvements to water quality may be of greater benefit to waterbirds in freshwater or sheltered coastal systems than in more dynamic open coastal systems. A provisional list of wintering waterbird interest features of freshwater and coastal sites classified as Special Protection Areas (SPAs) in England under the EC Directive on the conservation of wild birds (79/409/EEC) that may be affected by improvements to water quality is provided.
9. Although there is evidence of impacts between changes in water quality and waterbird numbers at a local within-site scale, evidence for impacts at a site-level may be harder to discern, due to the number of other factors that might affect populations at this scale. Nevertheless, recent research has linked temporal changes in waterbird populations to changes in fertiliser use at national and European scales (Møller et al. 2015, Møller & Laursen 2015). To build on this research, it is recommended that the spatial and temporal variation in non-breeding waterbird numbers (both freshwater and coastal), derived from Wetland Bird Survey (WeBS) data, should be explored in relation to spatial and temporal patterns in broad-scale metrics relating to fertilizer usage and water quality.