Understanding the impacts of dogs and birds on faecal pollution of bathing waters in Dublin Bay

Dublin Bay is a shallow bay located on the east coast of Ireland in Irish Sea. The water body is bounded to the west by Dublin City and to the east by the Irish Sea, with its northern and southern extents being defined by Howth Head and Dalky, respectively.  The southern side of the Bay includes the designated bathing waters of Sandymount Strand and the non-designated (but monitored) bathing waters of Merrion Strand. The water quality of these bathing areas remains vulnerable to numerous microbial pollution inputs, and these continue to present risks to recreational and economic activities that underpin much of the ecosystem service provision in the area, particularly during the bathing water that extends from June to September each year. Microbial pollutants are known to derive from agricultural diffuse sources in upland catchments and from point discharges from the wastewater drainage network, specifically during wet weather events when combined sewer overflows (CSOs) are active.  However, while accepted as being problematic in the overall pollution ‘mix’, concentrations of faecal indicator bacteria (FIB) from the faeces of dogs (dog fouling) and from local bird communities are less well understood – Dublin Bay was designated a 'biosphere reserve' by UNESCO in 2015 and remains home to numerous species of seabirds, many of which are present in internationally important numbers. Here we present an assessment of the significance of FIB inputs from dogs and birds in their contribution to total faecal pollution in Dublin Bay.  The extent of dog fouling was assessed through five daily ‘beach sweeps’ on both Sandymount and Merrion Strands from 2019 to 2021. Eighty-one dog fouling events (30 and 51 on Sandymount and Merrion Strands, respectively) were observed, equating to an average of six fouling ‘events’ per day at Sandymount and 10 ‘events’ per day at Merrion. Laboratory testing was undertaken to determine average Escherichia coli (E. coli) and enterococci concentrations in the dog faeces.  BirdWatch Ireland (an independent bird protection organisation in Ireland) data from the Dublin Bay Birds Project (2013 to 2016) was used to quantify E. Coli and enterococci pollution loadings to Dublin Bay bathing waters deriving from the presence of both migratory and non-migratory bird populations during the bathing water season. A coupled hydrodynamic and water quality model was integrated with sediment-bacteria interaction model which was further developed to simulate the inputs from dogs and birds. The model was then calibrated and validated with extensive water quality and ADCP (current speed and direction) measurements collected in nearshore areas around Dublin Bay to simulate the transport and fate of FIB in the study area.  The model included the freshwater (river) inputs carrying diffuse agricultural pollutants to the Bay, and the known point source pollution releases from within wastewater drainage network.  A dynamic decay rate, which included the effects of  temperature and light intensity was included in the model. This research (Acclimatize) was part funded by the European Regional Development Fund through the Ireland Wales Cooperation Programme.