Abstract
Zebra mussels (Dreissena polymorpha) are recognised biomonitors in determining the presence and viability of the human waterborne pathogens Cryptosporidium parvum, C. hominis, Giardia intestinalis and microsporidia in surface waters. This study investigated whether the size of zebra mussels is a significant factor in the concentration of protozoan Cryptosporidium oocysts, Giardia cysts and microsporidian spores. Zebra mussels were collected in Lough Arrow, a small Irish lake, which is utilized for drinking water abstraction and is subject to agricultural and human wastewater pollution drivers, both recognised risk factors for human waterborne pathogens. Zebra mussels were cleaned, divided into size (5 mm) interval classes based on their shell length and made up to 150 g samples (wet weight with shell). Combined fluorescence in situ hybridization (FISH) and immunofluorescent antibody (IFA) techniques were utilized as biomolecular techniques to assess the presence and concentration of the pathogens. PCR analysis provided source-tracking information on human and animal pollution sources. There was no significant relationship between the size of D. polymorpha and pathogen loads in similar sized samples, indicating that different sites in the same or different waterbody can be compared in terms of relative concentrations of human waterborne parasites irrespective of the zebra mussels' size. Cryptosporidium was the most abundant species, with lower counts of Giardia and the microsporidian Encephalitozoon hellem, respectively. Cryptosporidium oocysts and Giardia cysts were detected in zebra mussel samples at all three lake water abstraction points. A lake transect showed a decline in Cryptosporidium with increasing distance from a stream discharging sewage. Samples from agricultural sites indicated faecal inputs contaminated with these pathogens. Species identification implicated both human and animal faecal inputs to the lake from treated effluent, septic tanks, and agriculture. The research demonstrates the efficacy of zebra mussels as sentinels of water quality i rrespective of their size.
Highlights
Cryptosporidium and Giardia were found in Dreissena attached to the screens of all three drinking water plants (Table 1), with E. hellem (Site 1) and E. intestinalis (Sites 2 and 3) found at different plants
The numbers of potentially viable human pathogens found in zebra mussels attached to drinking water abstraction screens in Lough Arrow did not vary significantly according to pathogen or drinking water plant
The results indicate that based on the same sample mass, the size of the zebra mussels did not influence the pathogen loadings
Summary
Ireland (54°01′N, 08°19′W) is a small freshwater limestone lake (12.5 km2), renowned as a fishery for brown trout Salmo trutta Linnaeus, 1758 It is used for drinking water abstraction (three abstraction points) and has two small municipal wastewater treatment plants discharging treated effluent into inflowing streams (Figure 1). The Irish invasion of zebra mussels Dreissena polymorpha (Pallas, 1771) is recent (McCarthy et al 1998; Lucy 2009), and was first recorded in this lake in 2003 (Lohan 2004). While long-term assessments have classified the lake as mesotrophic; this trophic status has been reassessed as oligotrophic (2004-2006 assessment) due to low chlorophyll levels and increased transparency following the zebra mussel invasion (EPA 2008a)
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