Reassessing the risk of microbial contamination from roosting cormorants in source water supply reservoirs

Abstract

A previous water quality risk assessment of source water supply reservoirs in subtropical southeast Queensland (Australia) evaluated little black cormorants ('Phalocrocorax sulcirostris') roosting on intake infrastructure as potentially posing an extreme risk of microbial contamination through direct deposition of fecal matter to the aquatic environment. To evaluate this risk rating, we assessed populations of little black cormorants occupying 3 intake structures across 2 reservoirs, enumerated 'Escherichia coli' ('E. coli') levels collected from fecal matter, and estimated a daily 'E. coli' load to the reservoir for each population. Concurrently, we supplemented the existing routine monthly water quality monitoring program with targeted water sampling to measure 'E. coli' concentrations in water at the 3 water intake points and at 2 sites without extant cormorant populations. Up to 3.9 x 10¹⁴ 'E. coli' organisms were estimated to be produced per day by the largest population surveyed. Cormorants were present at intake sites and absent from reference sites; however, concentrations of 'E. coli' were not significantly higher in water at intake sites compared with reference sites (p = 0.793 vs. p = 0.1069, respectively), and there was no significant relationship (p = 0.9671) between cormorant numbers and water column concentrations of 'E. coli'. The inability to quantify significant differences in microbial concentrations among sites suggests a more intensive sampling regime is required to clarify the relative contribution of contamination sources. Populations of roosting cormorants in our study reservoirs are unlikely to pose an extreme risk to source water quality when compared to other catchment-based inputs that dominate microbial pollution.