The First Generation Magnox Storage Pond (FGMSP) is situated on the Sellafield Ltd site in the UK, and is an extremely inhospitable environment comprising of significant levels of radioactivity coupled with a high pH of pH 11.4. Despite such extreme conditions, microorganisms are known to colonise the pond and can form dense microbial blooms in the summer months. The blooms can restrict the visibility within the pond which hinders plant operations. The FGMSP is currently undergoing decommissioning and waste retrieval operations, as a priority on site, therefore any plant downtime increases both the cost and timeframe for decommissioning. Here we describe the microbial community that colonises the FGSMP, including during two bloom periods. In addition efforts to determine the adaptive mechanisms that key microorganisms use to colonise the pond and their interactions with Sr are described. Over the course of the sampling period Proteobacteria were the dominant phylum in the pond, with variations seen at the lower taxonomic levels. In addition, a single cyanobacterium, affiliated with a Pseudanabaena species, was identified as the dominant photosynthetic microorganism from samples taken from two bloom periods, comprising up to 30% of the phylotypes detected. While the FGMSP was dominated by prokaryotes, a hydraulically linked auxiliary pond was more abundant in eukaryotic organisms. Comparisons between the two pond communities suggested that the auxiliary pond was not seeding the FGMSP, as the elevated pH and radiation levels inhibited such colonisation. Data supplied by Sellafield Ltd. showed that the onset of the bloom periods coincided with increases in the residence time of the purge water, used to maintain the elevated pH of the pond. Once the residence time of the purge water was reduced the visibility was restored in the pond, indicating that this was an effective means of removing the bloom-forming microorganisms. Laboratory-cultures of Pseudanabaena catenata were used to investigate the adaptive responses to ionizing radiation. The culture was found to consist of 9 other operational taxonomic units, 5 of which were affiliated with genera identified in the FGMSP. Detailed investigations indicated that X-irradiation treatment (95Gy) had no significant impact on the growth rate of the culture, however there was an increase in polysaccharide production and a reduction in protein and chlorophyll-a production. Increases in polysaccharides could be of importance in the FGMSP as this could influence the fate of radionuclides present in the water. Sr was used to determine whether P. catenata could influence the fate of radionuclides. P. catenata cells could be seen to accumulate Sr associated with polyphosphate bodies, whilst SrPO4 and calcium containing SrCO3 minerals were formed. The colonisation of FGMSP by organisms closely related to those studied here, including the cyanobacterium Pseudanabaena catenata requires careful consideration. The results presented here suggest that elevated levels of polysaccharides could potentially be generated within the pond as part of an adaptive strategy, and this could have impacts not only on the radionuclide inventory of the pond but could also facilitate the colonisation of the pond by other heterotrophic microorganisms. This project also provides significant first steps into the development of effective control strategies to prevent further bloom events, supporting the hypothesis that purging cycles are able to control bloom formation. Finally the results presented provide an insight into the potential use of organisms identified in such facilities for use in the remediation of contaminated aquatic environments and other engineered facilities.