The Colne Estuary: A Long-Term Microbial Ecology Observatory

Research spanning over 40 years has examined many aspects of the microbial ecology of the Colne estuary (Essex, United Kingdom) and it is arguably the most comprehensively understood temperate estuary in the Northern hemisphere. The hypernutrified Colne estuary exhibits strong gradients of nutrient concentrations (nitrate, ammonium, phosphate, dissolved organic nitrogen) from river and treated sewage inputs at the top of the estuary, with concentrations decreasing towards the estuary mouth. These strong concentration gradients facilitate the study of the biogeochemical process rates and the microbial communities responsible. Planktonic primary production is at an oligotrophic level because of light limitation in the turbid water, but a mixed water column maintains planktonic photosynthesis despite low light. Dense microphytobenthic biofilms occur throughout the estuary, with high rates of annual primary production typical of NW European nutrient-rich estuaries, but benthic primary production accounts for only a small proportion of the N load to the estuary. Organic matter degradation is focussed in the estuarine sediments, with greatest organic content in the upper estuary, decreasing towards the mouth. Benthic biogeochemical processes, leading to organic mineralisation and element recycling, including O2 uptake, sulphate reduction, methanogenesis, nitrate reduction, denitrification and anammox have all been quantified along the estuary, together with chemolithotrophic nitrification. Benthic denitrification removes a significant proportion of the N load to the estuary. Molecular techniques, including profiling, identification and quantification of 16S rRNA and key functional genes, have provided an understanding of the microbial communities in relation to position and biogeochemical activity. The concentrations and emissions of biological volatile compounds (nitrous oxide, methane, hydrogen sulphide and isoprene) have also been measured, and their ecological significance elucidated. Tidal exchange between the saltmarshes and the main estuary channel shows seasonal variation. When nitrate in tidal water is high in spring, the marsh sediments remove it, but tidally export ammonium and small particles of organic nitrogen to the estuary in summer when nitrogen in coastal water is low. Overall, the saltmarshes show a balanced nitrogen cycle suggestive of a stable climax community, but capable of responding to and removing increased nitrate concentrations in estuarine waters. These data, originating from the long-term study of a single system, are unique and this paper highlights how the Colne estuary microbial ecology observatory has contributed to our understanding of estuarine microbial ecology and biogeochemistry.