Determining Drivers of Community Structuring and Denitrification Potential Within a Shallow Coastal Lake/Lagoon (Lake Ellesmere) Exposed to Anthropogenic Nutrient Deposition

Abstract

As a result of human agricultural activity, large amounts of the potent greenhouse gas nitrous oxide (N2O) are entering our atmosphere. In NZ, urine from dairy cows contributes large loads of nitrogen to the surrounding land and waterways via leaching or surface runoff. Lake Ellesmere, a coastal lake draining farmland, maintains lower nitrogen levels in its waters than inflowing rivers suggesting a nitrogen removal mechanism, present within the lake. It is likely that much of the incoming nitrogen could be emitted from the lake as N2 or N2O microbial denitrification. We investigated the microbial community structure and denitrification potential across 18 sites in Lake Ellesmere by using 16S ribosomal rRNA gene sequencing, denitrification enzyme assays and quantification of key nitrogen cycling genes, with a focus on denitrification genes, by quantitative PCR. Physical and chemical analyses (water salinity, temperature, clarity, total nitrogen, nitrate/nitrite, dissolved reactive phosphorous, ammonia, sediment organic matter, and sediment texture class classification) were also carried out simultaneously on the same sites. Analyses identified sediment texture class and organic matter content as the strongest drivers of bacterial community composition within the lake. Water column nutrients such as Total Nitrogen and Phosphorous were only weakly correlated to community structure suggesting they functioned as weaker modifiers of the system at the time of sampling. Analysis of discrete functional populations by qPCR (e.g. nitrogen fixers, denitrifiers) demonstrated significant variation of gene copy number within the lake sites, however no significant drivers of these populations were identified for the denitrifiers. Nitrogen fixers were found to be enriched in high silt sediments. These results demonstrate the difficulty in analyzing these diverse, variable systems. Nevertheless they restate the importance of sediment texture class as a strong determinant for microbial community structure. Further work will need to focus on temporal variability within the system and the production of supporting evidence through controlled testing of denitrification.