BIOGEOCHEMICAL NITROGEN TRANSFORMATIONS UNDER CONTRASTING MIXING REGIMES IN TWO COASTAL WATER COLUMNS

Abstract

The biogeochemical cycling of nitrogen (N) in coastal water columns is of high importance due to anthropogenic N input and its role in fueling primary production and eutrophication. Using highly resolved field observations of a wide variety of environmental parameters, this thesis explores how microbiological, geochemical and physical variables control N cycling in two contrasting coastal water columns. In Chapter 2, metagenomes and the 16S rRNA phylogenetic marker gene were used in conjunction with geochemical measurements to characterize the microbial community and its functional capacities along an oxic-suboxic-sulfidic-ferruginous-methanogenic gradient in permanently stratified Powell Lake, British Columbia. This revealed potential micro-aerobic nitrifiers, facultative anaerobic potential in the widespread freshwater lineage acI (Actinobacteria) and evolutionarily divergent microbial lineages in the bottom water containing ancient seawater. Chapters 3 and 4 focus on the Bedford Basin (BB), Nova Scotia, a seasonally stratified coastal basin. In the dark BB bottom water (60 meters), accumulated ammonium (~20 μmol kg-1) is oxidized to nitrate during stratified periods. Chapter 3 describes results from four years (2014–2017) of weekly time series measurements in BB bottom water of geochemical concentrations as well as functional and phylogenetic marker genes. Analyses in conjunction with a biogeochemical model suggested strong physical mixing in some years caused dilution of nitrifier biomass, which led to nitrification delay and temporary nitrite accumulation by delaying the regrowth of ammonia oxidizer and nitrite oxidizer populations. In Chapter 4, the N isotopic signatures (δ15N) of particulate N, ammonium, nitrite and nitrate associated with these nitrification patterns were analyzed, using two distinct “natural experiments”: in 2014, ammonia oxidation was performed almost entirely by archaea and nitrite did not accumulate beyond 0.5 μmol kg-1. In 2017, bacterial ammonia oxidation played a role in nitrite accumulation to about 8 μmol kg-1. N isotope fractionation patterns were analyzed with the support of a numerical model and high-confidence, in situ isotopic enrichment factors (15ε) were determined. Overall, this work identified key controls on the aquatic N cycle, its interaction with other biogeochemical cycles, microbial communities and climate, as well as controls on N isotope distributions in the context of different stratification regimes.