ALBERT

Description: The nitrogen cycle is fundamental to Earth's biogeochemistry. Yet major uncertainties of quantification remain, particularly regarding the global oceanic nitrogen fixation rate. Hydrogen is produced during nitrogen fixation and will become supersaturated in surface waters if there is net release from diazotrophs. Ocean surveys of hydrogen supersaturation thus have the potential to illustrate the spatial and temporal distribution of nitrogen fixation, and to guide the far more onerous but quantitative methods for measuring it. Here we present the first transect of high resolution measurements of hydrogen supersaturations in surface waters along a meridional 10,000 km cruise track through the Atlantic. We compare measured saturations with published measurements of nitrogen fixation rates and also with model-derived values. If the primary source of excess hydrogen is nitrogen fixation and has a hydrogen release ratio similar to Trichodesmium, our hydrogen measurements would point to similar rates of fixation in the North and South Atlantic, roughly consistent with modelled fixation rates but not with measured rates, which are lower in the south. Possible explanations would include any substantial nitrogen fixation by newly discovered diazotrophs, particularly any having a hydrogen release ratio similar to or exceeding that of Trichodesmium; under-sampling of nitrogen fixation south of the equator related to excessive focus on Trichodesmium; and methodological shortcomings of nitrogen fixation techniques that cause a bias towards colonial diazotrophs relative to unicellular forms. Alternatively our data are affected by an unknown hydrogen source that is greater in the southern half of the cruise track than the northern.

Description: Cephalopods are an important prey resource for fishes, seabirds, and marine mammals, and are also voracious predators on crustaceans, fishes, squid and zooplankton. Because of their high feeding rates and abundance, squids have the potential to exert control on the recruitment of commercially important fishes. In this review, we synthesize the available information for two intrinsic markers (δ15N and δ13C isotopic values) in squids for all oceans and several types of ecosystems to obtain a global view of the trophic niches of squids in marine ecosystems. In particular, we aimed to examine whether the trophic positions and trophic widths of squid species vary among oceans and ecosystem types. To correctly compare across systems, we adjusted squid δ15N values for the isotopic variability of phytoplankton at the base of the food web provided by an ocean circulation–biogeochemistry–isotope model. Studies that focused on the trophic ecology of squids using isotopic techniques were few, and most of the information on squids was from studies on their predators. Our results showed that squids occupy a large range of trophic positions and exploit a large range of trophic resources, reflecting the versatility of their feeding behavior and confirming conclusions from food-web models. Clear differences in both trophic position and trophic width were found among oceans and ecosystem types. The study also reinforces the importance of considering the natural variation in isotopic values when comparing the isotopic values of consumers inhabiting different ecosystems.

Description: Nitrogen is an essential nutrient for life. Its low abundance throughout much of the sunlit surface ocean limits the growth of primary producers that form the base of ocean ecosystems. Phytoplankton also consume surface ocean CO2 during growth, preventing this greenhouse gas from outgassing to the atmosphere where it will influence climate. Since the source and sink processes that control the balance of the bio-available nitrogen inventory, N2 fixation and denitrification/anammox (N-loss), respectively, are sensitive to climate, they may have an important feedback on atmospheric CO2 during climate change. N2 fixation and N-loss processes leave a distinguishable imprint on the ratio of stable nitrogen isotopes, δ15N, making it a useful tracer to constrain their patterns and rates. This dissertation incorporates δ15N into an Earth System Climate Model to better understand and quantify important N-cycling processes in the ocean. The two stable nitrogen isotopes, 14N and 15N, are included as prognostic tracers into the ocean biogeochemistry component of an Earth System Climate Model. A global database of δ15NO3− observations is compiled from previous studies and compared to the model results. The model is able to qualitatively and quantitatively reproduce many of the observed patterns such as high subsurface values in water column denitrification zones, low values in the North Atlantic attributed to N2 fixation, and the meridional and vertical gradients in the Southern Ocean caused by phytoplankton NO3− assimilation. Experiments show the most important isotope effects that drive the global distribution of δ15N are phytoplankton NO3− assimilation, N2 fixation, and denitrification/anammox. Nitrogen isotopes trends across the Pacific Ocean support that aeolian iron deposition is an important factor regulating the distribution of N2 fixation. N2-fixers have high structural iron requirements in their N2-fixing enzyme, which could restrict their growth since iron is a limiting micronutrient. Model experiments with and without Fe limitation of N2 fixation are compared to meridional δ15NO3− observations in the central and western Pacific Ocean. Only the model with Fe limitation of N2 fixation could reproduce the observed trends. This suggests that atmospheric iron deposition is important for relieving iron limitation of N2-fixers. Water column δ15NO3− and seafloor δ15N observations are used to constrain the rates of N2 fixation, water column N-loss, and benthic N-loss in the ocean. Experiments investigating uncertainties associated with the isotope effects of N-loss in the water column and sediments led to estimates for N-loss that varied by a factor of 3. Two sensitive processes affecting the large range of these estimates in the model are NO3− utilization in suboxic zones and the net fractionation factor associated with benthic N-loss. Sensitivity experiments that best reproduce observations in the suboxic zone and seafloor sediments estimate rates of N2 fixation, water column N-loss, and benthic N-loss are in the range 220–370, 70–90, and 150–280 Tg N yr-1, respectively, assuming a balanced bio-available nitrogen budget in the pre-industrial ocean. This model result suggests rates of N2 fixation have been previously underestimated and the residence time of bio-available nitrogen in the ocean is between 1,500 and 3,000 years.

Description: Over much of the ocean’s surface, productivity and growth are limited by a scarcity of bioavailable nitrogen. Sedimentary δ15N records spanning the last deglaciation suggest marked shifts in the nitrogen cycle during this time, but the quantification of these changes has been hindered by the complexity of nitrogen isotope cycling. Here we present a database of δ15N in sediments throughout the world’s oceans, including 2,329 modern seafloor samples, and 76 timeseries spanning the past 30,000 years. We show that the δ15N values of modern seafloor sediments are consistent with values predicted by our knowledge of nitrogen cycling in the water column. Despite many local deglacial changes, the globally averaged δ15N values of sinking organic matter were similar during the Last Glacial Maximum and Early Holocene. Considering the global isotopic mass balance, we explain these observations with the following deglacial history of nitrogen inventory processes. During the Last Glacial Maximum, the nitrogen cycle was near steady state. During the deglaciation, denitrification in the pelagic water column accelerated. The flooding of continental shelves subsequently increased denitrification at the seafloor, and denitrification reached near steady-state conditions again in the Early Holocene. We use a recent parameterization of seafloor denitrification to estimate a 30–120% increase in benthic denitrification between 15,000 and 8,000 years ago. Based on the similarity of globally averaged δ15N values during the Last Glacial Maximum and Early Holocene, we infer that pelagic denitrification must have increased by a similar amount between the two steady states.