title: The Distribution and Vertical Transport of Resources in the Upper Ocean
Creators: name: Rigby, Shaun
Publisher: name: University of Liverpool Repository
Year: 2021

description: Marine phytoplankton support higher trophic levels and are a key component of the biological carbon pump. The growth of marine phytoplankton is supported by the availability of bio-essential resources and incident light in the upper ocean. Over long periods, the biological carbon pump is sustained by the replenishment of depleted resources. In winter, the deepening of the mixed layer entrains underlying waters, transferring resources between the seasonal thermocline and mixed layer. The transfer of properties by entrainment is augmented by other physical processes, such as diapycnal diffusion and aeolian deposition. This thesis aims to synthesise and exploit new datasets in the Atlantic Ocean and Equatorial Pacific Ocean to quantify mixed-layer resource availability and physical resource transfers into the upper ocean. The availability of resources in the winter mixed-layer is quantified by combining observational data from the GEOTRACES programme with mixed layer estimates from a global data assimilation model. Basin-scale patterns in the availability of nitrate, phosphate, silicic acid, cadmium, zinc, cobalt, iron and manganese throughout the Atlantic Ocean mixed-layer are identified. Relative to phosphate, we show that the subtropical North Atlantic is depleted in nitrate and cadmium, while enriched in silicic acid, zinc, cobalt, iron and manganese, with the reverse true in high latitudes. Intermediate conditions in relative resource availability are located in mid latitudes. Differences in the availability of each resource are linked to the vertical structure, where mixed-layer resource stoichiometry is governed by offsets in nutricline depths between resources. We note a coupling of silicic acid and zinc vertical profiles in the subtropical North Atlantic, in contrast to recent works highlighting the rapid recycling of zinc compared to silicic acid; however, we suggest that reversible scavenging plays a crucial role in setting the zinc vertical profile in the deep water column, causing an alignment with silicic acid. Winter-time entrainment increases the availability of nutrient-type resources, such as nitrate, while surface stocks are eroded for those resources with scavenged-type resources, such as manganese, due to their vertical distributions, inducing a transfer of these resources from the mixed layer into the seasonal thermocline. In the mixed-layer, singular nitrogen limitation is identified in low latitudes, while singular iron limitation is identified at high latitudes, highlighting the potential for high latitude iron availability to influence low latitude biogeochemistry. Inter-annual variability in the depth of winter mixing causes changes in the winter mixed-layer resource stoichiometry, most notably in the low latitude North Atlantic where the mixed layer becomes richer in silicic acid, zinc, cobalt, iron and manganese relative to phosphate under a shoaled winter mixed-layer scenario. Changes to winter mixed-layer resource stoichiometry has important ecological implications. For example, in the equatorial Atlantic, changes to the distributions of nitrate and iron expand the diazotroph niche and hamper the success of non-diazotrophs. To further understand the importance of winter-time entrainment, this thesis applied the helium ‘flux gauge’ approach to estimate physical mixing in the upper ocean during two seasonally different field campaigns. Results demonstrate that active entrainment increases total physical mixing by a factor of ~7 compared to regions where entrainment is relatively weak. Vertical resource fluxes are also controlled by gradients in vertical resource profiles. Vertical gradients in resource profiles are linked to oxygen gradients, as expected from current knowledge of trace element redox chemistry, however, there are differences relationships with oxygen between resource and region. In the subtropical North Atlantic, we demonstrate that variability in resource fluxes is governed by mixing, while in the equatorial Pacific, variability in resource gradients and mixing equally controls resource flux variability. The vertical resource flux stoichiometry is compared to the cellular stoichiometry of in-situ biota to show there are mismatches between external resource supply and biological demand. Finally, an investigation into the effect of seafloor topography on resource transport showed that mixing in the upper 1000 m is a factor ~2 greater over shallow topography (Rainbow hydrothermal vent site, ~2700 m depth) compared to a deeper topographic site (Trans-Atlantic Geotraverse hydrothermal vent site, ~3600 m depth) along the Mid-Atlantic Ridge. Vertical resource fluxes are inferred by combining data from vertical microstructure profilers with resource profiles based on the geographic position and external forcing by wind and tides. Generally, nutrient-type and scavenged-type resources demonstrated upwards and downwards diapycnal fluxes, respectively. Vertical diffusivity at the shallow topographic site was estimated as a factor ~2 larger when compared to the deep topographic site. The increase in mixing at the shallow topographic site was not matched by the magnitude of resource fluxes, as gradients in vertical resource profiles were weaker at the shallow site, mitigating against the increase in mixing. Differences in the vertical resource profiles are linked to differences in the mixing rates, water mass contributions and regeneration rates between the sites. The contrasting vertical diffusivity observed at the shallow and deep topographical sites may be used to gain insights into a future ocean where vertical diffusivity is reduced, and stratification increased. In such a scenario, vertical resource profiles may adjust to a reduction in mixing and therefore mitigate change to the overall vertical resource flux. Thus, the first-order view that a reduction in diffusivity drives a proportional decrease in the resource flux is challenged when concurrent changes to resource profiles are considered.