Go with the flow: Timescales of biogeochemical and ecological ocean connectivity

Abstract

Ocean circulation can govern relationships between physical, biogeochemical and ecological processes and determines the connectivity between regions. As such, a variety of oceanographic problems can be addressed using a Lagrangian modelling approach. This thesis utilises velocity output from a high resolution ocean general circulation model and a Lagrangian particle tracking programme to address three topics: the global-scale efficiency of regional geoengineering by iron fertilisation; the role of natural iron fertilisation in phytoplankton blooms; and the connectivity of Marine Protected Areas (MPAs) to upstream anthropogenic impacts.

Artificial ocean iron fertilization (OIF) enhances phytoplankton productivity and is being explored as a means of sequestering anthropogenic carbon within the deep ocean for an extended period (e.g., the Intergovernmental Panel on Climate Change's standard 100 year time horizon). This study assessed the impact of deep circulation on sequestered carbon in the Southern Ocean, a high-nutrient low-chlorophyll region known to be iron stressed. The Lagrangian particle tracking approach was employed to analyze water mass trajectories over a 100 year simulation. By the end of the experiment, for a sequestration depth of 1000 m, 66% of the carbon had been re-exposed to the atmosphere, taking an average of 37.8 years. These results emphasized that successful OIF is dependent on the physical circulation, as well as the biogeochemistry.

Following on from the long-term impact of the wider Southern Ocean circulation, the local circulation around three Southern Ocean islands was considered. In exception to the typically High Nutrient, Low Chlorophyll conditions of the Southern Ocean, phytoplankton blooms occur annually downstream of the Kerguelen Plateau, Crozet Islands, and South Georgia, fertilized by iron-rich shelf waters. The Lagrangian particle tracking approach was used to investigate if advection could explain the inter-annual variability observed in the blooms in satellite ocean colour data. The results suggest that advection can explain the extent of each island's annual bloom, but only the inter-annual variability of the Crozet bloom, therefore suggesting that other factors, such as silicate limitation or the timing of mixed layer deepening, may also determine the inter-annual variability of the downstream blooms.

Finally, the method was applied to assess the remoteness of four MPAs: Pitcairn,
South Georgia, Ascension, and the British Indian Ocean Territory (BIOT), which were established to conserve important ecosystems. However, MPAs may be at risk of `upstream' human activity, such as marine pollution. Thus, improved understanding of exactly where upstream is, and on what timescale it is connected, is important for monitoring and future planning of MPAs. By reverse Lagrangian particle tracking, circulation `connectivity footprints' are produced for each MPA, revealing on annual timescales, that Pitcairn was not connected with land, whereas there was increasing connectivity for waters reaching South Georgia, Ascension, and BIOT. Such footprints are an inherent property of all MPAs, and need to be considered for all current and future MPAs.

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