Environmental drivers and advective transport of harmful phytoplankton in North West European shelf seas
Harmful phytoplankton occur naturally in British waters. However, little is known about the environmental drivers that lead to the formation and advection of harmful algae blooms (HABs). To minimise adverse effects of HABs it is necessary to improve our understanding of links between advection, environmental changes and HAB development. The aims of this PhD project were therefore to: 1) Provide a better understanding of the relationship between harmful phytoplankton, environmental drivers and key hydrodynamic features such as the shelf edge; 2) Determine the role of advection in bloom transport and development; 3) Utilise computational modelling to study environmental drivers and advection of HABs. To achieve these aims, field data was collected from two cruises and a glider mission. During the cruises, data on phytoplankton community was collected alongside physical data with a focus on key features such as the shelf edge and seasonal coastal fronts. This data provided an updated, detailed assessment of phytoplankton across the Hebridean and Malin Shelves. Field data showed that the European Slope Current (ESC) and Islay front can separate phytoplankton communities on the shelf. Lack of community differences along the ESC suggested stronger transport and exchange of phytoplankton within the ESC than adjacent shelf waters. Nutrients, nutrient ratios and light conditions were also found to play major structuring roles in determining phytoplankton assemblage. Data collected during field work can also be useful for regulatory assessment of shelf seas by providing baseline information about phytoplankton communities in the area. The glider mission provided an additional high resolution dataset of biological and physical water column properties across the Malin shelf. Vertical resolution showed that phytoplankton distribution was strongly linked to thermal stratification and temperature changes. Horizontal resolution was highly patchy, suggesting that scientific cruises could easily miss high density blooms with small spacial extent. The glider successfully monitored a high density HAB, suggesting that gliders could potentially be used for phytoplankton surveillance and detection of high biomass blooms. In addition to field data, a bio-physical individual based model (IBM) was used to simulate HAB progression. The IBM was coupled with a hydrodynamic ocean model to show the role of advection and importance offshore seed populations in coastal HAB development. Model output was compared to coastal count data and satellite images whenever possible. Running the model under different conditions for phytoplankton growth and behaviour, suggest that it was crucial to include biological processes to simulate HABs. The IBM could be initialised with data from satellite images, field data or discrete seed populations. Model simulations with different initial cell concentrations and locations could help to explain observed bloom pathways and suggest possible offshore origins for observed exceptional HABs. The results from field work and model simulations showed the role of the ESC in structuring phytoplankton community and transporting seed populations of HABs along the Scottish west coast. This suggests that future cruises and oshore monitoring should focus on the ESC and shelf break region. The bio-physical model could hindcast HAB pathways along the ESC, suggesting that modelling of HAB pathways of known seed populations could be integrated into an early warning system for aquaculture sites along the Scottish west coast in the future. Such an early warning system would allow the protection, relocation or early harvesting of affected aquaculture sites.
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