Long-Term Trends and Variability of North Atlantic Phytoplankton Populations

Marine phytoplankton are microscopic in size, but have substantial roles in the marine environment, structuring the base of the marine food web and facilitating marine biogeochemical cycles. Changes to their community therefore have important consequences on the functioning of marine ecosystem services. This thesis, describes long-term variability in the phytoplankton community at a taxonomic scale resolution over the North Atlantic region, using in-situ data from the Continuous Plankton Recorder (CPR) survey and output from the MIT Integrated Global Systems Model (IGSM).An innovative approach assesses the stability and disorder of the phytoplankton community in the North-East Atlantic using the CPR survey. Phytoplankton were found to return to its original structure after a period dominated by two diatom species, indicating its resilience to perturbation. Changes were not significantly linked to the physical environment, however, the results from novel approach emphasise that a rigid community structure may make the plankton population more prone to collapse under the extreme changes of a future climate. Generalised additive models (GAMS) were employed in a unique attempt to demonstrate the importance of multiple driving mechanisms. While phytoplankton continued to show very few links to the physical environment, zooplankton had a positive relationship with phytoplankton throughout the North Atlantic and had a strong negative response to deeper mixed layers and colder temperatures. It was concluded that a traditional bottom-up approach is an over simplification at inter-annual scales and future analysis should reflect this. In a final analysis, the MIT IGSM is used to explore the variability of hytoplankton in the 21st century under the pressures of increasing CO2 and global climate change. Analysis demonstrated a regional decline in biomass, with 94% of the North Atlantic demonstrating significant change. Change-point analysis shows abrupt shifts in biomass and richness occurred earlier in the sub-tropical regions (2030s) than the higher latitudes (2050s) and functional analysis showed these changes would result in a shift to smaller phytoplankton types in the higher latitudes. This study highlights the importance of maintaining spatially and temporally extensive datasets in the assessment of marine phytoplankton ecology. It highlights the potential impact of future changes to the marine environment, and the consequences this will have on the phytoplankton communities while reflecting on the broader implications this may have on the wider marine ecosystem.