Dynamics, control and variability of plankton in the northeast Atlantic and North Sea between 1958-2014

Abstract

Marine plankton provide essential support to all life on the planet, through the production of oxygen, contribution to nutrient cycling in the ocean, and by being the base of the marine food web. Due to their small size and limited motility, plankton are also highly susceptible to changes in the environment. Monitoring and understanding how the plankton responds to different climatic and anthropogenic effects is imperative in order to predict and prevent damaging outcomes to the ecosystem.
In this thesis, I focused on key plankton indicators in the Northeast Atlantic and North Sea, between 1958 and 2014, with aim to understand how and why these populations have been changing through time, what drives the variability observed in the plankton, and which are the best approaches to model plankton abundance from monitoring data.
I first explored the relationship between two major phytoplankton groups, diatoms and dinoflagellates, with a index for chlorophyll concentration. Differences in trend and abrupt transitions, combined with statistical modelling, implied that a non negligible amount of the greenness in the water came from groups of smaller phytoplankton, which were inferred to have increased in abundance in the recent past.
I proceeded to look into bottom-up versus top-down control of phytoplankton variability, from an interannual perspective. Through vector autoregressive modelling, I analysed interactions among the phytoplankton, zooplankton, and environmental indicators. Sea surface temperature emerged as a significant driver of variability, and some evidence for bottom-up control was found from a lack of dependency of diatoms on the other plankton variables. However, the plankton seemed to be mostly regulated by serial correlation and the seasonal cycle.
The serial correlation suggested that there could be nonlinearity in the system, and led to the question of whether linear approximations were suitable for plankton dynamics. I investigated the presence of nonlinearity, stochasticity and deterministic chaos in the plankton. The seasonal cycle was key to stabilise the fluctuations within plankton populations, which appeared to be regulated by a nonlinear stochastic dynamics rather than by chaos. The lack of signi6cant links between the plankton and the two environmental indicators reinforced the complexity of the plankton system, and implied that it is not likely to exist a single factor as the main driver of plankton variability.
The complexity of species interactions and environmental effects, combined with a strong dependency on the seasonal cycle, evidence how plankton communities are directly impacted by a changing climate. Changes to temperature, nutrient availability, ocean currents and mixing, for example, will have a direct impact on plankton phenology, with consequences likely to propagate across the global ecosystem.

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