Plankton size diversity in UK coastal waters: observations and modelling

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Cell size is regarded as a 'master trait' in plankton ecology as it best summarises an organism's ability to provide key ecosystem functions, such as nutrient absorption, energy transfer, carbon export, and primary production rates [1,2,3]. Individual plankton organisms are generally microscopic in size, and so their ecosystem contributions are a result of their combined abundance. Here, we aim to understand how the size diversity of plankton communities affects their overall ecosystem functioning, and what drives changes in plankton community structure. This can be achieved through the construction of size-structured ecosystem models describing plankton community dynamics, and utilising long-term time series data from three plankton observatories around the UK coast (Plymouth, Stonehaven, and Loch Ewe).Of particular interest is the relationship plankton size diversity has with ecosystem functioning. The importance of size structure in a plankton community is supported by the fact that mean cell size is associated with the general behaviours of a plankton assemblage in the ocean. Phytoplankton communities with a small mean cell size often occur in nutrient-depleted waters [4], with energy retained near the surface due to intense grazing pressure from higher trophic levels. Conversely, communities of larger phytoplankton are not under such intense grazing pressure, and larger cells can sink to the deep ocean [5]. Through this process, carbon is transported away from the atmosphere in a process known as the biological pump. Similarly, some existing work has shown that plankton size diversity is associated energy transfer in plankton communities [6], however, the scope of this work is limited as it is simply an observation of correlated variables. By taking a mechanistic modelling approach we can better understand how and why size diversity impacts energy transfer, carbon export and primary production in plankton communities.[1] Kiorboe, T. & Hirst, A. Am. Nat. 183.4 (2014), [2] Litchman, E. & Klausmeier, A. Annu. Rev. Ecol. Evol. Syst. 39.1 (2008), [3] Litchman, E. et al. J. Plankton Res. 35.3 (2013), [4] Acevedo-Trejos, E. et al. Sci. Rep. 5.1 (2015), [5] Marañón, E. Ann. Rev. Mar. Sci. 7.1 (2015), [6] Garcia-Comas, C. et al. Proc. Royal Soc. B 283.1824 (2016)