Mechanisms Influencing the Stoichiometry of Phytoplankton Carbon to Phosphorus Uptake in a Shelf Sea from Spring to Fall

The seasonal cycle of resource availability in shelf seas has a strong selective pressure on phytoplankton diversity and the biogeochemical cycling of key elements, such as carbon (C) and phosphorus (P). Shifts in carbon consumption relative to P availability, via changes in cellular stoichiometry for example, can lead to an apparent ‘excess’ of carbon production. We measured P-uptake in parallel to C-fixation by phytoplankton communities in the Celtic Sea (NW European Shelf) in spring (April 2015), summer (July 2015) and fall (November 2014). Short-term (<6h) P-uptake coupled with dissolved organic phosphorus (DOP) release, in parallel to short-term C-fixation, dissolved organic carbon (DOC) release and net (24 h) primary production, were all measured across an irradiance gradient designed to typify vertically and seasonally varying light conditions. Surface rates of phosphate uptake ranged from 1.2 to 5.1 nmol P l^-1 h^-1 in spring, 0.5 to 2.1 nmol P l^-1 h^-1 in summer and 0.2 to 0.4 nmol P l^-1 h^-1 in autumn. In terms of the percentage of extracellular release, DOP was lowest during summer (2-11%), low prior to the bloom in spring and increasing afterwards (from 2 to 50%), and high in autumn (22-62%). When examining these rates in the context of C-fixation, we find that spatially and seasonally resolved C:P stoichiometry is sensitive to: the time-scale of integration; release of DOP and DOC; gross or net primary production; and light and substrate availability. Redfield-like ratios of C:P were only observed when we consider C- and P-uptake on daily time-scales, include release of DOP and DOC, consider daily primary production, examine intermediate depths (irradiances) in the euphotic zone (or integrals), and the turnover of the P pool is relatively slow. We conclude that Redfield represents an ‘average’ of both cellular phytoplankton physiological processes and the community’s growth dynamics in an unstable environment.