Impact of light, turbulent mixing, and nutrients on phytoplankton photoinhibition in the euphotic zone from the subtropical to subpolar North Atlantic

Abstract. Photoinhibition, a condition caused when high photon fluxes lead to the rate of electron transport from light receptors in photosystem II exceeding a cell’s photochemical capacity, causes damage to the photosynthetic apparatus and reduces photosynthesis. The balance between damage and repair processes in cells determines the degree of photoinhibition (PI). PI is here shown to be ubiquitous in natural phytoplankton communities in the North Atlantic. PI effects were determined as the difference between the maximum photosynthetic quantum yield (F_v/F_m) recorded at the time of sampling and following a 4 h low-light incubation. Samples from the euphotic zone in the North Sea showed significant daily variation of PI extending to depths of 20 m. Principal component analysis identified a strong correlation between PI and light availability, whereas nutrient distributions and other environmental variables were less influential on PI. Furthermore, an almost immediate response of PI to changes in light intensity was demonstrated. A light-dependent linear regression model of PI for the northern North Sea explained a significant increase in PI (R² = 0.4) with intensity of insolation. PI in samples from the euphotic zone in the subpolar Irminger Sea and the subtropical Sargasso Sea also showed significantly elevated PI at higher light levels. Our results show that PI in natural phytoplankton communities is closely related to, and occurs almost immediately, in response to changes in insolation. The sensitivity of phytoplankton communities’ photo-chemical capacity to insolation changes may help to explain the wide variation in photosynthetic parameters reported in nature and should be taken into account when estimating ocean primary production.