Vertical mixing and the effects of solar radiation on photosystem II electron transport by phytoplankton in the Ross Sea Polynya

Detailed studies were made in the springtime Ross Sea Polynya of the interaction between near-surface mixing processes, inferred from Thorpe scales and acoustic backscatter, and the quantum efficiency and rate of PSII electron transport of phytoplankton photosynthesis measured at fine depth and time scales by active fluorometry. The phytoplankton assemblage was dominated by Phaeocystis antarctica. Surface conditions alternated between windy episodes producing Langmuir circulation (LC) or calm periods favoring internal waves, in both cases there was significant vertical transport of phytoplankton. During LC episodes, strong near-surface (<10 m) vertical gradients in quantum efficiency and inhibition of electron transport were observed despite evidence of vigorous vertical exchange. Under sunny mid-day conditions, profiles of PSII electron transport had sub-surface peaks and electron transport at PAR irradiances >40 μmol photons m−2 s−1 was generally lower compared to early morning profiles on the same day. In time-series deployments, intrinsic quantum efficiency (Fv′/Fm′) was relatively constant under calm conditions at 12 m, but highly variable at 6 m under LC conditions. Simultaneously measured time-series of acoustic backscatter and Fv′/Fm′ during LC conditions were negatively correlated, i.e. high backscatter arising from the downward advection of surface originating microbubbles coincided with low intrinsic efficiency. Transport also occurred in connection with finer-scale features, possibly arising from breaking surface waves. These results show that LC in the Ross Sea Polynya, in combination with changes in quenching driven by near surface UVR and PAR exposure, reduce photosynthetic electron transport over much of the upper mixed layer. Further observations and modeling will be needed to establish the extent to which this decreases water column productivity.

► Both Langmuir circulation and internal waves are important in the Ross Sea Polynya surface layer. ► These transport phytoplankton through the near-surface photoactive zone. ► Exposure in the photoactive zone is sufficient to inhibit photosynthetic electron transport. ► Affected phytoplankton are transported on the downwelling limbs of Langmuir cells. ► This propagates near-surface effects to the whole mixed layer.