Assessment of phytoplankton photosynthetic efficiency based on measurement of fluorescence parameters and radiocarbon uptake in the Kara Sea

Abstract

Spatial distribution of relative electron transport rate (rETR) values, the maximum quantum efficiency of PSII (Fv/Fm), and biomass-specific primary production (PB, mg C (mg Chl a)−1 h−1) were described for the eastern part of the Kara Sea in the autumn (September). A characteristic feature of this period was a noticeable decrease in the length of the day and the elevation angle of the sun, leading to a significant decrease in surface photosynthetically active radiation (PAR). Despite low light in the euphotic zone, the phytoplankton maximum quantum efficiency of PSII (Fv/Fm) was high (0.5–0.7) in the upper 30-m layer of the water column, which indicates a potentially active state of phytoplankton. At the same time, the main productive activity of phytoplankton was linked to the surface 0–3 m and was closely related to the daily incident PAR and the share of diatoms in the total phytoplankton biomass. Despite a decrease in light level and following a reduction in the values for productive characteristics of the phytoplankton community, diatoms continued to play a major role in primary productivity in the eastern Kara Sea at the end of the vegetative season. Comparative analysis of data obtained by two different techniques – fluorescence measurements and experimental carbon fixation estimations – demonstrated a close relationship between rETR and PB. However, surface values of PB changed more strongly than rETR, which may, apparently, reflect different efficiency in the use of absorbed light energy in the synthesis processes. Photosynthetic efficiency, reflecting the extent of use of the light energy caught in processes of organic matter synthesis, can be expressed through the ratio between PB and rETR values. The PB/rETR ratio increased in areas characterized by drastic gradients of hydrophysical conditions: over the external coastal shelf, central shelf and lower continental slope of the St Anna Trough.