Gross photosynthesis and lake community metabolism during the spring phytoplankton bloom

Abstract

Daily productivity determinations of linear photosynthetic electron transfer and of net and gross inorganic CO2 uptake were determined in situ throughout a 6-week sampling period of the spring phytoplankton bloom in Esthwaite Water in the English Lake District. Photosynthetic electron transfer rates, expressed as gross O2 evolution, were determined from fast repetition rate (FRR) fluorometry and discrete laboratory measurements of the photosynthetic unit size. These gross O2 evolution determinations were also made free from contemporaneous dark-adapted FRR measurements and the need for sample blanks. Net and gross CO2 uptake was determined from changes in total inorganic carbon calculated from in situ pH measurements. Two phases to the bloom were observed. An initial bloom was dominated by the diatom Asterionella formosa, while a secondary phase was characterized by an increase in flagellates and cyanobacteria. For both phases, daily FRR-based gross O2 evolution and pH-based gross CO2 uptake were tightly coupled, suggesting that daily gross O2 production was driving daily changes in CO2 assimilation and thus in community metabolism. This is the first investigation to show closely related rates of gross O2 production and CO2 uptake measured in situ.