Gross photosynthesis and lake community metabolism during the spring phytoplankton bloom

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. Daily phytoplankton production is determined routinely in aquatic studies to examine ecosystem energetics. Typically, daily production is estimated from discrete incubations of natural material. However, this approach may give rise to considerable error because realistic environmental conditions may not be reproduced in the incubation bottles (Wilhelm et al. 2004). Various techniques are now used to provide direct, in situ measures of primary productivity. These include diel O2 (Gelda and Effler 2002) and CO2 (Maberly 1996) exchange, and fast repetition rate (FRR) fluorescence (Kolber et al. 1998; Suggett et al. 2001; Raateoja et al. 2004). Direct in situ O2 and CO2 measurements yield a measure of the net daily planktonic community metabolism during the day and of net community respiration at night (del Giorgio et al. 1999; Cole et al. 2000), whereas FRR fluorescence yields the instantaneous gross phytoplankton photochemical activity (Falkowski and Kolber 1995; Gorbunov et al. 2001). We combined these techniques to examine the transfer of energy from photons to carbon dioxide fixation in situ. Continuous in situ measurements of FRR fluorescence and pH were used to estimate daily phytoplankton productivity at a single depth throughout a 6-week sampling period of the 2003 spring bloom in Esthwaite Water in the English Lake District. We describe continuous FRR-based productivity determinations that are free from additional dark-adapted fluorescence and sample blank measurements and incorporate photosynthetic unit size measurements from natural samples. Here we show that the gross photosynthesis measured by FRR and gross daily community productivity estimated from diel changes of TCO2 covaried in two phases of the spring bloom. The first phase was almost exclusively dominated by the diatom Asterionella formosa, whereas the second phase was characterized by increased numbers of flagellates, in particular Rhodomonas spp. and Chrysochromulina parva, and the cyanobacterium Anabaena circinalis. Our results demonstrate tight coupling of gross photosynthetic electron transfer rates with gross community metabolism. Different photosynthetic quotients were observed for the two phases of the spring bloom corresponding with the change in phytoplankton community structure.