Abstract
Chlorophyll fluorescence, primarily used to derive phytoplankton biomass, has long been an underutilized source of information on phytoplankton physiology. Diel fluctuations in chlorophyll fluorescence are affected by both photosynthetic efficiency and non-photochemical quenching (NPQ), where NPQ is a decrease in fluorescence through the dissipation of excess energy as heat. NPQ variability is linked to iron and light availability, and has the potential to provide important diagnostic information on phytoplankton physiology. Here we establish a relationship between NPQsv (Stern-Volmer NPQ) and indices of iron limitation from nutrient addition experiments in the sub-Antarctic zone (SAZ) of the Atlantic Southern Ocean, through the derivation of NPQmax (the maximum NPQsv value) and αNPQ (the light limited slope of NPQsv). Significant differences were found for both Fv/Fm and αNPQ for iron versus control treatments, with no significant differences for NPQmax. Similar results from CTDs indicated that changes in NPQ were driven by increasing light availability from late July to December, but by both iron and light from January to February. We propose here that variability in αNPQ, which has removed the effect of light availability, can potentially be used as a proxy for iron limitation (as shown here for the Atlantic SAZ), with higher values being associated with greater iron stress. This approach was transferred to data from a buoyancy glider deployment at the same location by utilising the degree of fluorescence quenching as a proxy for NPQGlider, which was plotted against in situ light to determine αNPQ. Seasonal increases in αNPQ are consistent with increased light availability, shoaling of the mixed layer depth (MLD) and anticipated seasonal iron limitation. The transition from winter to summer, when positive net heat flux dominates stratification, was coincident with a 24% increase in αNPQ variability and a switch in the dominant driver from incident PAR to MLD. The dominant scales of αNPQ variability are consistent with fine scale variability in MLD and a significant positive relationship was observed between these two at a ~10 day window. The results emphasise the important role of fine scale dynamics in driving iron supply, particularly in summer when this micronutrient is limiting.
Highlights
Chlorophyll fluorescence has previously been adopted as a proxy for chlorophyll concentration (Lorenzen, 1966), accurately measuring and interpreting these data is not trivial, in particular during daytime periods of high irradiance when fluorescence is depressed and no longer correlated with chlorophyll (Yentsch and Ryther, 1957; Slovacek and Hannan, 1977; Iron Limitation From Buoyancy GlidersOwens et al, 1980; Abbott et al, 1982; Falkowski and Kolber, 1995)
To estimate whether changes in nonphotochemical quenching (NPQ) variability can be directly linked to nutrient availability, iron limitation, a series of fluorescence light curves (FLCs) measurements were performed on nutrient addition incubation experiments
Results from these experiments displayed a seasonal development of iron limitation in the Atlantic sector of the sub-Antarctic zone (SAZ) (Ryan-Keogh et al, 2018b), with the addition of iron in December showing no significant differences in Fv/Fm, the photochemical efficiency of phytoplankton, [where Fv is the dark adapted variable fluorescence level, calculated as (Fm – Fo) and Fo is the minimum dark adapted fluorescence level], whereas in January and February the addition of iron resulted in significant increases in Fv/Fm
Summary
Chlorophyll fluorescence has previously been adopted as a proxy for chlorophyll concentration (Lorenzen, 1966), accurately measuring and interpreting these data is not trivial, in particular during daytime periods of high irradiance when fluorescence is depressed and no longer correlated with chlorophyll (Yentsch and Ryther, 1957; Slovacek and Hannan, 1977; Iron Limitation From Buoyancy GlidersOwens et al, 1980; Abbott et al, 1982; Falkowski and Kolber, 1995). Derivation of NPQ in situ has typically consisted of measurements of active chlorophyll fluorescence alongside the derivation of the maximum quantum yield of photosystem II, Fv/Fm, which has long been established as a key physiological indicator of phytoplankton, and in particular the significant decreases that occur under conditions of iron limitation (Geider, 1993; Moore et al, 2007). Measurements of both NPQ and Fv/Fm are routinely performed using active fluorometers, these are currently not readily available to be deployed on autonomous platforms. Whilst Fv/Fm cannot yet be routinely measured on autonomous platforms, if quenching is corrected the degree of quenching can provide useful information on NPQ
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call