Deriving a Proxy for Iron Limitation From Chlorophyll Fluorescence on Buoyancy Gliders

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 nonphotochemical 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 NPQ sv (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 NPQ max (the maximum NPQ sv value) and α NPQ (the light limited slope of NPQ sv ). Significant differences were found for both F v /F m and α NPQ for iron versus control treatments, with no significant differences for NPQ max . 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 α NPQ , 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 utilizing the degree of fluorescence quenching as a proxy for NPQ Glider , which was plotted against in situ light to determine α NPQ. Seasonal increases in α NPQ 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 α NPQ variability and a switch in the dominant driver from incident PAR to MLD. The dominant scales of α NPQ 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 emphasize the important role of fine scale dynamics in driving iron supply, particularly in summer when this micronutrient is limiting.

Iron Limitation Drives the Globally Extreme Fluorescence/Chlorophyll Ratios of the Southern Ocean

“…We also observe a significant positive relationship between F/Chl and NPQ max (second‐ranked candidate predictor; Table S3 in Supporting Information ). While NPQ max has not conclusively been shown to be a proxy for Fe limitation, Ryan‐Keogh and Thomalla (2020) showed similar patterns in NPQ max and α NPQ when glider and CTD data from different seasons (with different levels of Fe limitation) were analyzed. Likewise, these two variables are correlated in our BGC‐Argo dataset (conditional r 2 = 0.44, p < 0.001).…”

“…The NPQ‐corrected fluorescence was used to estimate average fluorescence between 10 and 40 dbar ( F mean ), which could then be related to the average Chl Kd over the same depths, yielding F/Chl. Additionally, NPQ‐corrected fluorescence was used to estimate a proxy for Fe limitation of the phytoplankton, following the approach by Ryan‐Keogh and Thomalla (2020). Building on earlier work relating NPQ and the Fe limitation status of phytoplankton (Schallenberg et al., 2020; Schuback & Tortell, 2019; Schuback et al., 2015), these authors found that the initial slope of the NPQ‐PAR relationship (α NPQ ) was an indicator of Fe limitation.…”

Diel quenching of Southern Ocean phytoplankton fluorescence is related to iron limitation

“…Ultimately, this approach to fluorescence has the potential to be applied to data from autonomous floats in the Southern Ocean, such as the biogeochemical (BGC) Argo fleet. A recent application to glider data from the SAZ in the Atlantic Ocean has allowed identification of regions and time periods where Fe limitation was relieved (Ryan-Keogh and Thomalla, 2020). Studies of this kind will improve our understanding of biogeochemical cycles in the understudied Southern Ocean.…”

Single-Turnover Variable Chlorophyll Fluorescence as a Tool for Assessing Phytoplankton Photosynthesis and Primary Productivity: Opportunities, Caveats and Recommendations

“…Furthermore, the absorption cross-section of PSII photochemistry (σ PII ) often increases under iron-limiting conditions, necessitating rapidly inducible NPQ under fluctuating light availability (e.g., Schuback and Tortell, 2019). Increased NPQ levels under iron-limited conditions can be extrapolated to observations of diurnal quenching of ChlF from "standard" in-vivo ChlF fluorometers deployed in continuous flow through and in-situ systems (Roesler and Barnard, 2013;Ryan-Keogh and Thomalla, 2020;Schallenberg et al, 2020).…”