Deriving a proxy for iron limitation from chlorophyll fluorescence sensors on buoyancy gliders

Chlorophyll fluorescence from standard fluorometers, 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), decreases in the fluorescence quantum yield through the dissipation of excess energy as heat. NPQ variability is linked to both iron and light availability, and as such can provide important diagnostic information on phytoplankton physiology. Here we establish an in situ relationship between the Stern-Volner derivation of NPQ (NPQ_sv) and iron limitation from a series of nutrient addition experiments in the Sub-Antarctic zone, 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 none found for NPQ_max. Similar results from CTD profiles indicated that changes in NPQ were driven by increasing light availability from July to December, but both iron and light from January to February. However, α_NPQ variability, which has removed the effect of light availability, can be used as a proxy for iron limitation, with higher values being associated with greater iron stress. This approach was transferred to a buoyancy glider deployment at the same location by utilising the degree of fluorescence quenching as a proxy for NPQ_glider, which was plotted against in situ photosynthetically active radiation (PAR) 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 is 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 (r² = 0.62) is observed between α_NPQ and MLD at a ~10 day window. Results emphasise the important role of fine scale dynamics linked to synoptic storms in driving iron supply, particularly in summer when this micronutrient is especially limiting.