Quantum efficiency of phytoplankton photochemistry measured continuously across gradients of nutrients and biomass in Lake Erie (Canada and USA) is strongly regulated by light but not by nutrient deficiency

Abstract

Unattended sensor networks are a cost-effective strategy to enhance the resolution of environmental datasets and are required to understand how large aquatic ecosystems respond to complex stressors (e.g., climate change). We made unattended and continuous measurements of the quantum yield of photosystem II ([Formula: see text]) photochemistry in the surface mixed layer of Lake Erie during three lake-wide cruises to observe how phytoplankton physiology varied across nutrient and taxonomic gradients. Three prominent diel [Formula: see text] patterns were noted. The diel maximum consistently occurred at sunrise or sunset, nocturnal measurements were consistently lower than diel maxima, and daytime values were strongly diminished by nonphotochemical quenching. The diurnal pattern was modeled as a function of irradiance to a mean accuracy of 0.03 to 0.04. Contrary to previously published reports in Lake Erie, [Formula: see text] was largely insensitive to indices of nutrient deficiency through space and time. This finding was consistent with much recent literature about [Formula: see text] and suggests that Lake Erie phytoplankton, like many others, can tune their photosynthetic machinery to maintain relatively high efficiency of photochemistry in photosystem II even when deficient in phosphorus or nitrogen.