Controls on phytoplankton physiology in Lake Ontario during the late summer: evidence from new fluorescence methods

Abstract

Fast repetition rate fluorescence (FRRF) and spectral fluorescence, together with measures of nutrients and pigments, were used to characterize the composition and photosynthetic physiology of Lake Ontario phytoplankton in late summer and relate them to environmental conditions. Two stations demonstrated effects from relatively heavy anthropogenic disturbance and showed that the response of phytoplankton physiology to different impacts is highly variable. Other stations were more similar in phytoplankton composition, and in situ fluorescence yields ([Formula: see text]) in the lower surface mixed layer suggested good physiological condition (0.45–0.50). Nutrient ratios and mean irradiance indicated a general state of light saturation and slight phosphorus (P) deficiency, but physiological variations among stations were unrelated to measures of P deficiency. Fluorescence yields often decreased when surface layer samples were held in the dark, consistent with an induction of chlororespiration and prior exposure to supersaturating levels of irradiance. Comparative estimates of photosynthesis by FRRF and ¹⁴C revealed disparities suggestive of substantial differences between in situ and incubation methods, while spectral fluorescence appeared to underestimate cyanobacterial abundance. FRRF parameters, particularly [Formula: see text], were effective in identifying higher-impact stations and showed promise as an efficient means of characterizing variations in phytoplankton condition that may underlie phenomena such as taste and odour production.