On phytoplankton growth and loss rates to microzooplankton in the epilimnion and metalimnion of Lake Ontario in mid-summer

Abstract

Size-fractionated (20–153μm; 2–20μm; and 0.2–2μm) chlorophyll-a (Chl-a) specific growth and loss rates of phytoplankton size due to microzooplankton grazing were determined in the metalimnion and epilimnion at pelagic hydrographic stations across Lake Ontario in July 2008, in addition to rates of growth and grazing of phycoerythrin (PE)-rich picocyanobacterial populations measured using flow cytometry. Eight of the nine stations contained pronounced deep chlorophyll maxima (DCM) in the metalimnion, which corresponded to the base of the euphotic zone (approximately 17m). DCM were composed primarily of the Heterokontophyta and Pyrrophyta, and frequently also contained smaller metalimnetic peaks of PE-rich cyanobacteria and Cryptophyta. Specific growth and grazing rates of all plankton size fractions suggested close coupling of growth and grazing rates. Using dilution assays, Chl-a-specific growth and grazing rates (per day; mean±standard deviation) for total Chl-a (0.2–153μm) were 0.42±0.30 and 0.53±0.24, respectively, in the epilimnion and 0.58±0.74 and 1.03±0.96 in the metalimnion. Rates for cell specific growth and grazing for PE-picocyanobacteria, the numerically dominant phytoplankton (107.1 to 108.1cells/L), were much higher overall with growth exceeding grazing at all stations and depths when measured by both dilution assays and antibiotic assays. The results of this study suggest that the DCM is as active a layer as the epilimnion during the period of summer thermal stratification and should be included in estimates of primary and secondary production.