Physiological characteristics and food‐web dynamics of Synechococcus in Lakes Huron and Michigan

Single Synechococcus cells accounted for an average of 10% (range 1–26%) of surface mixed‐layer primary production in Lakes Huron and Michigan in 1986–1988. Maximal photosynthetic rates (Pmax) were relatively low (range = 1.9–6.0 fg C cell−1 h−1) and no significant photoinhibition was found at irradiances as high as 3.0 Einst m−2 h−1. Synechococcus growth rates estimated by four techniques (ampicillin, 14C uptake, dilution, and small inocula) ranged from 0.1 to 0.9 d−1 with a mean of 0.37. Although substantial variability was noted among techniques on any one date, on average all estimates were in reasonable agreement with the exception of the dilution estimates which were significantly lower (P < 0.01). Three techniques for estimating grazing loss rates (ampicillin, dilution, and 14C‐labeling of Synechococcus) provided similar estimates ranging from 0.1 to 0.7 d−1. On specific dates, grazing loss rates were 33–120% of growth rates, suggesting that grazing was the major loss for Synechococcus populations in these lakes. Most of the grazing loss (68%) was attributable to small (4–10 µm), heterotrophic flagellates and ciliates. Crustaceans and rotifers accounted for only a small percentage of total grazing loss (5–21%) even when Daphnia accounted for 40% of crustacean biomass.