Organic nitrogen mineralization and substrate limitation of bacteria in Lake Michigan

Labile organic nitrogen mineralization and the apparent degree of bacterial substrate limitation were examined to consider seasonal relationships between substrate availability and bacterial activity in Lake Michigan. Accumulation rates of ammonium nitrogen in amino acid fortified and unfortified samples of epilimnetic Lake Michigan water, incubated in the dark, provided reasonable estimates of potential and actual rates of organic nitrogen mineralization. The labile organic nitrogen demand (LOND), defined as the difference between these respective rates, provided an index of heterotrophic potential. LOND ranged from ∼1–3 ng‐atoms N liter h −1 (during May–June and November) to 3–9 ng‐atoms N liter h −1 (during July–October) as compared to actual organic nitrogen mineralization rates of < 1 ng‐atom N liter h −1 in some unfortified samples. The high LOND, relative to actual turnover, observed in late summer is consistent with the hypothesis that growth rates of epilimnetic Lake Michigan bacteria are strongly limited by organic substrate during late stratification.