Ammonium recycling supports toxic Planktothrix blooms in Sandusky Bay, Lake Erie: Evidence from stable isotope and metatranscriptome data.

•Planktothrix blooms in Sandusky Bay relied on regenerated NH4+ in late summer.•Summer regeneration provided bioavailable N equivalent to 77% of annual N load.•Planktothrix was N-starved and mobilized intracellularly stored N in late summer.•Planktothrix in Sandusky had higher NH4+ affinity than Microcystis in Maumee Bay.