Nitrogen cycling in Sandusky Bay, Lake Erie: oscillations between strong and weak export and implications for harmful algal blooms

摘要

Recent global water quality crises point to an urgent need for greaterunderstanding of cyanobacterial harmful algal blooms (cHABs) and theirdrivers. Nearshore areas of Lake Erie such as Sandusky Bay may becomeseasonally limited by nitrogen (N) and are characterized by distinct cHABcompositions (i.e., Planktothrix over Microcystis). Thisstudy investigated phytoplankton N uptake pathways, determined drivers of N depletion,and characterized the N budget in Sandusky Bay. Nitrate(NO3−) and ammonium (NH4+) uptake, N fixation, and N removalprocesses were quantified by stable isotopic approaches. Dissimilatory N reduction was a relatively modest N sink, with denitrification, anammox, andN2O production accounting for 84, 14, and 2 % of sediment N removal,respectively. Phytoplankton assimilation was the dominant N uptake mechanism,and NO3− uptake rates were higher than NH4+ uptake rates.Riverine N loading was sometimes insufficient to meet assimilatory anddissimilatory demands, but N fixation alleviated this deficit. N fixationmade up 23.7–85.4 % of total phytoplankton N acquisition and indirectlysupports Planktothrix blooms. However, N fixation rates weresurprisingly uncorrelated with NO3− or NH4+ concentrations.Owing to temporal separation in sources and sinks of N to Lake Erie, SanduskyBay oscillates between a conduit and a filter of downstream N loading to LakeErie, delivering extensively recycled forms of N during periods of lowexport. Drowned river mouths such as Sandusky Bay are mediators of downstreamN loading, but climate-change-induced increases in precipitation and Nloading will likely intensify N export from these systems.