Abstract. Recent global water quality crises point to an urgent need for greater
understanding of cyanobacterial harmful algal blooms (cHABs) and their
drivers. Nearshore areas of Lake Erie such as Sandusky Bay may become
seasonally limited by nitrogen (N) and are characterized by distinct cHAB
compositions (i.e., Planktothrix over Microcystis). This
study 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 removal
processes were quantified by stable isotopic approaches. Dissimilatory N reduction was a relatively modest N sink, with denitrification, anammox, and
N2O 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 and
dissimilatory demands, but N fixation alleviated this deficit. N fixation
made up 23.7–85.4 % of total phytoplankton N acquisition and indirectly
supports Planktothrix blooms. However, N fixation rates were
surprisingly uncorrelated with NO3- or NH4+ concentrations.
Owing to temporal separation in sources and sinks of N to Lake Erie, Sandusky
Bay oscillates between a conduit and a filter of downstream N loading to Lake
Erie, delivering extensively recycled forms of N during periods of low
export. Drowned river mouths such as Sandusky Bay are mediators of downstream
N loading, but climate-change-induced increases in precipitation and N
loading will likely intensify N export from these systems.
N2O production in the eastern South Atlantic: Analysis of N2O stable isotopic and concentration data
