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

Salk, Kateri R.; Bullerjahn, George S.; McKay, Robert Michael L.; Chaffin, Justin D.; Ostrom, Nathaniel E.

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 (inline-formulaN) and are characterized by distinct cHAB compositions (i.e., Planktothrix over Microcystis). This study investigated phytoplankton inline-formulaN uptake pathways, determined drivers of inline-formulaN depletion, and characterized the inline-formulaN budget in Sandusky Bay. Nitrate (inline-formula M5inlinescrollmathml chem normal NO normal 3 - 25pt16ptsvg-formulamathimg91b2e19ca239409a7665981c17575147 bg-15-2891-2018-ie00001.svg25pt16ptbg-15-2891-2018-ie00001.png ) and ammonium (inline-formula M6inlinescrollmathml chem normal NH normal 4 + 24pt15ptsvg-formulamathimg6ca56caa63735c5009fe6b299c1a126b bg-15-2891-2018-ie00002.svg24pt15ptbg-15-2891-2018-ie00002.png ) uptake, inline-formulaN fixation, and inline-formulaN removal processes were quantified by stable isotopic approaches. Dissimilatory inline-formulaN reduction was a relatively modest inline-formulaN sink, with denitrification, anammox, and inline-formulaN2O production accounting for 84, 14, and 2 inline-formula% of sediment inline-formulaN removal, respectively. Phytoplankton assimilation was the dominant inline-formulaN uptake mechanism, and inline-formula M15inlinescrollmathml chem normal NO normal 3 - 25pt16ptsvg-formulamathimgecc3e6dd5af0ffb1da8bfbfcb16b8e8b bg-15-2891-2018-ie00003.svg25pt16ptbg-15-2891-2018-ie00003.png uptake rates were higher than inline-formula M16inlinescrollmathml chem normal NH normal 4 + 24pt15ptsvg-formulamathimg97c709e7ff43e05afce356dc3f53b497 bg-15-2891-2018-ie00004.svg24pt15ptbg-15-2891-2018-ie00004.png uptake rates. Riverine inline-formulaN loading was sometimes insufficient to meet assimilatory and dissimilatory demands, but inline-formulaN fixation alleviated this deficit. inline-formulaN fixation made up 23.7–85.4 inline-formula% of total phytoplankton inline-formulaN acquisition and indirectly supports Planktothrix blooms. However, inline-formulaN fixation rates were surprisingly uncorrelated with inline-formula M23inlinescrollmathml chem normal NO normal 3 - 25pt16ptsvg-formulamathimg9d4fcb6817fb360ca45e4c6c5f1d67f7 bg-15-2891-2018-ie00005.svg25pt16ptbg-15-2891-2018-ie00005.png or inline-formula M24inlinescrollmathml chem normal NH normal 4 + 24pt15ptsvg-formulamathimgae42caf6f031f0c6641dd1d09f6cb28c bg-15-2891-2018-ie00006.svg24pt15ptbg-15-2891-2018-ie00006.png concentrations. Owing to temporal separation in sources and sinks of inline-formulaN to Lake Erie, Sandusky Bay oscillates between a conduit and a filter of downstream inline-formulaN loading to Lake Erie, delivering extensively recycled forms of inline-formulaN during periods of low export. Drowned river mouths such as Sandusky Bay are mediators of downstream inline-formulaN loading, but climate-change-induced increases in precipitation and inline-formulaN loading will likely intensify inline-formulaN export from these systems.

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Salk, Kateri R. / Bullerjahn, George S. / McKay, Robert Michael L. / et al: Nitrogen cycling in Sandusky Bay, Lake Erie: oscillations between strong and weak export and implications for harmful algal blooms. 2018. Copernicus Publications.

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