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

Abstract

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 (NO − 3 ) and ammonium (NH + 4 ) 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 N 2 O production accounting for 84, 14, and 2 % of sediment N removal, respectively. Phytoplankton assimilation was the dominant N uptake mechanism, and NO − 3 uptake rates were higher than NH + 4 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 NO − 3 or NH + 4 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.

Seasonal and Spatial Variation in the Location and Reactivity of a Nitrate‐Contaminated Groundwater Discharge Zone in a Lakebed

“…Denitrification and anammox remove fixed N by reducing NO 3 − or nitrite (NO 2 − ) and producing nitrogen gas (N 2 ), while DNRA alters the mobility of the fixed N, without removing it, by reducing NO 3 − to NH 4 + . Nitrification and denitrification have been widely studied in rivers and streams as important mediators of N transport (Böhlke et al, 2009;Duff et al, 2008;Groffman et al, 2005;Hinkle et al, 2001;Mulholland et al, 2009;Peterson et al, 2001), and all of these processes have been found in groundwater and lake sediments (Hampel et al, 2018;Li et al, 2017;Miller & Smith, 2009;Rissanen et al, 2013;Rysgaard et al, 1993;Salk et al, 2018;Smith et al, 2015;Smith & Duff, 1988;Smith et al, 1991b;Stoliker et al, 2016;Washbourne et al, 2011). Abiotic reactions, such as the oxidation of iron (Fe +2 ) by NO 2 − producing nitrous oxide (N 2 O), can also contribute to N-species cycling (Klueglein et al, 2014;Klueglein & Kappler, 2013).…”

A Hybrid Lagrangian-Eulerian Particle Model for Ecosystem Simulation

“…This largely agricultural watershed leads to high nitrogen 370 and phosphorus entering Sandusky Bay. Combining these high nutrient loads with the 371 physical aspects leads to very high concentrations of nitrogen and phosphorus within 372 Sandusky Bay, thus resulting in these cyanobacteria blooms (Planktothrix agardhii) 373 [Davis et al, 2015;Salk et al, 2018] …”

A Hybrid Lagrangian-Eulerian Particle Model for Ecosystem Simulation

“…Since the mid-1990s, harmful algal blooms (HABs) have become the new norm for summer months in the Lake Erie ecosystem [29]. Harmful algal blooms occur in the system when cyanobacteria are provided the right temperature, light, and nutrient conditions to proliferate [28]. When these blooms transpire, they have many adverse impacts.…”

“…"Whereas the colonial cyanobacterium Microcystis dominates the cHAB community in offshore regions of western Lake Erie, filamentous Planktothrix has been shown to persist in N-limited bays and tributaries" [28], such as in Sandusky Bay [27,28,32]. Situated on Lake Erie's southwestern coast, Sandusky Bay borders Ohio's Ottawa, Erie, and Sandusky counties ( Figure 1).…”

“…Combining these high nutrient loads with the physical aspects leads to very high concentrations of nitrogen and phosphorus within Sandusky Bay, thus resulting in these cyanobacteria blooms (Planktothrix agardhii) [27,28,32].…”