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Abstract Efforts to reduce the frequency, extent, and toxicity of harmful algal blooms (HABs) require knowledge about drivers of algal growth, toxin production, and shifts in phytoplankton community composition to cyanobacterial dominance. Although labile nitrogen (N) and phosphorus (P) fuel primary production, micronutrients also play roles as the enzymatic engines that facilitate rapid and efficient growth and toxin production. Macro‐ and micronutrient availability can shape community composition and function by selecting for particular taxa. To address how phytoplankton in two Great Lakes subbasins respond to macro‐ and micronutrients, we conducted bottle incubation enrichment experiments using water collected from two blooming and two nonblooming sites in Lakes Erie and Michigan during late summer (August). Three of the four sites exhibited multi‐nutrient limitation of growth. Both blooming sites responded strongest to enrichment. Both nonblooming sites responded the strongest to enrichment, and three of the four sites responded in some way to a mix of micronutrients (Fe, Mn, Mo, Ni, and Zn).Microcystis aeruginosarelative abundance increased most with N enrichment, while P enrichment increased the abundance of diatoms and chlorophytes. At the Fox River, N‐enriched communities grew 10%–20% more than non‐N enriched communities (measured as chlorophylla), and N‐enriched communities had, on average, over twice as much microcystin (non‐N communities average MC = 2.45 μg · L⁻¹, +N communities MC = 5.35 μg · L⁻¹). These overarching trends support the idea that control of HABs may not be effective with a P‐only approach. 

Metals are used in primary producer metabolic pathways, such as photosynthesis and the acquisition of macronutrients nitrogen (N) and phosphorus (P), yet we often do not know their potential as limiting nutrients in freshwaters. In the Great Lakes, metals have sometimes been identified as limiting the acquisition of macronutrients, mostly in off-shore waters that are relatively isolated from tributary inputs and sediment interactions. We hypothesized that another area where metals might be important was within harmful algal blooms (HABs). Harmful algal blooms are more likely to occur where N and P loads are elevated due to human activities, but short-term growth assays still often find summer bloom communities are N or P limited due to high biotic demand. This high biotic is associated with rapid nutrient recycling which may increase demand for trace metals beyond the available supply. A relatively common cyanotoxin (microcystin) has also been hypothesized to have a role in trace metal management, so trace metal demand may also influence the toxicity of bloom communities. Here, we used nutrient diffusing substrates to measure the magnitude of macronutrient and trace metal effects on growth and toxicity of biofilms suspended in 10 nearshore sites in Lake Michigan and Lake Erie (5 with and 5 without perennial HABs). We measured microcystin, chlorophyll a, ash free dry mass and community composition on the experimental biofilms. 

To address how phytoplankton in the Great Lakes respond to macro- and micronutrients, we conducted a bottle incubation enrichment experiment using water collected from blooming (Maumee Bay and Fox River) and non-blooming sites (Detroit River and Ford River) in Lakes Erie and Michigan, respectively, during late summer. Surface water from these locations was collected and taken to Kent State University either via overnight shipping (Lake Michigan sites) or driven directly after collection (Lake Erie sites). Chlorophyll a (an index of overall biomass), community composition and toxicity were all measured as responses to treatments of labile inorganic nitrogen (N), phosphorus (P) and a mixture of micronutrients (chemical symbols: Fe, Mn, Mo, Ni, Zn).