Urea‐based fertilisers have grown in popularity over the past half‐century and are now the dominant nitrogen (N) form applied to agricultural landscapes. The widespread use of urea fertilisers has favoured its export to local waterways, and urea pollution may be contributing to the propagation and maintenance of cyanobacteria harmful algal blooms in fresh waters. The relative success of cyanobacteria in response to urea and inorganic N additions was studied to understand whether the recent changes in the magnitude and composition of N loading have created a scenario that now favours the dominance of cyanobacteria in fresh waters. Growth and photosynthetic efficiency of three bloom‐forming freshwater cyanobacteria (Microcystis, Dolichospermum and Synechococcus) grown on nitrate (NO3−), ammonium (NH4+) and urea (CO(NH2)2) as the sole N form were monitored. We hypothesised that N substrates that require the lowest energetic investment or offer the highest energetic return would favour optimal growth and photosynthetic performance. We predicted that urea would result in higher cellular growth and pigment production relative to NH4+ or NO3−, as urea provides twice the amount N and an additional carbon (C) source making it more energetically efficient. Cyanobacteria biomass was not significantly enhanced on urea relative to inorganic N forms. Growth on urea was matched by NO3− for all species, whereas growth on NH4+ was halved compared urea or NO3−. However, cyanobacteria cells had higher pigment concentrations when grown on urea relative to inorganic N sources. These findings suggest that the additional nutrient building blocks supplied from the hydrolysis of urea were not directed towards active growth, but rather accumulated in secondary pools to increase production of N‐rich compounds, such as pigments. Although urea did not influence cyanobacteria quantity “(i.e. biomass)” compared to inorganic N sources, it produced higher “quality” cells by enhancing pigment synthesis and potentially giving cyanobacteria a competitive advantage in light‐limiting conditions. Furthermore, when supplied in excess, cyanobacteria rapidly consumed urea in excess of their biosynthetic requirements suggesting a form of urea “gluttony.” These results demonstrate the importance of N speciation on cyanobacteria physiological responses and reinforce the emerging links between urea and cyanobacteria harmful algal blooms in inland waters.

Meromictic lakes provide a physically stable environment in which proxies for potentially harmful cyanobacteria are exceptionally well-preserved in the sediments. In Sunfish Lake, a meromictic lake that has recently become the focus of citizen concern due to the apparent rise in cyanobacteria blooms, we used a multi-proxy paleolimnological approach pairing novel spectral (i.e., VNIRS) and molecular (i.e., qPCR) assessment tools to explore long-term cyanobacteria trends. We hypothesized that climate change over the past 50 years altered the Sunfish Lake environment to favour cyanobacteria dominance, resulting in an increased incidence of bloom events. Spectral and genetic results aligned to reveal an unprecedented abundance of cyanobacteria in modern times and coincided with warmer and wetter climatic conditions in the region. Our findings offer evidence for climate-driven shifts in cyanobacteria abundance and suggest that a shift towards warmer and wetter conditions supports the rise of cyanobacteria in lakes.

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Increasing inputs of dissolved organic matter (DOM) to northern lakes is resulting in ‘lake browning.’ Lake browning profoundly affects phytoplankton community composition by modifying two important environmental drivers—light and nutrients. The impact of increased DOM on native isolates of red and green-pigmented cyanobacteria identified as Pseudanabaena, which emerged from a Dolichospermum bloom (Dickson Lake, Algonquin Provincial Park, Ontario, Canada) in 2015, were examined under controlled laboratory conditions. The genomes were sequenced to identify phylogenetic relatedness and physiological similarities, and the physical and chemical effects of increased DOM on cellular performance and competitiveness were assessed. Our study findings were that the isolated red and green phenotypes are two distinct species belonging to the genus Pseudanabaena; that both isolates remained physiologically unaffected when grown independently under defined DOM regimes; and that neither red nor green phenotype achieved a competitive advantage when grown together under defined DOM regimes. While photosynthetic pigment diversity among phytoplankton offers niche-differentiation opportunities, the results of this study illustrate the coexistence of two distinct photosynthetic pigment phenotypes under increasing DOM conditions.

The risk of human exposure to cyanotoxins is partially influenced by the location of toxin-producing cyanobacteria in waterbodies. Cyanotoxin production can occur throughout the water column, with deep water production representing a potential public health concern, specifically for drinking water supplies. Deep cyanobacteria layers are often unreported, and it remains to be seen if lower incident rates reflect an uncommon phenomenon or a monitoring bias. Here, we examine Sunfish Lake, Ontario, Canada as a case study lake with a known deep cyanobacteria layer. Cyanotoxin and other bioactive metabolite screening revealed that the deep cyanobacteria layer was toxigenic [0.03 μg L −1 microcystins (max) and 2.5 μg L −1 anabaenopeptins (max)]. The deep layer was predominantly composed of Planktothrix isothrix (exhibiting a lower cyanotoxin cell quota), with Planktothrix rubescens (exhibiting a higher cyanotoxin cell quota) found at background levels. The co-occurrence of multiple toxigenic Planktothrix species underscores the importance of routine surveillance for prompt identification leading to early intervention. For instance, microcystin concentrations in Sunfish Lake are currently below national drinking water thresholds, but shifting environmental conditions (e.g., in response to climate change or nutrient modification) could fashionan environment favoring P. rubescens, creating a scenario of greater cyanotoxin production. Future work should monitor the entire water column to help build predictive capacities for identifying waterbodies at elevated risk of developing deep cyanobacteria layers to safeguard drinking water supplies.