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Chemically reduced nitrogen forms are increasing in aquatic systems and beginning to reach concentrations not previously measured. Despite this, little research has examined the potential of reduced nitrogen forms to encourage excess nitrogen storage and promote algal bloom longevity compared to oxidised forms. A 2‐week field, pulse‐application experiment was carried out using 1,100‐L plastic limnocorrals to examine cyanobacterial community response to three nitrogen forms, including nitrate, ammonium, and urea (added as 600 µg N/L). Cell pigments and counts were used to calculate cell‐specific pigment concentrations, and cell‐associated microcystin concentrations were also measured to examine toxin response to a shift in nitrogen source. Results showed that, upon nitrogen introduction, extracellular nitrogen quickly decreased in accordance with an increase in cellular phycocyanin 72 hr after fertilisation. Ammonium and urea treatments had more phycocyanin/cell than nitrate or control treatments at 72 hr. After 72 hr, phycocyanin content quickly decreased, consistent with the use of nitrogen from phycobiliproteins. Despite the decrease in light‐harvesting pigments, the total number of cyanobacterial cells increased in the ammonium and urea treatments after 2 weeks. Cyanobacterial particulate toxin (microcystin) quotas were not affected by nitrogen additions. Results show that reduced nitrogen forms encourage greater nitrogen storage as pigments and increase bloom longevity compared to oxidised forms. Findings support previous studies that suggest reduced nitrogen forms encourage greater cell density and algal bloom persistence. Results further point to excess nitrogen storage as another mechanism that allows cyanobacteria to dominate freshwater systems despite variable environmental conditions.

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Reliable remote sensing platforms and methods for monitoring phytoplankton are needed for mitigating the detrimental impacts of cyanobacterial harmful algal blooms on small inland waterbodies. Commercial unoccupied aerial systems (UASs) present an affordable high-resolution solution for the rapid assessment of cyanobacterial abundance in small aquatic systems by recording the reflectance of photosynthetic pigments found in all phytoplankton (i.e., chlorophyll a [Chl a]) and those related to cyanobacteria (i.e., phycocyanin). This study evaluates the performance of four sensors, including visible light spectra (red, green, blue -[RGB]) sensors on the Phantom 4 and Phantom 4 Professional platforms, the MAPIR Survey3W modified multispectral (i.e., near-infrared, green, blue) sensor, and the Parrott Sequoia multispectral (i.e., green, red, near-infrared, red-edge) sensor for estimating cyanobacterial abundance. The performance of each sensor was determined by comparing 26 vegetation indices to Chl a and phycocyanin measurements of 54 ponds that varied in size and productivity. Vegetation indices that included the red and near-infrared wavelengths generated from Parrot Sequoia aerial images provided the best Chl a (i.e., Normalized Difference Vegetation Index, r 2 = 0.79, p < 0.0001) and phycocyanin (i.e., Green Normalized Difference Vegetation Index, r 2 = 0.64, p < 0.0001) estimates. The RGB sensors were moderately effective for estimating Chl a, whereas the MAPIR Survey3W generated poor estimates of both pigments due to differences in recorded wavelengths. Results suggest commercial multiband multispectral UAS sensors provide a low-cost, plug-and-play alternative for managers and researchers interested in integrating remote sensing tools for quantitatively estimating phytoplankton abundance in small inland systems.Small inland waterbodies are ubiquitous and provide important ecosystems services, including, but not limited to, drinking water supply, food production, aquatic recreation, and climate regulation (Biggs et al. 2017). Blooms dominated by cyanobacterial harmful algal blooms (CyanoHABs) are becoming more frequent and intense as a result of climate change and cultural eutrophication (Huisman et al. 2018). CyanoHABs impair freshwater systems through the production of toxins (i.e., cyanotoxins) linked to illness and fatalities in livestock, pets, and humans, as well as off-flavor compounds (e.g., geosmin and 2-methylisoborneol) that impart a musty scent and flavor on drinking water and farm-raised fish leading to significant economic losses to aquaculture and drinking water industries (Merel et al. 2013;Tucker and Schrader 2020). CyanoHABs are traditionally monitored via cell counts or by measuring the concentration of photosynthetic pigments found in all phytoplankton (i.e., chlorophyll a [Chl a]) relative to accessory pigments abundant in cyanobacteria (i.e., phycocyanin). However, traditional methods are constrained by processing times, training, cost, and may not be representative of the entire waterbody (...

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