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Evidence suggests that low cobalt (Co) may limit heterocyst frequency (HF) in filamentous diazotrophic cyanobacteria. Therefore, the effect of three Co concentrations on HF in four species in the genera Aphanizomenon and Dolichospermum was examined in N‐depleted (N was not added) culture. Long‐term HF data were also analysed in experimentally fertilised N‐limited Lake 227.HF ranges and means at 0.17, 17, and 170 nmol Co/L were 4.1%–5.7% (4.6%), 5.4%–7.4% (6.4%), and 5.9%–9.3% (7.4%), respectively, after 11 days of growth in batch cultures suggesting that Co plays a role in regulating HF. HF at 0.17 nmol/L was significantly different from HF at the two higher concentrations in all species implicating Co in heterocyst differentiation. However, growth rates and final biovolume yields were not significantly affected by Co in any of the species.Applying the ratio of total phosphorus (TP)/Co in culture media (106) at 0.17 nmol Co/L to an N‐limited system implies that dissolved Co would have to be less than 1 pmol/L to severely limit HF at 1 μmol TP/L. This Co concentration is much lower than dissolved Co in Lake 227 during an Aphanizomenon skujae bloom with 1 μmol TP/L in 2017. Furthermore, Co increased from 0.7 to 2.0 nmol/L simultaneously increasing with HF and cyanobacteria biomass suggesting that Co probably did not limit HF and bloom biomass in Lake 227.Mean summer HFs in Lake 227 during 2000–2020 were 3.4% (epilimnion) and 4.0% (metalimnion), similar to HF at the lowest Co in the cultures. However, HF was significantly higher after 2015 following a shift in dominant bloom species from A. schindlerii to the smaller A. skujae. Calculations suggest that the decrease in cell size may have prompted a higher HF in A. skujae in order to fix an amount of N per unit biomass similar to that fixed by the larger A. schindlerii. Thus, two previously unidentified factors, Co and cell size, can be added to the list of factors that affect HF. [ABSTRACT FROM AUTHOR]

Hydrologic management of shallow lakes is often undertaken to prevent fluctuations in lake level, and to ensure sufficient water volume for economic, domestic, and recreational uses, but there is inconsistent evidence of whether lake‐level stabilisation through hydrological management promotes or hinders eutrophication.Here we used multi‐proxy paleolimnological assessments of water quality (sedimentary carbon, nitrogen, total phosphorus, fossil pigments), and zooplankton community ecology (fossil Cladocera assemblages), combined with Landsat‐derived estimates of lake surface area in two shallow eutrophic lakes, in the Prairies of southern Saskatchewan, Canada, to quantify how 8 decades of contrasting hydrological management strategies (continuous or intermittent) affect primary production and phytoplankton composition.Analysis revealed that irregular hydrological management of Pelican Lake led to sharp increases in primary production concomitant with lake‐level decline. In contrast, continuously managed Buffalo Pound Lake, a drinking water reservoir for regional cities, exhibited slow, persistent eutrophication over decades despite active regulation of water levels. In both lakes, strong correlations of δ 15N values with pigments from diazotrophic cyanobacteria (canthaxanthin) showed that N2‐fixation increased during eutrophication irrespective of the timing of change. Finally, variation in fossil cladoceran density and composition reflected changes in pelagic and littoral habitats (e.g., reduced macrophyte cover) due to changes in both lake level and water quality.Basin comparison shows that while hydrologic management can moderate water quality degradation due to lake‐level change, it does not prevent eutrophication when nutrient influx remains high.Given that regional water availability is forecast to decline in coming decades, we anticipate that continued hydrological management will be unavoidable and will be unable to improve water quality unless nutrient influx is also controlled. [ABSTRACT FROM AUTHOR]