Environmental optima for common diatoms from Ontario lakes along gradients of lakewater pH, total phosphorus concentration, and depth

Diatoms are powerful biomonitoring indicators for contemporary and past limnological conditions, provided that their environmental optima are well-defined. Surface-sediment-calibration (or training) sets are widely used to develop quantitative transfer functions linking species distributions to limnological variables. Accurate estimates of each taxon’s environmental optimum benefit from a large dataset spanning a broad gradient of the variable of interest. Here, we compiled data from surface-sediment-diatom analyses from over 450 lakes in Ontario (Canada) to identify ecological optima for the province’s most common taxa. The data were collected from the same laboratory using similar methods and spanned more than three decades of research (1987–2019) on 546 samples (representing 464 unique sites). We describe the ecological responses of 52 common diatom taxa to three limnological variables that are key to lake management: pH, total phosphorus (TP), and maximum lake depth. The gradients were broad (pH: 4.2–9.3; TP: 1.1–96.8 µg L⁻¹; maximum depth: 0.7–95.0 m), enabling robust statistical analyses. We used hierarchical logistic regression modelling to identify statistically significant ecological response curves for all common taxa, and weighted averaging to identify the optima. In our dataset, 96% of common diatom taxa had significant ecological responses and optima for pH, 81% for TP, and 83% for depth. Aside from the benthic cosmopolitan Achnanthidium minutissimum complex (occurring in 88.1% of samples), the most frequently recorded taxa were planktonic, including Discostella stelligera/pseudostelligera (83.2%), Asterionella formosa (77.8%), and the Lindavia bodanica complex (76.6%). Our results illustrate the responses of diatom taxa across key limnological gradients, providing important autecological information to advance their use as bioindicators in environmental assessments. These data can be used to address management and knowledge gaps in understudied areas of Ontario where calibration sets are not available and can support research in regions with similar limnological environments and diatom communities.