ABSTRACT For more than 50 years, Lake Ontario has experienced continuous ecological change. The causes and consequences of these changes are only partially understood. Existing monitoring and management programs collect lake‐wide data and provide results and recommendations on perceived lake‐wide processes, potentially overlooking regionally important dynamics. We propose a framework that focuses on evaluating regional dynamics in food web structures by partitioning a large lake into six smaller ecological regions (i.e., ecoregions) based on multiple abiotic properties (bathymetry, thermal structure, tributary influence, trophic state, and anthropogenic influence). To evaluate this ecoregion approach, we investigated differences in trophic position (δ 15 N), carbon sources (δ 13 C), resource partitioning (isotopic niche size), and community metrics for round goby ( Neogobius melanostomus ), alewife ( Alosa pseudoharengus ), and lake trout ( Salvelinus namaycush ) in Lake Ontario sampled between 2009–2014 and 2018. We observed that despite having different trophic roles and using different carbon pathways, the three species responded similarly to differences in bathymetry, nutrient levels, and thermal dynamics, resulting in greater variation among than within an ecoregion. Estimates of trophic position were low (3.2 ± 0.1) for round goby in ecoregions that had higher nutrient loading compared to those with lower (4.1 ± 0.2). While trophic position for alewife (3.1–3.7) and lake trout (4.4–5.2) followed similar patterns among ecoregions, indicating these species respond similar to abiotic variation within the lake. Macrohabitat use derived using δ 13 C was either benthic ( α 0.30) for all three species in ecoregions that had higher nutrient loading and less heterogeneous bathymetry. Fish from ecoregions with lower nutrient loading and more heterogeneous bathymetry used a mix of benthic and pelagic carbon sources, had higher trophic positions, and smaller isotopic niches that resulted in less trophic redundancy. This study demonstrates that food webs in Lake Ontario operate in more discrete heterogenous spatial units (i.e., ecoregions) than as a single, large homogenous mixture. An ecoregion spatial framework, as our study developed and deploys, provides the specificity to support regional resource decision‐making (i.e., watershed and fisheries) and reduces the biases created by a whole lake approach.
Hlina et al. (Mon,) studied this question.