Fishes consume prey from across both benthic and pelagic habitats, thereby stabilizing the food webs of freshwater lakes. How fish exploit vertical and horizontal heterogeneity within the benthic environment, however, remains unclear. We characterized spatial variation in the C and N stable isotopes of a dominant benthic grazer (snails) along a water depth gradient in an oligotrophic, western Alaska lake to assess the effects of water depth on isotope characteristics in the benthic food web. Importantly, carbon stable isotopes in snails become substantially more depleted in ¹³C with increasing water depth. We compared this distribution of snail isotope values to those found in Arctic char (Salvelinus alpinus) diets to estimate the vertical location of char foraging on benthic resources. We found that Arctic char tended to prey on snails at greater than 10 m depth; however, individual fish specialized at a narrower range of water depths spanning from 5 to > 20 m. We used an isotope mixing model to assess the validity of using only shallow-water benthic invertebrates to represent all benthic resources in food web reconstructions and found that mixing models that only used shallow benthic prey underestimated the importance of benthic resources by about 50%. These results can help refine our understanding of how benthic prey contribute to the stability of aquatic food webs and highlight the need to account for spatial variation in the isotope composition of benthic resources in models of lake food webs.

鱼类从底栖和远洋栖息地捕食猎物，从而稳定淡水湖泊的食物网。然而，鱼类如何利用底栖环境中的垂直和水平异质性仍不清楚。我们描述了阿拉斯加西部贫营养湖中主要底栖食草动物（蜗牛）的 C 和 N 稳定同位素的空间变化特征，以评估水深对底栖食物网中同位素特征的影响。重要的是，随着水深的增加，蜗牛中的碳稳定同位素在¹³ C 中变得更加耗尽。我们将蜗牛同位素值的这种分布与在北极红点鲑（Salvelinus alpinus) 饮食来估计炭在底栖资源上觅食的垂直位置。我们发现北极红点鲑倾向于捕食深度超过 10 m 的蜗牛；然而，个别鱼类专长于 5 至 > 20 m 的较窄水深范围。我们使用同位素混合模型来评估仅使用浅水底栖无脊椎动物代表食物网重建中所有底栖资源的有效性，发现仅使用浅水底栖猎物的混合模型低估了底栖资源的重要性约 50%。这些结果有助于加深我们对底栖猎物如何促进水生食物网稳定性的理解，并强调需要在湖泊食物网模型中考虑底栖资源同位素组成的空间变化。