Stable isotopes ¹³C and ¹⁵N are often used in lake ecosystems to assess energy sources and trophic positions, respectively. However, δ¹³C and δ¹⁵N are also influenced by internal biogeochemical processes in epilimnetic and hypolimnetic habitats in lakes, but the extent to which biogeochemical processing mediates isotope values between these two habitats, and whether these patterns are influenced by lake productivity is not known. We sampled δ¹³C and δ¹⁵N in epilimnetic mussels, Chaoborus, cisco (Coregonus artedi), and seston and zooplankton in the epilimnia and hypolimnia of 22 Minnesota (USA) lakes ranging from oligotrophic to eutrophic. We also measured lake temperature–oxygen profiles and light levels to assess factors influencing isotope patterns. Isotope samples were baseline‐corrected using epilimnetic mussels in each lake (sample—mussel) to control for watershed‐level differences in isotope values. Results showed δ¹³C in epilimnetic and hypolimnetic zooplankton, hypolimnetic seston, Chaoborus, and cisco became more depleted in δ¹³C relative to epilimnetic mussels in low‐productivity lakes where light penetrated into the hypolimnion, while epilimnetic seston δ¹³C stayed similar to mussel δ¹³C in all lakes. This pattern was likely due to hypolimnetic phytoplankton in clearwater lakes incorporating more respired CO₂, which is depleted in δ¹³C, and subsequently passing depleted δ¹³C values up the food chain. Results also showed habitat differences in δ¹⁵N with epilimnetic and hypolimnetic zooplankton, hypolimnetic seston, Chaoborus, and cisco becoming more enriched relative to epilimnetic mussels in low‐productivity lakes with higher O₂ levels in the hypolimnion, while epilimnetic seston δ¹⁵N remained similar to mussel values. The δ¹⁵N pattern is consistent with the idea that denitrification and microbial degradation enriched hypolimnetic seston relative to epilimnetic seston in low nutrient lakes, while enhanced epilimnetic primary production enriched epilimnetic δ¹⁵N seston relative to hypolimnetic seston in high nutrient lakes. Our results indicate isotopic differences between epilimnetic and hypolimnetic organisms that change along productivity gradients and suggest that microbial processes and the light regime are important drivers.

中文翻译：

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湖泊食物网中稳定的同位素模式反映了生产率梯度

稳定的同位素¹³ C和¹⁵ N常用于湖泊生态系统，以分别评估能源和营养位置。然而，δ ¹³ C和δ ¹⁵ N被还通过在湖泊epilimnetic和hypolimnetic栖息地内部生物化学过程的影响，但在何种程度上生物地球化学处理介导同位素这两个生境之间的值，以及是否这些图案通过湖影响生产率不众所周知。我们采样δ ¹³ C和δ ¹⁵在epilimnetic贻贝N，Chaoborus，思科（白鲑artedi），以及明尼苏达州（美国）的22个湖泊的上浮和下浮中的浮游动物和浮游动物，从贫营养型到富营养化。我们还测量了湖泊的温度-氧气分布和光照水平，以评估影响同位素模式的因素。使用每个湖泊中的表皮贻贝（样品-贻贝）对同位素样品进行基线校正，以控制同位素值的分水岭水平差异。结果表明：δ ¹³下在epilimnetic和hypolimnetic浮游动物，hypolimnetic浮游物，Chaoborus和Cisco变得更贫δ在¹³相C到epilimnetic贻贝在低生产率湖泊，光渗透到均温，而epilimnetic浮游物δ ¹³ Ç住类似于贻贝δ ¹³C在所有湖泊中。这种模式可能是由于在清水湖hypolimnetic浮游植物将更多的呼出CO ₂，这是在δ耗尽¹³ C，和随后使耗尽δ ^13个C值在食物链。结果也显示在δ栖息地的差异¹⁵ n，其中epilimnetic和hypolimnetic浮游动物，hypolimnetic浮游物，Chaoborus和Cisco变得相对更富集到epilimnetic贻贝在低生产率湖泊具有较高ö ₂在均温的水平，而epilimnetic浮游物δ ¹⁵ Ñ保持类似于贻贝的价值。该δ ¹⁵Ñ图案的想法，脱氮和微生物降解富集相对于在低营养湖泊epilimnetic浮游物hypolimnetic浮游物相一致，而增强epilimnetic初级生产富集epilimneticδ ¹⁵ Ñ浮游物相对于在高营养湖泊hypolimnetic浮游物。我们的结果表明，沿生产力梯度变化的上层和下层生物之间的同位素差异表明微生物过程和光照制度是重要的驱动力。