Great Lakes coastlines are mosaics of wetland, stream, and lake habitats, characterized by a high degree of spatial heterogeneity that may facilitate the co-occurrence of seemingly incompatible biogeochemical processes due to variation in environmental factors that favor each process. We measured nutrient limitation and rates of N₂ fixation and denitrification along transects in 5 wetland–stream–lake ecotones with different nutrient loading in Lakes Superior and Huron. We hypothesized that rates of both processes would be related to nutrient limitation status, habitat type, and environmental characteristics including temperature, nutrient concentrations, and organic matter quality. We found that median denitrification rates (914 μg N m⁻² h⁻¹) were 166 × higher than N₂ fixation rates (5.5 μg N m⁻² h⁻¹), but the processes co-occurred in 48% of 83 points measured across all 5 transects and habitat types. N₂ fixation occurred on sediment and macrophyte substrate, while denitrification occurred mostly in sediment. Nutrient-diffusing substrate experiments indicated that biofilm chlorophyll-a was limited by N and/or P at 55% and biofilm AFDM was limited at 26% of sample points. N₂ fixation and denitrification rates did not differ significantly with differing nutrient limitation. Predictive models for N₂ fixation and denitrification rates both included variables related to the composition of dissolved organic matter, while the model for N₂ fixation also included P concentrations. These results demonstrate the potential for heterogeneity in habitat characteristics, nutrient availability, and organic matter composition to lead to biogeochemical complexity at the local scale, despite overall N removal at broader scales.

五大湖海岸线是湿地、溪流和湖泊栖息地的镶嵌体，其特点是高度的空间异质性，由于有利于每个过程的环境因素的变化，可能会促进看似不相容的生物地球化学过程的同时发生。我们测量了苏必利尔湖和休伦湖5个具有不同养分负荷的湿地-溪流-湖泊交错带沿横断面的养分限制以及N ₂固定和反硝化速率。我们假设这两个过程的速率都与养分限制状态、栖息地类型和环境特征（包括温度、养分浓度和有机质质量）有关。我们发现反硝化率中值（914 μg N m ⁻²  h ^{−1 ）比 N} ₂固定率（5.5 μg N m ⁻²  h ⁻¹ ）高 166 倍，但这些过程在 83 个点中的 48% 中同时发生对所有 5 个样带和栖息地类型进行了测量。N ₂固定发生在沉积物和水生植物基质上，而反硝化作用主要发生在沉积物中。营养物扩散底物实验表明，生物膜叶绿素-a受 N 和/或 P 限制为 55%，生物膜 AFDM 受采样点的 26% 限制。N ₂固定和反硝化率随养分限制的不同而没有显着差异。N _{2固定和反硝化率的预测模型都包括与溶解有机物组成相关的变量，而N 2}固定模型还包括P浓度。这些结果表明，尽管在更广泛的尺度上总体氮去除，但栖息地特征、养分可用性和有机质组成的异质性可能导致局部尺度的生物地球化学复杂性。