Benthic and planktonic primary production along a nutrient gradient in Green Bay, Lake Michigan, USA

Primary production in lakes occurs in both planktonic (water column) and benthic (bottom) habitats. How whole-lake primary production is distributed between these 2 habitats-referred to as autotrophic structure-is a key ecosystem property. Empirical research examining the balance between benthic and planktonic primary production in lakes is scarce, and how autotrophic structure changes across depth, nutrient, water clarity, and biological invasion gradients is unclear. Therefore, we are ill equipped to anticipate ecosystem-level responses to environmental change. We assessed the magnitude of offshore planktonic, nearshore planktonic, and nearshore benthic gross primary production (GPP) along a gradient of nutrients, water clarity, and Cladophora biomass in Green Bay, Lake Michigan, USA, during summer 2010, 2011, and 2012. Benthic and planktonic GPP varied strongly along the trophic gradient. Planktonic GPP increased with nutrients status, whereas benthic GPP decreased. From shore to 10 m depth, autotrophic structure shifted from planktonic dominance near the mouth of the Fox River (95% planktonic) to a mix of benthic and planktonic GPP 35 km from the mouth of the Fox (similar to 40% benthic). The steep bathymetry at more-distant sites reduced the relative importance of benthic GPP at the whole-ecosystem level. Our work highlights the dual-pathway (i.e., benthic and planktonic) nature of lentic food webs from the perspective of GPP, and shows how both trophic status and bathymetry affect autotrophic structure.