Lotic and lentic ecosystems are traditionally viewed as dominated by either benthic or water column processes. However, mid-sized rivers represent a transition zone where both benthic and water column processes may both contribute substantially to ecosystem dynamics. Ecosystem processes such as gross primary production (GPP), ecosystem respiration (ER), or nutrient uptake, and the relative contribution of the water column to these processes at the reach scale, are poorly understood in non-wadeable, mid-sized rivers. To clarify the role of the water column at the reach-scale, and to quantify controls on water column processes, we measured GPP, ER, and uptake of nitrate (NO₃⁻), ammonium (NH₄⁺), and soluble reactive phosphorus (SRP) in the water columns of 15 mid-sized rivers (discharge: 13.5–83.3 m³ s⁻¹) spanning nutrient and total suspended solids gradients. We compared water column metabolic and nutrient uptake rates to reach-scale rates to estimate the contribution of the water column to the entire river. Water column metabolism was autotrophic on the day when measured, GPP increased with nutrient availability, and the water column contributed more to whole river GPP than to ER. Water column nutrient uptake increased with GPP across solutes, and there was a positive relationship between human land use and water column uptake of NO₃⁻–N and SRP. The water column accounted for a substantial proportion of reach-scale metabolism and nutrient uptake, but this contribution depended on suspended material and nutrient availability. Integrating the water column into theory describing lotic ecosystem function should clarify mechanisms controlling metabolism and nutrient processing and enhance management of non-wadeable rivers.

传统上，狮子座和透镜状生态系统被认为是底栖或水柱过程所主导。然而，中型河流代表了一个过渡带，底栖和水柱过程都可能对生态系统的动态起很大作用。在无人涉水的中型河流中，人们对诸如初级生产总值（GPP），生态系统呼吸（ER）或养分吸收等生态系统过程以及水柱在这些过程中的相对贡献知之甚少。为了阐明在到达规模水柱的作用，并量化对水柱过程控制，我们测量GPP，ER，和硝酸的摄取（NO ₃ ^- ），铵（NH ₄ ⁺）和15条中型河流（流量：13.5–83.3 m ³  s ^-1）的水柱中的可溶性反应性磷（SRP），其分布在养分和总悬浮固体梯度之间。我们将水柱的代谢率和养分吸收率与达标率进行了比较，以估算水柱对整条河的贡献。在测量当天，水柱的代谢是自养的，GPP随着养分利用率的增加而增加，并且水柱对整条河GPP的贡献大于对ER的贡献。水柱养分吸收与GPP增加的跨过溶质，并有间人类土地利用和NO的水柱摄取一种积极的关系₃ ^--N和SRP。水柱在比例代谢和养分吸收中占很大比例，但是这种贡献取决于悬浮物质和养分的可利用性。将水柱纳入描述水生生态系统功能的理论中，应阐明控制新陈代谢和养分加工的机制，并加强对不可过河的河流的管理。