Basin-Scale Consequences of Agricultural Land Use on Benthic Light Availability and Primary Production Along a Sixth-Order Temperate River

The emerging spatial scale of interest for fluvial ecosystem studies and applications is the river basin. Although much focus has been directed toward habitat availability and nutrient cycling across the fluvial landscape, light availability has received considerably less attention and has not been assessed quantitatively at the basin-scale despite it being the primary energy source for aquatic ecosystems. We developed a basin-scale light availability model that couples readily available broad spatial data with easily measured synoptic data using a GIS framework and the principles of hydraulic geometry. We used this model to (i) quantify benthic light availability (E bed) along a 160-km river in central Wisconsin, USA, (ii) predict gross primary production (GPP) along the same river, and (iii) assess the effects of agricultural land use on E bed and GPP. Overall, E bed decreased in the downstream direction due primarily to increased turbidity, and there was considerable local variation caused by changes in topography, riparian vegetation, and channel orientation. These local variations in E bed caused GPP to fluctuate greatly over short distances, as much as 2.1 g C m⁻² d⁻¹ over 260 m. When summed over the entire channel length, present-day, post-agricultural GPP (635 kg C d⁻¹) was about eight times lower than estimated pre-agricultural GPP (4992 kg C d⁻¹). Model simulations revealed that agricultural land use can cause an order of magnitude change in GPP, reduce or increase inter-sectional variability in GPP, and significantly alter broad spatial trends in GPP. Our basin-scale benthic light availability model is a tool that researchers can use to investigate relationships between light availability and ecosystem processes at broad spatial scales and also one that practitioners can use for more holistic fluvial ecosystem assessments.