Abstract
AbstractSpatial variation in fish densities across river networks suggests that the influence of food and habitat resources on assemblages varies greatly throughout watersheds. Conceptual models predict that in situ primary production should vary with river characteristics, but the influence of autochthonous resource availability on the capacity for river reaches to support fish is poorly understood. We estimated primary production throughout the South Fork and Middle Fork of the John Day River, Oregon, by measuring diel cycles in dissolved oxygen (DO) during July 2013. Using these data, we (1) evaluated the extent to which juvenile salmonid abundance and resource limitation correlated with areas of high gross primary production (GPP), (2) developed models to predict GPP from both site‐level measurements and remotely sensed data, and (3) made predictions of GPP across the entirety of the Middle Fork John Day River (MFJD) network and assessed the utility of these spatially continuous predictions for describing variation fish densities at broad scales. We produced reliable estimates of GPP at sites where DO loggers were deployed using measurements of solar exposure, water temperature, and conductivity measured at each site, as well as surrogates for these data estimated from remote sensing data sources. Estimates of GPP across fish sampling sites explained, on average, 58–63% of the variation in juvenile salmonid densities during the summer sampling period, and 51–83% during the fall sampling period, while continuous network predictions of GPP explained 44% of the variation in fish densities across 29 km of the MFJD. Further, GPP explained nearly half of the variation in juvenile steelhead dietary resource limitation, as inferred from bioenergetics modeling results. These results comprise a first effort at quantifying variation in autochthonous production across an entire river network and, importantly, provide a much‐needed food‐web context for guiding more effective fish and habitat management.
Highlights
Primary production is a principle source of energy for consumers and a dominant control on the structure and function of ecosystems globally
Estimates of gross primary production (GPP) were similar in the Middle Fork John Day River (MFJD) and Murderers Creek
GPP increased by 0.211 g O2ÁmÀ2ÁdÀ1 with each 1-m increase in bankfull width (t = 5.65, P < 0.001)
Summary
Primary production is a principle source of energy for consumers and a dominant control on the structure and function of ecosystems globally. The seminal river continuum concept (Vannote et al 1980), for example, posits that aquatic primary production forms the dominant energy base in mid-sized streams, relative to smaller headwater streams or larger lowland rivers in which allochthonous resources prevail These predictions have some empirical support (Finlay 2011, but see Winterbourn et al 1981), riverscape-scale models of food-web structure and ecosystem function remain largely conceptual and provide little insight into productivity in specific numerical terms, despite the fact that well-developed tools exist for quantifying aquatic production and ecosystem metabolism at local scales (e.g., Demars et al 2015). This is in stark contrast to the state of knowledge for terrestrial ecosystems, for which a wealth of data on primary production exists and supports a variety of biological assessments at different spatial and temporal scales (see Sımova and Storch 2017 for a recent review)
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