Abstract
Similar to many large river valleys globally, the Sacramento River Valley has been extensively drained and leveed, hydrologically divorcing river channels from most floodplains. Today, the former floodplain is extensively managed for agriculture. Lack of access to inundated floodplains is recognized as a significant contributing factor in the decline of native Chinook Salmon (Oncorhynchus tshawytscha). We observed differences in salmon growth rate, invertebrate density, and carbon source in food webs from three aquatic habitat types—leveed river channels, perennial drainage canals in the floodplain, and agricultural floodplain wetlands. Over 23 days (17 February to 11 March, 2016) food web structure and juvenile Chinook Salmon growth rates were studied within the three aquatic habitat types. Zooplankton densities on the floodplain wetland were 53x more abundant, on average, than in the river. Juvenile Chinook Salmon raised on the floodplain wetland grew at 0.92 mm/day, 5x faster than fish raised in the adjacent river habitat (0.18 mm/day). Two aquatic-ecosystem modeling methods were used to partition the sources of carbon (detrital or photosynthetic) within the different habitats. Both modeling approaches found that carbon in the floodplain wetland food web was sourced primarily from detrital sources through heterotrophic pathways, while carbon in the river was primarily photosynthetic and sourced from in situ autotrophic production. Hydrologic conditions typifying the ephemerally inundated floodplain—shallower depths, warmer water, longer water residence times and predominantly detrital carbon sources compared to deeper, colder, swifter water and a predominantly algal-based carbon source in the adjacent river channel—appear to facilitate the dramatically higher rates of food web production observed in the floodplain. These results suggest that hydrologic patterns associated with seasonal flooding facilitate river food webs to access floodplain carbon sources that contribute to highly productive heterotrophic energy pathways important to the production of fisheries resources.
Highlights
The benefits of annual inundation of floodplains to riverine ecosystems and fish populations are well recognized in relatively unaltered tropical river systems [1, 2]
Discharge in the floodplain perennial drainage canal ranged from 0.82 m3s-1 to 68.0 m3s-1, while discharge in the Sacramento River ranged from 314.3 m3s-1 to 1,427.2 m3s-1
Stage in the floodplain agricultural wetland was stable throughout the study, while stage varied by 2.75 m in the canal and 5.00 m in the Sacramento River
Summary
The benefits of annual inundation of floodplains to riverine ecosystems and fish populations are well recognized in relatively unaltered tropical river systems [1, 2]. The impacts of anthropogenic interruption of natural flood pulses in temperate river systems, have only more recently begun to be characterized [3]. Extensive leveeing diminishes the expression of the full range of life history strategies of native fish, thereby weakening the resilience of populations to natural and anthropogenic disturbance. This landscape-scale hydrologic divorce of river channel and floodplain has only recently been widely recognized and case studies of community responses, such as this paper, are not yet common [7]
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