LAUSR

We used compound specific isotope analysis of carbon isotopes in amino acids to determine the biosynthetic source of amino acids in fish from major tributaries to California's Sacramento-San Joaquin river delta (i.e., Sacramento, Cosumnes, and Mokelumne rivers). Using samples collected in Winter and Spring between 2016 and 2019, we confirmed that algae are a critical component of floodplain food webs in California's Central Valley. Results from bulk stable isotope analysis of carbon and nitrogen in producers and consumers were adequate to characterize a general trophic structure and identify potential upstream and downstream migration into our study site by American shad Alosa sapidissima and rainbow trout Oncorhynchus mykiss, respectively. However, owing to overlap and variability in source isotope compositions, our bulk data were unsuitable for conventional bulk isotope mixing models. Our results from compound specific carbon isotope analysis of amino acids clearly indicate that algae are important sources of organic matter to fish of conservation concern such as Chinook salmon Oncorhynchus tshawytscha in California's Central Valley. But, algae were not the exclusive source of energy to metazoan food webs. We also revealed that other sources of amino acids such as bacteria, fungi, and higher plants contributed to fish as well. While consistent with the well-supported notion that algae are critical to aquatic food webs, our results highlight the possibility that detrital subsidies might intermittently support metazoan food webs. This article is protected by copyright. All rights reserved.