How do small dams alter river food webs? A food quality perspective along the aquatic food web continuum

Damming of rivers poses a significant threat to freshwater ecosystems. Previous studies about the impact of damming on river ecosystems have mostly focused on large dams, with the impact of small dams largely unknown. Further, while the impacts of dams on aquatic communities have been widely studied, the effect on energy flow across river food webs remains unclear. In recent years, long-chain polyunsaturated fatty acid analysis (LC-PUFA) has emerged as a promising technique for assessing food quality and trophic interactions. In this study, LC-PUFA was applied to explore the nutritional effects of small dams on river food webs. A field investigation was conducted at upstream and downstream areas of three small dams in the headwaters of Dongjiang River, China, to evaluate the impact of small dams on the nutritional quality of basal food sources, and their consequent impacts on aquatic consumers and trophic links. Basal food sources (i.e., submerged leaves, macrophytes and periphyton) and aquatic consumers (i.e., macroinvertebrates and fish) were collected, and their fatty acid (FA) composition was measured. Our results showed that periphyton, rather than submerged leaves and macrophytes, was the primary high-quality food source for aquatic consumers, providing them with LC-PUFA, irrespective of whether sites were upstream or downstream. Damming the streams induced changes in aqueous nutrient concentrations (TP, PO4-P, DIN, and TN) from upstream to downstream of the dams, leading to significant variation in periphyton FA content. Compared with periphyton collected at downstream sites, periphyton at upstream sites contained higher LC-PUFA, but lower short-chain PUFA. Differences in periphyton LC-PUFA between the upstream and downstream areas of dams were reflected in the FA profiles of invertebrate grazers and filterers, and further transferred to fish. Furthermore, decreased periphyton nutritional quality at the downstream of the dams was one of the reasons for the simplification of stream food webs. Our results indicated that small dams negatively affected food webs, emphasizing the importance of high-quality food sources for stream ecosystems. We suggest that the trophic integrity of river food webs hinges on the dietary availability of periphyton supplying physiologically highly required nutrients for consumers and must thus not be compromised by damming of streams or other alterations.