Abstract
Understanding the carbon sources supporting aquatic consumers in large rivers is essential for the protection of ecological integrity and for wildlife management. The relative importance of terrestrial and algal carbon to the aquatic food webs is still under intensive debate. The Yangtze River is the largest river in China and the third longest river in the world. The completion of the Three Gorges Dam (TGD) in 2003 has significantly altered the hydrological regime of the middle Yangtze River, but its immediate impact on carbon sources supporting the river food web is unknown. In this study, potential production sources from riparian and the main river channel, and selected aquatic consumers (invertebrates and fish) at an upstream constricted-channel site (Luoqi), a midstream estuarine site (Huanghua) and a near dam limnetic site (Maoping) of the TGD were collected for stable isotope (δ13C and δ15N) and IsoSource analyses. Model estimates indicated that terrestrial plants were the dominant carbon sources supporting the consumer taxa at the three study sites. Algal production appeared to play a supplemental role in supporting consumer production. The contribution from C4 plants was more important than that of C3 plants at the upstream site while C3 plants were the more important carbon source to the consumers at the two impacted sites (Huanghua and Maoping), particularly at the midstream site. There was no trend of increase in the contribution of autochthonous production from the upstream to the downstream sites as the flow rate decreased dramatically along the main river channel due to the construction of TGD. Our findings, along with recent studies in rivers and lakes, are contradictory to studies that demonstrate the importance of algal carbon in the aquatic food web. Differences in system geomorphology, hydrology, habitat heterogeneity, and land use may account for these contradictory findings reported in various studies.
Highlights
Understanding the relative importance of terrestrial and aquatic sources of the carbon that supports food webs in large rivers is essential for floodplain management and for wildlife conservation
The river continuum concept (RCC) proposes that the major source of organic matter supporting the large river food webs originates from terrestrial plants from the headwater and mid-streams, while in-stream primary production is limited by turbidity and light attenuation associated with depth [41,47]
The flood pulse concept (FPC) emphasizes the importance of lateral river floodplain exchanges and proposes that river food webs are more dependent on production derived from the floodplain than on organic matter transported from upstream [26]
Summary
Understanding the relative importance of terrestrial and aquatic sources of the carbon that supports food webs in large rivers is essential for floodplain management and for wildlife conservation. The river continuum concept (RCC) proposes that the major source of organic matter supporting the large river food webs originates from terrestrial plants from the headwater and mid-streams, while in-stream primary production is limited by turbidity and light attenuation associated with depth [41,47]. Thorp et al [46] examined these three food web theories within a floodplain and a constricted-channel reach of the Ohio River. The RPM and the importance of in-stream autotrophs were further verified in a study on the Upper Mississippi River using stable isotope analysis [46] They argued that the nutritionally poor, and sometime recalcitrant terrestrial organic matter cannot be the major source of carbon to the river food web. Other studies identify algal carbon as the dominant energy source, fueling the river and lacustrine food webs around the world [3,4,22,43,45]
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