Paying tribute to production hot-spots

Posted on July 2, 2014

Few studies have examined how physical discontinuities in rivers affect aquatic productivity. Large tributaries create such discontinuities by introducing large amounts of sediment and organic matter into the main stream. Does this process modify the river food web by increasing the local importance of detritus-based pathways? To find out, over a three-year period, researchers monitored physical conditions, invertebrates and fish close to the junctions of six major tributaries along a 386-km stretch of the Colorado River, U.S.A. Based on diet analyses, they were able to construct food webs above and below each tributary. Compared to upstream food webs, the downstream webs contained more species, including more native forms. In downstream webs, fish consumed a much larger fraction of the invertebrate production (i.e., ecological efficiency was higher), and the dominant fish species changed from trout to suckers. This corresponded to a change from a near-reliance on algal-based feeding pathways upstream to a more balanced use of both algae and detritus pathways downstream. Downstream food webs appeared to be more stable and more resistant to flooding disturbance. The results showed that cobble–gravel habitats derived from tributary flooding can be hot-spots of invertebrate production that are ultimately responsible for the vast majority of the food used by river fish.

Reference: Cross, W.F. et al. 2013. Food-web dynamics in a large river discontinuum. Ecological Monographs 83(3), 311–337.