Subsidy of a downstream river food web by phytoplankton growth from a run-of-the-river reservoir

Background/Question/Methods

River food webs can be supported by both allochthonous and autochthonous material. Rivers also commonly have reservoirs which can change the base of the food web. The Kalamazoo River is a eutrophic river with a run-of-the-river reservoir that produces a large phytoplankton subsidy. Natural abundance carbon isotopes were used to examine changes to the base of the food web above and below a run-of-the-river reservoir in a river. Common taxa such as filter feeding caddisflies (Hydropsychidae) and benthic feeding mayflies (Heptageniidae) were collected along the river for natural abundance isotopes. Riparian spiders (Tetragnathidae) also were collected to assess the subsidy to the terrestrial ecosystem. Along with macroinvertebrate natural abundance isotope samples, we measured chlorophyll a in 2008, 2010 and 2012 along the river to determine the changes in water column algae. Benthic algal chlorophyll a was also measured. Discharge in 2008 and 2010 was high, while 2012 was a year with low flow.

Results/Conclusions

Taxa collected below the reservoir were found to be more depleted in carbon-13. Phytoplankton tends to be more depleted than terrestrial organic matter and benthic algae. We use this signal to determine how far this phytoplankton affects food webs downstream. We found that in years where flow was higher, the phytoplankton subsidy from the reservoir traveled further downstream. The food web returned to the base of the food web sooner under periods of low flow. This matches with chlorophll a patterns. Chlorophyll a returned to levels comparable to above the reservoir under periods of low flow. Under higher flow conditions, the decrease in chlorophyll a was not as large. Differences between filter feeding taxa and benthic feeders will be discussed.  This work is important for understanding the effects of reservoirs in subsidizing downstream river and riparian food webs