River food webs: Spatial and Temporal

Spatial Changes in River Food Webs

Our lab has a long history of studying food webs in rivers, but we are just beginning to explore how hydrogeomorphic fluctuations alter food web relationships in rivers.

(1) What is the relative importance of ecosystem size, productivity, disturbance, and habitat complexity in controlling food chain length (FCL)?; (2) What is the relative importance of autochthonous vs allochthonous carbon and how does that vary by habitat?; and (3) What are the effects of anthropogenic impacts on FCL and web complexity?




Temporal Changes in River Food Webs

Rivers are arguably the most abused ecosystems in North America as a result of damming, levee construction, species introductions, and pollution, but our historical knowledge of ecosystem change is limited when compared to research on lake and terrestrial ecosystems. The Upper Mississippi and Ohio rivers have some of the best long-term records in the USA and provide a unique opportunity to examine food web drivers over long time scales. We evaluated historical changes in trophic position of fish using nitrogen amino acid stable isotope analysis of museum specimens. We found shifts in trophic positions in the Ohio and Upper Mississippi rivers over a 50-yr period that were correlated with major alterations to habitat structure (e.g., abundance of side channels) resulting from construction of lowhead dams. Discharge, gage height, and temperature were not correlated with the shift in trophic position. The two rivers possess different hydrogeomorphic complexity (anastomosing vs constricted), and their dams generally operate differently (seasonal vs yearly operation in some cases). It is not surprising, therefore, that the effects of dams on trophic position differed between rivers.


Figure 1. Land cover and land use maps of the Mississippi River at Pool 8, located near La Crosse, Wisconsin (GIS data from USGS.gov). The 1891 map provides a look at the river prior to the development of the lock and dam system in the 1930’s. The 1989 map displays the significant changes impoundment of the river introduced into the system. Figure 2. (A.) A shift in the trophic positions of invertivorous and piscivorous fish of the Upper Mississippi River occurred in relation to major hydrological engineering construction of the 1930’s. (B.) Average trophic position of invertivorous and piscivorous fish of the Upper Mississippi significantly increased from an average of 2.776 before (white background) to 3.037 after (gray background) the construction of the lock and dam system in the 1930’s. (C.) Trophic position was unrelated to the corresponding average annual changes in discharge of the Upper Mississippi River, showing that single measures of hydrology may not capture the driver of change in river systems. (D.) Annual fluctuations and overall changes in discharge, temperature, and gage height showed increasing trends over time, but were not correlated with trophic position of the fish. Figure 3. Graphical representation of the results of the change point analysis of fish trophic positions of the Upper Mississippi River. The gray shaded background represents the interval (1927-1940) between which a significant change occurred with 99% confidence.




This material is based upon work supported by the National Science Foundation under Grant Numbers: NSF EAGER grant #1249370 to J.H.T., NSF DDIG grant #1502017 to R.E.B. and J.H.T., NSF Macrorsystem Biology grant #1442595 to J.H.T. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.