Reweaving river food webs through time

Abstract

Our project sought to determine ecological effects of adding low‐head dams and levees to large rivers by examining potential changes to aquatic food webs over a 70‐year period in the Lower Ohio River (LOR) and Upper Mississippi River (UMR). We employed museum collections of fish and compound specific stable isotope analysis of amino acids to evaluate long‐term changes in primary food sources for multiple species of fish in each river. Fishes in both rivers depended more on autochthonous than allochthonous carbon sources throughout the 70‐year period (based on measurements of isotopic signatures of algae, C3 plants, C4 plants, cyanobacteria, and fungi), but the relative use of different carbon sources differed between the UMR and LOR. Significant but opposite shifts in trophic positions (TP) between rivers over time (higher TP in the UMR; lower in the LOR) were correlated with major anthropogenic changes to habitat structure (e.g. slight decrease in abundance of side channels in the UMR; increase in pool water depth in the LOR) resulting from low‐head dam construction. They may also have been influenced by likely increased primary productivity in the UMR from agricultural nitrogen inputs and by possible shifts in the importance of phytoplankton versus benthic algae in the LOR from changes in water depth. Shifts in trophic position and reliance on various food sources were not correlated with variation in discharge, gage height, or temperature. Although these two rivers have contrasting hydrogeomorphic complexity (UMR is an anastomosing river, while the LOR is a constricted channel river) and different discharge patterns (seasonal versus yearly operation in some cases), both differ substantially from rivers having hydrogeomorphic changes resulting from construction of high dams (>15 m). It is not surprising, therefore, that factors controlling trophic position and reliance on different carbon sources vary among different types of dams and river structures.

Long-term fish assemblages of the Ohio River: Altered trophic and life history strategies with hydrologic alterations and land use modifications

“…Yet, in the Ohio River, similar correlations between hydrologic alterations and changing food webs were not observed. Bowes [23] found that mean trophic positions of fishes decreased after dam construction, but trophic position was not correlated with hydrologic variables. Delong and Thoms [20] identified major changes in carbon sources for the Ohio River, and increased variation in mean nitrogen stable isotope ratios of fishes, following flow modifications that occurred from 1950–55.…”

“…Delong and Thoms [20] identified major changes in carbon sources for the Ohio River, and increased variation in mean nitrogen stable isotope ratios of fishes, following flow modifications that occurred from 1950–55. Bowes [23] identified carbon sources for fishes from the Ohio River collected from 1931–1970 and found that autochthonous algae was the major contributor. Food webs can also be influenced by biotic factors such as aquatic invasive species [24] that replace or supplement native species and can shift energy sources.…”

Long-term fish assemblages of the Ohio River: altered trophic and life history strategies with hydrologic alterations and landuse modifications

“…Delong & Thoms [20] identified major changes in carbon sources, and increased variation in mean nitrogen stable isotope ratios of fishes, following flow modification of the Ohio River in 1950-1955. Bowes [23] found decreased trophic position of Ohio River fishes over the past century. Piscivores may switch from traditional planktivore prey (Gizzard Shad) to larval fish, zooplankton, or benthic invertebrates following decreased turbidity; this provides a potential explanation for recent decreased trophic level in piscivores.…”

“…However, in the Ohio River, similar correlations between hydrologic alterations and changing food webs were not observed. Bowes [23] found that mean trophic positions of fishes decreased after dam construction, but trophic position was not correlated with hydrologic variables. Delong & Thoms [20] identified major changes in carbon sources for the Ohio River, and increased variation in mean nitrogen stable isotope ratios of fishes, following flow modifications that occurred from 1950-55.…”

“…Delong & Thoms [20] identified major changes in carbon sources for the Ohio River, and increased variation in mean nitrogen stable isotope ratios of fishes, following flow modifications that occurred from 1950-55. Bowes [23] identified carbon sources for Ohio River fishes collected from 1931-1970 and found that autochthonous algae was the major contributor. Food webs can also be influenced by biotic factors such as aquatic invasive species [24] that replace or supplement native species and can shift energy sources.…”