Influence of freshwater macrophytes on the littoral ecosystem structure and function of a young Colorado reservoir

The presence of rooted macrophytes, mostly the milfoil Myriophyllum sibiricum, was manipulated in enclosures in the littoral zone of a Colorado reservoir. The presence of macrophytes significantly increased the abundance of major invertebrate taxa by 70–1725% and increased the emission of methane 127%. The increase in abundance of most invertebrates was probably due to the habitat and surfaces provided by milfoil as stable isotope analyses indicated that milfoil was an insignificant carbon source for all of the invertebrate taxa, except for the milfoil midge Cricotopus myriophylli. Cricotopus is known to specialize on milfoil (other members of the genus specialize on Hydrilla or are generalists), had an isotopic signature that indicated a diet of milfoil, and was about 15 times more abundant when milfoil was present than when it was absent. Milfoil had no detectable effect on the total particulate phosphorus (TPP), soluble reactive phosphorus (SRP), total dissolved phosphorus (TDP), dissolved organic phosphorus (DOP), and Chl a of water within the enclosures. However, enclosures containing milfoil had higher concentrations of SRP in the pore water of surface sediments than enclosures that had milfoil removed. SRP in pore water dropped below 2 μg/L at >2 cm sediment depth and DOP increased progressively from nearly zero at the surface to about 150 μg/L at 15 cm depth, regardless of vegetation. Thus, milfoil had significant effects on many, but not all, measures of littoral ecosystem structure and function that were monitored.