Will Stream Periphyton Respond to Increases in Light Following Forecasted Regional Hemlock Mortality?

ABSTRACT

The hemlock woolly adelgid invasion is expected to cause widespread mortality of eastern hemlock (Tsuga canadensis) throughout much of New England. Light levels in streams with hemlock riparian zones are anticipated to increase as hemlock trees are replaced by deciduous trees. We sought to: (1) quantify differences in light reaching streams with hemlock and deciduous riparian zones, (2) determine if increases in light result in higher periphyton biomass, and (3) explore the role of macroinvertebrate grazing on periphyton biomass as light increases in an attempt to help predict stream ecosystem responses to hemlock mortality. Light measurements were taken along 100–800 m stream reaches with riparian zones of healthy hemlock and deciduous trees in Massachusetts and Connecticut. In addition, a 2 × 2 factorial experimental design with five replicates was executed on a deciduous reach of Egypt Brook in central Massachusetts, in which light (high light vs. low light) and grazing (high grazing vs. low grazing) were manipulated. Light measurements were significantly higher for streams with deciduous riparian zones than hemlock riparian zones. Periphyton biomass was significantly reduced by controlled shading, but was not influenced by grazer exclusion.