Stream Periphyton Response to Hemlock Mortality in Central Massachusetts 2006

The Hemlock Wooly Adelgid (HWA) invasion is expected to cause widespread mortality of eastern hemlock [Tsuga canadensis (L.) Carriere] throughout much of New England. Light levels in streams with hemlock riparian zones are anticipated to increase as hemlock 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 100-800m stream reaches with riparian zones of healthy hemlock and deciduous trees in MA and CT in order to document an integrated light profile for each stream. In addition, a 2 x 2 factorial experimental design with five replicates was executed on a deciduous reach of Egypt Brook in central MA, 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. Controlled shading reduced chlorophyll a, while excluding grazing yielded inconclusive results. Periphyton biomass in Egypt Brook was found to be light limited, and grazing did not suppress periphyton biomass. As hemlocks die, in-stream light will be significantly augmented, and periphyton biomass will increase. A challenge for stream ecologists will be to incorporate multiple physical, chemical, and biological controls on biota in order to fully understand how regional hemlock mortality will alter stream periphyton biomass.