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.

铁杉球蚜（Hemlock Wooly Adelgid, HWA）入侵预计将导致新英格兰大部分地区的东部铁杉[加拿大铁杉Tsuga canadensis (L.) Carriere]大范围死亡。随着河岸带的铁杉被落叶树替代，对应溪流的光照水平预计将有所提升。本研究旨在达成三项目标：1）量化河岸带为铁杉与落叶树的溪流之间的光照差异；2）探明光照增强是否会导致周丛生物（periphyton）生物量升高；3）探索光照增加条件下，大型无脊椎动物牧食对周丛生物生物量的调控作用，以期预测溪流生态系统对铁杉死亡的响应。研究人员在马萨诸塞州（MA）与康涅狄格州（CT）内、河岸带为健康铁杉或落叶树的100-800米溪流河段开展光照测量，以获取每条溪流的综合光照剖面。此外，研究团队在马萨诸塞州中部的埃及溪（Egypt Brook）落叶河段设置了包含5个重复的2×2析因实验设计，对光照（高光vs低光）与牧食强度（高牧食vs低牧食）两个因子进行操控。实验结果显示：落叶河岸带溪流的光照测量值显著高于铁杉河岸带溪流；控制性遮光处理会降低水体叶绿素a含量，而排除牧食的实验结果未获得明确结论；埃及溪的周丛生物生物量受光照限制，且牧食并未抑制周丛生物生物量。随着铁杉陆续死亡，溪流内光照将显著增强，周丛生物生物量也会随之升高。溪流生态学家面临的一项核心挑战是，整合对生物群的多重物理、化学与生物调控机制，从而全面理解区域铁杉死亡将如何改变溪流周丛生物的生物量。