Virginia Institute of Marine Science, College of William and Mary, P.O. Box 1346, Gloucester Point, Virginia 23062, USA.
Ecology. 2013 Feb;94(2):510-20. doi: 10.1890/12-0156.1.
In coastal marine food webs, small invertebrate herbivores (mesograzers) have long been hypothesized to occupy an important position facilitating dominance of habitat-forming macrophytes by grazing competitively superior epiphytic algae. Because of the difficulty of manipulating mesograzers in the field, however, their impacts on community organization have rarely been rigorously documented. Understanding mesograzer impacts has taken on increased urgency in seagrass systems due to declines in seagrasses globally, caused in part by widespread eutrophication favoring seagrass overgrowth by faster-growing algae. Using cage-free field experiments in two seasons (fall and summer), we present experimental confirmation that mesograzer reduction and nutrients can promote blooms of epiphytic algae growing on eelgrass (Zostera marina). In this study, nutrient additions increased epiphytes only in the fall following natural decline of mesograzers. In the summer, experimental mesograzer reduction stimulated a 447% increase in epiphytes, appearing to exacerbate seasonal dieback of eelgrass. Using structural equation modeling, we illuminate the temporal dynamics of complex interactions between macrophytes, mesograzers, and epiphytes in the summer experiment. An unexpected result emerged from investigating the interaction network: drift macroalgae indirectly reduced epiphytes by providing structure for mesograzers, suggesting that the net effect of macroalgae on seagrass depends on macroalgal density. Our results show that mesograzers can control proliferation of epiphytic algae, that top-down and bottom-up forcing are temporally variable, and that the presence of macroalgae can strengthen top-down control of epiphytic algae, potentially contributing to eelgrass persistence.
在沿海海洋食物网中，小型无脊椎草食动物（中型食草动物）长期以来一直被假设处于一个重要的位置，通过竞争优势的附生藻类的放牧来促进生境形成的大型藻类的优势地位。然而，由于难以在野外操纵中型食草动物，因此它们对群落组织的影响很少得到严格记录。由于全球范围内海草的减少，了解中型食草动物的影响变得更加紧迫，部分原因是广泛的富营养化有利于海草过度生长，藻类生长更快。本研究使用两个季节（秋季和夏季）的无笼野外实验，提供了实验证实，中型食草动物的减少和营养物质可以促进鳗草（Zostera marina）上附生藻类的爆发。在这项研究中，养分添加仅在秋季中型食草动物自然减少后增加了附生藻类。在夏季，实验性的中型食草动物减少刺激了附生藻类增加了 447％，似乎加剧了鳗草的季节性衰退。通过结构方程模型，我们阐明了夏季实验中大型藻类、中型食草动物和附生藻类之间复杂相互作用的时间动态。从调查相互作用网络中出现了一个意想不到的结果：漂移藻类通过为中型食草动物提供结构间接减少了附生藻类，这表明藻类对海草的净效应取决于藻类的密度。我们的结果表明，中型食草动物可以控制附生藻类的增殖，自上而下和自下而上的强迫作用是时间变化的，藻类的存在可以增强对附生藻类的自上而下的控制，这可能有助于鳗草的持续存在。
