Nutrient enrichment is a major threat to subtidal macroalgal forests. Several studies have shown that nutrient inputs can enhance the ability of opportunistic algal species to acquire space freed by disturbance, at the expense of architecturally complex species that form forests. However, competition between canopy- and turf-forming macroalgae is not limited to the aftermath of disturbance. Canopy-forming macroalgae can provide suitable substratum for diverse epibiont assemblages, including both algae (epiphytes) and sessile invertebrates (epizoans). Despite evidence of enhanced epiphyte loading under eutrophic conditions, few experimental studies have assessed how nutrient enrichment influences the structure of epibiont assemblages on canopy-forming macroalgae at the edge versus inside forests. In oligotrophic waters of the NW Mediterranean, we experimentally tested the hypothesis that nutrient-driven proliferation of opportunistic epiphytic algae would affect the performance of the fucoid, Carpodesmia brachycarpa, and reduce the richness and abundance of the epizoan species they support. We predicted negative effects of nutrient enrichment to be greater at the edge than inside forests and on thalli that had recovered in cleared areas than on those within undisturbed canopy stands. Nutrient enrichment did not affect the photosynthetic efficiency and reproductive output of C. brachycarpa. By contrast, it enhanced herbivore consumption and decreased the cover and diversity of epizoans at forest edges, likely by stimulating the foraging activity of Arbacia lixula, the most abundant sea urchin in adjacent encrusting coralline barrens. Fertilization of areas inside forests had no effect on either C. brachycarpa or epibiont assemblages. Finally, nutrient enrichment effects did not vary between cleared and undisturbed areas. Our results show that moderate nutrient enrichment of oligotrophic waters does not necessarily cause the proliferation of epiphytes and, hence, a strengthening of their competitive effects on canopy-forming macroalgae. Nevertheless, enhanced herbivory damage to fertilized thalli at forest edges suggests that fragmentation could reduce the resilience of macroalgal forests and associated epibiont assemblages to nutrient enrichment.

中文翻译：

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营养丰富刺激食草动物并改变边缘的表生群落，但不在潮下带大藻林内

营养丰富是对潮下带大型藻林的主要威胁。几项研究表明，养分输入可以增强机会藻类物种获得被干扰释放的空间的能力，但代价是形成森林的结构复杂的物种。然而，形成冠层和草皮的大型藻类之间的竞争并不限于干扰的后果。形成冠层的大型藻类可以为各种表生生物组合提供合适的基质，包括藻类（附生植物）和无柄无脊椎动物（表生动物）。尽管有证据表明在富营养化条件下附生植物负荷增加，但很少有实验研究评估养分富集如何影响边缘与森林内部形成冠层的大型藻类的附生植物组合结构。在地中海西北部的贫营养水域，我们通过实验测试了以下假设：条件性附生藻类的营养驱动增殖会影响褐藻糖胶 Carpodesmia brachycarpa 的性能，并降低它们支持的表皮动物物种的丰富度和丰度。我们预测，边缘养分富集的负面影响比森林内部更大，对在清除区域恢复的菌体的负面影响比对未受干扰的树冠林分内的影响更大。养分富集不影响 C. brachycarpa 的光合效率和繁殖产量。相比之下，它增加了食草动物的消费并减少了森林边缘表皮动物的覆盖率和多样性，这可能是通过刺激 Arbacia lixula 的觅食活动来实现的，Arbacia lixula 是邻近珊瑚礁贫瘠环境中最​​丰富的海胆。森林内部区域的施肥对 C. brachycarpa 或表生菌组合没有影响。最后，在已清除和未受干扰的区域之间，养分富集效果没有差异。我们的结果表明，贫营养水域的适度养分富集并不一定会导致附生植物的增殖，因此，它们对形成冠层的大型藻类的竞争作用增强。然而，对森林边缘受精菌体的食草性损害加剧表明，破碎可能会降低大型藻类森林和相关表生群落对养分富集的恢复力。我们的研究结果表明，贫营养水域的适度养分富集并不一定会导致附生植物的增殖，因此，它们对形成冠层的大型藻类的竞争作用增强。然而，对森林边缘受精菌体的食草性损害加剧表明，破碎可能会降低大型藻类森林和相关表生群落对养分富集的恢复力。我们的结果表明，贫营养水域的适度养分富集并不一定会导致附生植物的增殖，因此，它们对形成冠层的大型藻类的竞争作用增强。然而，对森林边缘受精菌体的食草性损害加剧表明，破碎可能会降低大型藻类森林和相关表生群落对养分富集的恢复力。