Seagrass meadows with more than two species co-occurring at a small scale are unusual in temperate regions, and such multispecies seagrass beds are undocumented in the Northeastern Pacific. Seagrasses in multispecies beds may coexist through trait differentiation in body size, life history types and phenology, especially when competitive exclusion is interrupted by environmental variability. Here, we survey and manipulate a multispecies seagrass meadow in Willapa Bay, Washington (USA), containing Zostera marina, Zostera japonica and Ruppia maritima; our 13-month survey is the first formal documentation of multispecies seagrass bed occurrence in the Northeastern Pacific. Z. japonica, a non-native species, reached an end-of-summer biomass that was an order of magnitude greater than either native seagrass. To test competition, we experimentally removed Z. japonica and found competition occurred disproportionately on the smaller R maritima relative to larger Z. marina. R. maritima germinated later and senesced earlier than the two Zostera species, and the removal of Z. japonica positively affected R. maritima biomass, supporting previous studies that R. maritima is an opportunistic species. In addition to species-specific phenology and body size, all species presented annual life histories and thus began each annual cycle under high resource availability, such as light, space and nutrients, which may contribute to coexistence.

在温带地区，两种以上的海草草甸小规模共生是不常见的，而且东北太平洋没有记录到这种多物种海草床。多物种床中的海草可能通过体型、生活史类型和物候的特征分化而共存，尤其是当竞争排斥被环境变化中断时。在这里，我们调查和操作多种类海草在Willapa湾，华盛顿（美国）的草地，包含大叶藻，甘藻和川蔓藻; 我们为期 13 个月的调查是对东北太平洋多物种海草床发生的第一个正式记录。粳稻，一种非本地物种，达到了夏末生物量，比任何一种本地海草都高一个数量级。为了测试竞争，我们通过实验去除了Z. japonica，发现与较大的Z. marina相比，较小的R maritima上发生的竞争不成比例。R. maritima比两个Zostera物种萌发晚和衰老早，并且Z. japonica的去除对R. maritima生物量产生了积极影响，支持了先前的研究，即R. maritima是一个机会主义的物种。除了特定物种的物候和体型外，所有物种都呈现出年度生活史，因此在高资源可用性（例如光、空间和营养物质）下开始每个年度周期，这可能有助于共存。