Competition and coexistence in a rare Northeastern Pacific multispecies seagrass bed

Abstract

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.