Seagrass and epiphyte seasonality in a biogeographic transition zone

Abstract

Abstract North Carolina, USA is an ecotone that includes two seagrass species located at the edge of their distributional ranges: Zostera marina, a temperate species, and Halodule wrightii, a tropical species. Both support dynamic epiphyte communities, yet seagrass and epiphyte species composition and biomass relationships are not well documented in mixed-species transition zones. This study investigates the seagrass and epiphyte biomass and epiphyte community composition by seagrass species by using monthly collections over one year from Topsail Sound, NC. H. wrightii biomass peaked in the fall and declined to seasonal minimums in winter. Z. marina biomass peaked in the spring and was lowest in fall. Maximum epiphyte biomass per leaf area occurred during periods of thermal stress for both species, summer for Z. marina and winter for H. wrightii. Epiphyte community composition differed between seagrass species within all seasons except for spring, which is a period of concurrent growth for both species, while there was no difference in community across the entire year. Variation in epiphyte communities within NC can be explained by divergent periods of thermal stress, leaf growth, and staggered periods of peak biomass between H. wrightii and Z. marina. Mixed temperate and tropical species meadows ensured continuous substrate for macroalgal epiphytes throughout the year with the seasonality of the epiphyte community closely tied to the growth cycles of their seagrass hosts. This research contributes to our understanding of mixed-species seagrass meadows in biogeographic transition zones and informs conservation and management strategies needed for a rapidly changing climate.