Variation in Epiphyte Assemblage within the Friday Harbor Laboratories Zostera marina Meadow on San Juan Island, Washington

Abstract

Eelgrass (Zostera marina) is an important foundation species in the Salish Sea, providing habitat, stabilizing sediment, cleaning water, and sequestering carbon. These ecosystem services are under threat in the San Juan Islands, as seagrass wasting disease (SWD) and heat stress have contributed to major declines in intertidal eelgrass meadow extent in the past decade. Deeper meadows experience lower SWD levels and heat stress, suggesting that these sites may act as refugia amidst continued climate change. An additional stressor is epiphyte load (epiphyte species richness and abundance on an eelgrass leaf). The presence of epiphytes can be influenced by leaf age, but influence from depth has not been reported. The present study investigates the relationships between depth, leaf age, and epiphyte load for one subtidal eelgrass meadow at Friday Harbor Laboratories, San Juan Island, Washington. Eelgrass leaves (n=29) were collected across a belt transect via subtidal snorkel survey and analyzed for epiphyte abundance and species richness. Epiphyte abundance and species richness were found to be consistent across depth. This suggests that depth relationships, such as with disease, act independent of epiphyte load. There was also a significant relationship between leaf age and epiphyte abundance and species richness. The difference in epiphyte load between young and old leaves raises further questions about varying stress levels between leaves of different ages. These findings can be used to guide further efforts in understanding eelgrass community dynamics, as well as contextualizing environmental stressors most relevant to eelgrass meadows on a site-specific basis.