Eelgrass (Zostera marina L.) systems provide numerous ecological services that benefit commercial fisheries, water quality, nutrient cycling, and carbon sequestration. These benefits depend on how eelgrass communities function: a product of abiotic factors (e.g., fluctuations in water temperature and nutrient regimes and seasonal cycles of sedimentation) and biotic factors (e.g., dispersal and trophic interactions), which vary across local, regional, and temporal scales. In upwelling-influenced estuaries of the Pacific Northwest coast of the United States, eelgrass systems are exposed to latitudinal variability in oceanographic inputs, but the degree to which these systems are controlled by processes at regional and local scales is largely unknown.
In this study, we consider the relationships between primary producers (i.e., eelgrass, ulvoid macroalgae, and epiphytes), epifaunal mesograzers, and fish predators within and across three Oregon estuaries (Netarts Bay, Yaquina Bay, and Coos Bay) during a spring-summer season in order to examine the role of multiple scales in structuring eelgrass communities. Specifically, we ask the following questions.
1) What is the relative importance of local (within estuary) versus regional (across estuaries) scale patterns to eelgrass community structure (i.e., primary producers, epifaunal mesograzers, and fishes) in upwelling-influenced estuaries in Oregon?
2) What is the potential role of regional oceanography versus trophic interactions in regulating eelgrass community structure, and is this dependent on the spatial scale?
3) What are the management implications for eelgrass communities when regional and local scales are considered?
We found that regional (across estuary) scale patterns strongly influence community structure in eelgrass communities, providing support that regional bottom-up forcing dominates eelgrass communities in PNW estuaries. Additionally, we found support for some local effects driving patterns of primary producers, epifauna, and fish at particular sites. These results suggest that variation in bottom-up forcing at the regional scale of the California Current is an important driver of community structure and that top-down effects are limited. Such results have important implications for ecosystem-based management as it relates to climate-induced changes in upwelling frequency and intensity and the consequences for eutrophication and ocean acidification.