- Foundation species are important components of ecosystems because they provide habitat and ameliorate stressful conditions for residents. This thesis considers the role of surfgrasses (Phyllospadix spp.) as dynamic foundation species on the coast of Oregon in two studies. Chapter 2, which presents an observational survey of two Phyllospadix congeners, investigates the ways in which surfgrass morphology and observed oceanographic conditions influence associated macroinvertebrate communities throughout a calendar year. Chapter 3, which presents an experimental manipulation of the system, considers the ways in which macroinvertebrates and surfgrass interact.
Surfgrasses (Phyllospadix spp.) are ubiquitous foundation species on the coast of Oregon, USA, protecting resident invertebrates from waves and providing them access to sandy substrate in an otherwise rocky habitat. We investigated whether native congeneric species function similarly by comparing the two species' resident macroinvertebrates, plant morphology, and sediment accretion at three capes that vary in oceanographic conditions. The results showed that while the macroinvertebrate abundance was the same between surfgrass species, species richness, composition, and functional groups varied considerably. P. serrulatus also had fewer tillers, rhizomes, and lower biomass per given area but greater sediment accretion than its congener P. scouleri. There was a large difference in macroinvertebrate abundance among capes, with Cape Perpetua having 2.5-3 times more animals per given area than Cape Foulweather or Cape Blanco. Overall, we found that while the two co-occurring surfgrass congeners provided functionally different habitat for resident macroinvertebrate species, regional oceanographic processes (upwelling, productivity) were more influential in determining the overall abundance and productivity of these highly diverse animal communities.
The interaction between surfgrass (P. scouleri) and resident macroinvertebrates may be an example of mutual facilitation if surfgrass provides shelter and macroinvertebrate residents reciprocate with nutrient input. We hypothesized that this positive interaction could vary depending on oceanographic conditions. Sites with intermittent upwelling, low retention of upwelled waters, and high nutrient delivery to the intertidal are often dominated by macroalgae and seagrasses, while sessile invertebrates dominate intermittent upwelling sites with high water higher retention and relatively low nutrient delivery to the intertidal. We expected the positive influence of macroinvertebrates upon surfgrass to be strongest at a site with possible nutrient limitation stress. We used manipulative field experiments to examine the interaction between Phyllospadix and macroinvertebrate residents at sites known to differ in coastal oceanography. Treatments were applied in situ to vary presence/absence of residents and nutrient conditions. Results indicate that the community of macroinvertebrates varies greatly in both abundance and composition between the two sites (ca. three fold higher abundance at Strawberry Hill than at Fogarty Creek), but the amount Phyllospadix habitat available to macroinvertebrates does not vary between the two sites. This variation is therefore driven by something external to local habitat, and is likely related to regional oceanography. Furthermore, there is some evidence that macroinvertebrates do have some positive nutrient effect upon surfgrass productivity.
Together, these studies provide critical descriptive information about community structure associated with the Phyllospadix system on the Oregon coast. This work contributes to the ever-increasing body of literature suggesting importance of regional oceanography in structuring coastal communities.