- Ecological communities are connected in space and time through the transfer of energy, materials, and organisms, together known as ecological subsidies. These ecological subsidies can have substantial effects on community structure, function, and services, especially when the connections are between communities with contrasting productivity. At the ocean-land interface, low productivity coastal beaches and dunes are regularly exposed to subsidies from the ocean. These marine subsidies likely provide an important nutrient source for dune vegetation that might otherwise be nutrient-limited. Given that dune vegetation plays a key role in dune formation and stabilization, it is imperative to understand the role of marine subsidies to the nutrient dynamics and plant productivity patterns of these coastal systems.
In this dissertation, I investigate the role of marine subsidies to dune ecosystem functions from local to meta-ecosystem scales along the U.S. Pacific Northwest coast. Coastal beaches and dunes make up approximately 45% of the Oregon and Washington coasts and are adjacent to many highly productive estuaries and rocky reefs. The plant communities at these sites experience variable ocean productivity, marine subsidies, and coastal sediment supply. The most common plant species are two non-native invasive grasses, the European beachgrass, Ammophila arenaria, and the American beachgrass, Ammophila breviligulata. Since their introduction in the early 1900’s, the Ammophila congeners have spread and rapidly transformed what was once an open, shifting, and sparsely vegetated habitat into one with continuous, stable, and densely vegetated foredunes. Despite this ecosystem-level transformation over the last century, little is known about the role of marine subsidies to the nutrient dynamics and primary production of dune grass communities. Here I use a combination of observations, laboratory analyses, and experiments to understand the role of ecological subsidies to dune grass production at local to regional scales along the U.S. Pacific Northwest coast. In Chapter 2, I explore the role of marine subsidies to dune grass production at 15 dune-backed beach sites in Washington and Oregon and over three years (2015-2017). Specifically, I ask 1) Is there a relationship between ocean upwelling, distance to wrack source habitat, and beach width to macrophyte wrack amount and composition across large regional (meta-ecosystem) spatial scales and over time (2015-2017)?, 2) Is there a relationship between ocean upwelling, sand supply, and/or marine macrophyte wrack and the marine nutrient dynamics at local and regional scales and over time?, and 3) Is there a relationship between marine nutrients and dune grass production at local to regional spatial scales and over time? I provide evidence that marine subsidies are common and predictable sources of nutrients to these beaches and dunes and that they influence dune grass ecosystems in significant ways. Specifically, I found that macrophyte wrack and sand nitrates, which were positively related to ocean upwelling and sand supply, were important determinants of dune grass production across sites and foredune profile locations. Beaches with more macrophyte wrack and greater sand supply had greater dune grass shoot density and biomass, especially at the toe of the dune where sand nitrate concentrations were high. Beaches with lower macrophyte wrack and sand supply had greater shoot density and biomass of dune grasses at the crest of the dune, where sand deposition and nitrate concentrations were high. Taken together, these results suggest that marine subsidies are important contributors to the nutrient dynamics and productivity of dune grasses, potentially influencing the sand capture and dune building capabilities of these ecosystem engineering grass species. In Chapter 3, I determine whether dune plant species in the U.S. Pacific Northwest use marine subsidies, and if so, how this use varies at local to regional spatial scales. In particular, I ask 1) Does the amount (%N) and marine source (δ15N) of nitrogen in four dune plant species (two non-native invasive beachgrass species Ammophila arenaria and A. breviligulata, the native dune grass, Elymus mollis, and the native shrub Cakile edentula) vary across sites (coast-scale) and foredune profile locations (dune-scale)? and 2) Does the amount (%N) and marine source (δ15N) of nitrogen of beachgrasses correlate with measures of marine subsidies (macrophyte wrack and sand nitrates) and/or beach and foredune geomorphology (beach width and foredune crest height) on coastal foredunes? I found that marine nutrients were an important source of nitrogen for dune plants in the U.S. Pacific Northwest, and that the amount and source of nitrogen in these plants varied among species, sites, and foredune profile locations. Regression analyses showed that, for the most part, macrophyte wrack biomass and sand nitrate concentrations were unrelated to %N or δ15N of the grasses. However, I did find that wider beaches and shorter foredunes had beachgrasses with slightly lower %N tissue content across foredune profile locations but the source of that nitrogen at the crest and heel had a higher marine signature. The results presented in this chapter corroborate earlier studies that beach nutrients, especially those with marine origin, are an important source of nitrogen for foredune plants. Finally, in Chapter 4, I test whether beachgrasses in the U.S. Pacific Northwest are nutrient limited and whether this potential limitation varies along foredune profile locations and sites with different sand nitrate conditions. Specifically, I used two manipulative experiments to ask: (1) How does nutrient addition affect dune grass production at sites and foredune profile locations that vary in natural sand nitrate concentrations? and (2) Does dune grass production and plant tissue elemental composition vary with increasing nitrogen additions and do dune grasses experience nitrogen and phosphorous co-limitation? I found that dune grasses were nutrient-limited, with nutrient additions positively affecting dune grass production at sites and foredune profile locations that have higher wrack deposition and background sand nitrate concentrations. I also found that increased nitrogen and phosphorous increased grass shoot density, total biomass, and plant tissue %N. However, grass shoot biomass and shoot length either did not change, or declined, with phosphorus additions suggesting that there was no phosphorus co-limitation. These results suggest that U.S. Pacific Northwest dune grasses are nutrient limited, especially at the foredune crest and heel, where background sand nitrate concentrations are lower. In summary, my dissertation fills a critical gap in our understanding of beach and dune nutrient dynamics and the role of nutrients to coastal dune plant community production from a meta-ecosystem perspective. By considering local and regional relationships, this meta-ecosystem approach provides an ideal framework to understanding the role of marine subsidies to dune grass production. This approach also allows us to better understand how coastal dune systems might change under varying climate so that we may better manage their functions and services in the future.