An organism can modify its abiotic environment in ways that can determine its own distribution across a landscape. Specifically, positive feedbacks between plants and their abiotic environment can structure patchy vegetated landscapes, and the strength of the feedback may determine the resilience of the system to disturbance. Eelgrass, a submerged, marine angiosperm, spreads primarily by branching rhizomes and can form patches and meadows, all of which can ameliorate waves and currents and stabilize sediment. Some studies suggest that these modifications to the abiotic environment can facilitate eelgrass persistence and/or spread by reducing stressful hydrodynamic conditions, but contention remains regarding the exact relationship between eelgrass abundance, spatial arrangement, and habitat modification in the field.  Data on the full feedback loop could resolve this issue, but to date, studies have only investigated eelgrass-abiotic or abiotic-eelgrass interactions. Understanding the natural and dynamic process of eelgrass spread and decline may inform management and restoration practices of this declining seagrass.

Results/Conclusions

In Spring 2007, we surveyed intertidal eelgrass abundance and sediment characteristics (as a proxy for hydrodynamic conditions) at 2 tidal elevations at 20 sites across Washington State. Preliminary analysis shows that eelgrass abundance is more heterogeneous in sandier sites, suggesting eelgrass is patchier under hydrodynamic stress. Furthermore, no seedlings were observed to colonize sandier sites, which may have negative consequences for recovery/spread of patches after removal disturbance.  Next, we are quantifying the magnitude of the potential feedback loop between eelgrass and its abiotic environment (with disturbance from winter storms). In Summer 2007, we manipulated eelgrass abundance within a meadow in Willapa Bay, WA by thinning/planting eelgrass shoots (0 - 140 shoots/m² and rhizome removal in 25m² plots) and creating gaps within the meadow (0 - 25m²). Preliminary results show that increasing eelgrass abundance decreases water movement, and larger gaps in an eelgrass meadow are more susceptible to sediment removal during Winter 2008 storms, with a threshold size of 25m². Our next steps: tracking the demography of the experimental plots, planting patches of eelgrass in an otherwise unvegetated area, and manipulating water flow over eelgrass on a tide flat to observe effects of flow on eelgrass abundance.