Possible Geomorphic Effects Of Invasive Spartina alterniflora in the San Francisco Estuary

1. Possible Geomorphic EffectsOf Invasive Spartina alterniflorain the San Francisco Estuary Joshua N. Collins, Ph.D San Francisco Estuary Institute Josh@sfei.org

2. Acknowledgments • US EPA, California Coastal Conservancy • Debra Smith, Katy Zaremba, Shannon Klohr of the Spartina Project • Donna Morton, SFEI Laurel Collins, Watershed Sciences • Debra Ayers and Donald Strong, UC Davis • Andrew Cohen, SFEI Karl-Malamud-Roam, CCMAD

3. Train of Study Assumptions • Function follows form. • Form of the intertidal zone is a consequence of interactions between water supply and sediment supply, as mitigated by vascular vegetation. • Vascular vegetation is a major structural component of the intertidal zone of the Estuary.

4. Study Assumptions (cont’d) • The geo-ecological effects of plants vary with their architecture through space and over time. • NIS Spartina (S. alterniflora and hybrids) has different architecture than any intertidal native plants. • Dominance by NIS Spartina will alter the form and function of the intertidal zone.

5. Fundamental Study Questions Where does NIS Spartina dominate the intertidal zone? Based on its dominant location, what might be its effects on geomorphic form? Given these effects on form, what might be effects on function (questions to answer)?

6. Synthesis of Study Assumptions and Questions Interactions between water, sediment, and plants comprise a dynamic physical template for geo-chemical and ecological processes.

7. Study Approach • Map the distribution of NIS Spartina throughout the Estuary. • Map its distribution relative to other dominant plants along salinity gradients. • Map its vertical intertidal distribution.

8. Regional Distribution and Abundance

14. Regional Abundance and Distribution of NIS Spartina by Patch Size Class

15. Distribution Relative toAqueous Salinity:Basic Methods • Choose local creeks near verified NOS benchmarks. • Conduct surveys of minimum elevations of NIS Spartina and co-dominant plants throughout local salinity gradient. • Verify genotype for each NIS Spartina patch surveyed.

17. Extent of aggradation upstream of cement apron

18. “Saline” “Brackish” “Fresh”

19. Site Elevation Relative to Local MHW (ft) Colma Creek n = 23 -4.24  0.9 Alameda Flood Channel n = 14 -3.85  0.7 San Leandro Creek n = 13 -3.31  0.6 Minimum elevations of NIS Spartina relative to MHW for fresh-brackish tidal reaches of three creeks of South San Francisco Bay

20. Distribution vs Aqueous Salinity:Basic Results • Minimum elevation of NIS Spartina intersects creek bed at about Mean Tide Level. • NIS Spartina grows lower than other plants throughout salinity gradient. • Pure S. alterniflora is restricted to fresher conditions near the head of tide.

21. Vertical Distributionin Saline Intertidal: Basic Methods • Choose sites with verified NIS Spartina and NOS benchmarks. • Map distribution of NIS Spartina relative to intertidal physiography. • Randomly survey minimum elevations of NIS Spartina.

22. Elevations of NIS Spartina

23. Plan Form Details of Distribution Distribution of NIS Spartina within Arrowhead Marsh

25. Arrowhead Marsh IR Photo

26. Arrowhead Marsh IR Photo

27. Plan Form Details:Basic Results • Due to channel cross-section form and preferred elevation range, NIS Spartina colonizes channel beds of existing saline tidal marshes mainly in middle reaches of the drainage networks. • Colonization of an existing marsh by NIS Spartina disconnects headward channels from the rest of the marsh drainage network.

28. Saline DistributionRelative to Tidal Datums

31. Conclusions The vertical distribution of NIS Spartina relates to the duration of tidal inundation, or conversely, the duration of exposure above the tides.

32. Conclusions NIS cordgrass is likely to invade the upper half of the saline tidal flats and will tend to invade a smaller proportion of the tidal flats in Far South Bay than in South Bay or Central Bay, but plant evolution may change the rules.

33. Conclusions NIS cordgrass will probably not dominate the saline high marsh above MHW, but plant evolution may change the rules.

34. Conclusions The invasion of existing saline marshes will tend to isolate the headward reaches of their channel networks.

35. Conclusions NIS cordgrass can cause the headward channels of marsh drainage systems to retrogress, thus shortening and simplifying intertidal channel networks and the shoreline of the Estuary as a whole.

36. Conclusions NIS cordgrass can obstruct tidal flow and upland runoff in the upper tidal reaches of fluvial drainages.

37. Conclusions NIS Spartina tolerates fresher conditions than native Spartina, grows lower upstream than downstream in local creeks, and grows lower than other intertidal plants along local salinity gradients.

38. Conclusions The upper tidal reaches of local streams can serve as refugia for non-hybrid S. alterniflora and as sources of new recruits for continued invasion around San Francisco Bay.

39. Major Questions How low will NIS Spartina grow in Suisun and perhaps the western Delta? Will NIS Spartina meadows evolve into high marsh with native plants? How will native plants and animals adjust to the Invasion by NIS cordgrass? How will the simplification of the intertidal drainage system affect estuarine filtration and material flux?