California Sea Grant College Program

Vascular plants structure wetland ecosystems. To examine mechanisms behind their influence, plants were studied under different scenarios of change: experimental manipulation of cover, invasion, and response to flushing regimes. I tested the hypothesis that wetland plants alter benthic communities through modification of abiotic factors, with cascading effects on microalgae and invertebrate communities. Major plant effects were observed in all systems studied, but the magnitude of, mechanisms behind, and exact consequences of plant alterations depended on the particular combination of physical and biological stresses within the habitat along the marine to terrestrial continuum. Manipulation of plant cover and light regime, combined with natural abundance isotope studies in a mid-elevation salt marsh of Mission Bay, CA revealed how two dominant plants, Spartina foliosa and Sarcocornia pacifica (formally Salicornia virginica ), regulate light, temperature, and moisture, thereby influencing the abundance of benthic diatoms and the relative importance of microalgal-feeding invertebrates. Tamarisk ( Tamarix spp.), normally a freshwater invader that has recently colonized the salt marsh in Tijuana Estuary, was studied in 3 marsh zones with mensurative benthic assessment techniques and stable isotope enrichment experiments. Results demonstrate that this plant has (1) impacted the mid-marsh environment most, (2) accelerated salt marsh succession towards a more terrestrial environment by creating drier, less organic-rich sediments and an altered macroinvertebrate community (decreased densities of gastropods and marine oligochaetes, more insects) and (3) entered the food web through a broad range of invertebrate consumers. Using similar approaches, the ephemeral seagrass, Ruppia maritima , abundant in lagoons during periods of inlet closure, was also shown to play a key trophic role in structuring wetlands in southern California. Results of faunal characterization and isotope enrichment studies within San Dieguito Lagoon suggest that food webs in these environments are driven by detrital and epiphytic production. Increased representation of detritivores in R. maritima habitats relative to unvegetated mudflat appears linked to animal feeding preferences and the ability of consumers to utilize R. maritima . In summary, this research developed several experimental methods by which to isolate structuring mechanisms of vascular plants in wetlands and allowed us to make generalizations across abiotic gradients in salt marsh ecosystems.