Abstract
doi: http://dx.doi.org/10.15447/sfews.v13iss3art6 Using stable isotopes of carbon (δ13C) and nitrogen (δ15N) and mixing models, we investigated the trophic levels and carbon sources of invertebrates and fishes of a large tidal marsh in the San Francisco Estuary. Our goal was to better understand an estuarine food web comprised of native and alien species. We found the following: (1) the food web was based largely on carbon from phytoplankton and emergent-aquatic and terrestrial vegetation, but carbon from submerged aquatic vegetation and phytobenthos was also used; (2) alien species increased the complexity of the food web by altering carbon-flow pathways and by occupying trophic positions different from native species; and (3) most consumers were dietary generalists.
Highlights
The aquatic ecosystem in the San Francisco Estuary before European colonization contained a relatively small number of species and a presumably simple food web (Cohen and Carlton 1998; Orsi and Ohtsuka 1999; Kimmerer 2004)
Terrestrial vegetation was depleted in both δ13C and δ15N (3.9 ± 1.6‰); emergent aquatic vegetation was depleted in δ13C (-27.8 ± 1.0‰) but more enriched in δ15N (7.6 ± 1.2‰); submerged aquatic vegetation (SAV) was much more enriched in δ13C (- 19.4 ± 2‰) and, except for Eurasian milfoil, more enriched in δ15N (8.9 ± 3.4‰)
There were two broad groups of primary producers along the carbon isotope axis: a δ13C-depleted group comprised of phytoplankton, terrestrial vegetation, and emergent aquatic vegetation; and a δ13Cenriched group comprised of phytobenthos and SAV
Summary
The aquatic ecosystem in the San Francisco Estuary (estuary) before European colonization contained a relatively small number of species and a presumably simple food web (Cohen and Carlton 1998; Orsi and Ohtsuka 1999; Kimmerer 2004). New species continue to invade and persist in the system (Emmett et al 2002; Schroeter and Moyle 2006) Some of these alien invasions have resulted in significant community and food-web alterations. The overbite clam (Potamocorbula amurensis) has reduced pelagic productivity by feeding heavily on both phytoplankton and small zooplankters (Nichols et al 1990; Alpine and Cloern 1992; Kimmerer et al 1994; Kimmerer and Orsi 1996). Fishes such as juvenile striped bass (Morone saxatilis) that once subsisted primarily on native invertebrates
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