MEPS 200:77-91 (2000)  -  doi:10.3354/meps200077

Relative importance of benthic microalgae, phytoplankton and the detritus of smooth cordgrass Spartina alterniflora and the common reed Phragmites australis to brackish-marsh food webs

Sam C. Wainright1,*, Michael P. Weinstein2, Kenneth W. Able3, Carolyn A. Currin4

1Institute of Marine and Coastal Sciences, 71 Dudley Road, Rutgers University, New Brunswick, New Jersey 08901-8521, USA
2New Jersey Marine Science Consortium, Bldg. 22, Fort Hancock, New Jersey 07732, USA
3Rutgers University Marine Field Station, 800 Great Bay Blvd., Tuckerton, New Jersey 08087, USA
4NOAA Beaufort Laboratory, 101 Pivers Island Road, Beaufort, North Carolina 28516-9722, USA
*Present address: US Coast Guard Academy, Science Depart-ment, 27 Mohegan Ave., New London, Connecticut 06320, USA. E-mail:

ABSTRACT: We conducted a study to determine the trophic pathways leading to juvenile fish in 2 mesohaline tidal marshes bordering Delaware Bay. The relative roles of the major primary producers in supplying energy, ultimately, to the mummichog Fundulus heteroclitus were assessed by measuring the stable isotopic compositions of juveniles (21 to 56 mm total length, TL; most of which were young-of-the-year) and those of macrophyte vegetation, phytoplankton, and benthic microalgae at each site. We collected samples of primary producers and F. heteroclitus, the dominant fish species in this and other marshes along the east coast of the USA, in June and August 1997, at 2 study sites (upstream and downstream) within Mad Horse Creek (a Spartina alterniflora-dominated site) and Alloway Creek (a Phragmites australis-dominated site), for a total of 4 study sites. Our results indicate that F. heteroclitus production is based on a mixture of primary producers, but the mixture depends on the relative abundance of macrophytes. In S. alterniflora-dominated marshes, C and S isotope ratios indicate that F. heteroclitus production is supported by S. alterniflora production (ca 39%, presumably via detritus), while in P. australis-dominated marshes, secondary production is based upon P. australis (73%). To our knowledge, this finding provides the first evidence that P. australis may contribute to aquatic food webs in tidal marshes. Benthic microalgae also contribute to the food chain that leads to F. heteroclitus in both marsh types, while phytoplankton may be of lesser importance. Benthic microalgal biomass was lower in the P. australis-dominated system, consistent with a greater effect of shading in P. australis- versus S. alterniflora-based creek systems. Based on the difference in nitrogen isotope values between F. heteroclitus and the primary producers, the trophic level of F. heteroclitus appears to be similar in the 2 marsh types, despite the differing vegetation types. In summary, the relative roles of the primary producers in supplying energy to F. heteroclitus varies locally and, in particular, with respect to the type of marsh macrophyte vegetation.

KEY WORDS: Stable isotopes · δ13C · δ15N · δ34S · Fundulus heteroclitus · Food web

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