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

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 benthlc 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), whde 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 E 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 E 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.

Does the invasive plant Elymus athericus modify fish diet in tidal salt marshes?

“…Other authors have found that fish species composition was also not affected by common reed invasion (Meyer et al, 2001) or that nekton densities are similar to those in some non-P. australis dominated wetlands (Hanson et al, 2002). Moreover, Wainright et al (2000) provided evidence that P. australis may contribute to the food chain in tidal marsh systems. Other studies showed differences, such as Posey et al (2003), who reported a slightly higher abundance of fauna in Spartina alterniflora marshes than in P. australis marshes.…”

“…Several functions of the salt marshes may therefore be modified by such an invasion: the export capacity of the salt marsh could be modified and in turn could affect mussel and oyster culture with resulting economic effects; the habitat value for Anatidae and other game birds could be changed Valéry et al, 2004). The role of invasive plants on the alteration of trophic function for larvae and juveniles of fish and crustaceans has been studied along the North American Atlantic coast with common reed Phragmites australis (see for example Able and Hagan, 2000;Wainright et al, 2000;Hanson et al, 2002). In comparison, the effects of E. athericus on trophic function for European fish have rarely been studied, although European salt marshes play important roles for fish communities.…”

Effects of anthropogenic shoreline hardening and invasion by Phragmites australis on habitat quality for juvenile blue crabs (Callinectes sapidus)

“…This higher abundance of infauna adjacent to natural marsh habitats as compared to anthropogenically altered shorelines also occurs in the York and Elizabeth-Lafayette Rivers (Seitz et al, 2006). This is most likely driven by differences in allochthonous carbon inputs between the marshes and the hardened shorelines (Quan et al, 2007;Roman and Daiber, 1989;Seitz et al, 2006;Wainright et al, 2000), though differences in water flow (which we did not measure) may also have contributed (Seitz et al, 2006). Additionally, bulkheads are often made of CCA-treated wood, which can have a negative effect on benthic organisms (Weis et al, 1998); however, in this system, riprap, not bulkheads, had the lowest density of benthic macrofauna, and its benthic community differed more from the marshes.…”

“…In the case of bulkheads, this may be due to toxic chemicals leaching from treated lumber (Weis et al, 1998). However, because marshes can supply substantial amounts of allochthonous carbon to subtidal habitats (Quan et al, 2007;Roman and Daiber, 1989;Wainright et al, 2000), replacing them with riprap or bulkheads, which cannot supply such resources, may lower macrofaunal densities (Seitz et al, 2006). Additionally, hardened shorelines are frequently associated with lower densities and smaller sizes of nektonic species Peterson et al, 2000), though not in all cases (Seitz et al, 2006).…”

Macroinvertebrate diet in intertidal seagrass and sandflat communities: A study using C, N, and S stable isotopes

“…The 8 (Peterson et al 1986;Sullivan & Moncreiff 1990;Currin et al 1995;Wainright et al 2000). The 8 13 C signatures of MPB were more depleted at the highshore location than at the mid-shore location of both sites in March.…”