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1 Gordon College, Wenham, MA
2 Massachusetts Audubon Society, Wenham, MA
* Corresponding author: dshull{at}gordon.edu
Salt marshes are important and productive ecosystems. Marsh grasses fuel coastal ecosystem production, and marsh invertebrates convert abundant decomposing marsh grasses into biomass available to higher trophic levels. Changes in climate, land use, nutrient input, and introduced species potentially threaten this ecosystem, however. An accelerated rate of sea-level rise has allowed cord grass (Spartina alterniflora) to migrate shoreward (1). Meanwhile, the high-marsh invasive reed Phragmites australis has expanded seaward, reducing the extent of indigenous high-marsh grasses such as Spartina patens and Distichlis spicata (2). Although changes in invertebrate community structure have been observed following Phragmites invasion (3), less is known about its effect on ecosystem function. How would changes in the species composition of marsh grasses affect food supply for higher trophic levels? Are all species of marsh grass equally nutritious for the invertebrates that feed on them? These questions are closely related to some of the primary goals of the Plum Island Ecosystem Long Term Ecological Research (PIE-LTER) program, which is concerned with the processing of organic matter within the salt marsh.
To address some of the above questions, our study investigated the growth rates of the salt marsh amphipod Orchestia grillus, one of the most abundant and best-studied detritivores at our study site (4), feeding on four species of marsh grass. Two sites were studied, Clubhead Creek and Greenwood Creek; the latter is nutrient enriched due to its proximity to a sewage outfall. The purpose of our research was to examine the nutritional quality of the detritus in terms of O. grillus growth rates and nitrogen and carbon content.
The four plant species studied were Spartina alterniflora (cord grass), Spartina patens (marsh hay), Distichlis spicata (spike grass), and Phragmites australis (common reed). S. alterniflora is the dominant low-marsh species, S. patens and D. spicata are native high-marsh species, and P. australis is a high-marsh invasive.
Samples of one-year-old standing dead grasses and young new-growth grasses were collected from each site from 2 to 6 June 2003. We sampled two differently aged grasses to assess how the quality of detritus changes over time. Old samples were sorted, cut, and frozen at -20 °C. Young grass samples were dried at 70 °C for 24 h, soaked in seawater for 63 h, and then dried again at 70 °C for 24 h before freezing, to simulate the formation of fresh detritus. Organisms collected from the marshes were allowed to acclimate to laboratory conditions in a large tank containing marsh wrack for 3 days. At the start of the experiment on 22 June 2003, single organisms were weighed and placed into petri dishes containing a single type of grass from each site. Ten replicates were designated for each grass-species/age/site treatment. These were placed at random over a numbered grid on a laboratory bench. Grasses were changed about once a week, and dishes were kept wet by the addition of filtered seawater from the marsh every few days. Growth data for O. grillus were collected weekly by removing, patting dry, and weighing individuals, and then returning them to their petri dish. These data were normalized by dividing measurements by the initial size of each individual. Mortality data were collected daily, but all organisms that died before the end of the experiment were eliminated from growth data sets. The experiment was allowed to run for 38 days.
Within a few weeks, nearly 80% of the organisms feeding on fresh grass were dead, and it became clear that those individuals remaining alive in these treatments were not growing. This pattern may have been due to the presence of high phenolic concentrations in the fresh marsh grass detritus. Phenolic concentrations (determined by absorbance of methanol extracts at 320 nm) were significantly higher in the fresh detritus, as demonstrated by two-factor ANOVA with grass species and age as fixed factors (F[1,24] = 331, P < 0.0001), and Orchestia mortality rates were significantly correlated with phenolic concentrations (R2 = 0.56, P = 0.01). For these reasons, we focused our analysis of growth rates on the year-old detritus.
The mean growth rates for O. grillus consuming year-old detritus varied with marsh grass species. Rates were highest for amphipods consuming D. spicata (O. grillus increased in size by 40% to 50%) and decreased in the order of S. alterniflora, S. patens, and P. australis (Fig. 1A). Growth rates were determined from the slope of organism size versus time plots by linear regression, and rates among sites and species were compared by two-factor analysis of variance, with collection site and marsh grass species as fixed factors. Data were log-transformed prior to analysis to correct for heteroscedasticity. No differences in growth rates between the two sites were found (F[1,49] = 0.088, P = 0.77). Variation in the growth rates of O. grillus on different marsh grass species was highly significant (F[3,49] = 5.7, P = 0.002), with no significant interaction (F[3,49] = 0.12, P = 0.95). Post hoc comparisons indicated that the growth rates of O. grillus on P. australis were significantly lower than those on S. alterniflora and D. spicata (Scheffé F-test, P < 0.01).
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Our results indicate that, when compared to the three native marsh plant species studied, the invasive species, P. australis, is a relatively poor food source for O. grillus. A diet of salt marsh species with higher nitrogen content, such as D. spicata, resulted in significantly higher growth rates for this organism. Furthermore, the observed seaward encroachment of P. australis is slowly pushing out D. spicata, the species that we found to have the highest nutritional quality (2). Given a continued shoreward migration of S. alterniflora due to sea-level rise and the seaward spread of P. australis, the overall food quality of marsh detritus for this invertebrate could decline. This suggests that changes in marsh grass species composition could affect higher trophic levels in this salt-marsh ecosystem.
This research was supported by an NSF-REU fellowship through the Plum Island Ecosystem LTER to AMA.
Literature Cited
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