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Gordon College, Wenham, Massachusetts 01984
1 Department of Biology, Gordon College, Wenham, MA.
2 Massachusetts Audubon Society, Wenham, MA.
Marsh plants may affect the structure of epibenthic communities in a variety of ways. They might reduce temperature stress on organisms by shading, provide refuge from predators, increase the rate of organic matter accumulation at the sediment surface, and serve as a food source. Thus, salt marsh plants might control the spatial distributions of epibenthic invertebrates within the marsh. An ongoing "experiment" on the impact of the periodic removal of plants has been carried out by New England farmers who have been haying some salt marshes for over 300 years. Earlier research by the Plum Island Estuary Long-Term Ecological Research project (PIE-LTER) has shown that haying causes a short-term increase in the growth of benthic algae (1), causes a shift in the diet of some marsh invertebrates to a greater percentage of algae (2), and results in a change in plant species composition (PIE-LTER, unpubl. results).
To determine the short- and long-term effects of haying on the epibenthic community and to identify the principal variables controlling the distribution of epibenthic invertebrates, we carried out two sets of experiments: a marsh-grass-removal experiment to mimic the effects of haying and a survey of invertebrates in areas of the marsh subjected to different haying schedules. Our work was conducted in the Plum Island Sound estuary in Rowley, Massachusetts, from June to August 2002.
For our marsh-grass-removal experiment, we clipped and removed the vegetation from three randomly selected circular sites of 3-m radius along a 50-m transect in a marsh that is hayed roughly every other year. Before clipping we counted organisms in four 0.25-m2 sampling areas, located 1.5 m from the center of the circle and equidistant from each other, within each of the three sites. We sampled the same four areas within each site immediately before clipping, and 1 and 4 days after clipping. Differences in abundances of organisms before and after clipping were evaluated by the Kruskal-Wallis test with time of sampling as treatment. Because differences among circular hayed sites were not significantly different, we pooled replicates from each site before our analysis.
For our surveys of invertebrates in areas subjected to different intensities of haying, we examined randomly selected sites in recently hayed and reference marshes along 12 previously established transects for which GPS elevation data were available. We sampled 40 sites between 1 and 3 July, and 40 more between 16 and 18 July. At each site, we visually estimated percent cover of different plant species. A count of organisms was made after clipping all vegetation from 0.25-m2 plots. A 177-cm2 plant sample was collected, live biomass was separated from standing dead biomass and weighed separately to determine above-ground biomass. Commonly encountered plants included Spartina patens, Spartina alterniflora, Distichlis spicata, Salicornia europaea, and Juncus gerardi. To evaluate the effect of plant cover on predation rate, we tethered amphipods, Orchestia grillus, to thin-diameter line and deployed 5 per site at most sites for 24 h before clipping. We used stepwise multiple regression to describe the relationship between measured environmental variables and abundance of epifauna for the 16–18 July sampling period (criteria P = 0.1 to accept, P = 0.25 to reject). Environmental variables used in stepwise multiple regression were haying frequency, elevation, temperature, humidity, dried live plant biomass, dried standing dead, total dried weight, percent live, percent dead, percent bare, percent S. alterniflora, percent S. patens, percent D. spicata, percent S. europaea, percent Atriplex patula, percent Triglochin maritima, percent Iva frutescens, proportion tethered live amphipods, and proportion eaten tethered amphipods. Collinearity among environmental variables was assessed by examining the variance inflation factor for each included variable. All variance inflation factors were less than 2, which indicated that collinearity among independent variables did not significantly affect our results. A few sampling sites were omitted from the regression analysis due to missing data. Analyses were performed using SPSS software.
The amphipod Orchestia grillus, leafhoppers (Order Homoptera), and the isopod Philoscia vittata were the most common invertebrates encountered in the marsh. In the marsh-grass-removal experiment we found a significant decrease in abundance of leafhoppers (
2[2] = 26.9, P < 0.0001), as well as spiders (Order Araneae) (2[2] = 8.72, P = 0.013), greenhead fly larvae (Tabanus nigrovittatus) (2[2] = 14.23, P = 0.001), and O. grillus (2[2] = 26.9, P < 0.0001) after clipping (Fig. 1). P. vittata was not significantly affected (2[2] = 3.32, P = 0.19, Fig. 1).
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Spiders were positively related to dead above-ground biomass, and percent bare space (F[2,34] = 8.25, P = 0.001), which is consistent with their reduced abundance in our experimental marsh-grass-removal treatments. The measured environmental variables related to the abundance of T. nigrovittatus larvae were percent Juncus gerardi, sediment surface temperature, and proportion of tethered amphipods eaten (F[3,33] = 8.04, P < 0.0001). The abundance of T. nigrovittatus larvae was negatively correlated with sediment surface temperature. It is likely that temperatures in the newly clipped patches were higher than in the unclipped marsh. Abundances of T. nigrovittatus larvae were also positively correlated with the proportion of tethered amphipods eaten, which is consistent with our observations of predation by T. nigrovittatus larvae on tethered O. grillus specimens.
O. grillus abundance was not significantly correlated with any variables, and the amphipod was widely distributed throughout the marsh. The lack of a significant correlation with aboveground biomass or haying frequency is noteworthy since the amphipods experienced a strong drop in abundance in the marsh-grass haying experiment. This finding suggests that the long-term effects of haying on O. grillus abundances are much less severe than the immediate response. Many invertebrate species may experience a strong reduction in abundance just after haying; however, many environmental variables in addition to haying had an effect on patterns of invertebrate abundance in the long-term.
We thank Carl Noblitt, our research assistant. This research was supported by an REU fellowship through the Plum Island Sound LTER program to JPL.
Literature Cited
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