Impact of Fertilization on a Salt Marsh Food Web in Georgia

Abstract

We examined the response of a salt marsh food web to increases in nutrients at 19 coastal sites in Georgia. Fertilization increased the nitrogen content of the two dominant plants, Spartina alterniflora and Juncus roemerianus, indicating that added nutrients were available to and taken up by both species. Fertilization increased Spartina cover, height, and biomass and Juncus height, but led to decreases in Juncus cover and biomass. Fertilization increased abundances of herbivores (grasshoppers) and herbivore damage, but had little effect on decomposers (fungi), and no effect on detritivores (snails). In the laboratory, herbivores and detritivores did not show a feeding preference for fertilized versus control plants of either species, nor did detritivores grow more rapidly on fertilized versus control plants, suggesting that changes in herbivore abundance in the field were driven more by plant size or appearance than by plant nutritional quality.Community patterns in control plots varied predictably among sites (i.e., 17 of 20 regression models examining variation in biological variables across sites were significant), but variation in the effects of fertilization across sites could not be easily predicted (i.e., only 6 of 20 models were significant). Natural variation among sites was typically similar or greater than impacts of fertilization when both were assessed using the coefficient of variation. Overall, these results suggest that eutrophication of salt marshes is likely to have stronger impacts on plants and herbivores than on decomposers and detritivores, and that impacts at any particular site might be hard to distinguish from natural variation among sites.

High-marsh invertebrates are susceptible to eutrophication

“…My results contrast with those of McFarlin et al (2008) who found no fertilization effects on Melampus densities in a Georgia (USA) salt marsh. The authors noted that their plot size (1 m 2 ) may have been insufficient to elicit a strong aggregative response (McFarlin et al 2008).…”

Invertebrates in field margins: taxonomic group diversity and functional group abundance in relation to age

“…Biodivers Conserv (2010) 19:3255-3268 3263 biomass being available to provide food for more individuals (e.g., McFarlin et al 2008) and more shelter against predators, which appear to be less abundant in these situations anyway (this study; Harvey et al 2008). More than half the herbivores counted were Gastropoda, but Cicadellidae and Aphidoidea were also caught in high numbers.…”

Cordgrass Biomass in Salt Marshes

“…In addition, drought markedly increased the total biomass of S. alterniflora and the live biomass of S. patens, perhaps by alleviating waterlogging of sediments (Charles & Dukes, 2009). On the other hand, Spartina biomass www.intechopen.com Biomass 8 also depends on nutrient availability, especially nitrogen, as it has been reported for S. alterniflora (Darby & Turner, 2008a;McFarlin et al, 2008). Biotic direct and indirect interactions also control biomass accumulation of Spartina populations.…”

“…Among cordgrasses with a "guerrilla" growth-form, S. alterniflora in western Atlantic low salt marshes accumulates between 100 and 1100 g DW m -2 . Changes in S. alterniflora biomass between populations are related mainly with its short and tall forms, varying its shoot height between 20 and 140 cm, also with a highly variable shoot density that changes markedly between 100 and 4000 shoot m -2 (Craft et al, 1999(Craft et al, , 2002(Craft et al, , 2003Proffitt et al, 2005;Culbertson et al, 2008;Darby & Turner 2008b;McFarlin et al, 2008;Sala et al, 2008;Tyrrell et al, 2008;Buchsbaum et al, 2009;Gonzalez Trilla et al, 2009;Krull & Craft, 2009;Michel et al, 2009;Holdredge et al, 2010). One year after invading Chinese marshes, S. alterniflora accumulated ca.…”