Anthropogenic modification of New England salt marsh landscapes

Abstract

Salt marshes play a critical role in the ecology and geology of wave-protected shorelines in the Western Atlantic, but as many as 80% of the marshes that once occurred in New England have already been lost to human development. Here we present data that suggest that the remaining salt marshes in southern New England are being rapidly degraded by shoreline development and eutrophication. On the seaward border of these marshes, nitrogen eutrophication stimulated by local shoreline development is shifting the competitive balance among marsh plants by releasing plants from nutrient competition. This shift is leading to the displacement of natural high marsh plants by low marsh cordgrass. On the terrestrial border of these same marshes, shoreline development is also precipitating the invasion of the common reed, Phragmites, by means of nitrogen eutrophication caused by the removal of the woody vegetation buffer between terrestrial and salt marsh communities. As a consequence of these human impacts, traditional salt marsh plant communities and the plants and animals that are dependent on these habitats are being displaced by monocultures of weedy species. E lucidating the causes and consequences of human modification of natural ecosystems is one of the most pressing ecological issues of our times (1). Although the direct impacts of habitat destruction are usually obvious, the indirect effects of many human activities, such as nitrogen loading, have been more difficult to detect, but are likely just as pervasive and critical to understand (2). In this paper, we show how a mechanistic understanding of the organization of salt marsh communities reveals that these ecologically and economically important habitats (3) are currently being dramatically degraded by localized shoreline development and human-induced nitrogen enrichment.Salt marshes have attracted the attention of scientists because their simple structure makes them amenable for studying the mechanisms that generate ecosystem and community patterns (4-6). New England salt marsh plant communities have a striking vertical zonation (Fig. 1) that has long interested researchers (7, 8). The cordgrass Spartina alterniflora dominates low marsh elevations that are flooded daily by tides as a dense, clonally propagated monoculture. On the seaward border of the high marsh, cordgrass is replaced by marsh hay, Spartina patens, whereas the terrestrial border of the high marsh is dominated by a monoculture of the black rush, Juncus gerardi. A band of the shrub Iva frutescens typically demarcates the terrestrial border of these marshes. A number of less-abundant halophytic forbs commonly coexist with the high-marsh dominants (9).This striking spatial segregation of plants in salt marshes is the product of both plant competition and the strong, physical gradients characteristic of these habitats. Physical stress on plants in salt marshes (e.g., water-logging and salinity) generally decreases with increasing tidal height (10, 11), and there is an inverse relationship between the ...

Nitrogen and phosphorus enrichment alter the composition of ammonia-oxidizing bacteria in salt marsh sediments

“…In New England, nearly 80% of salt marshes in this region have been altered dramatically by human activities (Bromberg and Bertness, 2005). Field experiments in this region show that nitrogen enrichment alters plant composition (Levine et al, 1998;Emery et al, 2001) and reduces the diversity of native plant species while promoting the invasion of non-native plant species (Bertness et al, 2002).…”

Nitrogen Uptake by Native and Invasive Temperate Coastal Macrophytes: Importance of Dissolved Organic Nitrogen

“…Over the past century, the common reed, Phragmites australis, (hereafter Phragmites) has become pervasive in Atlantic coast tidal marshes, displacing Spartina spp. and other intertidal species; its unprecedented expansion is a concern for wetland ecologists working in both tidal and non-tidal wetlands due to changes in faunal use (Meyerson et al 2000a, Angradi et al 2001, Buchsbaum et al 2006, biogeochemical cycles (Windham and Lathrop 1999, Meyerson et al 2000b, Windham and Ehrenfeld 2003, and reductions in species richness and biodiversity (Chambers et al 1999, Bertness et al 2002. Phragmites is thought to be one of the world's most widely distributed angiosperms (Holm et al 1977, Mal andNarine 2004).…”

Relationship between anthropogenic sewage discharge, marsh structure and bird assemblages in an SW Atlantic saltmarsh

“…Salt marshes could be vulnerable if the nutrient delivery is discharged without 368 management, given that it can ultimately degrade the structure of the marsh itself 369 (Levine et al, 1998;Bertness et al, 2002;Crain, 2007). The sewage discharge into the 370 Bahía Blanca estuary is being done without any treatment.…”