Eutrophication and coastal wetlands: Linking nutrient enrichment to tidal freshwater marsh ecosystem structure and function

The Chesapeake Bay watershed has been affected by human activities for over 300 years, causing an increase in nutrients entering its coastal aquatic ecosystems. Yet most of the efforts identifying the consequences of coastal eutrophication have not observed its effects on the marginal tidal wetlands of the Bay. The tidal freshwater marshes of Broad Creek and Marshyhope Creek, two tidal tributaries of the Nanticoke River (Delmarva Peninsula, USA), have been exposed to different levels of nutrient input, that appear to be adversely affecting Broad Creek. The Broad Creek watershed has had historically higher fertilizer application rates and more animal production facilities than Marshyhope Creek, both of which have been linked to increased availability of nutrients in coastal ecosystems. This study collected emergent macrophytes and aquatic macrofauna of tidal freshwater marshes in these two creeks from 2000 through 2002. Analysis of plant community composition indicated that Broad Creek had fewer plant species than Marshyhope Creek, yet greater overall plant biomass. Comparisons of nekton in the two creeks determined that there were more fish and macroinvertebrate species, individuals and biomass in Marshyhope Creek. Multivariate analysis identified strong seasonal patterns that extended across both creeks in floral and faunal distributions, but also suggested that animal abundance patterns were related to the creeks. Ecological network analysis suggested both creeks appear to be resistant to environmental stressors, but probably lack resilience. Broad Creek, however, had higher levels of total ecosystem activity than Marshyhope Creek, although ecosystem organization and development was similar between both creeks, suggesting nutrient enrichment in Broad Creek but not necessarily eutrophication. Stable isotope analysis indicated that the nitrogen circulating through Broad Creek is more enriched in 15N than Marshyhope Creek, although both creeks have enriched nitrogen signatures. Nevertheless, the high d15N in Broad Creek is indicative of larger nitrogen inputs to the system originating from animal waste. These results, however, must be tempered by an acknowledgement of the effects of a severe drought that caused an increase in salinity from October 2001 through August 2002, affecting animal and plant abundance throughout 2002