The pace of ecosystem development of constructed Spartina alterniflora marshes

Ecological attributes were measured along a chronosequence of 1- to 28-yr-old, constructed Spartina alterniflora marshes to identify trajectories and rates of ecosystem development of wetland structure and function. Attributes related to biological productivity and diversity (Spartina, epiphytic and sediment algae, benthic invertebrates), soil development (sediment deposition, organic C, N, P, organic matter quality), and microbial processes (C mineralization) were compared among eight constructed marshes and eight paired natural reference marshes. Most ecological attributes developed in a predictable manner over time, and most achieved equivalence to natural marshes 5-15 yr after marsh construction. An exception was soil organic C and N pools (0-30 cm) that, after 28 yr, were significantly lower in constructed marshes. Development of habitat structure (Spartina stem height and density) and biodiversity (algae and invertebrates) developed concurrently with functional characteristics such as biomass, chlorophyll a, and invertebrate density. Processes related to hydrology, sediment deposition and soil C and N accumulation, developed almost instantaneously with the establishment of Spartina, and young (1- to 3-yr-old), constructed marshes trapped sediment and sequestered N at higher rates than comparable reference marshes. Development of heterotrophic activity (C mineralization, invertebrate density) was strongly linked to surface (0-10 cm) soil organic C content. Ecosystem development of constructed (and natural) salt marshes depended on a minimum of 100 9 N/m(2) (0.05-0.1% N) to support emergent vegetation and 1000 g C/m(2) (0.5-1% C) to sustain the heterotrophic community.